V Regard
email: regard@lmtg.obs-mip.fr

D Hatzfeld

M Molinaro

C Aubourg

R Bayer

O Bellier

F Yamini-Fard

M Peyret

M Abbassi

à la diffusion de documents scientifiques de niveau recherche, publiés ou

Keywords: subduction, collision, Iran, Zagros, Makran, transform fault Running title: Transition Zagros-Makran

non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

The transition between Makran subduction and the Zagros collision: recent advances in its structure and Active deformation 1 Introduction

Spatial transition between subduction and continental collision is by itself unstable and often a transform fault will develop to accommodate the differences in tectonic setting as the Chaman fault in central Asia does [START_REF] Lawrence | Chaman Fault, Pakistan-Afghanistan[END_REF]. Interestingly, the Hormoz strait area in Iran (26.5°N; 56.5°E, Figure 1) displays such a setting, but in a juvenile stage. At this point Arabia converges northward towards Eurasia at a velocity of 23-25 mm yr -1 according to GPS measurements [START_REF] Bayer | Active deformation in the Zagros-Makran transition zone inferred from GPS measurements in the interval 2000-2002[END_REF][START_REF] Mcclusky | GPS constraints on Africa (Nubia) and Arabia plate motions[END_REF][START_REF] Vernant | Contemporary Crustal Deformation and Plate Kinematics in Middle East Constrained by GPS Measurements in Iran and Northern Oman[END_REF][START_REF] Bayer | Active deformation in Zagros-Makran transition zone inferred from GPS measurements[END_REF][START_REF] Masson | Large-scale velocity field and strain tensor in Iran inferred from GPS measurements: new insight for the present-day deformation pattern within NE Iran[END_REF]. The Arabian plate is oceanic to the east in the Oman Gulf whereas it is continental to the west in the Arabian platform (Figure 1). As expected, the Molinaro et al., 2005a). The arrows represent the velocity relative to Eurasia [START_REF] Vernant | Contemporary Crustal Deformation and Plate Kinematics in Middle East Constrained by GPS Measurements in Iran and Northern Oman[END_REF]. Note that the convergence accommodated through the Zagros Fold-and-Thrust Belt (ZFTB) is only ~9mm/y, and there is no value currently available for the part of the Arabia-Eurasia convergence accommodated through the Makran between Arabia and Lut (except using [START_REF] Vernant | Contemporary Crustal Deformation and Plate Kinematics in Middle East Constrained by GPS Measurements in Iran and Northern Oman[END_REF]'s results with some hypotheses). MZT: Main Zagros Thrust; SSZ: Sanandaj Sirjan Zone.

This area has been studied for some times by geologists and geophysicists. The latter described a sharp boundary between the eastern and western domains called the 'Oman Line'. Despite the associated low seismicity its sharpness was interpreted as evidence of a transform fault [START_REF] Kadinsky-Cade | Seismotectonics of Southern Iran: the Oman Line[END_REF].

In order to provide a better understanding of this newly-evolving transform fault system, the area has been recently studied with different geological and geophysical techniques in the framework of an Iranian-French collaboration including palaeomagnetism [START_REF] Aubourg | Post-Miocene shortening pictured by magnetic fabric across the Zagros-Makran syntaxis[END_REF][START_REF] Smith | Kinematics of a sigmoidal fold and vertical axis rotation in the east of the Zagros-Makran syntaxis (southern Iran): Paleomagnetic, magnetic fabric and microtectonic approaches[END_REF][START_REF] Aubourg | Tertiary block rotations in the Fars Arc (Zagros, Iran)[END_REF], tertiary and active tectonics [START_REF] Molinaro | The origin of changes in structural style across the Bandar Abbas syntaxis, SE Zagros (Iran)[END_REF][START_REF] Regard | Accommodation of Arabia-Eurasia convergence in the Zagros-Makran transfer zone, SE Iran: A transition between collision and subduction through a young deforming system[END_REF]Molinaro et al., 2005a;Regard et al., 2005a), seismology [START_REF] Yamini-Fard | Sismotectonique et structure lithosphérique de deux zones de transition dans le Zagros (Iran): la zone de Minab et la zone de Qatar-Kazerun[END_REF]2007;in prep), and geodesy [START_REF] Vernant | Contemporary Crustal Deformation and Plate Kinematics in Middle East Constrained by GPS Measurements in Iran and Northern Oman[END_REF][START_REF] Bayer | Active deformation in Zagros-Makran transition zone inferred from GPS measurements[END_REF][START_REF] Masson | Large-scale velocity field and strain tensor in Iran inferred from GPS measurements: new insight for the present-day deformation pattern within NE Iran[END_REF]. The purpose of this paper is to provide a synthesis of these acquired data and to discuss this subduction-collision transition. Thus, after introducing the general setting of the area, the present-day and recent deformation and the upper crustal structure from balanced cross-sections, we discuss the deep crustal structure revealed by seismological studies and finally present the actual deformation rates from GPS/Geodesy.

Geological Setting

Zagros

The Zagros mountain belt is a NW-trending fold-and-thrust belt, consisting of a 6-15km thick sedimentary pile which overlies Precambrian metamorphic basement [START_REF] Mccall | Minab quadrangle map 1:250,000 and explanatory text[END_REF][START_REF] Mccall | The geotectonic history of the Makran and adjacent areas of southern Iran[END_REF]. The sedimentary cover can be divided into three successive sequences. First, at its base, it is comprised of thick late Precambrian evaporitic deposits (the so-called 'Hormoz Salt') which constitute the main regional décollement for most of the larger folds within the Zagros fold-and-thrust belt (ZFTB). This layer is the origin of numerous salt diapirs that have pierced the overlying sedimentary cover and risen to the surface. A ~4000 m thick Cambrian to Eocene sequence forms the so-called Competent Group. Apart from the initial Cambrian-Carboniferous clastic formations, the majority of this group until Upper Cretaceous consists of massive platform carbonate rocks [START_REF] James | Stratigraphic nomenclature of Iranian oil consortium agreement area[END_REF][START_REF] Faure-Muret | Aperçu de l'évolution structurale de l'Iran[END_REF][START_REF] Szabo | Permian and Triassic stratigraphy, Zagros Basin, southwest iran[END_REF][START_REF] Sharland | Arabian plate sequence stratigraphy[END_REF]. The remainder of the stratigraphic sequence is represented by the Miocene to Recent clastic sediments of the Incompetent Group. These molasse-type sediments, derived from the uplift and erosion of the Zagros Mountains, show a typical coarsening-up evolution from marine-to-continental clastics to coarse proximal conglomerates at the top [START_REF] James | Stratigraphic nomenclature of Iranian oil consortium agreement area[END_REF][START_REF] Edgell | Salt tectonism in the Persian Gulf Basin[END_REF][START_REF] Hessami | The significance of strike-slip faulting in the basement of the Zagros fold and thrust belt[END_REF].

Makran

The Makran accretionary wedge stretches from Iran to central Pakistan and off the south coast of this area [START_REF] Schluter | The Makran accretionary wedge: sediment thicknesses and ages and the origin of mud volcanoes[END_REF]. It has been formed by the subduction of the oceanic portion of the Arabian Plate beneath Eurasia and is built up by sediments scraped off the Arabian Plate since early Tertiary [START_REF] Berberian | Towards a paleogeography and tectonic evolution of Iran[END_REF][START_REF] Harms | The Makran coast of Pakistan: its stratigraphy and hydrocarbon potential[END_REF][START_REF] Kopp | Structure of the Makran Subduction zone from wide angle and reflection seismic data[END_REF]. Subduction was probably initiated during Palaeocene [START_REF] Platt | Slip vectors and fault mechanics in the Makran accretionary wedge, southwest Pakistan[END_REF] and accretion started during Eocene times [START_REF] Byrne | Great Thrust Earthquakes and Aseismic Slip Along the Plate Boundary of the Makran Subduction Zone[END_REF]. The modern Makran accretionary prism has developed since Late Miocene [START_REF] Platt | Large-scale underplating in the Makran accretionary prism, southwest Pakistan[END_REF][START_REF] Platt | Slip vectors and fault mechanics in the Makran accretionary wedge, southwest Pakistan[END_REF], and is still propagating seaward at a rate of ~10 mm yr -1 [START_REF] White | Deformation of the Makran accretionary sediment prism in the Gulf of Oman (north-west Indian Ocean). In "Trench and Fore-Arc Geology: Sedimentation and Tectonics on Modern and Ancient Active Plate Margins[END_REF]. Two features make this accretionary wedge unusual: (1) the sediment thickness on top of the oceanic crust is extremely high (at least 6 km); and (2) the dip angle of subduction is extremely low (~5 degrees, [START_REF] Jacob | The Makran region of Pakistan and Iran: Trencharc system with active plate subduction[END_REF][START_REF] Byrne | Great Thrust Earthquakes and Aseismic Slip Along the Plate Boundary of the Makran Subduction Zone[END_REF][START_REF] Carbon | Tectonique post-obduction des montagnes d'Oman dans le cadre de la convergence Arabie-Iran[END_REF].

