Philippe Yamato
email: philippe.yamato@univ-rennes1.fr

Boris J P Kaus

Frédéric Mouthereau

Sébastien Castelltort

Dynamic constraints on the crustal-scale rheology of the Zagros fold belt, Iran

Thin-skinned fold-and-thrust belts are generally considered as the result of contractional deformation of a sedimentary succession over a weak decollement layer. The resulting surface expression frequently consists of anti-and synclines, spaced in a fairly regular manner. It is thus tempting to use this spacing along with other geological constraints to obtain insights in the dynamics and rheology of the crust on geological time scales. Here we use the Zagros Mountains of Iran as a case study as it is one of the most spectacular, well-studied thin-skinned fold-andthrust belts in the world. Both analytical and numerical models are employed to study what controls fold-spacing and under which conditions folding dominates over thrusting. The models show that if only a single basal décollement layer is present underneath a brittle sedimentary cover, deformation is dominated by thrusting which is inconsistent with the data of Zagros Fold Belt. If we instead take into account additional décollement layers that have been documented in

INTRODUCTION

It is often assumed that fold belts can be explained by folding of a sedimentary layer above a basal detachment formed by a weak layer. As the spacing between folds in such belts is quite regular, we can consider them as a large-scale natural experiment of crustal deformation.

Ideally, it should be possible to combine fold spacing with other geological data and theory to constrain parameters such as crustal rheology that are difficult or impossible to constrain from field observations alone. The classical explanation of folds in fold belts is that they are due to a folding instability, which is well known for a homogeneous sedimentary sequence with either a power-law viscous or an elastic rheology [START_REF] Schmalholz | Control of folding by gravity and matrix thickness: Implications for large-scale folding[END_REF][START_REF] Burg | Dome structures in collision orogens: Mechanical investigation of the gravity/compression interplay[END_REF][START_REF] Schmalholz | Scaled amplification equation: A key to the folding history of buckled viscous single-layers[END_REF]. The dominant wavelength λ dom , for a viscous power-law layer of viscosity and with exponent n overlying a linear viscous layer of viscosity , is given by [START_REF] Schmalholz | Control of folding by gravity and matrix thickness: Implications for large-scale folding[END_REF]:

(1)

where H sed and H salt are the thicknesses of the sedimentary cover and of the salt, respectively.

The growth rate (q dom ) of this instability non-dimensionalized over the background strain rate is given by [START_REF] Schmalholz | Control of folding by gravity and matrix thickness: Implications for large-scale folding[END_REF]:

q dom  ε = 2.5n η nη salt ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ 1 3 H salt H sed = 2.5n λ dom 3.1H sed ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ 2 (2)
and a combination of numerical and analytical studies have shown that q dom / should be larger than ~20 for folding to form observable folds, rather than homogeneous thickening (e.g., [START_REF] Kaus | From passive continental margin to mountain belt: Insights from analytical and numerical models and application to Taiwan[END_REF].

The Zagros Fold Belt of Iran constitutes a classical example of such a folded belt that is geologically (e.g., [START_REF] Stocklin | Structural history and tectonics of Iran; A review[END_REF][START_REF] Alavi | Regional stratigraphy of the Zagros fold-thrust belt of Iran and its proforeland evolution[END_REF][START_REF] Mcquarrie | Crustal scale geometry of the Zagros fold-thrust belt[END_REF][START_REF] Sherkati | Variation of structural style and basin evolution in the central Zagros (Izeh zone and Dezful embayment): Iran[END_REF][START_REF] Mouthereau | Tertiary sequence of deformation in a thin-skinned/thick skinned collision belt: The Zagros Folded Belt (Fars, Iran): Tectonics[END_REF] and geophysically (e.g., [START_REF] Jahani | The eastern termination of the Zagros Fold-and-Thrust Belt, Iran: Structures, evolution, and relationships between salt plugs, folding, and faulting: Tectonics[END_REF][START_REF] Hatzfeld | Comparisons of the kinematics and deep structures of the Zagros and Himalaya and of the Iranian and Tibetan plateaus and geodynamic implications[END_REF][START_REF] Nissen | The vertical separation of mainshock rupture and microseismicity at Qeshm island in the Zagros fold-and-thrust belt[END_REF] well studied due to excellent exposure and extensive seismic and borehole data from exploration. The main tectonic and stratigraphic units are summarized on Figure 1 and show that a particular feature of the Zagros Fold Belt is a consistent spacing of folds with a wavelength (λ dom ) of 14 ± 3 km. These folds are generally explained as detachment folding of the post-Cambrian sedimentary sequence above a basal weak layer constituted by the Hormuz salt.

