Fault & Fracture Development in Foreland Fold and Thrust Belts - Insight from the Lurestan Province, Zagros Mountains, Iran

Second Arabian Plate Geology Workshop Abu Dhabi, UAE, 24 - 27 January 2010The Simply Folded Belt of the Zagros Mountains, Iran, represents one of the best examples of foreland fold and thrust belt. A regional fault and fracture analysis of the Cenomanian ¿ Coniacian Ilam and Sarvak formations, exposed in southern Lurestan Province, is presented as a case study for fault and fracture development in folded belts. The area is characterised by the occurrence of gentle to tight anticlines and synclines whose NW-SE axial traces are parallel to the general trend of the belt. Fold style is intimately related to both vertical and lateral facies distribution. The two formations belong to the Bangestan Group and, in this area, they represent the oldest strata exposed in the core of most anticlines outcropping at surface. Distribution, kinematics and timing of faults and fractures have been characterised through extensive fieldwork and interpretation of orthorectified QuickBird imagery and 3-D virtual outcrop models based on LiDAR technology. Data have been collected from 10 anticlines covering an area of approximately 150 x 150 km. Key outcrops for fracture and fault kinematics interpretations are presented. Field observations and interpretation of QuickBird and 3-D photorealistic models suggest a complex fault and fracture geometry and timing relationship. Both fractures and faults record pre-folding to uplift-related deformations. Pre-folding structures are typically represented by small-scale, flat-ramp-flat geometry thrusts, systematic veins and stylolites, which are superimposed on inherited syn-sedimentary normal faults. Folding-related structures generally reactivated pre-existing fracture and fault planes. Strike-slip faulting is typically recorded as the last faulting event and is probably related to late stage of fold tightening. All structures are geometrically and kinematically consistent with the trend of the Arabian passive margin and its subsequent tectonic inversion. Uplift and stress release induced opening and propagation of through-going fractures. Faults and fracture orientations generally change accordingly with local fold trend. Symmetry between fracture and fold orientation, although commonly interpreted as evidence for folding-related fracture development, is here interpreted as evidence of syn- to post-folding local vertical axis passive rotation

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

Magnetostratigraphy of Miocene-Pliocene Zagros foreland deposits in the front of the Push-e Kush Arc (Lurestan Province, Iran)

The timing of the deformation in the Zagros Simply Folded Belt is poorly constrained because of the lack of an accurate absolute chronology of the syntectonic sedimentary sequences. The foreland basin infill at the front of the Push-e-Kush Arc is composed of fine-grained fluvial plain deposits (Agha Jari Fm.) and coarse conglomerates at the top of the section (Bakhtyari Fm.). A magnetostratigraphic study was carried out in a composite section spanning about 2800 m in order to date growth strata, and to constrain the timing of the deformation in the Mountain Front Flexure (MFF). Magnetostratigraphic correlation of the base of the Agha Jari Fm. with chron C5A yields an age of 12.8 to 12.3 Ma for this base. The transition to the conglomerates of the Bakhtyari Fm. correlates with the upper Gauss chron C2An at approximately 3 Ma. The deposition age of the top of the preserved Bakhtyari Fm. is extrapolated around the Pliocene-Pleistocene boundary. The base of the Agha Jari Fm. growth strata, and thus the beginning of the deformation in the front of the Push-e Kush Arc, is dated at 8.1-7.2 Ma. The topmost preserved Bakhtyari is folded in the NE flank of the Changuleh anticline and is unconformably overlying the SW flank of the Anaran anticline. This indicates that the tectonic deformation in the front of the Push-e-Kush Arc was active at least during 5 My. The MFF is a relatively long-lived structure active from 8.1 to 7.2 Ma to about the Pliocene-Pleistocene boundary, partly synchronous with the Changuleh anticline to the foreland. After MFF tectonic cessation, only the Changuleh anticline remained active. © 2004 Elsevier B.V. All rights reserved.This study has been financed by a collaborative project between the Institute of Earth Sciences “Jaume Almera”, CSIC of Barcelona (Spain) and the Norsk Hydro Research Centre of Bergen (Norway), with the partial support of project 2001 SGR 00339 Grup d'Estructura i Processos Litosfèrics. We also thank the support in the field of Hydro Zagros Oil and Gas Tehran and NPA people, and the National Iranian Oil Company (NIOC) for their collaboration during this project