Transition Zagros-Makran

The Zagros and the Makran domains are both bounded to the north by a continuous ophiolitic belt along the Main Zagros reverse fault and its eastern continuation along the Makran Thrust (Figure 1) [START_REF] Mccall | The geotectonic history of the Makran and adjacent areas of southern Iran[END_REF]. South of this suture, the Zagros and the Makran regions behave differently, highlighting how the subduction and collision settings differ. Indeed, while the convergence velocity accommodated by the Zagros collision increases progressively from northwest to southeast, the transition from the collision to the Makran subduction is marked by a jump from 9±2 mm/yr to ~19±2 mm/yr [START_REF] Vernant | Contemporary Crustal Deformation and Plate Kinematics in Middle East Constrained by GPS Measurements in Iran and Northern Oman[END_REF][START_REF] Masson | Large-scale velocity field and strain tensor in Iran inferred from GPS measurements: new insight for the present-day deformation pattern within NE Iran[END_REF]. The Zagros-Makran transition is thus expected to have a wrench motion of at least some 10 mm/yr. During late Cenozoic, Arabia and Eurasia continuously converged (figure 2; [START_REF] Mcquarrie | Cenozoic evolution of Neotethys and implications for the causes of plate motions[END_REF]. Palinspastic reconstructions show how this continuous convergence was accommodated in Zagros and Makran during the last 30 My (Figure 2) suggesting that the Zagros-Makran transition zone must have been structured during the last ~15 My. [START_REF] Hay | The Evolution of Cretaceous Ocean/Climate Systems[END_REF][START_REF] Soeding | ODSN Plate Tectonic Reconstruction Service[END_REF] [START_REF] Regard | Accommodation of Arabia-Eurasia convergence in the Zagros-Makran transfer zone, SE Iran: A transition between collision and subduction through a young deforming system[END_REF], which are distributed in two fault systems, the Jiroft, Sabzevaran (abbreviated as Sabz.) and Kahnuj faults in the Jiroft-Sabzevaran fault system (JS); the Minab, Zendan and Palami faults in the Zendan-Minab fault system (ZMP). A-A' and B-B' are the cross-sections (Figure 5) from [START_REF] Molinaro | The origin of changes in structural style across the Bandar Abbas syntaxis, SE Zagros (Iran)[END_REF]. C-C' is the local tomography cross-section (Figure 8), from [START_REF] Yamini-Fard | The diffuse transition between the Zagros continental collision and the Makran oceanic subduction (Iran): microearthquake seismicity and crustal structure[END_REF].

The transition takes place at the front of the mountainous Musandam peninsula (Hormoz Strait, Figure 1). Formed during Late Cretaceous, this mountain range not only magnifies the differences between the continent and the ocean, but also acts as heterogeneity within the colliding Arabia plate. It can be traced up to the Hormoz Strait, in a seismic profile running from Qeshm to Minab [START_REF] Ross | The Geology of the Persian Gulf-Gulf of Oman Region: a synthesis[END_REF]. Such a set-up suggests that the Oman peninsula may have interfered with the Zagros collision and be partly responsible for the curved shape of the Zagros Makran transition [START_REF] Ricou | Le Zagros[END_REF][START_REF] Kadinsky-Cade | Seismotectonics of Southern Iran: the Oman Line[END_REF][START_REF] Aubourg | Tertiary block rotations in the Fars Arc (Zagros, Iran)[END_REF].Several other mechanisms have been proposed to explain this curved shape of the Fars Arc: (1) rotation of basement faults [START_REF] Hessami | The significance of strike-slip faulting in the basement of the Zagros fold and thrust belt[END_REF]; (2) it could be produced by a change in the style of accommodation of shortening from head-on shortening in the south-east to oblique shortening to the north-west of the Zagros fold-and-thrust belt (ZFTB) observed by [START_REF] Talebian | A reappraisal of earthquake focal mechanisms and active shortening in the Zagros mountains of Iran[END_REF]; (3) a Jura style of deformation in which the arcuate shape is controlled by the progressive lateral pinch-out of the basal Hormoz evaporates upon which the Zagros folds detach (Molinaro et al., 2005a).

The transition between Zagros and Makran is also often described in the literature as the 'Oman Line' trending N20°E and running northward from the Musandam peninsula [START_REF] Kadinsky-Cade | Seismotectonics of Southern Iran: the Oman Line[END_REF]. It limits the continent to the east from the Oman Gulf oceanic crust to the west: it represents an inactive transform zone inherited from the Neotethys ocean opening [START_REF] White | Tectonics of the Western Gulf of Oman[END_REF]. This transition marks the boundary between a region of high seismicity located in the north-west (the Zagros domain) and a region of low-seismicity to the east and is highlighted by the Musandam Peninsula trend. Formerly the Oman line was suggested to represent the transition between Zagros and Makran (Kadinsky-Cade and [START_REF] Kadinsky-Cade | Seismotectonics of Southern Iran: the Oman Line[END_REF], but it is now thought that this role is played by the Minab-Zendan Fault system, whose southern prolongation has been seen on seismic profiles from the Oman Gulf [START_REF] Ravaut | The Sohar Basin, Western Gulf of Oman: description and mechanisms of formation from seismic and gravity data[END_REF].

The presented multidisciplinary study aims at better determining the structure and kinematics of this key area. In particular, the Zendan-Minab fault system (Figure 3) is studied to help us understand why, as it seems to play a major role in the accommodation of the deformation, the associated seismicity is low. We also discuss the deformation related to this zone and see if it could be the result of the indentation by the Musandam Peninsula.

Results

Deformation pattern

Tectonics

Using satellite images, and structural and geomorphic field observations, [START_REF] Regard | Accommodation of Arabia-Eurasia convergence in the Zagros-Makran transfer zone, SE Iran: A transition between collision and subduction through a young deforming system[END_REF] illustrated the study area's present-day deformation pattern accommodated by faulting. The study area shows a distributed deformation pattern covering a wide domain (Figure 3). Six N-to NW-trending major faults were identified, each one displaying clear evidences of late Quaternary reverse right-lateral slip. They constitute two fault systems. The first one encompasses, from west to east, the Minab, Zendan and Palami faults (the ZMP fault system; Figure 3). The Zendan fault corresponds to the lithological boundary between the Zagros and Makran. The second one, the Jiroft-Sabzevaran (JS) fault system, comprises the Jiroft, Kahnuj and Sabzevaran N-trending faults (Figure 3). The ZMP fault system transfers the Zagros (continental prism) deformation to the Makran accretionary prism, whereas the JS fault system transfers some motion northward to the Alborz/Kopet Dagh convergence zone in northern Iran (Figures 1 &3). Tectonic study and fault slip vector analyses indicate that two distinct tectonic regimes have occurred successively since the Miocene within a consistent regional NE-trending compression: (1) an upper Miocene to Pliocene tectonic regime characterised by partitioning between reverse faulting and en echelon folding; (2) a NEtrending σ 1 axis transpressional regime homogeneously affecting the region since upper Pliocene [START_REF] Regard | Accommodation of Arabia-Eurasia convergence in the Zagros-Makran transfer zone, SE Iran: A transition between collision and subduction through a young deforming system[END_REF]. The change is contemporaneous with a major regional tectonic reorganisation [START_REF] Allen | Late Cenozoic reorganization of the Arabia-Eurasia collision and the comparison of short-term and long-term deformation rates[END_REF]. This study provides evidence of active deformation that is not localised, but distributed across a wide zone. It accommodates the convergence and transfers it from collision to subduction by transpressional tectonics without any partitioning process in the present-day period.

