ABSTRACT Integration of geochronological analyses of in situ zircons with the petrological study of their microstructural growth microdomains (through petrography and phase diagram modeling) yields a more precise interpretation and understanding of any geological process than the study of ages from separate zircons. This is essential especially for those rocks affected by polymetamorphic histories. We present new in situ U-Pb zircon dating from metapelitic granulites exposed in the contact with the ultramafic massif of Beni Bousera (northern Morocco, Western Mediterranean). Geochronological data scatter from Paleoproterozoic (1508 ± 23 Ma) to Miocene (22.9 ± 0.7 Ma), though the majority of ages range between 100 and 400 Ma, with four main peak ages clustering around 390–319, 286–264, 193, and 105 Ma. The Middle Permian (ca. 286–264 Ma) and earliest Miocene (22.9 ± 0.7 Ma) ages are constrained from a thermobarometric point of view: The former ages represent the formation of the zircons at depth (higher pressure), and the latter ages represent the exhumation process (lower pressure). Combining these petrological results with previous geophysical and geodynamical studies from the literature in the area, we propose a four-step geodynamic model for the Beni Bousera orogenic peridotite since the end of the Paleozoic (i.e., Middle Permian) to the present, supporting an evolution linked to a subduction-related scenario since Late Cretaceous times.

Abstract Hydrothermal dolomite (HTD) bodies are known as high-quality hydrocarbon reservoirs; however few studies focus on the geometry and distribution of reservoir characteristics. Across the platform-to-basin transition of the Ramales Platform, fault-controlled HTD bodies are present. Three kinds of bodies can be distinguished based on their morphology, that is, elongated HTD corridors, a massive HTD body (Pozalagua body) and an HTD-cemented breccia body. The differences in size and shape of the HTD bodies can be attributed to differences in local structural setting. For the Pozalagua body, an additional sedimentological control is invoked to explain the difference in HTD geometry. A (geo)-statistical investigation of the reservoir characteristics in the Pozalagua body revealed that the HTD types (defined based on their texture) show spatial clustering controlled by the orientation of faults, joints and the platform edge. Porosity and permeability values are distributed in clusters of high and low values; however, they are not significantly different for the three HTD types. Two dolomitization phases (i.e. ferroan and non-ferroan) can be observed in all HTD bodies. In general, the HTDs resulting from the second non-ferroan dolomitization phase have lower porosity values. No difference in permeability is found for the ferroan and non-ferroan dolomites.

Abstract Field data in combination with interpretation of old seismic lines across the Pusht-e Kuh arc and northwest Dezful embayment in the Zagros fold belt allow the investigation of the geometry of folding at different structural levels. Folds in the Pusht-e Kuh arc are exposed along the upper part of the 7-km (4.34-mi)-thick Competent Group level, which is folded between the main detachment at or near the base of the cover sequence (lower Mobile Group) and the Gachsaran evaporites (upper Mobile Group). Intermediate detachment levels (Triassic Dashtak, middle Cretaceous Garau and Kazhdumi, Paleocene Amiran, and middle Miocene Kalhur formations) within the Competent Group control the geometry of anticlines at surface as well as their variations with depth. The Kabir Kuh anticline is the largest and highest anticline in the Pusht-e Kuh arc and has been selected to construct a geometrical model to explore the variations of folding style with depth. Results from this anticline constituted a backbone for both a regional study to link the mechanical stratigraphy to the structure and to build up a conceptual model for folding that may apply to the Pusht-e Kuh arc as well as to the Dezful embayment tectonic domains. The Kabir Kuh anticline in its central part displays box fold geometry, characterized by a wide and rounded crestal domain, which is slightly tilted to the southwest. This geometry developed above an intermediate detachment at the level of 1.3-km (0.81-mi)-thick Triassic Dashtak evaporites within the Competent Group. Below this detachment, the geometry of the fold changes to more acute with a narrower crestal region. Although conjectural, we propose low-angle thrusting forming a tectonic wedge to accommodate shortening at the deepest part of the anticline (Paleozoic sequence). Subsidiary thrusts, related to fold tightening, may reactivate axial surfaces as well as local detachment levels to propitiate displacements of the crest of the anticline above its forelimb as inferred along the southeastern segment of the Kabir Kuh anticline. A conceptual model of folding characterized by changes in fold geometry at depth is proposed. A significant outcome of this model is that anticlines in the Passive Group may be displaced, sometimes few kilometers, from anticlines in the Competent Group. In the same way, the internal mechanically weak layers of the Competent Group may form intermediate detachments that can produce a significant change in fold style with depth and displace the upper part of the structure shifting again the position of the anticline crests. The uplifted Pusht-e Kuh arc is an excellent natural laboratory to investigate potential relationships among mechanical stratigraphy, tectonic structure, time of oil generation, and time of trap development. Fold and thrust geometries investigated in this study are directly applicable to petroleum exploration of recently awarded exploration areas within the Pusht-e Kuh arc and may apply to the rest of the Zagros fold belt.