The centroid depths of waveform-modeled earthquakes indicate that faulting is restricted to two structural levels located in the competent sediment cover units at 5-6 km depth and within the Precambrian basement at depth larger than 11 km down to depths of 30 km (e.g., [START_REF] Talebian | Reappraisal of earthquake focal mechanisms and active shortening in the Zagros mountains of Iran[END_REF][START_REF] Nissen | The vertical separation of mainshock rupture and microseismicity at Qeshm island in the Zagros fold-and-thrust belt[END_REF]. Seismic reflection profiles [START_REF] Jahani | The eastern termination of the Zagros Fold-and-Thrust Belt, Iran: Structures, evolution, and relationships between salt plugs, folding, and faulting: Tectonics[END_REF] and field observations in the Fars region [START_REF] Mouthereau | Tertiary sequence of deformation in a thin-skinned/thick skinned collision belt: The Zagros Folded Belt (Fars, Iran): Tectonics[END_REF] show a lack of major thrust faults cutting the folded cover up to the surface. This confirms that detachment folding rather than thrusting is the dominant deformation mode in the Zagros Fold Belt. In this aspect, the Zagros Mountains differ from other fold-and-thrust belts such as the Jura Mountains, where large-offset faults are continuous across the stratigraphic sequence, well imaged through seismic studies [START_REF] Simpson | Mechanical modeling of folding versus faulting in brittle-ductile wedges[END_REF].

Detachment folding theory should thus be perfectly applicable to the Zagros Fold Belt.

Equations ( 1) and (2) show that fold spacing depends strongly on the rheology of the overburden and on the thickness of the basal salt layer. In the Zagros, a linear viscous overburden (n = 1) and a viscosity contrast of 100 between salt and overburden, requires a salt layer thickness of ~7.8 km to fit the observed spacing of folds (Equation 1). Yet, seismic data indicates that the thickness of the Hormuz salt is no more than 1 or 2 km [START_REF] Jackson | Salt diapirs of the Great Kavir, central Iran[END_REF][START_REF] Mouthereau | The Zagros folded belt (Fars Iran): Constraints from topography and critical wedge modelling[END_REF][START_REF] Jahani | The eastern termination of the Zagros Fold-and-Thrust Belt, Iran: Structures, evolution, and relationships between salt plugs, folding, and faulting: Tectonics[END_REF]. If the sedimentary cover has a power law rheology instead, its power law exponent should be n~23 (Equation 2) to explain the data, which is considerably larger than estimates from rock creep experiments [START_REF] Ranalli | Rheology of the Earth[END_REF]. Large power law exponents are often taken as evidence for a brittle rheology. Currently, however, there is no theory that can reliably predict the spacing of detachment folds in the case of a brittle overburden.

There is thus presently no satisfactory explanation for (1) why deformation in the Zagros Fold Belt is dominated by folding and not by thrusting and for ( 2) what controls the spacing of folds and how it is linked to crustal rheology. In order to address this, we performed thermomechanical numerical simulations to study the dynamics of detachment folding in the presence of a brittle sedimentary cover.

NUMERICAL MODEL

To study the effect of using visco-elasto-plastic rheologies on crustal dynamics, we have performed a series of numerical experiments using the finite element code MILAMIN_VEP (e.g., Kaus, 2010 and GSA Data Repository DR1). The viscosities of the weak layers are assumed to be linear and constant, which is a reasonable approximation for the rheology of salt.

The brittle layers have a temperature-dependent rheology of limestone (see DR1), which correspond to the majority of rocks within the sedimentary cover (Fig. 1C, [START_REF] Mouthereau | Tertiary sequence of deformation in a thin-skinned/thick skinned collision belt: The Zagros Folded Belt (Fars, Iran): Tectonics[END_REF]. A linear geotherm of 25 °C.km -1 is initially applied (see DR1 Finally, to initiate the folding, the interface between the salt and the overburden rocks has random noise with maximum amplitude of 100 m. Model simulations are performed for 5.5

Myrs, after which results are interpreted.