Late Cretaceous–Paleocene formation of the proto–Zagros foreland basin, Lurestan Province, SW Iran

Late Cretaceous emplacement of ophiolitic-radiolaritic thrust sheets over the Arabian passive margin was the first manifestation of the protracted closure of the Neotethys Ocean, which ended with the continental collision between Arabia and central Iran and the formation of the present Zagros fold belt. This tectonic stacking produced a flexural basin (the Amiran Basin: 400 x 200 km in size) in the northwest Zagros that was filled with a 1225-m-thick shallowing-upward detrital succession made up of the Amiran, Taleh Zang, and Kashkan Formations. This succession sits unconformably above the Late Cretaceous Gurpi Formation and is overlain by the Oligocene-Miocene Shahbazan-Asmari carbonate succession. Dating of the Amiran-Kashkan succession is based on detailed biostratigraphy using large foraminifera and calcareous nannoplankton. The Cretaceous-Tertiary (K-T) boundary is located within the uppermost 25–45 m of the Gurpi Formation. The overlying Amiran and Taleh Zang Formations have been dated as Paleocene in age. However, the base of the Paleocene within the Gurpi Formation lacks NP1 and NP2 zones, implying a hiatus of ~2 m.y. at ca. 65.5 Ma, which is inferred to correspond to an early folding phase near the Cretaceous-Paleocene boundary. The upper part of the Kashkan Formation is dated to the earliest Eocene by palynostratigraphy. A large hiatus (or very slow deposition) lasting about 15 m.y. occurs between the Kashkan and Shahbazan Formations in the studied region. The base of the prograding Shahbazan platform deposits is dated by 87Sr/86Sr stratigraphy at ca. 33.9 Ma. The upper part of the Asmari Formation is dated as early-middle Miocene using foraminifera associations. Reconstruction of the Amiran–Taleh Zang–Kashkan succession of the Amiran Basin indicates a thickening of the basin fill from the southern pinch-out along the SE flank of the Kabir Kuh anticline to SW of the Khorramabad anticline, where the flexure is at least 900 m. In contrast, the NE part of the basin underwent coeval contraction and uplift of ~1300 m. Superimposed smaller undulations onto the large-scale flexure are interpreted as Late Cretaceous–Paleocene folds. Regional comparisons (SE Zagros, Oman, and Turkey) indicate that Late Cretaceous–Early Tertiary deformation affected the entire NE margin of Arabia but that compression was not synchronous, being younger in Lurestan than in the NW Persian Gulf where inversion tectonics occurred from Turonian to mid-Campanian times. The long sedimentary hiatus spanning most of the middle and late Eocene must have been related to deep lithospheric processes linked to the initial events of the protracted closure of the Neotethys Ocean between Arabia and central Iran. The tectono-sedimentary history recorded in the Zagros Basin may help to understand early foreland basin growth in other orogens in which subsequent continental collision has obliterated these early events.This study is a contribution of the Group of Dynamics of the Lithosphere (GDL) within the framework of a collaborative project with the former Hydro Oil and Energy Research Centre in Bergen (Norway). We thank both Hydro Zagros Oil and Gas Tehran and the National Iranian Oil Company (NIOC) for support in the field as well as for permission to publish these results. We thank Eric Blanc for fruitful discussion on early growth folding during a field trip to the study area after the submission of the first paper. We also thank Gary Axen and two anonymous reviewers, as well as Associate Editor Jim Schmitt for their helpful comments that improved the paper. We are also grateful to Neil Pickard and Anna Travé for their help and discussion, and to J.M. McArthur for providing the last version of the statistical LOWES fit for 87Sr/86Sr stratigraphic dating.Peer reviewe