Magnetism

Several studies using magnetic fabric data were conducted in the Fars Arc and in the eastern Makran [START_REF] Bakhtari | Magnetic fabrics of tertiary sandstones from the Arc of Fars (Eastern Zagros, Iran)[END_REF][START_REF] Aubourg | Composite ferromagnetic fabrics (magnetite, greigite) measured by AMS and partial AARM in weakly strained sandstones from western Makran, Iran[END_REF][START_REF] Aubourg | Post-Miocene shortening pictured by magnetic fabric across the Zagros-Makran syntaxis[END_REF][START_REF] Smith | Kinematics of a sigmoidal fold and vertical axis rotation in the east of the Zagros-Makran syntaxis (southern Iran): Paleomagnetic, magnetic fabric and microtectonic approaches[END_REF]. In the Agha-Jari formation (Upper Miocene-Pleistocene silicoclastic rocks), the magnetic foliation is parallel to the bedding, whilst the magnetic lineation strikes perpendicular to the horizontal shortening or layer parallel shortening (LPS). Several studies from other thrust belts (e.g., in the Pyrenees, [START_REF] Averbuch | Magnetic fabric as a structural indicator of the deformation path within a fold thrust structure: a test case from the corbières (NE Pyrenees, france)[END_REF][START_REF] Pares | Evolution of magnetic fabrics during incipient deformation of mudrocks (Pyrenees, northern Spain)[END_REF] suggest that the magnetic lineation records LPS with anticipation before any detectable folding. [START_REF] Bakhtari | Magnetic fabrics of tertiary sandstones from the Arc of Fars (Eastern Zagros, Iran)[END_REF][START_REF] Aubourg | Post-Miocene shortening pictured by magnetic fabric across the Zagros-Makran syntaxis[END_REF][START_REF] Smith | Kinematics of a sigmoidal fold and vertical axis rotation in the east of the Zagros-Makran syntaxis (southern Iran): Paleomagnetic, magnetic fabric and microtectonic approaches[END_REF]; see text for explanations.

Figure 4. Sum of magnetic studies on the Fars Arc and western Makran

Near our study area, Aubourg et al. ( 2004) compared the magnetic fabric data with Paxes earthquakes focal mechanisms and found a good agreement west of the Zagros-Makran transition, contrasting with significant differences in the east. Data collected during the last ten years are summarised in Figure 4. To the west, in the Fars Arc, the general trend is a shortening direction (LPS) close to the GPS convergence direction (N-S), with slight anticlockwise rotations. Just west from and within the Minab-Zendan fault system, the shortening direction is roughly normal to the structures whereas clockwise rotations are recorded. To the east (within the Makran), data are sparse and do not show a clear signal, despite many LPS directed parallel to the N-S convergence direction. [START_REF] Smith | Kinematics of a sigmoidal fold and vertical axis rotation in the east of the Zagros-Makran syntaxis (southern Iran): Paleomagnetic, magnetic fabric and microtectonic approaches[END_REF] and [START_REF] Aubourg | Tertiary block rotations in the Fars Arc (Zagros, Iran)[END_REF] measured palaeomagnetic data in the Agha Jari formation (Mio-Pliocene) to document vertical axis block rotations. The pre-tilting palaeomagnetic component B documents the rotation that occurred between the age of Agha-Jari Fm. and recent time (Figure 4). At first glance, it is apparent that clockwise and counterclockwise rotations of small magnitude (typically less than 20°) occurred respectively in the eastern and western Zagros-Makran transition zone. If the block rotation is removed, the LPS directions are in good agreement with the overall convergence direction. This suggests that the present shape of the Zagros-Makran transition zone, oblique to the convergence direction, has been acquired only recently (since Mio-Pliocene).

In sum, palaeomagnetic and magnetic fabric data (LPS) document the ~N-S convergence, except near the Zagros-Makran transition zone, where the structures have experienced recent clockwise rotations that may continue at present (Figure 4). These rotations agree with a wrench zone that is not localised on a single structure but over an area that is some tens of kilometres wide. The Fars Arc exhibits slight anticlockwise rotations suggesting it has experienced some indentation since Middle Miocene (Figure 4). [START_REF] Molinaro | The origin of changes in structural style across the Bandar Abbas syntaxis, SE Zagros (Iran)[END_REF]Molinaro et al., 2005a). See Figure 3 for profile location. AA'-cross-section through the Minab-Zendan Fault System. BB' -cross-section from Bandar Abbas to Hajiabad, southeastern Zagros. MFF, Main Front Fault, HZF, High Zagros Fault, MZT, Main Zagros Thrust, SSZ, Sanandaj-Sirjan Zone.

Balanced cross-sections

Molinaro et al. constructed two cross-sections on each side of the Zagros-Makran transition [START_REF] Molinaro | The origin of changes in structural style across the Bandar Abbas syntaxis, SE Zagros (Iran)[END_REF]Molinaro et al., 2005a). The contribution of this work is not only to provide quantitative shortening values but also to reveal how the deformation is accommodated. The section on the eastern side of the transition indicates some 6 km shortening in front of the Zendan fault, affecting only the Tertiary cover, over a décollement approximately 6 km deep (Figure 5; section AA' in Figure 3). The shortening is measured perpendicular to the Zendan fault, i.e. along a N150°E-trend. In the south-east Zagros, the Bandar Abbas-Hadjiabad section (BB' in Figure 3) displays two different N-S shortening values of 10 and 45 km, respectively, for the basement and the cover (Figure 5). In particular, with this section the authors document two main steps in the evolution of the Zagros foldthrust belt. In the first Mio-Pliocene stage the deformation was thin-skinned in style, with a décollement lying approximately 8-9 km deep. In the second Pliocene to Recent shortening stage, the basement must be involved through major thrust faults, inferred from focal mechanisms and observation of steps in the general topography and structural elevation of the Zagros mountains. These faults ramp up through the cover and cut the former folds obliquely: the best example is the Kuh-e-Khush (Figure 2). This obliquity can be due either to rotation of folds, reactivation of old basement structures or stress rotation. Geographically, to the west, in the Zagros, deformation is involving the entire crust with folding in the cover controlled by the basal Hormoz salt layer and major thrusting in the basement. To the east on the other hand, folding is controlled by a décollement located at 6 km depth and the basement does not appear to be involved in the deformation.

Seismotectonics and deep structure

Seismotectonics

Yamini-Fard et al. ( 2007) installed a dense seismological mobile network of 24 stations for 8 weeks in 1999, in the area of Bandar Abbas, to locate precisely the microseismicity of the area and calculate focal mechanisms (Figure 6). In addition, they realised a tomographic traverse, with 25 stations from Hadjiabad to Minab and the Makran ranges (Figure 7).

The microearthquake distribution around the transition between the Zagros continental collision and the Makran subduction is restricted to the west of the Jaz Murian depression and the Jiroft fault (Figure 6). No earthquakes seem to be related to the Zendan-Minab-Palami fault system. Most of the shallow seismicity is related either to the Zagros mountain belt, located to the west, or to the Sabzevaran-Jiroft fault system, located to the north. The depth distribution of the microearthquakes increases northeastward to an unusual value for the Zagros of 40 km. Two dominant types of focal mechanisms are observed in this region: low angle thrust mostly restricted to the lower crust (at depths between 15 and 30 km) and strikeslip at shallow depth (10-20 km, Figure 6). Both are consistent with NS to NE-SW shortening.

Crustal seismic structure

Teleseismic P-wave travel times, from ~50 earthquakes, were calculated along a profile that crosses the Minab-Zendan Fault system at three different locations. The orientation of the profile was governed by accessibility and safety of the road. We observe a large delay in the travel time residual every time the profile crosses the ZMP (Figure 7). The inversion of the travel time residuals performed by [START_REF] Yamini-Fard | Sismotectonique et structure lithosphérique de deux zones de transition dans le Zagros (Iran): la zone de Minab et la zone de Qatar-Kazerun[END_REF] highlights perturbations relative to a homogeneously-layered velocity structure. In the first layer (associated with the crust), two low-velocity anomalies are related to the ZMP fault zone. The deeper layers related to the upper mantle show a slow velocity in the west relative to a fast velocity in the east of the ZMP. If we project the residual on a profile perpendicular to the ZMP function of the distance to the ZMP (Figure 7), we see a clear offset of up to 1 sec at the location of the ZMP. For stations located on the Makran block, we observe a decrease with the distance to the ZMP, which is also related to a south-eastward orientation of the profile. Because the crustal structure is 3D, and the slab is dipping northward beneath the Makran, it is likely that the offset decreases southward. The strong offset exactly related to the ZMP, however, indicates a strong velocity contrast between Zagros and Makran. Figure 6. Map of the microseismicity recorded for the period 17 November 1999 to 6 January 2000 by [START_REF] Yamini-Fard | Sismotectonique et structure lithosphérique de deux zones de transition dans le Zagros (Iran): la zone de Minab et la zone de Qatar-Kazerun[END_REF]2007). They computed 59 focal mechanisms that can be divided into two groups regarding their mechanism: strike-slip (horizontal T-axis) or thrusting (vertical T-axis). They are represented and lead, respectively, to ~N45°E-trending and ~N-trending mean P-axis. Interestingly, thrust mechanisms occur at depths (15-35 km) greater than strike-slip mechanisms (5-25 km); black and white dots are P-and T-axes, respectively (redrawn after [START_REF] Yamini-Fard | The diffuse transition between the Zagros continental collision and the Makran oceanic subduction (Iran): microearthquake seismicity and crustal structure[END_REF].

