Ondřej Krýza
email: kryza@natur.cuni.cz

Ondrej Lexa

Karel Schulmann

Alexandra Guy

Denis Gapais

John Cosgrove

Wenjiao Xiao

Oroclinal buckling and associated lithospheric-scale material flow -insights from physical modelling: Implication for the Mongol-Hingan orocline

Keywords: Analogue modelling, Mongolian orocline, Hingan orocline, oroclinal buckling

Oroclinal buckling model of a vertical belt separating

Introduction

The concept of orocline formation was described almost seventy years ago by [START_REF] Carey | The orocline concept in geotectonics-Part I[END_REF] as a product of secondary curvature of the originally linear orogen. The secondary curvature results from either passive oroclinal bending (e.g. [START_REF] Rosenbaum | Formation of arcuate orogenic belts in the western Mediterranean region[END_REF]Rosenbaum, 2012;[START_REF] Menant | 3D numerical modeling of mantle flow, crustal dynamics and magma genesis associated with slab roll-back and tearing: The eastern Mediterranean case[END_REF], active oroclinal bending or oroclinal buckling [START_REF] Edel | Palaeomagnetic and structural constraints on 90° anticlockwise rotation in SW Mongolia during the Permo-Triassic: implications for Altaid oroclinal bending[END_REF][START_REF] Lehmann | Structural Constraints of the Evolution of the Central Asian Orogenic Belt in sw Mongolia[END_REF][START_REF] Rosenbaum | Geodynamics of oroclinal bending: Insights from the Mediterranean[END_REF]. The passive bending mode is controlled by the geometry of the subducting plate such as curvatures produced by variations in rollback velocity. In contrast, the active oroclinal bending is controlled by horizontal indentation perpendicular to the subduction/accretionary orogen (e.g. the entry of an oceanic plateau or a continental block into a subduction zone), while oroclinal buckling results from buckling instability during the orogen-parallel shortening of a linear orogenic belt. The latter mechanism is commonly related to the advance of continental blocks parallel to the belt as exemplified by the Paleozoic Iberian, Kazakhstan and Mongolian oroclines (Pastor-Galán et al. 2012;Sengör et al., 1993;[START_REF] Lehmann | Structural Constraints of the Evolution of the Central Asian Orogenic Belt in sw Mongolia[END_REF]. Various aspects such as paleomagnetism, structural evolution and sedimentary records as well as mechanical processes related to orogen-parallel shortening are best-described and understood for the Iberian orocline [START_REF] Gutiérrez-Alonso | Buckling an orogen: The Cantabrian Orocline[END_REF][START_REF] Johnston | Oroclines: Thick and thin[END_REF][START_REF] Weil | Kinematic constraints on buckling a lithospheric-scale orocline along the northern margin of Gondwana: A geologic synthesis[END_REF][START_REF] Shaw | Orocline formation at the core of Pangea: A structural study of the Cantabrian orocline, NW Iberian Massif[END_REF]Shaw andJohnston, 2016a, 2016b). In contrast, processes associated with the formation of the Kazakhstan and Mongolian oroclinal systems are less constrained and consequently, mechanisms of orogen-parallel shortening of those two systems remain poorly understood. These oroclines constitute a major part of the 13,000 km long Central Asian Orogenic Belt (CAOB), which formed from Neo-Proterozoic to Mesozoic times [START_REF] Şengör | Evolution of the Altaid tectonic collage and Palaeozoic crustal growth in Eurasia[END_REF][START_REF] Wilhem | The Altaids of Central Asia: a tectonic and evolutionary innovative review[END_REF]. Both oroclines are composed of Precambrian ribbon J o u r n a l P r e -p r o o f continents surrounded by wide accretionary complexes and magmatic arcs (Fig. 1). Their origin has recently been hotly debated and suggestions include the buckling and strike-slip imbrication of two Late Proterozoic to Early Paleozoic intra-oceanic arcs [START_REF] Şengör | Evolution of the Altaid tectonic collage and Palaeozoic crustal growth in Eurasia[END_REF][START_REF] Şengör | Turkic-type orogeny and its role in the making of the continental crust[END_REF], the passive oroclinal bending related to the continuous roll-back of the Paleoasian ocean [START_REF] Xiao | Late Paleozoic to early Triassic multiple roll-back and oroclinal bending of the Mongolia collage in Central Asia[END_REF] and the pure buckling of a long ribbonlike system [START_REF] Lehmann | Structural Constraints of the Evolution of the Central Asian Orogenic Belt in sw Mongolia[END_REF]. While the tectonic evolution of the Kazakhstan system remains poorly constrained, recent investigations of the Mongolian orocline have allowed its deep crustal structure to be defined [START_REF] Guy | Late Paleozoic-Mesozoic tectonic evolution of the Trans-Altai and South Gobi Zones in southern Mongolia based on structural and geochronological data[END_REF]2015;2020) and the global architecture of the whole system prior its orogen-parallel shortening to be determined [START_REF] Jiang | Anatexis of accretionary wedge, Pacific-type magmatism, and formation of vertically stratified continental crust in the Altai Orogenic Belt[END_REF][START_REF] Yakubchuk | Evolution of the Central Asian Orogenic Supercollage since late Neoproterozoic revised again[END_REF]. A new model of buckling of the whole linear system has been proposed (Lehmann et al., 2010;Guy et al., 2020) and detailed structural investigations showing geochronologically constrained orthogonal superposition of deformation fabrics, have been presented (e.g. [START_REF] Lehmann | Structural Constraints of the Evolution of the Central Asian Orogenic Belt in sw Mongolia[END_REF][START_REF] Guy | Late Paleozoic-Mesozoic tectonic evolution of the Trans-Altai and South Gobi Zones in southern Mongolia based on structural and geochronological data[END_REF][START_REF] Zhang | Distinct deformational history of two contrasting tectonic domains in the Chinese Altai: Their significance in understanding accretionary orogenic process[END_REF][START_REF] Jiang | Structural and Geochronological Constraints on Devonian Suprasubduction Tectonic Switching and Permian Collisional Dynamics in the Chinese Altai, Central Asia[END_REF] In addition, [START_REF] Yakubchuk | Architecture and mineral deposit settings of the Altaid orogenic collage: a revised model[END_REF][START_REF] Yakubchuk | Re-deciphering the tectonic jigsaw puzzle of northern Eurasia[END_REF] proposed that the whole region between the Siberian and North China cratons can be defined by a double oroclinal fold based on the arcuate distribution of basement blocks which connect Precambrian ribbons in Mongolia with those in the far-east of China (Fig. 1a). This author thus defines the Mongolian orocline in the north and the Hingan orocline in the SE, both cored by Paleozoic oceanic units. Later, [START_REF] Yakubchuk | Evolution of the Central Asian Orogenic Supercollage since late Neoproterozoic revised again[END_REF] proposed a plate-tectonic scenario explaining a kinematic model related to the formation of this full wavelength oroclinal bend during Late Paleozoic amalgamation of Pangea. Based on Yakubchuk's approach, we used the IUGS geological map [START_REF] Parfenov | Preliminary northeast Asia geodynamics map[END_REF] of the Precambrian blocks and Paleozoic units of eastern central Asia, to draw a possible full oroclinal bend called here the Mongol-Hingan orocline (Fig. 1b). All these recent findings allowed the setting up of a new analogue model which satisfied the geological, structural, geophysical and paleomagnetic constraints of the Mongolian orocline acquired during the last two decades [START_REF] Lehmann | Structural Constraints of the Evolution of the Central Asian Orogenic Belt in sw Mongolia[END_REF][START_REF] Guy | Late Paleozoic-Mesozoic tectonic evolution of the Trans-Altai and South Gobi Zones in southern Mongolia based on structural and geochronological data[END_REF]Guy et al., , 2020;;[START_REF] Edel | Palaeomagnetic and structural constraints on 90° anticlockwise rotation in SW Mongolia during the Permo-Triassic: implications for Altaid oroclinal bending[END_REF] as well as the full wave-length Mongol-Hingan oroclinal structure proposed by [START_REF] Yakubchuk | Architecture and mineral deposit settings of the Altaid orogenic collage: a revised model[END_REF][START_REF] Yakubchuk | Evolution of the Central Asian Orogenic Supercollage since late Neoproterozoic revised again[END_REF]. Our analogue model follows the existing studies of Pastor-Galán et al. (2012) and [START_REF] Boutelier | Buckling of orogens: insights from analogue modelling[END_REF] who investigated the development and distribution of deformation structures of the upper crust, ductile deformation of the deep lithosphere and kinematic aspects related to the relative contribution of bending and buckling. The primary aim of our model is the analysis of the three dimensional lower crustal fluxes and surface deformation governed by oroclinal buckling of a vertical plate.

Geodynamic background

The Mongolian orocline can be divided into five principal zones (Fig. 1) which can be described from the interior of the orocline to its external parts as follows: 1) Silurian to Carboniferous oceanic sediments of the Mongol Okhotsk ocean at the orocline core called here the Mongol-Okhotsk Zone, 2) the Mongolian Precambrian continental blocks, 3) the Late Proterozoic accretionary complexes intruded by Cambrian-Ordovician arc forming the Lake Zone, 4) the Ordovician accretionary wedge (Mongol-Altai Zone) and 5) the Devonian to Carboniferous volcano-sedimentary sequences imbricated with Ordovician to Devonian intra-oceanic ophiolites of the East Junggar in NW China and Trans-Altai Zone in south Mongolia (East Junggar-Trans Altai Zone). In the south the South Gobi Zone occurs, formed by Precambrian basement [START_REF] Rojas-Agramonte | Detrital and xenocrystic zircon ages from Neoproterozoic to Palaeozoic arc terranes of Mongolia: significance for the origin of crustal fragments in the Central Asian Orogenic Belt[END_REF]) and its early Paleozoic cover which according to [START_REF] Yakubchuk | Architecture and mineral deposit settings of the Altaid orogenic collage: a revised model[END_REF] represents the southern limb of the Hingan orocline.

In order to understand the pre-buckling architecture of this orocline, we need to briefly characterize the principal orogenic events responsible for the accretion of oceanic complexes J o u r n a l P r e -p r o o f onto Precambrian blocks forming an ca. 3000 km long linear belt (Fig. 2a). Accretion started during the Early Cambrian obduction of Late Proterozoic intra-oceanic arcs onto the Mongolian Precambrian l blocks [START_REF] Štípská | Early Cambrian eclogites in SW Mongolia: evidence that the Palaeo-Asian Ocean suture extends further east than expected[END_REF][START_REF] Buriánek | Geochemical and geochronological constraints on distinct Early-Neoproterozoic and Cambrian accretionary events along southern margin of the Baydrag Continent in western Mongolia[END_REF], followed by the formation of a Cambro-Ordovician Japan type arc above the subducting Panthalasian oceanic plate [START_REF] Janoušek | Cambrian-Ordovician magmatism of the Ikh-Mongol Arc System exemplified by the Khantaishir Magmatic Complex (Lake Zone, south-central Mongolia)[END_REF]. Simultaneously an up to 2000 km long and several hundred kilometres wide volcano-sedimentary accretionary wedge of the Mongol-Altai Zone) developed outboard of the continental blocks (Barguzin -B, Tuva-Mongolia -TM, Dzavkhan-Baydrag -DB, Erguna -E) and the magmatic arc [START_REF] Jiang | Neoproterozoic-Early Paleozoic Peri-Pacific Accretionary Evolution of the Mongolian Collage System: Insights from Geochemical and U-Pb Zircon Data from the Ordovician Sedimentary Wedge in the Mongolian Altai[END_REF] together with a wide supra-subduction oceanic domain formed by oceanic arcs and back arcs of the East Junggar-Trans Altai Zone. This supra-subduction system continued to form the accretionary wedge until the end of the Silurian [START_REF] Soejono | Early Palaeozoic sedimentary record and provenance of flysch sequences in the Hovd Zone (western Mongolia): Implications for the geodynamic evolution of the Altai accretionary wedge system[END_REF].