Abstract The Barremian–Aptian upper Khami Group and Albian–Campanian Bangestan Group have been studied at outcrop in Lurestan, SW Iran. The upper Khami Group comprises a thin deltaic wedge (Gadvan Fm) transgressively overlain by shelfal carbonates (Dariyan Fm). The Dariyan Fm can be divided into lower and upper units separated by a major intra-Aptian fracture-controlled karst. The top of the Daryian Fm is capped by the Arabian plate-wide Aptian–Albian unconformity. The overlying Bangestan Group includes the Kazhdumi, Sarvak, Surgah and Ilam formations. The Kazhdumi Fm represents a mixed carbonate-clastic intrashelf basin succession, and passes laterally (towards the NE) into a low-angle Orbitolina- dominated muddy carbonate ramp/shoal (Mauddud Mbr). The Mauddud Mbr is capped by an angular unconformity and karst of latest Albian–earliest Cenomanian age. The overlying Sarvak Fm comprises both low-angle ramp and steeper dipping (5–10°) carbonate shelf/platform systems. Three regionally extensive karst surfaces are developed in the latest Cenomanian–Turonian interval of the Sarvak Fm, and are interpreted to be related to flexure of the Arabian plate margin due to the initiation of intra-oceanic deformation. The Surgah and Ilam Fm represent clastic and muddy carbonate ramp depositional systems respectively. Both The Khami and Bangestan groups have been affected by spectacularly exposed fracture-controlled dolomitization. Dolomite bodies are 100 m to several km in width, have plume-like geometry, with both fracture (fault/joint) and gradational diagenetic contacts with undolomitized country rock. Sheets of dolomite extend away from dolomite bodies along steeply dipping fault/joint zones, and as strata-bound bodies preferentially following specific depositional/diagenetic facies or stratal surfaces. There is a close link between primary depositional architecture/facies and secondary dolomitization. Vertical barriers to dolomitization are low permeability mudstones, below which dolomitizing fluids moved laterally. Where these barriers are cut by faults and fracture corridors, dolomitization can be observed to have advanced upwards, indicating that faults and joints were fluid migration conduits. Comparisons to Jurassic–Cenozoic dolomites elsewhere in Iran, Palaeozoic dolomites of North America and Neogene dolomites of the Gulf of Suez indicate striking textural, paragenetic and outcrop-scale similarities. These data imply a common fracture-controlled dolomitization process is applicable regardless of tectonic setting (compressional, transtensional and extensional).