Yamini-Fard (2003) also computed teleseismic receiver functions. The P-S converted phases usually image the Moho discontinuity well. Their interpretation is not straightforward: the receiver function data are complex and cannot be easily used to draw a Moho profile. This complexity probably comes from the departure of the flat-layered structure that is used as an initial model and generates complex wave propagations in the crust.

Finally, [START_REF] Yamini-Fard | The diffuse transition between the Zagros continental collision and the Makran oceanic subduction (Iran): microearthquake seismicity and crustal structure[END_REF] computed a local 3D crustal velocity structure and relocated simultaneously the local earthquakes inverting the travel times of local seismicity [START_REF] Yamini-Fard | The diffuse transition between the Zagros continental collision and the Makran oceanic subduction (Iran): microearthquake seismicity and crustal structure[END_REF]. The resulting velocity structure suggests a high velocity body dipping northeastward (Figure 8). An important result is that no seismicity appears to be associated with the Zendan-Minab-Palami fault system, suggesting that the transition between the Zagros collision and the Makran subduction is not associated with a sharp transform fault. Instead it is associated with a progressive transition located in the lower crust. The shallow right-lateral strike-slip faulting is the response of the upper crust to the shortening. This 'partitioning' in depth is likely related to the difference in the strength of the upper and lower crusts. [START_REF] Bayer | Active deformation in Zagros-Makran transition zone inferred from GPS measurements[END_REF]

. (a) GPS velocity and their 95 per cent confidence ellipses in Eurasia fixed reference frame. (b and c) close-up into the study area in, respectively, central Iran fixed reference frame and Arabia fixed reference frame; dots represent instrumental seismicity. (d and e) GPS velocity profiles, normal and perpendicular, respectively, to the Zendan-Minab fault trend (AA' in map a). Lines refer to best-fit deformation with a block model (see Bayer et al., 2006 for details).

Assuming a rigid block model, [START_REF] Bayer | Active deformation in Zagros-Makran transition zone inferred from GPS measurements[END_REF] computed the motion accommodated by the Minab-Zendan-Palami fault system. They found 15 mm/yr and 6 mm/yr for the motions parallel or perpendicular to the direction of the fault, i.e. N160°E (Figure 9). In addition, they estimated the strike-slip motion of the Jiroft-Sabzevaran fault system (N-trending) to be 3.1±2.5 mm/yr. The Minab-Zendan-Palami fault system motion is estimated in its southern part, where the entire Zagros-Makran motion must be accommodated [START_REF] Regard | Accommodation of Arabia-Eurasia convergence in the Zagros-Makran transfer zone, SE Iran: A transition between collision and subduction through a young deforming system[END_REF], whereas in its northern part, Zagros-Makran transition zone defomation is distributed over the two fault systems. The 15 mm/yr-MZP fault system displacement rate must therefore encompass the 3.1±2.5 mm/yr of the Jiroft-Sabzevarn fault system calculated by Regard et al. (2005a, see next paragraph).

Geodetic data show that almost all the convergence is accommodated in the East by the Makran subduction zone, while only half of it is accommodated in the west by the Zagros. The Zagros-Makran transition zone clearly accommodates ~15 mm/yr of differential motion; the transpressive character of the transition zone is because of the fault obliquity relative to the overall plate convergence direction. Geodesy does not provide evidence of a rigid indentation of the Musandan peninsula into Iran that would be represented by a velocity field pattern divergent from the Musandam Peninsula.

Tectonics and geomorphology

As described earlier, [START_REF] Regard | Accommodation of Arabia-Eurasia convergence in the Zagros-Makran transfer zone, SE Iran: A transition between collision and subduction through a young deforming system[END_REF] provided evidence of two fault systems accommodating the relative velocities in the northern part of the study area (whereas the results of [START_REF] Bayer | Active deformation in Zagros-Makran transition zone inferred from GPS measurements[END_REF]'s concern the southern part of the system). Tectonic and geomorphic analyses combined with cosmogenic nuclide-dating ( 10 Be) have revealed a total right-lateral slip rate of 4.7±2.0 mm yr -1 to 6.3±2.3 mm yr -1 for the ZMP fault system, depending on the ages of offsets, and 5.7±1.7 mm yr -1 for the JS fault system (table 3 in Regard et al., 2005a;[START_REF] Regard | 10 Be dating of alluvial deposits from Southeastern Iran (the Hormoz Strait area)[END_REF]. They evaluate the total motion accommodated across the area to 11.3±3.9 mm.yr -1 or 13.1±4.3 mm.yr -1 in a direction N10±20°E.

The total shortening at the west front of the ZMP fault system is estimated to be ~6 km (Figure 4) since the Mio-Pliocene (~5 My, Molinaro et al., 2005a), and implies an average shortening rate in the order of 1 mm.yr -1 .

Discussion

The work presented here is a compilation of data collected by different means. It gives a unique insight into a subduction-collision transition zone current dynamics. In particular, important questions arise, such as what is the lithospheric structure? where is the main structure? does the Musandam Peninsula act as an indentor?... In order to give a coherent view of this transition we first describe what we do know about its structuration, then we try to assess the question of its modern kinematics and the timing of its set-up before inferring its future through comparison with laboratory experiments and other subduction-collision transitions.

Crustal and lithospheric structure

Before Arabia and Iran began to collide, probably late in Oligocene, the area was occupied by a continuous subduction zone. The subduction of the continental Arabia to the west provoked a collision, whereas to the east the oceanic part of the Arabian plate still subducts. Under the Zagros collision the plate probably broke and the oceanic part may have sunk deep into the mantle [START_REF] Molinaro | Lithospheric structure beneath the south-eastern Zagros Mountains, Iran: recent slab break-off?[END_REF]. More to the east, in the study area, it is actually difficult to know if the nearby Zagros underwent such a slab break-off or if the subducted slab is still attached. In particular, the transitional area is wide, which pleads speculatively for a gentle transition at depth, compatible with a continuous deep slab (Regard et al., 2005a;[START_REF] Regard | Slab pull and indentation tectonics: insights from 3D laboratory experiments[END_REF]. Closer to the surface, seismological data display a clear view of a NE-dipping surface (Figure 8). This plane dips ~15° and is likely to originate at surface near the ZMP fault system (Figure 8) [START_REF] Yamini-Fard | The diffuse transition between the Zagros continental collision and the Makran oceanic subduction (Iran): microearthquake seismicity and crustal structure[END_REF]. This plane is associated with an important microseismicity with NE-trending P-axes, showing thrusting at depth (figure 6) [START_REF] Yamini-Fard | The diffuse transition between the Zagros continental collision and the Makran oceanic subduction (Iran): microearthquake seismicity and crustal structure[END_REF]. It would thus correspond to an active crustal-scale thrust, separating a Zagros-related part in its footwall from Makran formations in its hanging wall.

The system tectonics appears more complicated in the uppermost part. It is dominated by folds and faults organised in two fault systems: (1) the ZMP fault system to the southwest and (2) the JS fault system to the north-east. (1) The NNW-trending ZMP fault system is associated with en echelon folding and constitutes the lithological boundary between Zagros and Makran [START_REF] Regard | Accommodation of Arabia-Eurasia convergence in the Zagros-Makran transfer zone, SE Iran: A transition between collision and subduction through a young deforming system[END_REF]. It presents high-velocity anomalies (1s residuals, Figure 7) [START_REF] Yamini-Fard | Sismotectonique et structure lithosphérique de deux zones de transition dans le Zagros (Iran): la zone de Minab et la zone de Qatar-Kazerun[END_REF]. It is made of numerous and highly-segmented faults. The system's western faults dip ENE whereas the eastern ones dip WSW, giving a flower-structure-like superficial organisation [START_REF] Regard | Accommodation of Arabia-Eurasia convergence in the Zagros-Makran transfer zone, SE Iran: A transition between collision and subduction through a young deforming system[END_REF]. This system could act as a developing crustalscale strike-slip fault, with infilling by dense material, but these observations do not agree with the balanced cross-section that implies an 8km-deep décollement surface that should extend a couple of kilometres eastward from the ZMP fault system. This apparent discrepancy will be resolved later. (2) The N-trending Jiroft-Sabzevaran fault system does not appear to be marked by any seismicity alignment even if the local seismicity level is high [START_REF] Yamini-Fard | Sismotectonique et structure lithosphérique de deux zones de transition dans le Zagros (Iran): la zone de Minab et la zone de Qatar-Kazerun[END_REF]. The faults are strike-slip with a little component of vertical motion. The fault system is partly linked to the south with the tectonical Makran northern boundary, south of the Jaz Murian depression, and partly to the ZMP [START_REF] Regard | Accommodation of Arabia-Eurasia convergence in the Zagros-Makran transfer zone, SE Iran: A transition between collision and subduction through a young deforming system[END_REF]. To the north the system seems to pursue northward to the Nayband and Gowk faults which mark the boundary between Central Iran and the Lut Block [START_REF] Walker | Offset and evolution of the Gowk fault, SE Iran; a major intra-continental strike-slip system[END_REF].