The Mongol-Okhotsk Ocean started to open in the Late Silurian in the back-arc position to the ongoing subduction of the main oceanic system [START_REF] Wilhem | The Altaids of Central Asia: a tectonic and evolutionary innovative review[END_REF]Fig. 2b). During the Early Devonian, the Mongol Altai Zone accretionary sediments and volcanics were metamorphosed and underwent partial melting thereby producing granulitic-migmatitic lower crust accompanied by the massive intrusion of granitoids in the middle and upper crust [START_REF] Jiang | Anatexis of accretionary wedge, Pacific-type magmatism, and formation of vertically stratified continental crust in the Altai Orogenic Belt[END_REF]. These anomalous thermal conditions result either from the subduction of an active spreading oceanic ridge (e.g., [START_REF] Sun | Early Paleozoic ridge subduction in the Chinese Altai: insight from the abrupt change in zircon Hf isotopic compositions[END_REF][START_REF] Cai | Geochronological and geochemical study of mafic dykes from the northwest Chinese Altai: Implications for petrogenesis and tectonic evolution[END_REF] or the elevation of the asthenosphere due to Pacific-type back-arc spreading above a retreating oceanic plate (e.g., [START_REF] Jiang | Anatexis of accretionary wedge, Pacific-type magmatism, and formation of vertically stratified continental crust in the Altai Orogenic Belt[END_REF][START_REF] Jiang | Structural and Geochronological Constraints on Devonian Suprasubduction Tectonic Switching and Permian Collisional Dynamics in the Chinese Altai, Central Asia[END_REF]. During the Early Carboniferous, the mobile and partially molten Mongol-Altai Zone lower crust was redistributed beneath the East Junggar-Trans Altai Zone supra-subduction oceanic crust in the form of a 20 km thick felsic to intermediate relaminant [START_REF] Guy | Geophysical and J o u r n a l P r e -p r o o f Journal Pre-proof geochemical nature of relaminated arc-derived lower crust underneath oceanic domain in southern Mongolia[END_REF](Guy et al., , 2020;;[START_REF] Nguyen | Geochemistry and geochronology of Mississippian volcanic rocks from SW Mongolia: Implications for terrane subdivision and magmatic arc activity in the Trans-Altai Zone[END_REF]. At this stage, the pre-buckling architecture of the Mongolian collage system consisted of 1) the Mongol-Okhotsk oceanic subduction dipping beneath the 2) Mongolian Precambrian blocks forming dismembered continental ribbon attached to the 3) metamorphosed wedge sediments of the Mongol-Altai Zone relaminating the 4) oceanic East Junggar-Trans Altai Zone crust in the south (schematic cross-section in Fig. 2b).

During the Early Permian the whole Mongolian system suffered a major clockwise rotation [START_REF] Cocks | Siberia, the wandering northern terrane, and its changing geography through the Palaeozoic[END_REF][START_REF] Xiao | A tale of amalgamation of three collage systems in the Permian-Middle Triassic in Central Asia: Oroclines, sutures and terminal accretion[END_REF] at the end of which the Mongolian accretionary system formed a north-south oriented belt located between the subducting Mongol-Okhotsk Ocean in the east and the retreating "Paleo-Asian" Ocean in the west (e.g. [START_REF] Lehmann | Structural Constraints of the Evolution of the Central Asian Orogenic Belt in sw Mongolia[END_REF][START_REF] Wilhem | The Altaids of Central Asia: a tectonic and evolutionary innovative review[END_REF]. Theoretically, the (Buryea, Jiamisu, Khanka (BJK) Precambrian blocks in far east of China and the South Gobi (SG) basement ribbon, constituted a southern continuation of the future Mongolian-Hingan oroclinal system (Fig. 2d). The rapid northward movement of the Tarim-North China collage during the Permo-Triassic (e.g. [START_REF] Xiao | Late Paleozoic to early Triassic multiple roll-back and oroclinal bending of the Mongolia collage in Central Asia[END_REF][START_REF] Edel | Palaeomagnetic and structural constraints on 90° anticlockwise rotation in SW Mongolia during the Permo-Triassic: implications for Altaid oroclinal bending[END_REF] was responsible for the squeezing and buckling of this arcuate linear system about a sub-vertical axis between the northerly Siberian Craton and the southerly Tarim-North China collage (Fig. 2b-2c). During the closure of the Mongolian orocline, the Mongol-Okhotsk oceanic plate subducted both towards the south and north while the Mongol-Okhotsk Ocean closed in a scissor-like manner [START_REF] Donskaya | The Late Triassic Kataev volcanoplutonic association in western Transbaikalia, a fragment of the active continental margin of the Mongol-Okhotsk Ocean[END_REF][START_REF] Kelty | Detrital-zircon geochronology of Paleozoic sedimentary rocks in the Hangay-Hentey basin, north-central Mongolia: implications for the tectonic evolution of the Mongol-Okhotsk Ocean in central Asia[END_REF]. Subsequently, the Mongol-Hingan oroclinal system was affected by post-buckle flattening leading to the formation of deformation zones similar to axial planar crustal-scale cleavage (e.g. [START_REF] Lehmann | Structural Constraints of the Evolution of the Central Asian Orogenic Belt in sw Mongolia[END_REF]Guy et al., 2014a;Edel et al., 2014) (Fig. 2c-2d). This event was also accompanied by development of upright folds with horizontal axes in the Mongol-Altai Zone and the relaminated oceanic crust of the East Junggar-Trans Altai Zone (Guy et al., 2020).

J o u r n a l P r e -p r o o f

In order to understand the 3D deformation of the Mongol-Hingan oroclinal system, we designed a model setup that resembles the orogenic system described above prior to the oroclinal buckling (Fig. 2). The model setup corresponds to the Permo-Triassic orogenparallel shortening of the linear ribbon-like string of continental blocks and attached accretionary system between the Siberian and North China cratonic jaws [START_REF] Edel | Palaeomagnetic and structural constraints on 90° anticlockwise rotation in SW Mongolia during the Permo-Triassic: implications for Altaid oroclinal bending[END_REF]. The evolution of the system was simulated by means of eight experiments allowing analysis of material transfers for individual time steps and a better understanding of the final internal architecture and structural pattern of the Mongolian orocline.

In our experiments we used Fontainebleau sand (to model the brittle behavior of the upper crust), silicone putties (to represent the ductile crust and upper mantle segments) and plasticine (the buckling of which represented the deformation of the continental ribbon) with rheological properties similar to those of [START_REF] Cagnard | Modes of thickening of analogue weak lithospheres[END_REF] and [START_REF] Brun | Deformation of the continental lithosphere: Insights from brittle-ductile models[END_REF]. The rheological parameters of individual model layers are described in Table 1 and shown in Fig. 3. Our model does not contain the asthenosphere due to significant difficulties during model preparation related to the need to introduce complex horizontal and vertical layering in different domains of the model. The effect of this simplification is discussed later.

All experiments were conducted in the same analogue modelling box. The modelled domain is divided in three subdomains shortened between a mobile piston and a stationary wall representing the Siberian and North China cratons. The subdomain representing the Mongol-Okhotsk Ocean (MO) has a typical structure for an oceanic crust characterized by a thick segment of the upper mantle and relatively thin crust represented by silicon putty overlain by Fontainebleau sand, respectively. The subdomain corresponding to a Precambrian continental ribbon and attached accretionary wedge of the Mongol Altai Zone (CRAW) is represented by vertical plasticine and silicon putty blocks, respectively (Fig 3). It is this curved vertical plasticine layer which represents a controlling layer of the buckle folding. The EJTA subdomain represents an oceanic upper crust of the East Junggar-Trans Altai Zone underplated by a relaminant formed by the Mongol-Altai Zone metamorphosed rocks (Fig. 3b). It consists of silicone putty representing the high viscosity upper mantle and the low viscosity middle and lower crust (relaminant), overlain by Fontainebleau sand (brittle upper crust). Both the CRAW and EJTA subdomains are underlain by silicon putty representing the upper mantle. The EJTA subdomain is separated from the sandbox wall by silicone putty to minimize the boundary condition effects imposed by the immobile wall of the apparatus. We also reduced the friction between the modelling box walls and the model materials by using petroleum jelly. Model geometry, the mechanical properties of the materials and the rheological profiles are shown in Fig. 3. In the model the fold cored by the MO subdomain is called the Mongolian fold, and the fold cored by the EJTA subdomain is called Hingan fold. This terminology is used to compare the structural pattern and deep crustal architecture of both Mongolian and Hingan oroclines.

Model scaling

The scaling between natural and laboratory conditions constrain the size of the modelled domain, the selection of the specific analogue materials used and the strain-rate (e.g. [START_REF] Brun | Deformation of the continental lithosphere: Insights from brittle-ductile models[END_REF][START_REF] Cobbold | Gum rosin (colophony): A suitable material for thermomechanical modeling of the lithosphere[END_REF][START_REF] Davy | Indentation tectonics in nature and experiment. 1. Experiments scaled for gravity[END_REF][START_REF] Davy | Experiments on shortening of a 4-layer model of the continental lithosphere[END_REF][START_REF] Hubbert | Theory of scale models as applied to the study of geological structures[END_REF][START_REF] Ramberg | Gravity, Deformation and the Earth Crust[END_REF]. Our analogue experiments reproduce the indentation and shortening of lithospheric domains associated with faulting and surface elevation in the upper crust, the coupling and flow of the lower crust, and the decoupling of the lithospheric mantle. Based on previous studies (e.g. [START_REF] Brun | Deformation of the continental lithosphere: Insights from brittle-ductile models[END_REF][START_REF] Cagnard | Modes of thickening of analogue weak lithospheres[END_REF][START_REF] Gapais | Mountain building and exhumation processes through time: inferences from nature and models[END_REF], we set ) the ratio varies from 1.2 • 10 -17 to 3.8 • 10 -17 (the scaling factors for all layers are displayed in Tab. 1). From these scaling factors it is possible to obtain the factors for stress, time, strain-rate and velocity using the following relationships:

𝜎 * = 𝑃 * • 𝐿 * • 𝐺 *
(1)

𝑇 * = 𝑀 * 𝜎 * = 𝑡 𝑀 𝑡 𝑁
(2)

𝜀 * = 𝜎 * 𝑀 *
(3)

𝑉 * = 𝜀 * • 𝐿 * (4)
The accuracy of the dynamic scaling was tested by calculating the non-dimensional Ramberg numbers defined by the ratios between the forces acting on the model [START_REF] Ramberg | Gravity, Deformation and the Earth Crust[END_REF][START_REF] Sokoutis | Lithospheric-scale structures from the perspective of analogue continental collision[END_REF]. The appropriate scaling numbers for individual layers are shown in Tab. 2.

