Eduard Saura

Crustal-scale cross-sections across the NW Zagros belt: implications for the Arabian margin reconstruction

Quantified balanced and restored crustal cross-sections across the NW Zagros Mountains are presented in this work integrating geological and geophysical local and global datasets. The balanced crustal cross-section reproduces the surficial folding and thrusting of the thick cover succession, including the near top of the Sarvak Formation (~90 Ma) that forms the top of the restored crustal cross-section. The base of the Arabian crust in the balanced cross-section is constrained by recently published seismic receiver function results showing a deepening of the Moho from 42 ± 2 km in the undeformed foreland basin to 56 ± 2 km beneath the High Zagros. The internal parts of the deformed crustal cross-section are constrained by new seismic tomographic sections imaging a ~50° NE-dipping sharp contact between the Arabian and Iranian crusts. These surfaces bound an area of 10800 km 2 that should be kept constant during the Zagros orogeny. The Arabian crustal cross-section is restored using six different tectonosedimentary domains according to their sedimentary facies and palaeobathymetries, and assuming Airy isostasy and area conservation. While the two southwestern domains were directly determined from well-constrained surface data, the reconstruction of the distal domains to the NE was made using the recent margin model of Wrobel-Daveau et al . (2010) and fitting the total area calculated in the balanced cross-section. The Arabian continental–oceanic boundary, at the time corresponding to the near top of the Sarvak Formation, is located 169 km to the NE of the trace of the Main Recent Fault. Shortening is estimated at ~180 km for the cover rocks and ~149 km for the Arabian basement, including all compressional events from Late Cretaceous to Recent time, with an average shortening rate of ~2 mm yr −1 for the last 90 Ma.

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

Abstract: 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.

Sub-seismic fractures in foreland fold and thrust belts: insight from the Lurestan Province, Zagros Mountains, Iran

ABSTRACT The Simply Folded Belt of the Zagros Mountains, Iran, is a spectacularly well-exposed example of a foreland fold and thrust belt. A regional analysis of the Cenomanian–Coniacian Sarvak and Ilam Formations, exposed in the southern Lurestan Province, is presented as a case study for sub-seismic fracture development in this type of compressive setting. The area is characterized by gentle to tight anticlines and synclines parallel to the NW–SE trend of the belt. In the Lurestan Province, the Cenomanian–Coniacian interval is exposed in the core of most of the outcropping anticlines. Fold style is intimately related to both vertical and lateral facies distribution. Geometry, kinematics and timing of sub-seismic fractures were characterized through extensive fieldwork, interpretation of orthorectified QuickBird imagery and interpretation of 3D photorealistic models derived from LiDAR. Data were collected from 12 anticlines covering an area of approximately 150 × 200 km. Key outcrops for understanding fracture geometry, kinematics and timing are presented. Field observations and interpretation of QuickBird and 3D photorealistic models reveal the complexity of fracture geometry and timing. Fractures record pre-, syn- and post-folding stages of deformation. Pre-folding structures include synsedimentary normal faults, and subsequent small-scale thrusts, systematic veins and stylolites. During folding, pre-existing fracture planes were re-activated and through-going fractures and reverse faults developed. Strike-slip faults typically postdate pre- and syn-folding structures and are probably related to the late stages of fold tightening. All structures are geometrically and kinematically consistent with the trend of the Arabian passive margin and its subsequent tectonic inversion.

Syn- to post-rift diapirism and minibasins of the Central High Atlas (Morocco): the changing face of a mountain belt

The Atlas Mountains are classically regarded as a failed Mesozoic rift arm subject to Alpine inversion, folding and thrusting. Here, we present new integrated structural and sedimentological studies that have revealed numerous Early–Middle Jurassic diapiric ridges and minibasins, characterized by distinctive halokinetic structures. Diachroneity in halokinesis is observed across the Central High Atlas, waning first in the SW during the Early–Middle Jurassic (Jbel Azourki and Tazoult ridges) and continuing to late Middle Jurassic towards the NE (Imilchil region). The halokinetic structures are readily differentiated from the effects of later Alpine deformation, allowing a new picture of the Central High Atlas to emerge. The most pervasive deformation in the Central High Atlas is associated with Early–Middle Jurassic diapirism, whereas the impact of Alpine inversion is mostly focused at the basin margins. This new understanding helps explain previously problematic aspects of the Atlas Mountains, which we now recognize as an exceptionally well exposed natural laboratory for understanding the interactions between halokinesis, tectonics and sedimentation.

Basin architecture and growth folding of the NW Zagros early foreland basin during the Late Cretaceous and early Tertiary

Abstract: We present and use the chronostratigraphy of 13 field logs and detailed mapping to constrain the evolution of the early Zagros foreland basin, in NW Iran. Large foraminifera, calcareous nannofossil, palynological and 87Sr/86Sr analysis supplied ages indicating a Campanian–early Eocene age of the basin infill, which is characterizd by a diachronous, southwestward migrating, shallowing upwards, mixed clastic–carbonate succession. Growth synclines and local palaeoslope variations indicate syndepositional folding from Maastrichtian to Eocene time and suggest forelandward migration of the deformation front. We also illustrate the basin architecture with a synthetic stratigraphic transect. From internal to external areas, time lines cross the formation boundaries from continental Kashkan red beds to Taleh Zang mixed clastic–carbonate platforms, Amiran slope deposits and basinal Gurpi–Pabdeh shales and marls. The foreland basin depocentres show a progressive migration from the Campanian to Eocene (c. 83–52.7 Ma), with rates of c. 2.4 mm a−1 during the early–middle Palaeocene (c. 65.5–58.7 Ma) increasing to c. 6 mm a−1 during the late Palaeocene–earliest Eocene (c. 58.7–52.8 Ma). Coeval subsidence remained at c. 0.27 mm a−1 during the first 12.7 Ma and decreased to c. 0.16 mm a−1 during the last 4.2 Ma of basin filling. Finally, we integrate our results with published large-scale maps and discuss their implications in the context of the Zagros orogeny. Supplementary material: Tables with dating results are available at http://www.geolsoc.org.uk/SUP18439.