Surface structural setting and modern kinematics (Map view)

The current deformation at surface and close to the fault systems is shown to be caused by a NE-trending main compressional direction by seismology and active tectonics [START_REF] Regard | Accommodation of Arabia-Eurasia convergence in the Zagros-Makran transfer zone, SE Iran: A transition between collision and subduction through a young deforming system[END_REF][START_REF] Yamini-Fard | The diffuse transition between the Zagros continental collision and the Makran oceanic subduction (Iran): microearthquake seismicity and crustal structure[END_REF]. Active tectonics also indicate that convergence is accommodated nearly equally by the two fault systems, the deformation being distributed within each fault system instead of being localised (Regard et al., 2005a).

On a wider scale, the palaeomagnetic data show that rotations occurred both clockwise to the east and anticlockwise to the west of the transition zone. This could plead for an indentor role of the Musandam peninsula [START_REF] Aubourg | Post-Miocene shortening pictured by magnetic fabric across the Zagros-Makran syntaxis[END_REF][START_REF] Aubourg | Tertiary block rotations in the Fars Arc (Zagros, Iran)[END_REF]. On the other hand, the GPS velocity field does not show any divergence away from the 'indentor', and this would suggest there is no rigid indentation in the area (Figures 9 & 10). Tectonic observations also do not favour the Musandam Peninsula indentor hypothesis, since they indicate that the motion between Arabia and Makran is accommodated only by one transcurrent fault system (ZMP), on which all the relative motion is accommodated, as indicated by the similarity in GPS and tectonic velocities.

Indeed, the global motion accommodated by the transition zone fault systems is evaluated by GPS to be 15 mm/yr, close to the evaluation from tectonics from 11 to 13 mm/yr. GPS and tectonics agreement is not so good on the part accommodated by the Jiroft Sabzevaran fault system, whose transcurrent motion is evaluated to be 3.1±2.5 mm/yr or 5.7±1.7 mm/yr respectively (figure 10). The discrepancy comes from the loss of the elastic component away from the network, whose easternmost station is close to the Jiroft Fault, as highlighted by recent results (Peyret et al., in prep).

Two stages post-Miocene evolution

Some of the studies presented here highlight a two-stage scenario for the recent (since Miocene times) evolution of the area. First, magnetic studies indicate that the transition shape, oblique to the convergence, has been acquired recently, since the Mio-Pliocene. Before the transition structural set-up, the deformation recorded by magnetism was roughly coherent with the overall plate convergence. Superimposed on this, some moderate rotations, clockwise to the east and anticlockwise to the west, may account for a slight indentation by the Musandam Peninsula. Second, a major change in tectonics was also recorded. As emphasised before, the balanced cross-section of the Minab fold proposed by [START_REF] Molinaro | The origin of changes in structural style across the Bandar Abbas syntaxis, SE Zagros (Iran)[END_REF] appears not to be compatible with modern kinematics and structure. The Minab fold may have been formed during a former deformation stage. In addition, palaeostress determinations by [START_REF] Regard | Accommodation of Arabia-Eurasia convergence in the Zagros-Makran transfer zone, SE Iran: A transition between collision and subduction through a young deforming system[END_REF] show a stress-orientation change in the Pliocene. The authors suggested that it corresponds to a change from a partitioned convergence accommodation through folds and reverse faults (ZMP) to non-partitioned convergence accommodation through the ZMP transpressional faulting. The best way to combine these results (tectonics, structure, and kinematics) is to assume that the flat thrust shown by balanced cross-sections has been cut through by a more vertical fault, possibly evolving to a flower structure near the surface. This fault is likely to carry exotic slices like the Palami range lying between the Palami and Zendan faults.

Comparisons with comparable settings, both in laboratory

and in nature [START_REF] Cotton | Modeling of thrust fronts above ductile and frictional detachments: Application to structures in the Salt Range and Potwar Plateau, Pakistan[END_REF] proposed a sandbox experiment that interestingly mimics the study area even if it was not intended o reproduce it (Figure 11). The experiment comprises a sandbox undergoing N-S compression; the sand is lying over a frictional substrate in the western part whereas it is underlain by a ductile level in its eastern part (Figure 11). Even if it was actually designed for basin tectonics, we could interpret it in terms of lithospheric structure. The frictional vs. ductile substrate should be compared to the collision vs subduction convergence settings. In this experiment, the eastern range (over ductile substrate) extends more to the south than the western range. Between the two ranges a transfer zone is produced which is noticeably made of two N-trending strike slip faults: (i) a south-western one connecting the deformation zone of the western range (over a frictional substrate) to the frontal thrust of the eastern one; (ii) a northern one that connects the northern part of the experiment with the backstop of the eastern range (Figure 11). Comparison of this scheme with the structural setting of the Zagros-Makran transition zone highlights many similarities; in particular, the faults (i) and (ii) probably represent the ZMP and JS fault systems, respectively (Figure 11). Moreover the experiment cannot explain the curvature of the Fars Arc (south-eastern Zagros), whereas in the experiment the range over the frictional substrate has a linear trend (Figure 11). A possible explanation for this is that in the Fars Arc the curvature is due to the underthrusting of the Musandam Peninsula. Another transition between subduction and collision can be found at the other Makran end. Some of us have already initiated this discussion (Regard et al., (2005a). There, the transfer zone is constituted by three single faults, the Chaman, Ghazaband and Ornach-Nal faults (Figure 12). The system connects the Makran accretionary prism to the Pamirs (Panjshir Fault) [START_REF] Lawrence | Chaman Fault, Pakistan-Afghanistan[END_REF]. The overall system is thought to accommodate between 25 and 35 mm yr-1, following geological evidence [START_REF] Beun | Premières données quantitative relatives au coulissage du décrochement de Chaman (Afghanistan du sud-est)[END_REF], and 40 mm yr-1, after the NUVEL-1 model [START_REF] Demets | Current Plate Motions[END_REF]. The most important part of the deformation is transmitted to the inner Makran and there is no evidence of active deformation transmission to the frontal, offshore, Makran thrust. The transition zone is more mature than the Zagros-Makran transition zone. Indeed, it is known to be much older (20-25 Myr instead of 5 My or less) and its length (more than 1000 km) implies a considerable slab-stretching such that the slab must no longer be continuous between Markan and Pamirs. Consequently this system could be viewed as a possible future for our study area. In particular, it is noteworthy that the accommodation by two or three disconnected systems characterises the transition zone as well as in the [START_REF] Cotton | Modeling of thrust fronts above ductile and frictional detachments: Application to structures in the Salt Range and Potwar Plateau, Pakistan[END_REF] experiment and at the Zagros-Makran transition (Figures 11 & 12). Some partitioning also occurs, for example, between the Chaman strike-slip fault and the Sulaiman fold belt [START_REF] Lawrence | Chaman Fault, Pakistan-Afghanistan[END_REF][START_REF] Davis | Changing mechanical response during continental collision: active examples from the foreland thrust belts of Pakistan[END_REF]. In turn, the maturity is expressed by the localisation of the deforming structures that are long, low-segmented faults in contrast with the highly-segmented fault systems in the Zagros-Makran area (Regard et al., 2005a).

Figure 12. Landsat image of the Chaman fault (bands 7, 4 and 2) and its tectonic interpretation, after [START_REF] Lawrence | Chaman Fault, Pakistan-Afghanistan[END_REF]. Note that the transfer zone is constituted by three continuous but disconnected faults: the Chaman, Ghazaband and Ornach-Nal faults.

Synthesis

The Zagros-Makran transition zone evolution through time has probably been influenced by an inherited structural setting. For a better understanding of the way it recently evolved, a simplified scenario is proposed in Figure 13. Actually, the way this area is deforming is comparable to the Makran-Pamir transition and the laboratory experiment. Two characteristic fault systems are found to the south-west (Zendan-Minab-Palami fault system) and to the north-east (Jiroft-Sabzevaran fault system). To the east the Makran deformation expresses the same pattern as in the experiment and at the eastern Makran bound, but to the west, the Fars Arc shape disagrees with the experiment. We hypothesise that this is due to the complexity introduced by the Musandam Peninsula, topographically much higher than the surrounding parts of Arabia.