Experimental procedure

The experiments were repeated with an identical original setup (Fig. 3), for the same constant velocity of 2 cm/h and finite shortening of 25%, 37% and 50% (Fig. 4). The laboratory temperature during all experiments was 19-20 °C. Reproducibility was verified by visual inspection of topography features during repeated experiments. Only a few negligible differences were noted, which were a result of small variations in the shape and thickness of the plasticine layer. The sectioning of the deformed model domain was performed normal and parallel to the direction of shortening. The modelled domain was further frozen to eliminate undesirable deformation of silicon putty after the experiment. To obtain cross sections for different orientations and stages of model evolution, a series of identical experiments were carried out, each being stopped after different amounts of shortening.

Four cameras with different elevations and view angles recorded the images of the model surface every 5 or 10 minutes. One camera was situated directly above the experiment in order to observe the evolution of the modelled surface from the top view. This camera was used for 2D velocity field reconstructions (PIV -particle image velocimetry), while the other three cameras were used for the reconstruction of a 3D model topography using the MicMac software (e.g. [START_REF] Galland | Application of open-source photogrammetric software MicMac for monitoring surface deformation in laboratory models[END_REF]. For the detailed investigation of the upper crust deformation and topography evolution, the PIVlab software was used [START_REF] Thielicke | PIVlab-towards user-friendly, affordable and accurate digital particle image velocimetry in MATLAB[END_REF] to determine the velocity field and the horizontal strain components [START_REF] J O U R N A L P R E -P R O O F Krýza | Advanced strain and mass transfer analysis in crustal-scale oroclinal buckling and detachment folding analogue models[END_REF]. The divergence of the velocity field was used to identify zones of localized thickening (negative divergence) or thinning (positive divergence) of the uppermost part of the model. These data were superimposed on the results obtained from the topography reconstruction to evaluate the mass transfer in a vertical direction.

Results

J o u r n a l P r e -p r o o f

Journal Pre-proof

We present the first finite geometries after 50% shortening for sections perpendicular (Fig. 5) and parallel (Fig. 6) to shortening directions to identify the main structural features of the model. The progressive evolution of structures and deduced material fluxes were obtained from analyses of sections representing different deformation increments (Fig. 7), model topography and deformation of the model surface (Fig. 8).

4.1 Sections perpendicular to the shortening direction (i.e. parallel to the axial plane of the orocline)

The sections perpendicular to the shortening direction expose two contrasting domains (Fig. 5): (1) on the piston side of the model the Mongolian Orocline fold hinge amplifies and indents towards the EJTA subdomain, sections 1 and 2), and (2) in the backstop part of the model, the Hingan fold hinge amplifies towards the MO subdomain (sections 4 and 5). The inflection part of the fold is examined in section 3. Sections 1 and 2 show a similar pattern characterized by generalized thickening of the ductile part of the MO and EJTA subdomains.

The main feature is the underthrusting of the EJTA subdomain beneath the MO subdomain associated with the upright folding of the lower crust-mantle interface. This underthrusting is accompanied by the exhumation of the EJTA mantle beneath the propagating fold hinge.

Section 3 shows that horizontal movements are limited to the EJTA and MO mantle lithosphere, which are accompanied by minor extrusion of the EJTA mantle beneath the CRAW subdomain. Sections 4 and 5 show a considerable thinning and extension of the EJTA subdomain lower and middle crust. This is accompanied by the thrusting of the CRAW and EJTA mantle lithosphere over the MO subdomain.

Sections parallel to the shortening direction (i.e. perpendicular to the orocline axial plane)

Section 1 of Fig. 6, shows deformation of the MO subdomain and the tip of Hingan fold hinge. Section 4 shows the deformation of the EJTA subdomain and the tip of Mongolian fold hinge. Sections 2 and 3 present structures related to the deformation of the central parts of both folded domains. In order to characterize deformation related exclusively to movement of the piston, we also present sections 5 and 6 which are some distance from the folded domain. Here, we can observe a thickening of the mantle close to the piston (right) which decreases towards the back-stop (left). This is characteristic of a viscous deformation gradient typically developed in front of the moving piston (e.g. England andHouseman, 1986, 1989;[START_REF] Ježek | Strain partitioning in front of an obliquely convergent indenter[END_REF]. The EJTA crust-mantle interface (section 5) is irregularly deformed and shows upright cuspate folds further away from the piston.

Section 4 intersects the tip of the Mongolian fold hinge and exhibits an overall homogeneous shortening of the lithosphere. This is expressed by pop-down structures along the uppermiddle crust boundary and by the cuspate-lobate upright folding of both the crust-mantle and lower-middle crust interfaces. Section 3 is characterized by a contrasting response of the EJTA and MO subdomains. The EJTA subdomain fold hinge forms an antiformal domal structure formed by the middle crust. It is surrounded by marginal synforms and cored by the lower crust and the mantle associated with a cuspate-lobate folding of the crust-mantle interface. The MO crust-mantle boundary is characterized by the formation of pop-down structures while the interface of MO and CRAW-EJTA mantle forms a large scale synformal structure. The main feature of section 2 is the influx of the MO mantle into the EJTA subdomain below the amplified crustal antiformal domal structure. The core of the Hingan fold hinge area (section 2 on the left) also shows a décollement between overthrust crustal antiform and the EJTA mantle (see panels 4 and 5 in Fig. 5). Finally, section 1 reveals a homogeneous thickening of the MO subdomain accompanied by the deformation of the J o u r n a l P r e -p r o o f mantle-crust interface which exhibits pop-down structures similar to those described by [START_REF] Cagnard | Modes of thickening of analogue weak lithospheres[END_REF].

Two general features can be observed in these sections: 1) Indentation deformation gradients can be observed in regions outside the folded region in the mantle of both the EJTA and MO subdomains.

2) The folded region shows homogeneous deformation associated with the development of pop-down structures close to the axial plane of both the Mongol and Hingan fold hinges.

Incremental evolution

In order to better understand the significance of the general features described above we examine the evolution of the model along sections cross-cutting the folded region (section A-B) and in the EJTA subdomain (section C-D), Fig. 7. The evolution of the system is examined in three repeated identical experiments with different finite shortening 25%, 37% and 45%.

The folded region (sections A-B) reveals contrasting behaviour of the MO and EJTA subdomains and their interaction with the CRAW subdomain in the middle. The early shortening (25%) is accommodated mainly by the thickening of the MO mantle and its flux into the EJTA subdomain, which remains weakly deformed. In the second stage (37% shortening) a massive flux of MO mantle from the Mongol fold hinge area towards the central part of the EJTA subdomain can be observed. This flow causes the formation of an inclined antiformal structure in the region of the axial plane of the Hingan fold. The subsequent stage of deformation (45% shortening) shows an amplification of the antiformal dome structure cored by the EJTA lower crust that has been massively shortened by the MO mantle lithosphere indentation. Here, the crustal antiformal dome is decoupled from the EJTA mantle. The evolution described above is accompanied by the dynamic deformation of the upper crust which is progressively uplifted in the Hingan fold domain while in the Mongolian fold hinge area forms a synform which evolves into a large-scale pop-down structure.

The EJTA subdomain (sections C-D, Fig. 7) reveals a contrasting behaviour of the crust and the lithospheric mantle during the progressive shortening. The EJTA mantle exhibits a classical indentation thickening gradient in front of the piston [START_REF] England | Finite strain calculations of continental deformation: 2. Comparison with the India-Asia collision zone[END_REF]. As a consequence, a mantle dome structure develops with a thinned crust in the apical part of the dome. In contrast, both the upper and lower oceanic crust thickens at the back-stop side of the model. Here, the upper crust thickening occurs through folding while the middle and lower crust thickening reflects homogeneous shortening associated with the development of cuspate-lobate structures at the crust-mantle and lower-middle crust interfaces.

Evolution of model surface deformation

A time-resolved particle image velocimetry (PIV) software tool was used to assess the deformation of the uppermost crust represented in the models by a sand layer, which was removed before model sectioning (Fig. 8). The shear strain-rate, velocity field patterns (Fig. 8a) and the corresponding divergence of the velocity field (Fig. 8b) are analyzed for four shortening stages (3%, 25%, 37% and 45%). The analytical PIV post-processing approach is described in detail in [START_REF] J O U R N A L P R E -P R O O F Krýza | Advanced strain and mass transfer analysis in crustal-scale oroclinal buckling and detachment folding analogue models[END_REF].

In the early stages of the experiments (3% shortening), the EJTA subdomain is homogeneously deformed, while the folded region shows velocity field related to active buckling (Fig. 8a). This buckling vanishes at 37% shortening when a velocity field typical of a post-buckle flattening (i.e. parallel velocity vectors) is established. Another feature is the surface deformation partitioning expressed by narrow zones of deformation develop at 25%

J o u r n a l P r e -p r o o f and 37% of shortening in both the MO and EJTA subdomains (Fig. 8 blue zones). The divergence of the velocity field becomes characterized by high frequency negative and positive anomalies, which developed preferentially along the axial planes in the hinge zones of both the Hingan and Mongolian folds (Fig. 8b). These anomalies correspond to the development of thrust related folds, which initiate the pop-up structures and the narrow synclines associated with the development of pop-down structures at deeper crustal levels (Krýza et al., 2019) described above (Fig. 7). With ongoing deformation (45% shortening), the whole domain is governed by homogeneous deformation marking a post-buckle flattening mode of deformation (Fig. 8a). The final topography reconstruction after 50% shortening (Fig. 8c) shows well developed positive topography anomalies along the folded region and positive narrow linear ranges, which correspond to the thrust related folds in Fig. 7. The narrow and low topography anomalies correspond to the above mentioned negative values of divergence associated with the narrow synclines, while large topography anomalies correspond to positive values of divergence associated with the development of the pop-up structures. Wide plateaus of neutral to high topography are developed in both the MO and EJTA subdomains close to the piston side. Velocity profiles for various shortening values (Fig. 8d) show a linear decrease of velocity distribution typical of homogeneous deformation for small amounts of shortening. Starting from 25% of bulk shortening the velocities are characterized by variations typical of heterogeneous deformation. The highest strains are partitioned along the axial plane of the Hingan fold hinge.

