Tertiary sequence of deformation in a thin-skinned/thick-skinned collision belt: The Zagros Folded Belt (Fars, Iran)

International audienceWe describe how thin-skinned/thick-skinned deformation in the Zagros Folded Belt interacted in time and space. Homogeneous fold wavelengths (15.8 ± 5.3 km), tectono-sedimentary evidence for simultaneous fold growth in the past 5.5 ± 2.5 Ma, drainage network organization, and homogeneous peak differential stresses (40 ± 15 MPa) together point to buckling as the dominant process responsible for cover folding. Basin analysis reveals that basement inversion occurred ∼20 Ma ago as the Arabia/Eurasian plate convergence reduced and accumulation of Neogene siliciclastics in foreland basin started. By 10 Ma, ongoing contraction occurred by underplating of Arabian crustal units beneath the Iranian plate. This process represents 75% of the total shortening. It is not before 5 Ma that the Zagros foreland was incorporated into the southward propagating basement thrust wedge. Folds rejuvenated by 3–2 Ma because of uplift driven by basement shortening and erosion. Since then, folds grew at 0.3—0.6 mm/yr and forced the rivers to flow axially. A total shortening of 65–78 km (16–19%) is estimated across the Zagros. This corresponds to shortening rates of 6.5–8 km/Ma consistent with current geodetic surveys. We point out that although thin-skinned deformation in the sedimentary cover may be important, basement-involved shortening should not be neglected as it requires far less shortening. Moreover, for such foreland folded belts involving basement shortening, underplating may be an efficient process accommodating a significant part of the plate convergence

Building the Zagros collisional orogen: Timing, strain distribution and the dynamics of Arabia/Eurasia plate convergence

The Zagros Mountains are the result of the Arabia/Eurasia collision initiated at ~. 35. Ma as the rifted Arabian lithosphere was underthrusted beneath the Iranian plate due to its negative buoyancy. The onset of crustal thickening started at ~. 25. Ma, as recorded by the hinterland exhumation and foreland clastic deposition. Deformation throughout the Arabia/Eurasia collision zone and the uplift of the Iranian plateau occurred after 15-12. Ma, as a result of shortening/thickening of the thin Iranian crust. We emphasize that only 42% of the post-35. Ma convergence is partitioned by shortening within central Iran. Tomographic constraints show ongoing slab steepening or breakoff in the NW Zagros, whereas underthrusting of the Arabian plate is observed beneath central Zagros. The current subduction dynamics can be explained by the original lateral difference in the buoyancy of the distal margin that promoted slab sinking in NW Zagros and underthrusting in central Zagros. Critical wedge approach applied to the Zagros favors the hypothesis of strong brittle crust detached above a viscous lower crust. In contrast, the weak sedimentary cover deforms by buckling of a thick multilayered cover. Thrust faulting associated with folding occurs in the competent layers and is responsible for most of the earthquakes. There is evidence that the role of the slab pull force in driving the Arabian plate motion was reduced after ~. 12. Ma. Large-scale mantle flow induced by mantle upwelling at the Afar plume appears to be the main driver of the Arabia plate motion. We stress that the main kinematic change in the Zagros region occurred at 15-12. Ma as the Zagros uplifted, before the Arabian slab detached. The Zagros appears key to investigate coupling between continental rheology, plate driving forces and mountain building, in which the role of rift inheritance appears to be central. © 2012 Elsevier B.V.Peer Reviewe

Structure of orogenic belts controlled by lithosphere age

The structure of a mountain belt reflects the manner in which plate convergence is accommodated in Earth's lithosphere. However, the extent to which orogenic structure is preconditioned by the thermo-mechanical conditions of the converging plates is debated. Here we re-process and analyse existing data on the amount and style of contractional deformation in 30 orogens worldwide and compare this with the lithospheric strength and age of the colliding plates. We find a correlation between orogenic deformation, and specifically the depth at which the crust decouples from the underlying plate, and the age of the lithospheric plate at the time of collision. Orogens formed from Phanerozoic lithosphere, which has high geothermal gradients and weak mantle, are characterized by several under-thrust faults that form in the mid-to-lower crust and moderate amounts of deformation, at less than 35% crustal strain. In contrast, orogens formed on older lithospheric plates, which have greater strength and higher-viscosity mantle, are characterized by a large detachment fault and large amounts of deformation, at about 70% crustal strain. We conclude that inherited lithospheric strength influences the style and amount of plate-tectonic contraction during mountain building, and thus the stability of continental subduction. Our results emphasize the influence of the deep Earth on the structural style of collisional orogens. © 2013 Macmillan Publishers Limited. All rights reserved

Autogenic versus allogenic controls on the evolution of a coupled fluvial megafan–mountainous catchment system: numerical modelling and comparison with the Lannemezan megafan system (northern Pyrenees, France)