RESULTS FROM NUMERICAL SIMULATIONS

With a 1.5 km-thick single basal detachment layer underlying a homogeneous brittle sedimentary cover, the models develop faults rather than folds (Fig. 2B). Such faults develop at early stages with a spacing that is approximately twice the brittle layer thickness. Subsequent deformation is locked around these folds that have a box-fold geometry. Compared to the Zagros Fold Belt, we thus obtain a too large wavelength and an incorrect deformation style. Additional simulations where we varied the frictional parameters of the crust, or the viscosity of the salt layer gave similar results (see Fig. DR1). We thus infer that it is impossible to reproduce the observed finite wavelength of Zagros Fold Belt folds (Fig. 1) by considering only one weak basal décollement layer, unless this layer has an unrealistically large thickness.

A detailed look at the stratigraphic column, however, reveals that the sedimentary cover is not rheologically homogeneous. Instead there are several layers that are composed of relatively weak rocks such as evaporites or shales (Fig. 1B,C, see detailed descriptions in [START_REF] Mcquarrie | Crustal scale geometry of the Zagros fold-thrust belt[END_REF][START_REF] Sherkati | Central Zagros fold-thrust belt (Iran): New insights from seismic data, field observation, and sandbox modelling[END_REF][START_REF] Mouthereau | Tertiary sequence of deformation in a thin-skinned/thick skinned collision belt: The Zagros Folded Belt (Fars, Iran): Tectonics[END_REF]. A second set of simulations took this fine-scale rheological structure into account (Fig. 3). The results are remarkably different from the previous experiments: rather than being fault-dominated, deformation is now achieved by folding, with a final wavelength similar to the one observed in the Zagros Fold Belt (Fig. 3). An analysis of the simulation shows that the spacing of the folds is fixed at a very early stage, after which the individual structures grow without clear geometric pattern, in accordance with field constraints [START_REF] Mouthereau | Tertiary sequence of deformation in a thin-skinned/thick skinned collision belt: The Zagros Folded Belt (Fars, Iran): Tectonics[END_REF]. The initial fine-scale rheological stratification of the sediment cover of the Zagros Fold Belt thus has a first-order effect on the development of upper crustal-scale structures. These results are in full agreement with a recent study of active seismicity in the Zagros Fold Belt which showed that both the Hormuz salt layer and the intermediate layers are mechanically-weak zones that form barriers to rupture for active faults [START_REF] Nissen | The vertical separation of mainshock rupture and microseismicity at Qeshm island in the Zagros fold-and-thrust belt[END_REF].

CONSTRAINTS ON CRUSTAL RHEOLOGY

The simulations presented in this study highlight the different modes of deformation that might occur in fold-and-thrust belts. However, they give limited insights into the underlying physics, as it remains unclear how sensitive the spacing of structures is to the rheology of the crust. For this reason, we developed a semi-analytical methodology drastically reducing the computational requirements that allows us to predict the outcome of numerical simulations in a large parameter space (see details in DR2). The resulting wavelength versus growth rate diagrams have a single maximum as a function of non-dimensional wavelength (Fig. 4A).

Rigorously, these semi-analytical results are only valid for very small deformations. Yet, a comparison with numerical simulations reveals that they correctly predict the spacing of folds even after 5.5 Myrs, which confirms that fold-spacing is selected at a very early stage in the evolution of a fold and thrust belt (Fig. 4A).

Results for a homogeneous and brittle sedimentary cover reveal that the dominant growth rate is smaller than 20, which essentially means that folding will not be able to overcome background pure-shear thickening. Indeed, our numerical simulations indicate that this leads to fault-dominated deformation rather than folding (box folds, Fig. 2). If, on the other hand, weak layers are taken into account in the sedimentary sequence, the growth rate is significantly larger and the dominant wavelength is reduced (Fig. 4B). The addition of a single weak layer is sufficient to switch deformation styles from fault-to fold-dominated, and elasticity has a minor effect only.

Using the same semi-analytical methodology, we performed a large number of simulations and found that the two most important parameters are the viscosity of the salt/weak layers and the friction angle of the crust, whereas rock density plays little to no role. Plots of dominant wavelength and growth rate versus those two parameters show an approximate equal dependence on the two parameters (Fig. 4). The results also show that weak layers in all cases yields growth rates that are sufficiently large for the folding instability to dominate faulting.