Discussion

Model limitations related to boundary conditions

A limitation of our modelling is the absence of a dense viscous fluid used as a proxy to the asthenospheric mantle in the model of [START_REF] Cagnard | Modes of thickening of analogue weak lithospheres[END_REF]. Actually, due to the rather complex geometry of the modelled lithosphere, the technical realization and exploitation of an asthenospheric-scale model would have been impossible. The absence of dense fluid at the bottom of the model is the likely reason for the development of the deformation gradient in front of the piston in the EJTA subdomain (Figs. 6,7 and 8). This deformation feature, known as an indentation profile, is typical of numerical models based on a viscous, thin sheet approximation with the rigid floor [START_REF] England | Finite strain calculations of continental deformation: 2. Comparison with the India-Asia collision zone[END_REF][START_REF] Ježek | Strain partitioning in front of an obliquely convergent indenter[END_REF]. This imposes a vertical extrusion of material exponentially decaying away from the front of the indenter [START_REF] Lexa | Cretaceous collision and indentation in the West Carpathians: View based on structural analysis and numerical modeling[END_REF]. It should be noted that the indentation profile is only weakly developed in the folded region (Fig. 6) and disappears at high finite shortening (Figs. 7 and8). An important feature of models with a fluid asthenosphere is the absence of an indentation profile and the development of homogeneous thickening of mantle lithosphere across-strike of the modelled domain (Fig. 3 in [START_REF] Cagnard | Modes of thickening of analogue weak lithospheres[END_REF]. Therefore, our model resembles the homogeneously distributed deformation within the deep lithosphere either in the folded region (Sections A-B in Fig. 7) or for a high degree of the shortening (Fig. 8). The indentation profile developed in the EJTA subdomain is therefore considered as an artefact of the model setup and is not used in further geological interpretations. Therefore, we suggest that the mutual interactions between the MO and the EJTA subdomains during oroclinal buckling would be similar to that which is found to occur when a dense viscous fluid is present at the base of the model.

Another limitation of the model is the presence of high deformation gradients adjacent to the fixed lateral walls of the model box (Fig. 8). Complex structures developed in these regions Fig. 5), which are excluded from interpretations. Only the internal part of the modelled domain is considered.

J o u r n a l P r e -p r o o f

In addition, our model cannot produce heterogeneous mantle thickening in the hinge zone of the orocline as modelled by Pastor-Galán (2012). The reason is that the controlling layer for buckling in their model is oriented horizontally and the bulk deformation is restricted to the folded domain. In contrast, in our model, the controlling layer for buckling is a vertical thin beam surrounded by viscous silicone putty and therefore most of the displacement occurs within the horizontal plane.

Mantle and crustal fluxes related to oroclinal buckling

The analyses of finite strain patterns along the axial planes of the oroclines and on the sections parallel and orthogonal to the shortening direction together with the study of the incremental evolutions of these sections and surfaces, allow the relationships between deep lithospheric fluxes and surface deformation related to the oroclinal buckling to be determined. A major zone of mechanical decoupling is developed along the interface between the brittle upper crust (sand) and the viscous middle-lower crust (silicone putty). It is manifested by contrasting deformation patterns within the viscous and brittle parts which are apparent up to a shortening of ~40%. While the viscous part of the model exhibits a typical indentation profile in front of the piston, the brittle layer is detached and most of the deformation is localized along the axial plane of Hingan fold. During subsequent shortening, this independent behaviour gradually vanishes and both parts of the model exhibit a more coupled deformation.

However, the deformation pattern is different below the folded accretionary domain (CRAW). Here, the lateral fluxes parallel to the fold axial planes dominate the deformation of both types of mantle lithosphere, which are dragged by the migrating orocline hinges in a vortex manner (Fig. 9). In this way, the two contrasting mantle lithospheres are laterally juxtaposed in the region where the MO mantle flows towards the hinge domain of the Mongolian fold (sections 1 and 2 in Fig. 5), and the EJTA lithosphere flows towards the Hingan fold (section 4 in Fig. 5). We note that both oroclinal bends reveal more important displacement of the fold hinge in the crustal level compared to the crust-mantle boundary. This is expressed by the opposite general inclination of subdomain boundaries associated with thrusting of the MO over the EJTA lithosphere in sections 1 and 2 and the opposite thrusting in section 4 (Fig. 5). The inclination angle of the fold axes and associated thrusts of the lithospheric segments is governed by differences in vortex velocity between the mantle and upper crust as deduced from the serial sections and the PIV analysis of the surface deformation (Figs. 5,6,7 & 8). It can be shown that the vortex velocity at the surface is high compared to the slower flow of the viscous mantle at depth (Fig. 9). This velocity difference implies a progressive rotation over time of the orocline fold axes from vertical to less inclined orientations and the above-described thrusting of mantle lithosphere.

These redistributions of mantle and crustal materials are associated with the formation of an antiformal fold with a horizontal axis in the region of the Hingan fold and a horizontal synform in the Mongolian fold domain. These processes are also reflected in the surface where the pop-down structures dominate the Mongolian fold hinge, while the Hingan fold hinge shows the presence of a central antiform surrounded by narrow, marginal synclines (Fig. 7).

The consequence of oroclinal buckling is the development of a region dominated by lowermiddle crustal material in the Hingan fold hinge compared to the mantle dominated region in the Mongolian fold hinge zones. Mechanically, the Hingan fold hinge is represented by a multi-layer system formed by the stiff EJTA mantle at the bottom, overlain by extremely weak lower and middle crustal layers and the strong upper crust higher in the column. In J o u r n a l P r e -p r o o f contrast, the Mongolian fold hinge is formed exclusively by the MO oceanic mantle and a thin upper crust. Therefore, the integrated strength of the EJTA multilayer system [START_REF] England | Finite strain calculations of continental deformation: 2. Comparison with the India-Asia collision zone[END_REF][START_REF] England | Extension during continental convergence, with application to the Tibetan Plateau[END_REF][START_REF] Thompson | Thermally softened continental extensional zones (arcs and rifts) as precursors to thickened orogenic belts[END_REF] is significantly lower compared to the adjacent MO lithosphere. This difference implies that under horizontal compression the EJTA subdomain collapses and shortens preferentially. The deformation is controlled by the presence of a strong substratum overlain by a weak layer which is an ideal condition for detachment of a weak lower crust from the rigid upper mantle and the buckling of the stronger, middle-upper crustal layer. The formation of the detachment antiform in the Hingan fold hinge area is accompanied by the flux of MO mantle and the progressive development of material deficit in the EJTA subdomain close to piston leading to the formation of a large-scale synform filled with oceanic crust decorated by pop-down structures at the surface. [START_REF] Tikoff | Clutch tectonics and the partial attachment of lithospheric layers[END_REF] discussed the deformation of the lithosphere and argue that it occurs either by movements of the mantle layer (i.e. bottom driven tectonics) or by the rigid crust (i.e. top driven tectonics). In order to determine which of these two models is the more likely, it is necessary to determine which layer is mechanically dominant and controls the deformation of the whole system. We have shown that the primary response of the mantle lithosphere with a low mechanical anisotropy is the development of a typical indentation profile (Figs. 6 and 7lateral view; Fig. 8a). Both crustal sections and surface analysis show that during the early stages of the experiments the mantle governs the bulk lithospheric deformation. However, this behavior changes rapidly after ca. 5-10% shortening i.e. during the maximum amplification of the orocline within the crust. The significant crustal flow related to the fold amplification affects the mantle flow which is manifested by the development of vortex flow in the mantle (Fig. 9). During this stage, the folding of the CRAW subdomain governs the bulk material fluxes in its vicinity leading to the accumulation of oceanic mantle material in the Mongolian fold and of the EJTA crustal material in the Hingan fold. After ca. 30% shortening the active folding ceases and the postbuckle flattening, i.e. homogeneous flattening, takes over. This point is reached when the layering has rotated into an almost stable position with increased resistance to the deformation along the shortening direction. The deformation becomes partitioned between the two orocline axial domains because of the different integrated strengths of these two domains. During this stage, the lithospheric deformation progressively passes to a mechanically coupled behavior. The mechanical evolution of the system is transient and characterized by three stages starting with bottom driven flux of the mantle, followed by top driven stage during amplification of orocline and terminating with fully coupled deformation.

Is the deformation driven by top or bottom fluxes

Large scale geological implications for intra-oceanic oroclinal systems

The analyses of the finite strain patterns and lower crustal and mantle fluxes described above allow the first order features of the model to be defined: 1) redistribution of EJTA mantle beneath the Mongolian fold hinge, 2) important crustal thickening of the Mongolian fold hinge related to underthrusting of EJTA crust beneath the CRAW subdomain and 3) intensive deformation of the EJTA crust and formation of pop-down structures along the axial plane of the Hingan fold. An attempt is made to correlate these features with the main geophysical and geological observations of the Mongolian part of the CAOB.

J o u r n a l P r e -p r o o f

The CAOB is traditionally interpreted as a result of long-lived accretion of highly contrasting lithospheric terranes such as continental blocks, magmatic arcs, oceanic lithosphere and accretionary prisms [START_REF] Badarch | A new terrane subdivision for Mongolia: implications for the Phanerozoic crustal growth of Central Asia[END_REF][START_REF] Windley | Tectonic models for accretion of the Central Asian Orogenic Belt[END_REF]. In theory, these terranes should be composed of crust and lithospheric mantle that differ in thickness and composition. However, even if the geological expression of suspect terranes seems to be valid, the geophysical data show to a certain degree homogeneous lower crust beneath the EJTA and the CRAW [START_REF] Guy | Late Paleozoic-Mesozoic tectonic evolution of the Trans-Altai and South Gobi Zones in southern Mongolia based on structural and geochronological data[END_REF]2015;[START_REF] Yakubchuk | Evolution of the Central Asian Orogenic Supercollage since late Neoproterozoic revised again[END_REF]. In addition, the massive alkaline Permo-Triassic magmatism affecting this vast region reflects the same mantle source beneath both the EJTA and the MO domains (e.g. [START_REF] Kovalenko | Isotope provinces, mechanisms of generation and sources of the continental crust in the Central Asian mobile belt: geological and isotopic evidence[END_REF][START_REF] Yarmolyuk | Late Cretaceous-Early Cenozoic volcanism of Southern Mongolia: A trace of the South Khangai mantle hot spot[END_REF]. In our analogue model, these observations are supported by the redistribution of the EJTA mantle beneath the central part of the folded domain, in particular the hinge of the Mongolian orocline (section 3 in Fig. 6).