Reservoir characteristics of fault-controlled hydrothermal dolomite bodies: Ramales Platform case study

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.

Rising and falling diapirs, shifting depocenters, and flap overturning in the Cretaceous Sopeira and Sant Gervàs subbasins (Ribagorça Basin, southern Pyrenees)

This research was carried out with the aid of grants by CSIC-ESF 2007–2013 JAE-Doc postdoctoral research contract (E.S.) and with funding from the Spanish Research Agency through projects CGL2009-1355, CGL2011-26670, and CGL2010-15416. Additional funding was provided by Atlas Project and Statoil Research Center

Modeling the flexural evolution of the Amiran and Mesopotamian foreland basins of NW Zagros (Iran‐Iraq)

The evolution of the Amiran and Mesopotamian flexural basins of the Zagros belt is approached by coupled 2-D forward modeling of orogenic wedge formation, lithospheric flexural isostasy, and stream power erosion/transport/sedimentation. Thrust geometries and sequence of emplacement derived from geometric and kinematic models presented here are the inputs to our evolutionary model, constrained by basin geometry, sediment volume, and topography. Modeling results confirm that the Zagros flexural basins evolution is consistent with two stages of deformation: (1) the obduction stage involving the Kermanshah accretionary complex and a basement unit and (2) the collision stage, emplacing the Gaveh Rud and Sanandaj-Sirjan domains in the hinterland and forming a basement duplex in the outer part. Results provide quantitative insights into processes involved in mountain and basin building. The lithospheric equivalent elastic thickness (Te) changed from 20 km during the Amiran stage (~90–50 Ma) to 55 km during the Mesopotamian subsidence stage (last 20 Myr). The Amiran basin results from flexure of the Arabian plate below the load of the Kermanshah cover and basement thrust sheets. During this stage, material eroded in the inner parts was enough to fill the flexural trough. The Mesopotamian basin formed in front of the outermost basement units flexing the Arabian plate. During this latter stage, material eroded from the orogenic wedge was not enough to fill the Mesopotamian basin. An additional longitudinal sediment supply of up to 200 m/Myr is required to fill the flexural basin.

Diapiric growth within an Early Jurassic rift basin: the Tazoult salt wall (Central High Atlas, Morocco)†

The Central High Atlas (Morocco) constitutes a diapiric province that hosts a complex array of elongated diapirs and minibasins that formed during the Lower Jurassic rift of the Atlas Basin. This paper aims to study the structure and growth evolution of the Tazoult diapiric wall, located in the Central High Atlas, by means of structural and sedimentological fieldwork integrated with remote sensing mapping. The Tazoult salt wall is a 20 km long x 3 km wide NE-SW trending ridge that exposes Upper Triassic red beds and basalts along its core. The succession flanking the salt wall ranges from Hettangian to Bajocian ages displaying spectacular sedimentary wedges in the SE and NW flanks. The Hettangian-early Sinemurian carbonates mainly crop out as blocks embedded in the core rocks. The ~1-km thick Pliensbachian platform carbonates display large subvertical flap structures along the flanks of the Tazoult salt wall with unconformities bounding tapered composite halokinetic sequences. In contrast, the ~2.5-km thick late Pliensbachian-Aalenian mixed deposits form tabular composite halokinetic sequences displaying small-scale hook halokinetic sequences. Passive diapirism resulted in the lateral extrusion of the evaporite-bearing rocks to form an allochthonous salt sheet towards the adjacent SE Amezrai minibasin. The Bajocian platform carbonates partially fossilized the Tazoult salt wall and thus constitute a key horizon to constrain the timing of diapir growth and discriminate diapirism from Alpine shortening. The Pliensbachian carbonate platform evolved as a long flap structure during the early growth of the Tazoult salt wall, well before the onset of the Alpine shortening.

Fracture characterization in sigmoidal folds: Insights from the Siah Kuh anticline, Zagros, Iran

Fieldwork and remote-sensing data from the Siah Kuh anticline, simply folded belt, Zagros, Iran indicate that specific structures and fracture systems formed during the development of its sigmoidal shape, and that conceptual fracture models developed for cylindrical folds are inadequate for the correct evaluation and development of hydrocarbon accumulations in this type of anticline. The sigmoidal shape of the Siah Kuh anticline was achieved in the Pliocene due to vertical axis rotations of an already existing anticline. These rotations promoted the development of (1) two systems of normal faults in the outer arcs of the sigmoidal shape, (2) a low-angle thrust, and (3) the north–south Danan anticline in the inner arc of its easternmost bend. The passive margin to syn-folding structures, typically observed in nearby cylindrical and periclinal anticlines, predated the development of the sigmoidal shape and were passively rotated into the segments of the anticline. The sigmoid-related structures are spatially, geometrically, and kinematically related to the bends of the anticline trend, hence they can be predicted and modeled in the subsurface. The sigmoid-related normal faults have a great potential to preserve porosity and promote localized high flow rates or early water breakthrough. However, if they cut through thin reservoir and seal units, sigmoid-related thrusts and normal faults might compromise lateral reservoir continuity and seal integrity. The results of this study can help in reducing risks and uncertainty in the evaluation and development of business opportunities in secondary sigmoidal anticlines within the Zagros or any other fold-thrust belt.