In the case of Zagros Fold Belt, the effective viscosity of salt has been determined to be close to 10 18 Pa.s, a value consistent with scaled laboratory-derived values [START_REF] Spiers | Experimental determination of constitutive parameters governing creep of rocksalt by pressure solution[END_REF] and other modeling studies [START_REF] Van Keken | The effective viscosity of rocksalt: Implementation of steady state creep laws in numerical models of salt diapirism[END_REF][START_REF] Mouthereau | The Zagros folded belt (Fars Iran): Constraints from topography and critical wedge modelling[END_REF]. If we combine this with our method, we estimate that the effective friction angle for the crust in the Zagros Fold Belt on geological timescales is around 5°+/-5° (Fig. 4B).

DISCUSSIONS AND CONCLUSIONS

Contrary to the common view of fold belts that often consider a single major basal décollement only, we demonstrate through the example of the Zagros Mountains that the whole stratigraphic sequence might influence the dynamics of the belt. Heterogeneities within the sedimentary cover, and weak layers in particular, control whether deformation is dominated by crustal-scale folds or by thrusts. The stratigraphy of a fold belt plays a much larger role than previously appreciated and should thus be taken into account if one wishes to reconcile field observations with physically consistent models of geological processes.

Balancing geological cross-sections in fold-thrust belts is a difficult exercise that aims at providing a consistent structural and kinematic interpretation of usually independent structural data. Our method paves the way for developing future generations of 2D and 3D dynamic reconstruction models for fold and thrust belts (e.g., [START_REF] Lechmann | Dynamic unfolding of multilayers: 2D numerical approach and application to turbidites in SW Portugal[END_REF].

Moreover, we show that the regular spacing of folds puts constraints on the rheology of the crust on geological timescales. In the case of Zagros Fold Belt, the value for the friction angle we obtained in this manner is small (<10°), which indicates that the crust was rather weak, potentially due to large fluid pressures (e.g., [START_REF] Huismans | Effect of plastic-viscous layering and strain softening on mode selection during lithospheric extension[END_REF].

FIGURE CAPTIONS

Figure 1. Field constraints for the Zagros folded belt. A: Topography illustrates the regular spacing of folds with amplitude ~500-1000 m over on an area of ~80 000 km 2 . Fold crest length are of ~50 km in average. Inset shows the distribution of fold wavelengths measured for 88 anticlines from the Zagros Folded Belt. B: Cross-section (aa') across the Zagros Fold Belt based on field measurement [START_REF] Mouthereau | Tertiary sequence of deformation in a thin-skinned/thick skinned collision belt: The Zagros Folded Belt (Fars, Iran): Tectonics[END_REF]. λ corresponds to the average wavelength of the folds. This value is slightly smaller than the 15.8 +/-5.3 km from [START_REF] Mouthereau | Tertiary sequence of deformation in a thin-skinned/thick skinned collision belt: The Zagros Folded Belt (Fars, Iran): Tectonics[END_REF] that took into account the folds from the whole Fars area. MFF and SF correspond to the seismogenic Mountain Front Fault and the Surmeh Fault, respectively, associated with basement faulting.

Vertical fold velocity is 0.3-0.6 mm.yr -1 . C: Synthetic stratigraphic log where WL1, WL2 and WL3 correspond to the weak layers in the sedimentary sequence (Fm: Formation). respectively. This diagram was produced using the semi-analytical approach described in DR2.

A: Growth rate values obtained for given values of λ/H for 0, 1, 2 and 3 weak layers. For each case, the characteristic wavelength value corresponds to the highest value of growth rate (e.g., white star for the case with 3 weak layers). Insets show results of numerical simulations after 5.5

Myrs, which develop folds with a spacing that is in excellent agreement with the predicted characteristic wavelength. A single basalt décollement layer results in small folding growth rates and in thrust-dominated deformation. Addition of one or more weak layers to the brittle sedimentary cover results increases the growth rate significantly and leads to folding-dominated deformation. The ZFB brown area corresponds to the λ/H ratio of the Zagros Fold Belt. B: diagrams of characteristic wavelength (left) and corresponding growth rate (right) versus viscosity of the weak layers and friction angle of the crust. Thick white lines show the constraints for Zagros Fold Belt (average +/-1 standard deviation). As in the Zagros Fold Belt salt viscosity is constrained independently, the best-fit friction angle for the crust is 5+/-5°.The white star corresponds to the simulation of figure 3.

Figure 2 .

 2 Figure 2. Simulation with a basal décollement layer only. A: Initial setup with a sedimentary

Figure 3 .