The geophysically derived MOHO depth beneath the Mongolian CAOB reveals that maximum crustal thickness occurs beneath the hinge of the Mongolian orocline and the Altai mountains [START_REF] Guy | Moho depth model for the Central Asian Orogenic Belt from satellite gravity gradients[END_REF]. The periphery of this domain is also marked by presence of UHT Permian granulites suggesting major thermal perturbation in the mantle depths [START_REF] Burenjargal | Thermal evolution of the T seel terrane, SW Mongolia and its relation to granitoid intrusions in the Central Asian Orogenic Belt[END_REF][START_REF] Li | Discovery and genetic mechanism of basic granulite in the Altay orogenic belt, Xinjiang, NW China[END_REF]Broussolle et al., 2018a) related to the progressive growth of the orocline [START_REF] Jiang | Structural and Geochronological Constraints on Devonian Suprasubduction Tectonic Switching and Permian Collisional Dynamics in the Chinese Altai, Central Asia[END_REF]Guy et al., 2020). These observations indicate a major mobility of the hot felsic crust during Permian times and its redistribution associated with the growth of the hinge of Mongolian orocline. In our model, the crustal thickening in this domain could result from overthrusting of the Mongolian fold hinge over the EJTA crust during orocline amplification (sections 1 and 2 in Fig. 5). Indeed, the underthrusting of the East Junggar-Trans Altai Zone lower crust beneath the hinge of Mongolian orocline in the Chinese Altai was shown by both structural [START_REF] Jiang | Structural and Geochronological Constraints on Devonian Suprasubduction Tectonic Switching and Permian Collisional Dynamics in the Chinese Altai, Central Asia[END_REF] and geophysical data (Guy et al., 2020). The interpretation of this complex boundary is portrayed in Fig. 10 which shows a large gravity high beneath the East Junggar basement (western part of the East Junggar-Trans Altai Zone) continuing far beneath the Chinese Altai (western continuation of the Mongol Altai Zone). The gravity modeling is interpreted in terms of the emplacement of the dense East Junggar-Trans Altai Zone crust beneath the low density Mongol Altai Zone crust, and the thickening and exhumation of both Permian granulites and the Devonian migmatitemagmatite dome further north. The analogue model presented here thus explains the far travelling underthrusting of the East Junggar-Trans Altai Zone crust beneath the Chinese part of the Mongol Altai Zone deduced from gravity modelling and the thrusting of the Mongol Altai Zone sequences over the imbricated East Junggar-Trans Altai Zone oceanic crust (Briggs et al., 2007 (Fig. 10). The Permian thermal anomalies and related melting of the lower crust can be explained as a consequence of the elevation of EJTA mantle beneath the folded domain as shown in our model (Fig. 5).

Several structural and geophysical studies from south Mongolia report the existence of steep E-W trending deformation zones associated with localized elongated gravity highs in the East Junggar-Trans Altai and Mongol Altai zones (Fig. 11). Linear gravity highs have been interpreted as a result of the shortening of the upper crust and its downward penetration into the homogeneous lower crust along vertical zones as deep as 20 km [START_REF] Guy | Geophysical constraints for terrane boundaries in southern Mongolia[END_REF]. These gravity highs correspond to Permian-Triassic upright synforms with horizontal axes which folded Devonian ophiolites composed of dense mafic gabbroic and basaltic crust and peridotites (Guy et al., 2014a). These deformation synforms developed simultaneously with the oroclinal buckling in the East Junggar-Trans Altai and Mongol Altai zones (Guy et al., 2014a;[START_REF] Lehmann | Structural Constraints of the Evolution of the Central Asian Orogenic Belt in sw Mongolia[END_REF]. This period is also connected with the formation of E-W trending Permian volcanic rift zones [START_REF] Kovalenko | Isotope provinces, mechanisms of generation and sources of the continental crust in the Central Asian mobile belt: geological and isotopic evidence[END_REF] marked by important magnetic lineaments in the East Junggar-Trans Altai Zone in south Mongolia. We suggest that pop-J o u r n a l P r e -p r o o f down structures affecting the EJTA subdomain simulated in this study are an excellent explanation for linear gravity and magnetic anomalies affecting the East Junggar-Trans Altai Zone in south Mongolia [START_REF] Guy | Geophysical constraints for terrane boundaries in southern Mongolia[END_REF]. Both the upright synforms, vertical deformation and linear rift zones described is south Mongolia and NW China can be compared to linear pop-down and pop-up structures visualized in the surface model analysis (Fig. 8) and sections perpendicular to the axial plane of large oroclines (Fig. 6).

The global correspondence between the results of the analogue model and existing structural and geophysical data from southern Mongolia and NW China is shown in Fig. 12. The 3D geology model in Fig. 12a shows the final geometry the folded system while the sections perpendicular to the axial planes of the oroclines show possible geometries along their hinges and limbs (Fig. 12b). Both structures and thermal anomalies developed at the East Junggar-Trans Altai Zone and Mongol Altai Zone boundary in NW Chinaare compatible with the progressive emplacement of MO mantle beneath the hinge of the Mongolian orocline (Fig. 12b) and lower crustal fluxes parallel to the axial plane of both the Mongolian and Hingan oroclines. The main consequence is the redistribution of homogeneous EJTA mantle beneath the hinge of the Mongolian orocline and adjacent rear part of the Hingan orocline. The model shows both the development of pop down structures in the core of the Hingan orocline compatible with the development of deep narrow synforms and linear gravity highs in the East Junggar-Trans Altai Zone in south Mongolia (Fig. 11) and underthrusting of the East Junggar-Trans Altai Zone upper-middle crust beneath the exhumed hinge of the Mongolian orocline (Fig. 10).

Conclusions

This study represents a new approach to the simulation of the mechanical processes associated with the building of large oroclinal systems that are associated with a lateral compression. It investigates the mass transfer at the model surface, which is responsible for the deformation of the upper crust and for the development of topography. Furthermore, the ductile flows inside the model domains and the mechanical coupling in a lithospheric multilayer are studied from the model sections. The main modelling features can be summarized as follows:

• The lateral shortening of the pre-folded accretionary belt leads to the development of two regional-scale types of folds: the steeply plunging oroclinal folds and the smaller scale upright folds with sub-horizontal fold axes which are formed in the MO and EJTA subdomains.

• Generally, all curvilinear fold-thrust systems developed in the upper crust express the geometry of the axial planes of upright folds developed in deeper lithospheric layers. • The overthrusting of the oceanic MO lithosphere and the continental CRAW crust over the EJTA crust in the Mongolian orocline hinge is associated with the material vortex flow around the inflection of the orocline. This pattern decays towards deeper lithospheric layers, where the lateral flow of material is negligible.

• The analogue model explains the far travelling underthrusting of the East Junggar-Trans Altai Zone crust beneath the Mongol Altai Zone sequences in NW China deduced from gravity modelling and the geometry of East Junggar-Trans Altai Zone oceanic crust. • The redistribution of EJTA mantle material related to the oroclinal buckling may explain the presence of homogeneous mantle beneath both the Mongolian and Hingan oroclines. In addition, simulated pop-down structures in the analogue model are J o u r n a l P r e -p r o o f consistent with the presence of linear zones of gravity highs associated with vertical zones coinciding with synforms cored by dense ophiolites in the the East Junggar-Trans Altai Zone in south Mongolia.

Figure captions

Fig. 1: A simplified regional geological map of the Mongol-Hingan orocline (drawn after [START_REF] Parfenov | Preliminary northeast Asia geodynamics map[END_REF]. Colors represent the individual regional units of the Mongolian and Hingan orocline fold. The Precambrian continental blocks are marked by capital letters (B: Baydrag, TM: Tuva-Mongol, DB: Dzabkhan-Baydrag, E: Erguna, BJK: Burya-Jiamusi-Khanka, SG: South Gobi). The map of the CAOB is shown on the bottom right (modified after [START_REF] Han | Final amalgamation of the Tianshan and Junggar orogenic collage in the southwestern Central Asian Orogenic Belt: constraints on the closure of the Paleo-Asian Ocean[END_REF] with indicated oroclines (red: Kazakhstan orocline, blue: Mongol-Hingan orocline).

Table captions

Tab. 1: The model rheological parameters and scaling ratios.

Tab. 2: The scaling model parameters -Ramberg numbers.

J o u r n a l P r e -p r o o f OK co-designed and prepared all models including performing of the photogrammetry and velocimetry, analyzed the results (kinematic and dynamic behavior of the model), prepared figures 2-9 and 12, wrote the first draft of the manuscript and contributed to the general discussion about interpretations in a frame of the CAOB. OL co-designed model, contributed to the manuscript preparation including drawing of the Figure 1

  J o u r n a l P r e -p r o o f the model length to 30 cm, the model width to 42 cm and the model height to 4 cm implying a space dimensions scaling factor 𝐿 * = 𝐿 𝑀 𝐿 𝑁 = 8 • 10 -7 . We ran all experiments under normal gravity acceleration implying 𝐺 * = 1. The scaling factor for densities (𝑃 *

Fig. 2 :Fig. 5 :Fig. 6 :

 256 Fig. 2: A simplified sketch of development of the Mongolian orocline. (a) shows the Late Silurian subduction of the Paleo-Asian ocean underneath the East Junggar-Trans Altai Zone and the opening of a rift system. This is followed by the Carboniferous initialization of the Mongol-Okhotsk oceanic subduction (b). Note that the continental blocks and the Mongol Altai Zone were welded together prior to these two stages. Stages (c) and (d) show Permo-Triassic oroclinal buckling induced by the drift of the North China Craton towards the Siberian Craton and the compression of remnants of the accretionary system after slab breakoff. The buckling caused by the convergence of the two cratons results in a development of the Mongolian and Hingan oroclines (the subject of this study). The cross-sections that are shown in these four stages are illustrative and non-scaled. The sketches are based on figures published in Xiao et al. (2015) and (2018). The abbreviations of the Precambrian continental blocks are the same as in Fig. 1.

Fig. 7 :

 7 Fig. 7: The incremental evolution of the model domain. Three different stages of the shortening are shown on the left side with two lines for sectioning (red/yellow). The corresponding cross-sections on the right reveal the progressive development of major deformation features inside the model domain.

Fig. 8 :

 8 Fig. 8: The model surface analysis. For reconstruction of material transfer at the surface we analyzed the displacement field calculated by PIVlab software. Panel (a) represents a shear strain rate where arrows correspond to the velocity field and colors represent the second invariant of the deviatoric part of the strain-rate tensor (isovolumetric shear deformation). The divergence of the velocity field is shown in the panel (b) where major anomalies situated in the MO and EJTA part are related to the development of pop-ups (positive divergence) and pop-downs (negative divergence). A digital elevation model (DEM) is shown in panel (c). Narrow elevated parts in the EJTA and MO correspond to pop-ups. The velocity profiles across line A-B (for the various shortening stages shown in panel (b)) are displayed in diagram (d). These profiles show a transition of the model dynamics from an indentationsimilar evolution (blue curve) to a homogeneous thickening evolution (red curve).

Fig. 9 :

 9 Fig.9: A block diagram of the model's inner dynamics. At the top is shown the colour map of the vorticity that reveals a clockwise rotation of the orocline inflection and counterclockwise rotation of the orocline outer limbs. The black arrows correspond to material flow from the indenter side to the backstop while the red arrows show a decreasing vorticity in the material transfer from the surface to the basement. The red dashed curve represents the interface between the MO and the EJTA subdomains, which coincides with the CRAW.