The first fault system (ZMP) currently trends N160°E, oblique to the convergence. It is made of three highly-segmented faults that are the possible expression at surface of a flower structure that changes to an oblique thrust at its northern bound. Seismology indicates that it is correlated by a strong discontinuity at depth and that near its northern termination its deep structure is a north-east dipping plane. In this scheme some 6-km structure-normal shortening occurs that could be the result of a former strain distribution or of a slight deformation partitioning, with a frontal fault accommodating up to 5 mm/yr of shortening, as indicated by the recent results of Peyret et al (in prep). The 6 km-deep décollement proposed by [START_REF] Molinaro | The origin of changes in structural style across the Bandar Abbas syntaxis, SE Zagros (Iran)[END_REF] may therefore now branch at depth to the flower structure. We propose that its central fault (Zendan) represents the main boundary between Zagros and Makran. The second fault system (JS) trends N-S. It probably connects to the north to a wellknown strike-slip system bounding the Lut block to the west. To the south its deformation probably transfers partly to the northern Makran tectonic boundary and partly to the ZMP fault system, as suggested by the laboratory experiment of [START_REF] Cotton | Modeling of thrust fronts above ductile and frictional detachments: Application to structures in the Salt Range and Potwar Plateau, Pakistan[END_REF].

Of the ~19 mm/yr differential convergence rate, some 3 to 6 mm/yr are accommodated by the JS fault system and ~6-7 mm/yr in the ZMP fault system at the Minab latitude, increasing to ~15 mm/yr to the south; this increase corresponds to the progressive deformation transmission from the JS fault system to the ZMP fault system. Note that if one part of the JS accommodated deformation is transmitted to the Makran northernmost thrust, the 15 mm/yr strike-slip deformation observed to the south of the ZMP fault system is not fully explained by addition of the ZMP and JS motions (max 13 mm/yr). This discrepancy is not yet resolved. The data presented here provide evidence of post-Miocene system evolution (Figure 13). The overall oblique structure is from recent (less than 5 My) setting as attested by magnetism and tectonics. The set-up time is contemporaneous with a major change in the Middle East tectonics and with Zagros topography-building initiation [START_REF] Allen | Late Cenozoic reorganization of the Arabia-Eurasia collision and the comparison of short-term and long-term deformation rates[END_REF]. During this set-up, Zagros and Makran, originally continuous, differentiated. Interestingly this differentiation occurred at the time when deformation in Zagros passed from thin-skinned to thick-skinned [START_REF] Molinaro | The origin of changes in structural style across the Bandar Abbas syntaxis, SE Zagros (Iran)[END_REF], that is, very differently from the Makran, where only the sediment cover is scraped off. It is very tempting to relate the onset of the Zagros-Makran transition zone to the initiation of thin-skinned tectonic shortening in Zagros from then on very different from the Makran tectonic style, where only the sediments overlying the oceanic crust are affected by deformation. The faults and folds constituting the system are currently in a young stage and they sometimes undergo some change as attested by stress tensor orientation changes. Nowadays, the system is divided into two fault systems that will endure while their internal organisation will simplify to localise in a single and continuous structure, even if some partitioning is likely to continue. Obviously, such a transition is a lithosphericscale deformation zone.

Conclusion

This review of studies about the Zagros-Makran transition zone clarifies many points, giving a coherent overview of its structural setting and behaviour.

-The location of this transition is dictated by the past. Indeed, it corresponds to a coastline offset by a transform fault at the ocean opening time (Figure 13a). -There is no evidence that currently the two deformation zones, Zagros and Makran, are connected by a transform fault. Numerous faults accommodate the deformation, organised in two fault systems. Some of the faults could be of crustal extent. -The current stress state is transpressional with a NE-trending σ1; the associated strain is strike-slip with some transpressional component. -An important change in how the tectonics works occurred somewhere in the Mio-Pliocene, contemporaneous with other changes widely recorded in the Middle East, and in Zagros in particular. This may indicate the real set-up of the zone as a transform zone between the Zagros and Makran, from then on evolving differently (Figure 13b-d). This set-up may be closely related to the change from thin-skinned to thick-skinned tectonics at the same time in the Zagros; meanwhile in Makran tectonics remained unchanged. -At depth, a north-eastward dipping plane is linked to the Zagros, probably an underthrusted slice. This plane seems to connect to the Zendan fault at surface. -An important clue is whether Musandam (Oman) acts as an indentor. Our conclusion is that there is no first-order indentation (Figure 13e). In turn, it could be a second-order indentation explaining the modern Fars Arc curvature (Figure 13f). -Laboratory experiments and a real analogue help us to imagine the future of such a transform zone: the fault zones are likely to simplify in locating the deformation in a single low-segmented fault. There is no clear evidence at present, however, that the fault zones will coalesce into a single one.

6

  The transition between Makran subduction and the Zagros collision: recent advances in its structure and Active deformation Vincent Regard, Denis Hatzfeld, Matteo Molinaro, Charles Aubourg, R. Bayer, O Bellier, Farzam Yamini-Fard, M. Peyret, M. R. Abbassi To cite this version: Vincent Regard, Denis Hatzfeld, Matteo Molinaro, Charles Aubourg, R. Bayer, et al.. The transition between Makran subduction and the Zagros collision: recent advances in its structure and Active deformation. Special Publication -Geological Society of London, 2010, 330, pp.41-64. 10.1144/SP330.4. hal-00356532

Figure 1 .

 1 Figure 1. Map of South Iran/North Arabia (modified afterMolinaro et al., 2005a). The arrows represent the velocity relative to Eurasia[START_REF] Vernant | Contemporary Crustal Deformation and Plate Kinematics in Middle East Constrained by GPS Measurements in Iran and Northern Oman[END_REF]. Note that the convergence accommodated through the Zagros Fold-and-Thrust Belt (ZFTB) is only ~9mm/y, and there is no value currently available for the part of the Arabia-Eurasia convergence accommodated through the Makran between Arabia and Lut (except using[START_REF] Vernant | Contemporary Crustal Deformation and Plate Kinematics in Middle East Constrained by GPS Measurements in Iran and Northern Oman[END_REF]'s results with some hypotheses). MZT: Main Zagros Thrust; SSZ: Sanandaj Sirjan Zone.

Figure 2 .

 2 Figure 2. (a) Palinspastic reconstructions around the study area for the last 30 My, plates moving relative to Eurasia. The segmented lines with circles represent the displacement trajectories; see inset for stages division. The reconstruction is made after the ODSN plate reconstruction service: http://www.odsn.de/odsn/services/paleomap/paleomap.html. Reconstructions are based on block rotations calculated from magnetic anomalies[START_REF] Hay | The Evolution of Cretaceous Ocean/Climate Systems[END_REF][START_REF] Soeding | ODSN Plate Tectonic Reconstruction Service[END_REF]. Note that the ancient Makran shoreline cannot be estimated since it is constituted by an accretionary prism under construction; motion-segmented arrow is estimated by the Lut block reconstructed position. (b) Graph showing the position of a reference point (38°N, 48°E) representing Arabia with respect to Eurasia versus time, both including (grey boxes) and excluding (black diamonds) rotation describing the opening of the Red Sea, modified after[START_REF] Mcquarrie | Cenozoic evolution of Neotethys and implications for the causes of plate motions[END_REF].

  Figure 2. (a) Palinspastic reconstructions around the study area for the last 30 My, plates moving relative to Eurasia. The segmented lines with circles represent the displacement trajectories; see inset for stages division. The reconstruction is made after the ODSN plate reconstruction service: http://www.odsn.de/odsn/services/paleomap/paleomap.html. Reconstructions are based on block rotations calculated from magnetic anomalies[START_REF] Hay | The Evolution of Cretaceous Ocean/Climate Systems[END_REF][START_REF] Soeding | ODSN Plate Tectonic Reconstruction Service[END_REF]. Note that the ancient Makran shoreline cannot be estimated since it is constituted by an accretionary prism under construction; motion-segmented arrow is estimated by the Lut block reconstructed position. (b) Graph showing the position of a reference point (38°N, 48°E) representing Arabia with respect to Eurasia versus time, both including (grey boxes) and excluding (black diamonds) rotation describing the opening of the Red Sea, modified after[START_REF] Mcquarrie | Cenozoic evolution of Neotethys and implications for the causes of plate motions[END_REF].

Figure 3 .