 3 Figure 3. Simulation with intermediate crustal detachment layers. A: Initial setup as in figure 2, but with three additional weak crustal detachment layers with 10 18 Pa.s. B: Snapshots of

Figure 4 .

 4 Figure 4. Influence of multiple weak layers and elasticity on folding. λ, H, q and corresponds

  ). For the low-temperature conditions of the Zagros Fold Belt, stresses are such that the rocks effectively deform in the brittle regime. Our model domain is initially 200x7.225 km in size (see DR1). The top boundary is a free surface with no erosion (see DR1) and a constant background strain rate of 10 -15 s -1 is applied at the right of the model box, which results in 15% shortening after 5.5 Myrs consistent with geological constraints (see DR1). All other sides of the model have free slip conditions.

GSA Data Repository item

2011xxx, xxxxxxxx, is available online at www.geosociety.org/pubs/ft2011.htm, or on request from editing@geosociety.org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.

ACKNOWLEDGMENTS

This work benefits from fruitful discussions with L. Le Pourhiet, S. Schmalholz and Y. Podladchikov. Constructive reviews from D. van Hinsbergen and 2 anonymous reviewers have also largely contributed to improve the manuscript. BJP Kaus was partly supported by ERC Starting Grant 258830.


David Widory
email: d.widory@brgm.fr

Dorothea M Kujawinski

Nicole Baran

Laurence Amalric

Maik A Jochmann

Torsten C Schmidt

THE CARBON ISOTOPE COMPOSITION OF HERBICIDES IN GROUNDWATER: THE EXAMPLE OF GLYPHOSATE AND ITS DEGRADATION BYPRODUCT AMPA

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

The carbon isotope composition of herbicides in groundwater : the example of Glyphosate and its degradation by product AMPA Jaworska et al., 2002;Nowack, 2003).

Our project aims at showing that the carbon isotope composition (δ 13 C) of these two molecules could be of great help for identifying their respective sources, and to assess and characterize potential biodegradation processes that would link AMPA to either its parent pesticide glyphosate or phosphonate molecules.

Methodology-Results

The first step is to understand the carbon isotopes behavior and potential isotope fractionation during infiltration and degradation of glyphosate during its transfer in the unsaturated zone of aquifers. With that aim, we designed a lab experiment, in which degradation of glyphosate is tested and isotopically monitored (over a period of 100 days) in various types of soils (covering both biotic and abiotic degradation pathways).

After quantification of glyphosate and AMPA concentrations in the studied soils, corresponding δ 13 C of both glyphosate and AMPA are measured by LC-CFIRMS.

In parallel samples of commercial herbicide formulations containing glyphosate (from various manufacturers), as well as sewage effluents (representing potential sources of phosphonates from which AMPA could be produced by degradation) are isotopically characterized.

We will discuss results from these two parallel approaches, and assess the possibility to use carbon isotopes as a tool to track sources and processes controlling the budget of Glyphosate and AMPA in groundwater.

Jaworska J., Van

  International Symposium on Applied Isotope Geochemistry 1

	1. Introduction
	Glyphosate is the principal active substance of a
	weed-killer used worldwide. Its use and
	migration towards groundwater is of real
	concern. Both glyphosate and the aminomethyl
	phosphonic acid (AMPA), one of its degradation
	product, are among the 10-most observed
	pesticides or metabolites in France (SOeS,
	2009), usually leading to the classification of the
	corresponding groundwater as having a bad
	quality status (in regard to the Water Framework
	Directive).
	Moreover, the glyphosate and AMPA can have
	multiple origins. Glyphosate is used as pesticide
	in agricultural or non-agricultural contexts, and
	can be synthesized by different manufacturers.
	AMPA is either a glyphosate metabolite or is
	formed by the abiotic degradation of certain
	phosphonates	(present	in	particular	in
	detergents;
	Tarragona, Spain				19 th -23 rd September 2011

David Widory, Dorothea Kujawinski, Nicole Baran, Laurence Amalric, Maik Jochmann, Torsten Schmidt To cite this version: David Widory, Dorothea Kujawinski, Nicole Baran, Laurence Amalric, Maik Jochmann, et al.. The carbon isotope composition of herbicides in groundwater : the example of Glyphosate and its degradation by product AMPA. 9th International Symposium on Applied Isotope Geochemistry, Sep 2011, Tarragona, Spain. hal-00623077 th