Growth fold controls on carbonate distribution in mixed foreland basins: Insights from the Amiran foreland basin (NW Zagros, Iran) and stratigraphic numerical modelling

The evolution from Late Cretaceous to early Eocene of the well dated Amiran foreland basin in the NW Iranian Zagros Mountains is studied based on the reconstruction of successive thickness, palaeobathymetry and subsidence maps. These maps show the progressive forelandwards migration of the mixed carbonate-siliciclastic system associated with a decrease in creation of accommodation. Carbonate facies variations across the basin suggest a structural control on the carbonate distribution in the Amiran foreland basin, which has been used as initial constraint to study the control exerted by syndepositional folding in basin architecture and evolution by means of stratigraphic numerical modelling. Modelled results show that shallow bathymetries on top of growing folds enhance carbonate production and basin compartmentalization. As a consequence, coarse clastics become restricted to the internal parts of the basin and only the fine sediments can by-pass the bathymetric highs generated by folding. Additionally, the development of extensive carbonate platforms on top of the anticlines favours the basinwards migration of the depositional system, which progrades farther with higher fold uplift rates. In this scenario, build-ups on top of anticlines record its growth and can be used as a dating method. Extrapolation of presented modelling results into the Amiran foreland basin is in agreement with an early folding stage in the SE Lurestan area, between the Khorramabad and Kabir Kuh anticlines. This folding stage would enhance the development of carbonate platforms on top of the anticlines, the south-westward migration of the system and eventually, the complete filling of the basin north of the Chenareh anticline at the end of the Cuisian. Incremental thickness maps are consistent with a thin (0.4-2 km) ophiolite complex in the source area of the Amiran basin. © 2012 Blackwell Publishing Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists.Additional support was provided by Consolider-Ingenio 2010 Topo-Iberia (CSD2006-00041), MEC projects Atiza (CGL2009-1355) and Tecla (CGL2011-26670). Additional funding was also provided by a CSIC JAE-Doc postdoctoral research contract (E.C.).Peer Reviewe

Late Cretaceous to Present protracted convergence between Arabia and Iran: time constraints from Zagros foreland basins studies

European Geosciences Union General Assembly 2010 Vienna, Austria, 02 – 07 May 2010The Zagros orogeny took place during a protracted period of time, and its complete evolution is difficult to ascertain due to the multiple stages starting with oceanic obduction related processes and culminating with arc-continent and continent-continent collision. In addition to this long-lasting evolution, the Neogene shortening partially masked previous compressive histories. These earlier fold and thrust events are discontinuously preserved and thus authors working in different areas reached different but certainly complementary results. In this short review we summarize the recent results on the timing of the northwestern Zagros foreland evolution constrained by biostratigraphy, magnetostratigraphy and both bedrock and detrital AFT ages. Four main periods of denudation correspond to Early-Late Cretaceous at 91 Ma, to Maastrichtian at 66 Ma, middle-late Eocene at 38 Ma and Oligocene-early Miocene at 22 Ma. Both late Cretaceous (obduction related thrusting and folding reaching far regions of the foreland basin) and early Miocene collision processes produced bending of the Arabian plate and concomitant foreland deposition. Between the two major flexural foreland episodes, the middle-late Eocene phase mostly produced a long-lasting slow- or non depositional episode in the inner part of the foreland basin that is supported by the recognized 15 My long sedimentary hiatus between the Kashkan and Shahbazan formations in the study region. During this period the deformation migrated towards the Sanandaj-Sirjan domain and its Gaveh Rud fore-arc basin. Well constrained growth strata across the Zagros fold and thrust belt indicate a forward propagation of the folding wave starting in middle Miocene time along the footwall of the High Zagros Fault and reaching the front of the Pusht-e Kuh arc in latest Miocene to Pliocene times