 3 Figure 3. Geological map of the study area, modified after the 1:1 000 000 geological map of Iran. Faults in black are faults from the original drawing; in red are the active faults mapped by[START_REF] Regard | Accommodation of Arabia-Eurasia convergence in the Zagros-Makran transfer zone, SE Iran: A transition between collision and subduction through a young deforming system[END_REF], which are distributed in two fault systems, the Jiroft, Sabzevaran (abbreviated as Sabz.) and Kahnuj faults in the Jiroft-Sabzevaran fault system (JS); the Minab, Zendan and Palami faults in the Zendan-Minab fault system (ZMP). A-A' and B-B' are the cross-sections (Figure5) from[START_REF] Molinaro | The origin of changes in structural style across the Bandar Abbas syntaxis, SE Zagros (Iran)[END_REF]. C-C' is the local tomography cross-section (Figure8), from[START_REF] Yamini-Fard | The diffuse transition between the Zagros continental collision and the Makran oceanic subduction (Iran): microearthquake seismicity and crustal structure[END_REF].

Figure 5 .

 5 Figure 5. Balanced cross-sections, from Molinaro et al.[START_REF] Molinaro | The origin of changes in structural style across the Bandar Abbas syntaxis, SE Zagros (Iran)[END_REF] Molinaro et al., 2005a). See Figure3for profile location. AA'-cross-section through the Minab-Zendan Fault System. BB' -cross-section from Bandar Abbas to Hajiabad, southeastern Zagros.MFF, Main Front Fault, HZF, High Zagros Fault, MZT, Main Zagros Thrust, SSZ, Sanandaj-Sirjan Zone.

Figure 7 .

 7 Figure 7. Teleseismic travel-times residuals at station along the Minab profile. (a) Location of seismological stations. (b) Residual values, given by reference to the average residual (in s), vs. eastward distance to the Zendan fault. Redrawn from Yamini-Fard et al.(2003; in prep). Symbols colours are varied equally in the map and the residual graph.

Figure 8 .

 8 Figure 8. Cross-section of the 3D velocity structure trending SW-NE computed from local travel time residuals: see Figure 3 for localisation. Results are reliable for a spread function less than 5 (white contour). The hypocentres are reported. There is a clear indication of a northward dipping anomaly related to the seismicity[START_REF] Yamini-Fard | The diffuse transition between the Zagros continental collision and the Makran oceanic subduction (Iran): microearthquake seismicity and crustal structure[END_REF].

  . (2004; updated by Masson et al., 2007) used a network of 27 GPS sites to establish the current large-scale deformation rates within Iran. The network was measured three times, during September 1999, October 2001 and October 2005. This work provided a useful overview of Iranian geodynamics. The authors then divided Iran into various rigid blocks. They conclude that to the west, the Zagros is undergoing 9±2 mm/yr of shortening, in a direction close to N-S. To the east, the Makran subduction accommodates 19±2 mm/yr of N20°E-trending convergence. The Makran-Zagros transition zone should thus accommodate some 11±2 mm/yr of right-lateral movement. Bayer et al. (2006) used a denser network focused on the Zagros-Makran transition zone. It consists of 15 stations, with an average distance of ~60 km, which were measured in 2000 and 2002.The GPS-derived velocity field can be expressed in different useful frames: fixed Eurasia, fixed Central Iran or fixed Arabia (Figure9). When expressed in Central Iran fixed frame, the velocity directions are more or less parallel to the convergence velocity. There is no clear divergence from the Musandam peninsula (Oman), contrary to what would be expected if Musandam was acting as an indenter, as proposed by Kadinsky-Cade and Barazangi (1982; figure9).

Figure 9 .

 9 Figure 9. GPS data for the study area[START_REF] Bayer | Active deformation in Zagros-Makran transition zone inferred from GPS measurements[END_REF]. (a) GPS velocity and their 95 per cent confidence ellipses in Eurasia fixed reference frame. (b and c) close-up into the study area in, respectively, central Iran fixed reference frame and Arabia fixed reference frame; dots represent instrumental seismicity. (d and e) GPS velocity profiles, normal and perpendicular, respectively, to the Zendan-Minab fault trend(AA' in map a). Lines refer to best-fit deformation with a block model (seeBayer et al., 2006 for details).

Figure 10 .

 10 Figure 10. Sum of observations at surface. GPS is indicated in the Central Iran fixed reference. See text for deep-structure observations.

Figure 11 .

 11 Figure 11. Sandbox experiment realised by[START_REF] Cotton | Modeling of thrust fronts above ductile and frictional detachments: Application to structures in the Salt Range and Potwar Plateau, Pakistan[END_REF]. (a) The setup consists of a box filled with sand (3) over a rigid basement (2) which is partly replaced by a low viscosity layer (1). The sand layers experience convergence owing to piston push. (b) The experimental result shows that the structures propagate further over the viscous basement than over the rigid basement; a transition zone is created. (c) Structural scheme of our study area, with strain arrows, which is much like the experimental result (b).

Figure 13 .

 13 Figure 13. Sketch of the study area history. 1-Initial setting: a former passive margin south of the Thetys, cut by a transform fault. 2-Obduction occurred near the end of Cretaceous. 3-The Tethys northern margin was subducting for a while when the accretionary prism began to be built (Eocene). 4-Subsequently to ocean closure and emergence, Agha Jari deposited. 5-Zagros building during Pliocene; onset of Zagros-Makran syntaxis. 6-Modern setting; JS and ZMP are, respectively, the Jiroft-Sabzevaran and Zendan-Minab-Palami fault systems. The topographic Musandam high causes the Fars Arc curvature.

Acknowledgements

The tectonic study is indebted to ISIS program for SPOT satellite image acquisition (©CNES 2004 to 2007, distribution SPOT images S.A.). We wish to thank R. Walker and an anonymous reviewer for their constructive reviews. This work benefited from 10-year collaboration between Iranian and French scientists who all participated to data recollection and discussions.


Didier Graillot

Frederic Paran

Djamel Mimoun
email: mimoun@emse.fr

Gudrun Bornette

Janine Gibert

L Cadilhac

Coupling Groundwater Modeling with Biology to Identify Strategic Water Resources

The identification of hydraulic interactions between rivers and groundwater is part and parcel of assessing the capabilities of aquifers in response to the constraints induced by anthropic activities and climate changes on water management. On the extensive Rhône river basin in France, lack of measurements hinders groundwater modeling everywhere and simulating water management scenarios in every place. This work focuses on the advantages of coupling groundwater modeling with biological markers i.e. micro-and meso-invertebrates and aquatic vegetation on very well instrumented areas. A correlative analysis between hydraulic and biological properties of the aquifer provides a better understanding of the transfer of groundwater into or from the surface waters. This analysis makes the adjustment of boundary conditions easier and the calibration of groundwater models more efficient. Finally our approach enables to identify the conditions under which it could be possible to substitute biological indicators in lieu of a large field of piezometric measurements. This work proposes an interdisciplinary approach for the quantitative and qualitative characterisation of hydraulic interactions between rivers and shallow aquifers. Integration in a future decision support system should help water agencies to consider appropriate long term water policies like preservation of groundwater for potable water supplying and/or mitigation of pollutions risks.

INTRODUCTION

The assessment of interactions between the rivers and their floodplain via groundwater represents a relevant issue for the water resources management in the long term and for the preservation of biodiversity. This is accomplished by characterizing hydraulic exchanges existing between i) the Rhône River in France, its river branches and tributaries, ii) channels and diversion canal iii) alluvial watersheds and other connected aquifers. For this, new methodologies are required to provide watershed managers with decision support systems able to identify strategic reserves and their scope for water uses.

PURPOSE AND SCOPE

This research program is designed to estimate the groundwater recharge to the Rhône River and to identify their origin. It will be possible to delineate the sections of the aquifers which are the most influenced by the river and most threatened by a potential pollution in the river. It will be possible to identify those areas of surface water and the wetlands which are strongly affected by groundwater flows. This is an interdisciplinary approach which is based on hydraulics, groundwater mechanics and biology to characterize and assess potential or existing hydraulic interactions between the Rhône River and the connected aquifers from the Leman Lake in Switzerland all the way downstream to the Mediterranean Sea. Anthropic water use is considered as potable water supplying, irrigation and industrial water needs. Our methodology should contribute to the preservation of groundwater, often considered as an invisible resource as well as provide more generally to the preservation of water resources and biodiversity throughout the entire Rhône River basin.