Figure 10 :

 10 Figure 10: Gravity data showing the presence of East Junggar-Trans Altai Zone dense crust beneath the Mongol Altai Zone at the boundary of the East Junggar and the Chinese Altai regional units. A. Location of the gravity map and the modelling profile with respect to the hinge of the Mongolian orocline. B. Complete Bouguer anomaly map and location of the modelling profile. The color scale of the gravity anomalies is linear. The thick white line represents the surface contact between the East Junggar-Trans Altai Zone and the Mongol Altai Zone crust C. 2D forward model along profile A-A' cross-cutting the Mongol Altai Zone and the East Junggar-Trans Altai Zone boundary. The observed and calculated gravity data are shown in the upper panel. The lower panel shows the 2D structure of the crust in northwestern China: 1) Underthrusting of the dense East Junggar-Trans Altai crust beneath the Mongol Altai Zone, 2) extrusion of Permian granulites south of Altai city and 3) exhumation of the Devonian granite-migmatite dome in the central part of the Mongol Altai Zone. Geophysical model (Fig.10c) is modified afterGuy et al., (in review) where the geophysical modeling procedure and details of gravity data are explained.J o u r n a l P r e -p r o o f

Figure 11 :

 11 Figure 11: Gravity data showing the linear gravity highs in the East Junggar-Trans Altai upper crust in SW Mongolia. A. Location of the gravity map and the modelling profile crosscutting the core of the Hingan orocline. B. Isostatic residual gravity map and location of the profile. The color scale of the gravity anomalies is linear. C. 2D forward model along profile B-B' cross-cutting the Mongol Altai Zone, the East Junggar-Trans Altai Zone and the South Gobi Zone in southern Mongolia. The observed and calculated gravity data are shown in the upper panel. The second panel shows the global structure of the crust in southern Mongolia: 1) development of three 20 km deep vertical tabular upper crustal zones marked by high density penetrating into lower density lower crust, 2) intermediate density lower crust beneath the Mongol Altai and East Junggar-Trans Altai zones (representing the relaminant), 3) minor underthrusting of the East Junggar-Trans Altai Zone strong and dense upper crust beneath gently thickened Mongol Altai Zone upper-middle crust. Geophysical model (Fig.11c) is modified afterGuy et al., (in review) where the geophysical modeling procedure and details of gravity data are explained.

Fig. 12 :

 12 Fig. 12: (a) schematized 3D model of the Mongolian-Hingan oroclinal system combining map view with walls perpendicular and parallel to the axial plane of oroclines both experimental results and geological data from the studied region of Mongolia and China. The position of the main tectonic zones (Mongol Okhotsk Zone, Precambrain continents, Mongol Altai Zone and East Junggar-Trans Altai Zone) are shown together with modeled MO, CRAW and EJTA subdomains. (b) sections across the schematized 3D model perpendicular to the axial planes of oroclines show: 1) exhumation of the MO mantle material (dark red) beneath the core of the Mongolian orocline (central block), 2) general redistribution of EJTA mantle beneath the hinge of Mongolian orocline from the adjacent rear part of the Hingan orocline (light red), 3) underthrusting of EJTA crust beneath the CRAW in the hinge of the Mongolian orocline and 4) pop down structures affecting the EJTA subdomain in the core of the Hingan orocline.

  Figure1

Figure 12

 12 Figure12

Acknowledgments

This paper was supported by the Czech Science Foundation GAČR EXPRO grant GX19-27682X to KS. The Center for Geosphere Dynamics (UNCE/SCI/006) is acknowledged for the support of OL and OK. National Natural Science Foundation of China (41888101) for WX. We thank to J. J. Kermarrec for his help in the modelling laboratory at Géosciences Rennes, University of Rennes 1, France.

Primary data for this paper are available on https://doi.org/10.7910/DVN/QNFHDO, Harvard Dataverse

Conflict of Interests Statement

We declare that the submitted work was carried without the presence of any personal, professional or financial relationships that could potentially be construed as a conflict of interest.


Carsten Schradin
email: carsten.schradin@iphc.cnrs.fr

Successful reproduction of Trachemys scripta in the Altrhein of Kehl (Germany) and simultaneous increase in population estimate

INTRODUCTION

Invasive species are threatening native biodiversity worldwide. Neobiota are alien species that establish themselves outside their natural distribution range due to human action, i.e. they reproduce successfully in the areas in which they have been released and they maintain their population [START_REF] Geiger | Überlebensfähigkeit allochthoner Amphibien und Reptilien in Deutschland -Konsequenzen für den Artenschutz[END_REF]Wilson et al. 2009). They are regarded as being invasive if they have the capacity to spread into new areas and at the same time cause harm to the native fauna and / or flora by predation and/or competition [START_REF] Lowe | 100 of the World's Worst Invasive Alien Species. A selection from the Global Invasive Species Database[END_REF]Wilson et al. 2009).

Of the world`s 100 worst invasive alien species, only two species are reptiles, the brown tree snake (Boiga irregularis) from Australia and the red-eared slider (Trachemys scripta) from North America [START_REF] Lowe | 100 of the World's Worst Invasive Alien Species. A selection from the Global Invasive Species Database[END_REF]. Compared to invertebrates (26 of the 100 worst invasive species) and small mammals (10 of the 100 the worst invasive species), reptiles grow, mature, and reproduce slowly, which can be one reason why so few reptiles are invasive. However, a slow life history also makes it more difficult to monitor and predict the long-term consequences of an introduced alien species, i.e. whether it will establish itself and become invasive or not. From an ecological point of view it is not important whether an invasion takes a few years or 100 years, as the long-term consequences on the native flora and fauna would still be deleterious. The potentially invasive T. scripta has been released worldwide, including central and south America [START_REF] Böhm | Records of invasive Trachemys scripta elegans (WiEd-NEuWiEd, 1839), in cenotes of the Yucatán Peninsula, Mexico[END_REF], Asia [START_REF] Mo | Possible Records of Yellow-bellied Sliders (Trachemys scripta scripta) or River Cooters (Pseudemys concinna) in Hong Kong[END_REF], and Europe [START_REF] Standfuss | Threat or fiction: is the pond slider (Trachemys scripta) really invasive in Central Europe? A case study from Slovenia[END_REF]. While the slow life history of the species should theoretically allow a timely reaction by authorities, unfortunately the opposite has occurred. Small neobiota populations are often ignored and long-term monitoring programmes are only established many years after the population has grown to a size that makes population control difficult [START_REF] Sancho | Expansion of Trachemys scripta in the Valencian Community (Eastern Spain)[END_REF]). While T. scripta has become invasive in areas with tropical, sub-tropical, and Mediterranean climates [START_REF] Lowe | 100 of the World's Worst Invasive Alien Species. A selection from the Global Invasive Species Database[END_REF][START_REF] Foglini | Non-native turtles in a peri-urban park in northern Milan (Lombardy, Italy): species diversity and population structure[END_REF], the consequences in areas with temperate climates are unclear [START_REF] Cadi | Successful reproduction of the introduced slider turtle (Trachemys scripta elegans) in the South of France[END_REF][START_REF] Prevot | Pets and invasion risks: Is the slider turtle strictly carnivorous?[END_REF][START_REF] Kopecký | Establishment risk from pet-trade freshwater turtles in the European Union[END_REF][START_REF] Standfuss | Threat or fiction: is the pond slider (Trachemys scripta) really invasive in Central Europe? A case study from Slovenia[END_REF].

In 2014, the European Union published Regulation (EU) No 1143/2014, declaring an action plan to prohibit the import, breeding and release of invasive species (European Parliament 2014), including T. scripta (European Commission 2016). A regulation is a legal act of the European Union that becomes immediately enforceable as law in all member states. Thus, this regulation forces all member states to take actions to avoid the spread of the declared invasive species. As environmental conditions differ between members states, the individual states are allowed to respond differently. In Germany, the proposed actions against T. scripta are to increase public awareness, allowing non-commercial transfer of individuals that are already kept in captivity (to avoid these individuals being released into the wild), while the removal of individuals from wild habitats is not considered to be feasible as it is thought to be both ineffective and very costly (StA "Arten-und Biotopschutz" 2018a). Instead, the approach taken in Germany has been to assume that populations of this species are not stable (StA "Arten-und Biotopschutz" 2018a) since cold winter weather would result in high mortality of any released T. scripta [START_REF] Geiger | Überlebensfähigkeit allochthoner Amphibien und Reptilien in Deutschland -Konsequenzen für den Artenschutz[END_REF] and that successful reproduction would only be sporadic [START_REF] Pieh | Die Rotwangen-Schmuckschildkröte (Trachemys scripta elegans) in Baden-Württemberg -mit Hinweis auf eine Reproduktion im Freiland[END_REF] The overall aim was to establish one of the first long-term monitoring programmes in Germany to investigate the population dynamics of T. scripta in an oxbow lake, the Altrhein of Kehl [START_REF] Pieh | Die Rotwangen-Schmuckschildkröte (Trachemys scripta elegans) in Baden-Württemberg -mit Hinweis auf eine Reproduktion im Freiland[END_REF][START_REF] Laufer | Buchstaben-Schmuckschildkröte Trachemys scripta (Schoepff). Die Amphibien und Reptilien[END_REF]. This was undertaken to determine 1) If there is evidence of population decrease or increase, 2) Whether there is evidence of successful reproduction of this population, and 3) Whether there is evidence that additional individuals are released into the wild.

MATERIALS AND METHODS

Study area and study period

The study was conducted in the city of Kehl in Baden-Württemberg, Germany. Kehl is in the Upper Rhine Valley close to the French city of Strasbourg. Data were collected from the Altrhein of Kehl (48° 34'1.95"N, 7° 48'35.41"E), which is an oxbow lake formed over 100 years ago from the River Rhine. It is 690m long and between 25 and 80m wide. The climate in Kehl is temperate, but the Upper Rhine Valley is one of the warmest areas of Germany. During the study period, the minimum temperature in winter was around -12°C (2016), and daily maximum temperatures in summer reached 40°C in 2019. Periods of permanent frost in winter were relatively short with 1 -2 weeks in the years 2016 -2019, and no such period in 2020.

The study was implemented from May to July in the years 2016 to 2019 as part of a course that I gave at the Hector Kinderakademie Kehl, which provides additional education to highly gifted school children. In this course, I taught school children 8 -9 years old about ecology and nature conservation. Due to the COVID-19 crisis, this course was cancelled in 2020. In this year, I used the opportunity to collect data on two sunny days in March (at this stage it was unsure whether later data collection would be possible) as well as during six afternoons in May and June. There was no indication that the March data differed from the later data (peak counts of individuals and species were recorded in May and June, the normal period of monitoring), and excluding these data would not have changed the results.

Monitoring

In the years 2016-2018, the population of exotic pond turtles was monitored on six afternoons, but on seven in 2019, and eight in 2020. At five locations along the eastern shore of the lake, that were previously determined to have a high abundance of pond turtles, observations were made using binoculars. In addition, any pond turtle observed between these locations was recorded. For every individual, the carapace length was estimated to be in one of the following categories: 5cm, 10cm, 20cm or 30cm.