Abstract The Mountain Frontal Flexure shows a single step along the front of the Pusht-e Kuh Arc with about 3 km of structural relief. This front has been interpreted as being formed by a basement monocline above a blind crustal-scale and low-angle thrust with a ramp–flat geometry (the ramp dips 12–15° towards the inner part of the orogen and cuts the entire crust). The Anaran anticline on top of the Mountain Frontal Flexure shows an irregular geometry in map view and consists of four segments with diverse directions of which the SE Anaran, the Central Anaran and the NW Dome are culminations. The North–South Anaran segment may form a linking zone developed during the rise and amplification of single culminations, the NW Dome and the Central Anaran, above the Mountain Frontal Flexure. The asymmetric Anaran anticline is characterized by the existence of multiple normal faults, some of them with significant dip-slip displacements of up to 1000 m. These faults limit grabens located along the crests of the anticline segments. Cross-cutting relationships show that the normal faults along the Central Anaran are older than along the North–South Anaran, reinforcing the temporal constraints on the later growth of this segment of the anticline. The geometry of the Anaran anticline is asymmetric with the subvertical forelimb very little exposed. This forelimb is cut above and below by a thrust system that seems to develop along the fold hinges. The lower thrust, with a ramp–flat geometry, carries the entire anticline towards the foreland on top of slightly deformed rocks in the footwall. The thrust flattens in the Gachsaran evaporitic level forming a typical triangular zone filled with evaporites, which produce a strong fold disharmony between the overburden (Passive Group) and the underlying rocks (Competent Group). The growth of the Anaran anticline lasted for about 6 Ma and was the consequence of detachment folding that was subsequently thrust, rotated and uplifted above the Mountain Frontal Flexure with coeval reactivation of earlier crestal layer-parallel extension normal faults to accommodate the large increase of structural relief between the foreland and the tectonic arc. Three main results from analogue modelling have been combined with field data to resolve the geometry of the Anaran anticline as well as its evolution: (1) a thickening of intermediate evaporites (Gachsaran Formation) is produced above the flat segment of the thrust carrying the anticline on top of foreland strata; (2) growth strata deposited in the adjacent syncline modify the geometry of the anticline by increasing the dip and the length of its forelimb; (3) coeval erosion to anticline growth, as well as thick growth strata deposition, increases fold amplification rather than foreland propagation of deformation. The proposed fold model may be applied to other anticlines on top of this major basement-related thrust, such as the Siah Kuh and Khaviz anticlines in the Pusht-e Kuh Arc and Dezful Embayment domains.

Abstract Compressive basins show various geometries of growth structures. In this paper, we examine the effects of the rate and nature of synkinematic sedimentation by comparing analogue models and field examples. We performed different types of experiments in order to simulate various natural conditions, the parameters tested including the rate of synkinematic sedimentation and the presence of a potential décollement layer deposited during deformation. Modelling techniques are similar to those usually used for experiments on brittle-ductile systems made of sand and silicone putty. To study the influence of the synkinematic sedimentation rate, we used sets of experiments with different velocity ratios R between the rate of sedimentation ( V s ) and the rate of uplift of the top of the structure ( V u ). In standard experiments, we deposited only sand during deformation. In all experiments, growth folds and growth faults developed. The geometry of growth folds changes from a steep anticline for R = 1, to a rounded broken anticline when R exceeds 2. The geometry of growth reverse faults changes: from segments which initiate with a dip angle of about 30°, but then flatten or steepen, for R = 1/2 and for R = 1, respectively. For some sets of experiments, a thin silicone layer deposited instead of sand at some stage of sedimentation simulated the introduction of potential décollement layers. When a change in the nature of synkinematic sedimentation occurs between the two flanks of a fold, a fault develops in the flank where brittle material dominates. The direction of growth folds forming during compression in front of a ramp and above a décollement layer is parallel to the ramp and oblique to the direction of bulk shortening. Good correlations are observed between experimental geometries and field examples from the southern Pyrenees (Eastern Ebro Basin and Jaca Basin, Spain) and the Apennines (Italy).

Abstract In the outermost region of the southeastern Pyrenees, a suite of thrusted folds detach above an upper Eocene salt that was about 300 m thick before deformation. The foremost anticlines formed during the Oligocene and represent a small amount of shortening. In map view, they display a relay pattern slightly oblique to the margin of the salt layer, where deformation stops. The three-dimensional edge effects caused by the pinch-out of the Cardona salt play an important role in the development of the frontal structures in the southeastern Pyrenees. Fold evolution has been reconstructed by interpreting variations along the strike of the folds as an indicator of deformation sequence. Where the sedimentary pile contains an upper detachment, thrusts developed fishtail geometries in which thrusts of alternating vergence were stacked up. Where an upper detachment is lacking, a thrusted anticline formed, into whose core salt migrated during the early phases of folding. Whether or not an upper detachment is present, anticlines continued to amplify during and after thrusting. Folding blocked further slip on some thrusts and promoted the development of pop-up structures.