SITE LOCATION AND DESCRIPTION

The Rhône River flows from southwestern Switzerland in the Alps to the Mediterranean Sea along 545 kilometers in France. The surface of the drainage area is about 5 220 square kilometers. The land covering is composed of rocks and glaciers (38%), forest and pasture (46%), agricultural land (16%). The Rhône River crosses 3 administrative regions, 11 counties, 213 municipalities and 48 hydrographic zones (Figure 1). Along this river many different watersheds exist: --Free and confine sedimentary aquifers: limestone, alluvial aquifers with sand and gravel, glacial tills and deposits, Pliocene formations, Miocene molasses, glacial complex formations, --Impervious geological formations which can produce very small aquifers: silt and alluvium, limestone, marl and sandstones. --Aquifers in the fissured bedrock (granite and schist).

As many rivers, the Rhône is supplied by rain and run-off. The Rhone River also receives snowmelt upstream in the Alps and groundwater from the aquifers along the river. Groundwater influences strongly surface waters.

Many man-made structures have been built along this river downstream from the metropolis of Lyon.

The water supply of the entire urban area depends on groundwater from the alluvial aquifer close to the city. The wells are exploited up to an average of 350 000 m 3 per day using bank filtration from 3 tracts of the Rhône river. Direct recharge by the river can reach 95%. About 40 dams have been built to produce hydroelectric power, prevent floods, enhance navigation and provide water for irrigation. A diversion canal has been dug along the river with 300 kilometers of embankments. All these engineering works have a strong influence on the hydraulic interaction between the river, slow flowing former channels and aquifers. The average flow upstream Lyon is about 423 m 3 /s and varies from 160 m 3 /s to 1900 m 3 /s. Downstream, the average flow reaches 177 m 3 /s and varies from 540 m 3 /s to 9600 m 3 /s.

The European Water Framework Directive has defined quality objectives for ground and surface waters which have to be reached and satisfied in 2015 so that decision makers and water authorities have an obligation to protect rivers, aquifers and ecosystems along the Rhône River.

METHODS FOR ESTIMATING GAINS AND LOSSES OF WATER

This novel approach associates the well known physical methods i.e. groundwater modeling, geochemical and isotopic analysis with biological methods based on the sampling of micro-invertebrates and on aquatic plants [START_REF] Paran | Un territoire en 3 dimensions (T3D) et un acteur en 4 dimensions (A4D) pour une argumentation spatiale adaptée[END_REF][START_REF] Bornette | Evaluation des échanges nappes rivières et des apports souterrains dans l'alimentation des eaux de surface. Application au Rhône et à ses aquifères superficiels[END_REF]. Hydraulic interactions between the alluvium and the river are preliminarily estimated from gradients which are calculated from digitalized piezometric maps (Figure 2). Bank filtration is calculated with a Darcy's law considering the cross section in the river and the permeability of the aquifer. This estimation is weighted by a clogging factor to take into account the impervious banks on each side of the river. Sometimes channel drawdowns the groundwater flow on each side of the river so that the contribution of the aquifer to the river is removed at downstream.

Comparison with Biological Markers

This first diagnosis of the hydraulic exchanges (outflow, inflow, equilibrium) is compared to the population of micro-and meso -invertebrates which have been sampled from the river, into the wells drilled in the aquifer and into canals and channels. Obligate groundwater-dwelling species (i.e. stygobionts) as certain oligochaetes are especially influenced by phreatic conditions and significant of groundwater infiltration. A spatial analysis of the distribution of hyporheic microfauna and of stygobionts helps to determine the nature, the origin and the intensity of the flow circulating into sediments at the bottom of a river. The spatial distribution of the ecological groups: stygobionts, stygophiles (ubiquitous species) or stygoxenes (epigean species) should obtain a relevant description of hydrological exchanges.

A comparison with sampled aquatic vegetation is also made. Aquatic vegetation indicates the trophic degree of the rivers (oligotrophic, mesotrophic, eutrophic). The analysis of ecological successions and of particular vegetal species as macrophytes can help to determine groundwater infiltration. There is a correlation between the intensity of groundwater infiltration and the thermal variability of surface water in which vegetal species grow. However, these correlations are not always available at different latitudes. Other physical parameters such as conductivity and temperature can be used to check the coherence of the hydraulic exchanges which have been formerly calculated.

Finally, the study of the hydraulic exchanges is made in two steps. A first correlative analysis is made at the scale one single sampling station to identify the correlations between the different metrics. The second step tries to apply them to other cases study to validate these correlations or to identify others.

Comparison with Groundwater Modeling Results

A limited case study in the Rhône River basin has produced results which can be visualized by GIS mapping with geological hydrological and biological metrics. A comparison was performed with the results given by finite differences 2D models implemented with Modflow on this river section. The infiltration flow from the aquifer into the river is generally represented by a Cauchy condition in the model (de Lange 1999) [1]. A cross validation of each method (biologic and hydraulic gradients) improves the convergence of the calculated flow discharge between the aquifer and the river.

RESULTS AND DISCUSSION

From a hydrogeological view point the case study shows that the groundwater discharge is limited to 100 l/s and per kilometer (350l/s along 7.5 km) into the river. These very weak values mean that the aquifer doesn't represent a strategic reserve of groundwater (Figure 3). It is not possible to extend the catchment areas to dig other wells to supply water for irrigation or potable water use. Downstream of this section, the results show that the groundwater flows into the channel beneath the river are higher (660 l/s).

The biological assessment of micro-mesoinvertebrates and aquatic vegetation sampling leads to confirm a phreatic influence of the alluvial plain in this section. They bring more information about the origin and the role of groundwater flows on the eutrophication process in the river and in the aquatic medium [START_REF] Paran | Distribution of groundwater invertebrates along an environmental gradient in a shallow water-table aquifer[END_REF]. A first comparison between the hydraulic and biological metrics on this case study shows the interest of such an interdisciplinary approach. Complementary isotopic analysis and environmental tracers measurements (Cl -, δ 18 O and δ 2 H) should be made to confirm these results [START_REF] Fette | Assessing river-groundwater exchange in the regulated Rhone River (Switzerland) using stable isotopes and geochemical tracers[END_REF]. Several already-existing groundwater models have been applied on at least 3 main river sections. The results given by these very precise and documented models will be compared with biological and geochemical measurements and the calculation of the hydraulic interactions. This could lead to a new global modeling approach at the scale of such a river where it would be too prohibitive (in cost and time) to implement a gridded model.

METHOD LIMITATION AND ACCURACY

The accuracy of the results given by the method is different for each metric. In hydrogeology, we find the traditional sources of errors such as the depth of the aquifer, inhomogeneities in the alluvial aquifer, errors in the measurement of groundwater level and conductivity. In hydraulics, the depth of the channels has been estimated up to 5 meters depending on the data quality provided by the National Company of Rhône. The possible clogging of the banks has been estimated from the results given by models when available. Errors induced by the biological field sampling procedures may also arise even if identification of fauna and aquatic vegetation seems to be more deterministic. A spatial analysis of uncertainties has been achieved for each metric so that it could be possible to propose uncertainty mapping with a confidence interval for each calculation of flow exchanges.

FUTURE RESEARCH

The spatial correlation found between these different metrics has to be validated on another case study to take into account diverse latitude, climate, geological context and anthropic conditions. Transient conditions remain to be studied since GW/SW interactions can be modified during the fall low-flow period. Another field of research should be developed to predict the modification of these hydraulic exchanges induced by climate changes. The resulting thermal modification could have rough repercussions on the nuclear power plants which would need more and more stenothermal groundwater. Other rivers have been studied in the world and especially to identify hydraulic interactions with aquifers: Danube, Murray River, Snake River. Very few only have been studied with an integrated biological and physical approach.

CONCLUSIONS

Numerical modeling of groundwater/surface-water interactions is not always efficient for a large scale river basin. On very well instrumented areas the gains and losses between groundwater and surfacewater can be calculated from spatial analysis of piezometric gradients. They can be validated by results given by groundwater models which have been implemented at a local scale. The biological markers which are influenced or not by the phreatic conditions and the discharge fluctuations can also confirm these GW/SW interactions. This novel approach has been applied on two sections of the Rhône River and shows correlations between hydraulics and biology which permits under specific conditions to avoid extensive groundwater modeling on similar sections. Results will provide decision makers with relevant hydrological and biological information to identify by sight strategic reserves of groundwater.

Figure 1 :

 1 Figure 1 : Rhône River Basin (Coulet 1997) 545 kilometers from Geneva to the Mediterranean sea

Figure 2 :

 2 Figure 2: Mapping of the piezometric level in the alluvial aquifer

Figure 3 :

 3 Figure 3: Mapping of the hydraulic exchanges between the river and the alluvial aquifer

ACKNOWLEDGMENT

This research program is supported by the French Water Agency of Rhône-Méditerranée-Corse with the help of the National Company of Rhône for providing hydraulic data.