In 2016, only the number and size of observed pond turtles was noted. In this year it became evident that besides T. scripta several other species were present. In the following years, I took photographs of each of the individuals I observed from distances up to 30 m. From 2017 to 2019, photographs were taken with a Panasonic HC-V380EG-K camcorder with 50x optical zoom, and in 2020 with a Nikon Coolpix P1000 Digital camera with 125x optical zoom. Using characteristics of the carapace and especially the head, the genus, species and if possible sub-species of the photographed individuals was determined using a guide to the introduced turtles of Alsace [START_REF] Pellerin | Les tortues introduites[END_REF]). If the individual was in a position where it could not be identified it was scored as 'unknown genus'.

Measurement of hatchlings

I was contacted by kindergarten teachers near the lake when they found hatchlings of T. scripta.

These were measured and then sent to an animal shelter in Munich (Germany) that specialises in reptiles (https://www.reptilienauffangstation.de/). I was also informed about sightings of large females that were believed to have left the lake to lay eggs.

Data analysis

Data have been expressed as peak counts, the maximum number of live individuals observed in one survey afternoon of a particular year. Where the peak count of all turtle species is given then this is the sum of peak counts of several species but potentially from observations made on different afternoons; similarly for the peak counts of individuals in different carapace size categories. Data were analysed using Spearman's rank correlation and Fisher`s exact test in GraphPad Instat 3.05.

RESULTS

Turtle numbers

The annual total peak counts for all species increased continuously from 33 individuals in 2016 to 166 individuals in 2020 (Fig. 1), which resulted in a statistically significant correlation between year and peak count (Spearman rank correlation, r s =1.00, df=3, p=0.02). These findings were contrary to the expectation that the population should decline each year due to cold weather mortality. The expected ratio for a decline each year would be 4:0, when this is compared with the observed ratio of 0:4 the difference is statistically significantly (Fisher's exact test, p=0.03) Over the years, increasing numbers of very small pond turtles with a carapace of not more than 5cm were observed which was close to statistical significance (Spearman rank correlation, rs=0.90, df=3, p=0.08, Fig. 2). All photographed pond turtles in this category were T. scripta.

Additionally, the number of large pond turtles with a carapace of more than 20cm increased over the years (Spearman rank correlation, rs=0.90, df=3, p=0.08, Fig. 2). All pond turtles with a carapace of 30cm belonged to one of the three following species: T.scripta, Pseudemys concinna, or Pseudemys nelsoni.

Altogether six different species were observed, with three subspecies of T. scripta (Table 1). Over the years the number of observed individuals of T. scripta seemed to increase (Fig. 3).

The two species of Pseudemys and Graptemys pseudogeographica occurred in all years. A few individuals of Chrysemys picta and Mauremys sinensis were observed in all years except 2018 (Fig. 3). 1. Annual peak count for all turtle species in the Althrein of Kehl combined (including individuals for which the taxon could not be identified and which are not shown in Table 1) Figure

Breeding of Trachemys scripta

Altogether, 12 T. scripta hatchlings were found on land walking from a park towards the lake, more than 100 m away from the water. Eight were found in the period 12 th to 22 nd May 2019, one the 15 th of July 2019, one the 19 th of May 2020, one the 17 th of June 2020 and one the 30 th June 2020.

Two of these were found dead on a bicycle path between the kindergarten and the lake, one died in the aqua-terrarium of the kindergarten. These dead individuals were disposed of by the kindergarten and not measured. The remaining nine individuals were kept and I was able to measure them, confirming that these were all hatchlings of T. scripta (Fig. 4, Table 2). On 6 th June 2019 a female T. scripta was caught at the kindergarten, with a body mass of 3064g and a carapace length of 29cm. A female T. scripta elegans was seen laying eggs in the park behind the kindergarten on 15th June 2020. The clutch was excavated on 16 th of June, the top of which was 15cm below ground. There were a total of 24 eggs with a length of 32.8 + 1.2mm and a width of 23.9 + 0.7mm. The total clutch weighed 218g which represented 9.1g per egg.

DISCUSSION

In the current study I have shown that the population of exotic pond turtles in an oxbow lake, the Altrhein of Kehl, comprises six exotic species with the invasive T. scripta being the most abundant. Contrary to predictions that in the cold climate of Germany populations would decline [START_REF] Nehring | Die invasiven gebietsfremden Arten der ersten Unionsliste der EU-Verordnung[END_REF]StA "Arten-und Biotopschutz" 2018a;[START_REF] Geiger | Überlebensfähigkeit allochthoner Amphibien und Reptilien in Deutschland -Konsequenzen für den Artenschutz[END_REF], population size seemed to have increased steadily over the five study years. This could be partly due to the release of more individuals into the wild as evidenced by 1) A high proportion of the observed individuals being from genera other than Trachemys, indicating that species that are still available in pet shops are regularly released into the wild, and/or 2) The constant increase of the size category of carapace diameter 20cm and 30cm (Fig. 2), indicating that old individuals have been released, that might have been bought a long time ago and have become too big and as a consequence released by pet owners. Furthermore, the regular observation of hatchlings indicates that it cannot be assumed that the cold climate of Germany will prevent exotic pond turtles from breeding.

I had expected to observe mainly individuals from the three different subspecies of T. scripta in the lake and, based on previous reports, maybe some individuals of Graptemys pseudogeographica [START_REF] Pieh | Die Rotwangen-Schmuckschildkröte (Trachemys scripta elegans) in Baden-Württemberg -mit Hinweis auf eine Reproduktion im Freiland[END_REF][START_REF] Laufer | Buchstaben-Schmuckschildkröte Trachemys scripta (Schoepff). Die Amphibien und Reptilien[END_REF]. Instead it was a surprise to find a high exotic biodiversity with four commonly observed species (T. scripta, P nelsoni, P. concinna, and G. pseudogeographica) and few individuals of the two species Mauremys sinensis and Chrysemys picta (Table 1). This high diversity is most likely a consequence of the trade ban on T. scripta, leading to a trade in other species [START_REF] Maceda-Veiga | What's next? The release of exotic pets continues virtually unabated 7 years after enforcement of new legislation for managing invasive species[END_REF], which in turn have been released into the wild. Overall, the combined peak counts of all pond turtles increased consistently over the years (Fig. 1) which was due to an increased number of both small and large individuals (Fig. 2), especially of T. scripta (Fig. 3).

In southern France, clutch size of T. scripta varied between four and 11 eggs [START_REF] Cadi | Successful reproduction of the introduced slider turtle (Trachemys scripta elegans) in the South of France[END_REF], while here I found one clutch consisting of 24 eggs. I also observed one very large female of 3 kg that probably laid a clutch. Egg laying by itself is not evidence of successful reproduction since females will lay eggs whether or not they are fertilised and it is unknown whether fertilised eggs would develop successfully under the prevailing climatic conditions. In the current study, very small T. scripta were observed in four out of five years, making it likely that reproduction occurred repeatedly. The capture of hatchlings in both 2019 and 2020 showed that successful reproduction had occurred, especially as they were found in a park hundreds of meters away from the lake (any releases would have been into the lake). This is the second case of proven reproduction in Kehl with the first case having been in 2004 [START_REF] Pieh | Die Rotwangen-Schmuckschildkröte (Trachemys scripta elegans) in Baden-Württemberg -mit Hinweis auf eine Reproduktion im Freiland[END_REF]. The absence of a report of reproduction in Kehl for the years 2005 to 2016 can be explained by a lack of monitoring during these years. In 2019, the seven hatchlings found in Kehl within 10 days in May plus the single hatchling found in July indicates that these might be from two or three different clutches.

The hatchlings must likely originate from clutches laid in summer 2018, and probably overwintered in a nest burrow as suggested by [START_REF] Pieh | Die Rotwangen-Schmuckschildkröte (Trachemys scripta elegans) in Baden-Württemberg -mit Hinweis auf eine Reproduktion im Freiland[END_REF] Reproduction of released T. scripta has been reported in southern Europe [START_REF] Cadi | Successful reproduction of the introduced slider turtle (Trachemys scripta elegans) in the South of France[END_REF][START_REF] Perez-Santigosa | The reproductive ecology of exotic Trachemys scripta elegans in an invaded area of southern Europe[END_REF][START_REF] Sperone | Sightings and successful reproduction of allochthonous reptiles in Calabria[END_REF][START_REF] Sancho | Expansion of Trachemys scripta in the Valencian Community (Eastern Spain)[END_REF][START_REF] Foglini | Non-native turtles in a peri-urban park in northern Milan (Lombardy, Italy): species diversity and population structure[END_REF], but also in a part of Croatia with a continental climate [START_REF] Koren | The current distribution of pond slider Trachemys scripta (Reptilia: Emydidae) in Croatia[END_REF], as well as in central Europe in Slovenia [START_REF] Standfuss | Threat or fiction: is the pond slider (Trachemys scripta) really invasive in Central Europe? A case study from Slovenia[END_REF]. The regular reproduction of T. scripta reported here for Germany adds to previous records of successful reproduction in other temperate climates.

Even though successful reproduction of T. scripta in Europe has been observed, it has been argued that this will not make the species invasive, as sex determination in this species is temperature dependent [START_REF] Geiger | Überlebensfähigkeit allochthoner Amphibien und Reptilien in Deutschland -Konsequenzen für den Artenschutz[END_REF]. The sex ratio of clutches is influenced by soil temperature, and at low temperatures all hatchlings are male while both sexes can be produced from incubation temperatures between 28.3 and 30.6°C [START_REF] Cadi | Successful reproduction of the introduced slider turtle (Trachemys scripta elegans) in the South of France[END_REF]). Female hatchlings have been observed both in Spain (Perez-Santigosa, Díaz-Paniagua & Hidalgo-Vila 2008) and in southern France [START_REF] Cadi | Successful reproduction of the introduced slider turtle (Trachemys scripta elegans) in the South of France[END_REF]. So far, no soil temperature data are available for Kehl, but air temperatures above 33°C during the day and above 20°C at night are common in summer and often occur for periods of two weeks or more. The hatchlings captured in the current study were too small to be sexed. Climate warming will make it more likely that females will hatch in the future.

To make management decisions for the long-lived, slow reproducing T. scripta, it is essential to estimate how their populations will develop for several decades. Most T. scripta sold in pet shops are female, as breeders in the USA used high temperatures for faster hatching, and higher temperatures induce a female biased sex-ratio [START_REF] Prevot | Pets and invasion risks: Is the slider turtle strictly carnivorous?[END_REF]. The combination of a potential female biased sex ratio of released sliders [START_REF] Prevot | Pets and invasion risks: Is the slider turtle strictly carnivorous?[END_REF]) and a male biased sex ratio of hatchlings could cause an increased population growth in the medium-term (5-20 years), as more females will lay fertilized eggs. An overall increase in the number of hatchlings together with rising temperatures due to global warming makes it more likely that female hatchlings will occur in the coming decades. Heat waves, such as the one experienced in Kehl in 2019 when temperatures reached 40°C, will increase the number of female hatchlings [START_REF] Breitenbach | Using naturalistic incubation temperatures to demonstrate how variation in the timing and continuity of heat wave exposure influences phenotype[END_REF]. This scenario makes it likely that the T. scripta population in Kehl is now fully entrenched and that other populations will become established in other areas of the Upper Rhine Valley.