Alluvial megafans are sensitive recorders of landscape evolution, controlled by both autogenic processes and allogenic forcing, and they are influenced by the coupled dynamics of the fan with its mountainous catchment. The Lannemezan megafan in the northern Pyrenean foreland was abandoned by its mountainous feeder stream during the Quaternary and subsequently incised, leaving a flight of alluvial terraces along the stream network. We use numerical models to explore the relative roles of autogenic processes and external forcing in the building, abandonment and incision of a foreland megafan, and we compare the results with the inferred evolution of the Lannemezan megafan. Autogenic processes are sufficient to explain the building of a megafan and the long-term entrenchment of its feeding river on time and space scales that match the Lannemezan setting. Climate, through temporal variations in precipitation rate, may have played a role in the episodic pattern of incision on a shorter timescale. In contrast, base-level changes, tectonic activity in the mountain range or tilting of the foreland through flexural isostatic rebound do not appear to have played a role in the abandonment of the megafan

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 anticlines and synclines spaced in a fairly regular manner. It is thus tempting to use this spacing along with other geological constraints to obtain insights into 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-and-thrust belts in the world. Both analytical and numerical models are employed to study what controls fold spacing and under what conditions folding dominates over thrusting. The models show that if only a single basal decollement layer is present underneath a brittle sedimentary cover, deformation is dominated by thrusting, which is inconsistent with the data of the Zagros fold belt. If we instead take into account additional decollement layers that have been documented in the field, a switch in deformation mode occurs and crustal-scale folding is obtained with the correct spacing and time scales. We show that fold spacing can be used to constrain the friction angle of the crust, which is similar to 5 degrees in the Zagros fold belt. This implies that on geological time scales, the upper crust is significantly weaker than previously thought, possibly due to the effect of fluids

Incision dans le Bassin de l'Ebre (nord de l'Espagne) entre la fin du Pliocène et le Pléistocène

International audienceThe Ebro Basin constitutes the central part of the southern foreland of the Pyrenees. It was endorheic during the Cenozoic and accumulated sediments. By the end of the Miocene, erosion and river incision reconnected the basin to the Mediterranean Sea, establishing a post-opening drainage network. Those rivers left terraces that we study in this work. We first synthesize previous works on river terraces that are widely dispersed in the basin. We provide new age constraints, up to 3 Ma, obtained thanks to cosmogenic nuclides using both profile and burial methods. We derive a unified fluvial terrace chronology and a homogenized map of the highest terraces over the entire Ebro Basin. The dated terraces labeled A, B, C, D, and E are dated to 2.8±0.7 Ma, 1.15±0.15 Ma, 850±70 ka, 650±130 ka, and 400±120 ka, respectively. The chronology proposed here is similar to other sequences of river terraces dated in the Iberian Peninsula, around the Pyrenees, and elsewhere in Europe. The oldest terraces (A, B, C) are extensive, indicating they form a mobile fluvial network while from D to present, the network was stable and entrenched by 100 to 200 m-deep valleys. The transition from mobile to a fixed fluvial network is likely to have occurred during the Middle Pleistocene Transition (MPT, between 0.7 and 1.3 Ma), when long-period/high-intensity climate fluctuations were established in Europe. We estimate that between 2.8-1.15 Ma and present, the incision rates have tripled.Le bassin de l’Ebre constitue le centre du piémont sud-pyrénéen. Il a été endorhéique et a accumulé une épaisseur importante de sédiments au cours du Cénozoique. Vers la fin du Miocène, l'érosion et l'incision fluviale ont reconnecté le bassin à la mer Méditerranée, en établissant un nouveau réseau de drainage. Les rivières constituant ce réseau ont laissé des terrasses que nous étudions dans le cadre de ce travail. Nous faisons d'abord une synthèse des travaux antérieurs sur les terrasses fluviales, qui sont largement dispersées dans le bassin. Ensuite, nous fournissons de nouvelles contraintes d'âge, jusqu'à 3 Ma, obtenues grâce aux nucléides cosmogéniques utilisant à la fois des méthodes de profil d’exposition et d'enfouissement. Enfin, nous en déduisons une chronologie unifiée des terrasses fluviales et une carte homogénéisée des terrasses les plus hautes sur l'ensemble du bassin de l'Ebre. Les terrasses, que nous notons A, B, C, D et E, sont datées respectivement de 2,8±0,7 Ma, 1,15±0,15 Ma, 850±70 ka, 650±130 ka et 400±120 ka. La chronologie proposée ici est similaire à d'autres séquences de terrasses fluviales datées dans la péninsule ibérique, autour des Pyrénées, et ailleurs en Europe. Les terrasses les plus anciennes (A, B, C) sont étendues, ce qui témoignent d’un réseau fluvial mobile alors que depuis la formation des terrasses D jusqu’à aujourd'hui, le réseau était stable et canalisé le long de vallées de 100 à 200 m de profondeur. Le passage d'un réseau fluvial mobile à un réseau fluvial fixe s'est probablement produit pendant la transition du Pléistocène moyen (entre 0,7 et 1,3 Ma), lorsque des fluctuations climatiques de longue période et de forte intensité ont été établies, comme l'ont déjà proposé Gibbard et Lewin (2009) pour les rivières européennes. Nous estimons qu'entre 2,8 et 1,15 Ma et la période actuelle, les taux d'incision ont triplé