Tectonic Record of Ophiolite Obduction in the Sedimentary Sequence of the Pust-e-Kuh Arc, Simply Folded Belt, Zagros Mountains (Lurestan, Iran)

Second Arabian Plate Geology Workshop Abu Dhabi, UAE, 24 - 27 January 2010The emplacement of ophiolite and radiolarite obduction complexes (e.g. Kermanshah and Neyriz) on top of the north-eastern Arabian margin started during the Cenomanian ¿ Turonian in most of the Zagros Belt. In the Pusht-e Kuh Arc (northwest Zagros), where the latest thrust emplacement has been dated as Early Maastrichtian, the effects of the concomitant lithospheric loading can be observed in the Late Cretaceous ¿ Early Eocene sedimentary record. Lithofacies study of the carbonate-dominated Barremian ¿ Aptian Khami Group and Albian ¿ Campanian Bangestan Group, deposited during the transition from passive to active margin, show their evolution is associated to 13 major eustatic cycles. The deposition of the Cenomanian ¿ Turonian Upper Sarvak Formation is characterised by a northwards progadational platform-basin transition. Detailed paleofacies maps of this interval reveal a SE-elongated paleohigh with at least four periods characterized by normal faulting followed by karstification related to flexing of the foreland margin along the forebulge domain. This period is also characterized by incipient folding as determined in the Khorramabad region where the Upper Sarvak platforms show karstification and progradation away from the culmination of the anticline. Subsequently, the Campanian ¿ Maastrichtian Gurpi Formation recorded higher subsidence and filled the entire foreland basin. The Amiran fan deltas in the inner part of the basin also show deflection around the growing Khorram-abad anticline during Maastrichtian time. The Maastricthian to Early Eocene infill of the Amiran basin wedges out towards the southwest, with a maximum thickness of 1,500 m in its central part. This shallowing upwards mixed clastic-carbonatic basin infill is diachronous and progrades south-westward at a rate of c. 5.5 mm/year. The sedimentary record analyzed in Lurestan may be compared to the early foreland basin successions in Oman where chronology related to obduction and emplacement of the Semail Ophiolite is wellknown

Fold patterns and multilayer rheology of the Lurestan Province, Zagros Simply Folded Belt (Iran)

Anticlines of the Lurestan Province in the Zagros fold–thrust belt have been studied by integrating field-based analysis with the use of high-resolution satellite images and data available from the literature. The distribution of folds in the southeastern Lurestan Province, expressed in terms of axial length and wavelength distribution, shows a direct link with the characteristics of the sedimentary multilayer in which the folds developed. Within the carbonate deposits of the Late Cretaceous Bangestan Group the transition from pelagic to neritic facies determines a threefold increase in anticline spacing and promotes the development of thrust structures in the forelimb of anticlines. The Oligocene–Miocene Shahbazan–Asmari unit folds harmonically with the Bangestan Group, except in the areas where the Palaeogene deposits interposed between the two units exceed 1300 m of thickness. In these areas the Shahbazan–Asmari carbonates display short-wavelength folds indicating a complete decoupling from the underlying folds of the Bangestan Group. It is suggested that this decoupling occurs because the summed thickness of the incompetent units separating the two carbonate units exceeds the extension of the zone of effective contact strain of the Bangestan Group folds.This is a contribution of the Group of Dynamics of the Lithosphere (GDL), financed by a collaborative project between the Institute of Earth Sciences ‘Jaume Almera', CSIC of Barcelona (Spain) and StatoilHydro Research Centre, with the partial support of project Team Consolider-Ingenio 2010 no. CSD 2006-00041. We also thank StatoilHydro, and the National Iranian Oil Company (NIOC) for the support received in the field.Peer reviewe