Pseudemys nelsoni, P. concinna, G. pseudogeographica and M. sinensis have a low risk of becoming invasive in Europe [START_REF] Kopecký | Establishment risk from pet-trade freshwater turtles in the European Union[END_REF][START_REF] Masin | Risk of invasion by frequently traded freshwater turtles[END_REF]. In contrast, T. scripta is generally regarded as being invasive, as in many countries it can survive, breed, and colonise additional habitats with negative impact on the native biodiversity [START_REF] Lowe | 100 of the World's Worst Invasive Alien Species. A selection from the Global Invasive Species Database[END_REF].

However, detailed studies on its effect on the native fauna and flora are rare. In France, young T. scripta are omnivores, eating invertebrates, amphibians and fish, while older individuals have a more herbivorous diet [START_REF] Prevot | Pets and invasion risks: Is the slider turtle strictly carnivorous?[END_REF]. They have been observed to disturb the nests of water birds when searching for basking spots [START_REF] Laufer | Buchstaben-Schmuckschildkröte Trachemys scripta (Schoepff). Die Amphibien und Reptilien[END_REF]. They might have a negative impact on the endangered native European pond turtles (Emys orbicularis) by trying to mate with them [START_REF] Jablonski | Attempting courtship between Emys orbicularis and Trachemys scripta (Testudines: Emydidae)[END_REF] and by competing for basking spots [START_REF] Cadi | Competition for basking places between the endangered European pond turtle (Emys orbicularis galloitalica) and the introduced red-eared slider (Trachemys scripta elegans)[END_REF]. In Kehl, the oxbow lake is not known as an important site for native biodiversity, but there are ecologically important areas close by (less than 1km), especially the Rhine floodplains. Increasing population size in the lake increases the risk that these habitats would be colonized. Further, T. scripta have also been observed next to the Kehler nature reserve Sundheimer Grund (less than 3kms from the Altrhein), an ecologically important and vulnerable habitat (Schradin, personal observation).

Implications for German and European legislation

The regulation (EU) No 1143/2014 demands that all member states should take action against invasive species (European Parliament 2014) including T. scripta (European Commission 2016).

After the trading ban in 2006 it was expected that the release of T. scripta into the wild would decline in Germany. But the current study, undertaken 14 years later, does not support this assumption. In Germany, it has been believed that the only action needed was to advise the public not to release exotic pond turtles into the wild since it was too cold for the released individuals to survive for long periods or to reproduce [START_REF] Geiger | Überlebensfähigkeit allochthoner Amphibien und Reptilien in Deutschland -Konsequenzen für den Artenschutz[END_REF][START_REF] Pieh | Die Rotwangen-Schmuckschildkröte (Trachemys scripta elegans) in Baden-Württemberg -mit Hinweis auf eine Reproduktion im Freiland[END_REF][START_REF] Laufer | Buchstaben-Schmuckschildkröte Trachemys scripta (Schoepff). Die Amphibien und Reptilien[END_REF][START_REF] Nehring | Die invasiven gebietsfremden Arten der ersten Unionsliste der EU-Verordnung[END_REF]). Thus, populations in Germany are officially believed to be unstable [START_REF] Nehring | Die invasiven gebietsfremden Arten der ersten Unionsliste der EU-Verordnung[END_REF]StA "Arten-und Biotopschutz" 2018b), which has influenced the national action plan ( StA "Arten-und Biotopschutz" 2018a). In light of the facts uncovered by the current study that population size is not decreasing, there is no indication of high mortality and successful reproduction occurs more regularly than previously believed, the management plan in Germany should be reconsidered.

The results presented here add to several other studies indicating that T. scripta has the potential to become invasive in western, middle and central Europe [START_REF] Mačát | Good invasion ability is not enough: Predation on the pond slider (Trachemys scripta) by the wels catfish (Silurus glanis) in the Czech Republic[END_REF] sliders from wetlands has been suggested [START_REF] Cadi | Successful reproduction of the introduced slider turtle (Trachemys scripta elegans) in the South of France[END_REF], and this has been done in Spain [START_REF] Sancho | Expansion of Trachemys scripta in the Valencian Community (Eastern Spain)[END_REF]. While the invasion is slower in colder regions, the fact that the exotic populations there do not decline but reproduce and increase should be of concern. Additional actions are required; good examples have been suggested by [START_REF] Teillac-Deschamps | Management strategies in urban green spaces: Models based on an introduced exotic pet turtle[END_REF]). An obvious need is public education with a focus on pet owners such as a social media campaign targeting Facebook groups of owners of exotic pond turtles. Education is also needed for law enforcement (police, order enforcement offices, wardens), whose focus is often the poor pet owner who has 'lost' a pond turtle instead of the criminal that released it. Police statements to the local news outlets could then emphasize the unlawfulness of the release of exotic animals. Local order enforcement officers could check owners of exotic pond turtles to ensure no breeding takes place and animals kept in the garden cannot escape. An active search for advertisements posted by people selling hatchlings of exotic pond turtles followed by legal action would be useful. Alternatives to release should be made available which means that more financial state support is needed for animal shelters to create more space for unwanted pond turtles.

Other important actions would be 1) Destruction of clutches; 2) Removal of large females from the population when they are found on land before / after laying clutches, and 3) Active removal of released exotic pond turtles, especially from ecologically valuable waters. For this, local resources would be needed, especially staff being able to perform and coordinate these actions.

Collaboration between nature conservation officers of the local communities with nature conservation groups, hunters, and local fishery association are needed for this. Last but not least, the increased release of other exotic pond turtle species leads to the question which of these species should also be banned from trade, and how the trade is regulated to ensure that individuals will not be released. Individual marking (transponders) and registration of exotic pond turtles to allow previous owner to be identified may be an effective option.

Going forward

The population of exotic pond turtles at the Althrein of Kehl has not decreased since the ban of the trade of T. scripta in the European Union. On the contrary, the diversity of species being released has increased. In addition, the population size of T. scripta is increasing, and there is no indication of high mortality due to cold winters. In contrast, there seems to be regular successful reproduction in summer. This might lead to a more balanced sex ratio in the future and as such a further increase in reproduction and population size. It is now important to measure soil temperatures, to determine the sex ratio of hatchlings, and to combine this with climate predictions for the next 30 years. Furthermore, ecologically important wetlands close to waters into which pond sliders have been released must be monitored and actions must be taken to remove pond sliders from these areas before they can establish themselves there. The further growth of existing neobiotic pond slider populations must be stopped. The current action plan in Germany for T. scripta will have to change if an invasion is to be avoided. Other west and central European countries might have to apply similar action plans.

Figure 3 .

 3 Figure 2. Annual peak counts for all turtle species observed in the Althrein of Kehl by carapace size category from 2016 to 2020

Figure 4 .

 4 Figure 4. Three different T. scripta elegans hatchlings found at the Altrhein of Kehl in May 2019. The scale on the left photo is in cm.

  . Reproduction might have been possible due to the recently mild winters, as it is known that eggs easily survive temperatures as low as -4°C in the soil (Tucker & Packard 1998), which would represent much colder air temperatures. For example, in the winter 2019 / 2020, frost was uncommon in Kehl. In its northern range, mean summer temperatures are around 23°C (June to August, embryonic period) and winter temperatures 6-8°C (September to April, posthatching dormancy) (Willette, Tucker & Janzen 2005). While mean summer temperatures in Kehl are clearly colder (18.7°C), winters are within the reported range (7.9°C) (data from the closest weather station at Entzheim, France). As such, the climate in Kehl does not preclude successful reproduction. The size and mass of both hatchlings and eggs was similar to those reported from their natural habitat (Tucker 2000) as well as other populations in Europe (Perez-Santigosa, Díaz-Paniagua & Hidalgo-Vila 2008). The mean egg mass reported here was 9.1 g compared to 10.5 g in Illinois and 9.4 g in Spain (Perez-Santigosa, Díaz-Paniagua & Hidalgo-Vila 2008), while hatchling mass in the current study was 6.5 + 0.7 g compared to 6.8 + 0.8 g in Illinois (Tucker 2000) and 6.7 + 1.0 g in Spain (Perez-Santigosa, Díaz-Paniagua & Hidalgo-Vila 2008). As such, data from both eggs and hatchlings do not indicate constraints on reproduction of T. scripta in Kehl, Germany.

Table 1 .

 1 Annual peak counts for the turtle species observed in the Althrein of Kehl. Cloudy weather in 2018 made it difficult to determine the subspecies / species of both Trachemys and Pseudemys. Individuals for which the taxon could not be determined have been excluded.

Genus, species, or sub-species

	TOTAL		38	32	43	87
	Figure
				Peak counts
			2017	2018	2019	2020
	Trachemys	scripta
	undetermined	sub-	1	7	5	14
	species
	Trachemys s. elegans	8	5	11	24
	Trachemys s. scripta	4	1	6	4
	Trachemys s. troosti	1	0	5	1
	Pseudemys sp		3	12	2	1
	Pseudemys nelsoni	3	1	1	7
	Pseudemys concinna	10	3	7	26
	Graptemys pseudepgeographica	6	3	4	7
	Mauremys sinensis	1	0	1	2
	Chrysemys picta	1	0	1	1
				7

Table 2 .

 2 Measurements of nine hatchling T. scripta found at the Altrhein of Kehl (six from 2019 and three from in 2020) and of hatchlings from the USA* for comparison

	Trait (mm or g)	Mean	Standard deviation	Maximum	Minimum	Data from the USA (mean ± SD)*
	Carapace length	30.6	1.3	32.0	28.0	31.2 + 1.4
	Carapace width	30.4	1.3	32.0	29.0	-
	Carapace height	15.8	1.5	19.0	13.5	-
	Plastron length	28.7	2.0	31.0	26.0	-
	Plastron width	22.3	2.1	27.0	19.5	-
	Body mass (g)	6.5	0.7	7.8	5.3	6.9 + 0.8
	*Data from Illinois, USA (Tucker 2000)

ACKNOWLEDGEMENTS

This project was funded by the CNRS. The project was done as part of a course I do at the Hector Kinderakademie in Kehl, teaching 8 to 9 years old children about animal behaviour, population biology, animal welfare and nature conservation. I am very thankful to the Hector Kinderakademie in Kehl for their support. I am thankful to members of the Facebook group "Wasserschildkröten" which assisted me in identifying species from photographs, especially to N. Ziegenhagen, who runs an animal shelter for exotic pond turtles. I would also like to thank the city of Kehl (Dr. A.-M. Amui-Vedel, Bereich Stadtplanung/Umwelt) and the Regierungspräsidium Freiburg (S. Person, M.