Jean-Paul Callot

The eastern termination of the Zagros Fold‐and‐Thrust Belt, Iran: Structures, evolution, and relationships between salt plugs, folding, and faulting

[1]xa0The Zagros Mountains result from the ongoing collision between the Arabian and central Iran plates. The main features of the eastern Zagros are (1) numerous emerged or buried salt diapirs, made up of Late Precambrian Hormuz salt and (2) the irregular along-strike shape of the collision-related detachment folds with frequent bending. To understand this layout, four geological cross sections have been constructed from the Persian Gulf foreland basin to the inner part of the Zagros Fold-and-Thrust Belt. Shortening in the deformed parts of the sections is less than 10% and is mainly accommodated by detachment folding. We show that late Cenozoic folding occurred in a region that was already punctuated by salt domes and diapirs. In fact, almost continuous halokinesis developed since the earlier Paleozoic, i.e., just short time after the deposition of the Hormuz salt, and continued up to the Present. These preexisting salt structures and their relevant local thickening strongly influenced both the localization and the direction of folds.

Stress and strain patterns, kinematics and deformation mechanisms in a basement‐cored anticline: Sheep Mountain Anticline, Wyoming

[1]xa0In order to characterize and compare the stress-strain record prior to, during, and just after folding at the macroscopic and the microscopic scales and to provide insights into stress levels sustained by folded rocks, we investigate the relationship between the stress-strain distribution in folded strata derived from fractures, striated microfaults, and calcite twins and the development of the Laramide, basement-cored Sheep Mountain Anticline, Wyoming. Tectonic data were mainly collected in Lower Carboniferous to Permian carbonates and sandstones. In both rock matrix and veins, calcite twins recorded three different tectonic stages: the first stage is a pre-Laramide (Sevier) layer-parallel shortening (LPS) parallel to fold axis, the second one is a Laramide LPS perpendicular to the fold axis, and the third stage corresponds to Laramide late fold tightening with compression also perpendicular to the fold axis. Stress and strain orientations and regimes at the microscale agree with the polyphase stress evolution revealed by populations of fractures and striated microfaults, testifying for the homogeneity of stress record at different scales through time. Calcite twin analysis additionally reveals significant variations of differential stress magnitudes between fold limbs. Our results especially point to an increase of differential stress magnitudes related to Laramide LPS from the backlimb to the forelimb of the fold possibly in relation with motion of an underlying basement thrust fault that likely induced stress concentrations at its upper tip. This result is confirmed by a simple numerical model. Beyond regional implications, this study highlights the potential of calcite twin analyses to yield a representative quantitative picture of stress and strain patterns related to folding.

Burial and temperature evolution in thrust belt systems: Sedimentary and thrust sheet loading in the SE Canadian Cordillera

[1]xa0The southern Canadian foreland fold-and-thrust belt (FFTB) (SW Alberta–SE British Columbia) records the interplay between foreland basin evolution with the deforming wedge and thus controls the regional-scale overburden and exhumation history. Overburden estimates are typically based on the assumption that peak burials were reached by sedimentary burial prior to the emplacement of thrust sheets. This study combines organic maturity ranks from a newly compiled catalog with forward thermokinematic modeling to examine this assumption. The organic maturity rank trends correlate not only to sedimentary but also to tectonic burial histories. The forward thermokinematic modeling builds on this combined burial history scenario and shows how required peak burial and temperatures can be achieved with reduced sedimentary overburden when combined with tectonic loading. We thus consider that the overburden was reached during instead of prior to the contraction of the FFTB as result of an integrated sedimentary and tectonic burial history. Thermokinematic modeling also suggests that first-order steady state temperature conditions prevail during the development of the FFTB. Differently from earlier findings that considered a drop in paleotemperature gradient during the belts growth, steady state conditions reduce the amount of regional overburden required and, consequently, of eroded sediments. Besides tracing the regional evolution, organic maturity ranks are also affected by more local phenomena such as thrust-scale denudation patterns and fluid flow.

The Salt Diapirs of the Eastern Fars Province (Zagros, Iran): A Brief Outline of their Past and Present

The salt diapirs of the eastern Fars (Zagros, Iran) have been reexamined in light of their relationships to the regional geodynamic history. The present-day surface morphology of salt diapirs can be divided into six types, which we suggest represent different stages in a long and complex history: type A are buried diapirs, type B high relief active diapirs, type C with salt fountain and glacier, type D similar to type C but with more erosion and with out fountain, type E as dead diapirs and empty crater, and type F linear diapirs generally emerging along faults. We show that nearly all the diapirs of the study area were already active prior to Zagros folding either as emergent diapirs forming islands in the Paleogene to Neogene sea or as buried domes initiated at least by the Permian. They have been reactivated by subsequent tectonic events. At the initiation of Zagros folding, the abundance of emergent diapirs close to the present Persian Gulf coast line weakened the entire sedimentary cover and facilitated the preferential localization of the deformation in a narrow zone. Then, salt-cored detachment folding in the whole eastern Fars Zagros Fold-Thrust Belt reactivated the preexisting domes and allowed salt movement along faults.

Interactions between continental breakup dynamics and large-scale delta system evolution: Insights from the Cretaceous Ceduna delta system, Bight Basin, Southern Australian margin

[1]xa0The interpretation of two regional seismic reflection profiles and the construction of a balanced cross section through the southern Australian margin (Bight Basin) are designed to analyze the influence of the Australia-Antarctica continental breakup process on the kinematic evolution of the Cretaceous Ceduna delta system. The data show that the structural architecture of this delta system consists of two stacked delta systems. The lower White Pointer delta system (Late Albian-Santonian) is an unstable tectonic wedge, regionally detached seaward above Late Albian ductile shales. Sequential restoration suggests that the overall gravitational sliding behavior of the White Pointer delta wedge (∼45 km of seaward extension, i.e., ∼27%) is partially balanced by the tectonic denudation of the subcontinental mantle. We are able to estimate the horizontal stretching rate of the mantle exhumation between ∼2 and 5 km Ma−1. The associated uplift of the distal part of the margin and associated flexural subsidence in the proximal part of the basin are partially responsible for the decrease of the gravitational sliding of the White Pointer delta system. Lithospheric failure occurs at ∼84 Ma through the rapid exhumation of the mantle. The upper Hammerhead delta system (Late Santonian-Maastrichtian) forms a stable tectonic wedge developed during initial, slow seafloor spreading and sag basin evolution of the Australian side margin. Lateral variation of basin slope (related to the geometry of the underlying White Pointer delta wedge) is associated with distal raft tectonic structures sustained by high sedimentation rates. Finally, we propose a conceptual low-angle detachment fault model for the evolution of the Australian-Antarctic conjugate margins, in which the Antarctic margin corresponds to the upper plate and the Australian margin to the lower plate.

Salt tectonics in the Sivas Basin, Turkey: Outstanding seismic analogues from outcrops

The Sivas Basin in Central Anatolia is possibly the world’s finest open-air museum of salt tectonics structures. It is an elongated Oligo-Miocene sag basin that developed in an orogenic context above the Neotethys suture zone. A mid-Oligocene quiet period during convergence of the Arabian and Eurasian plates allowed the deposition of a thick sequence of evaporites. Erosion of the Taurus Mountains shed clastic sediments northwards over the evaporitic basin. Sediments and deformation propagated from the south, forming mini-basins and associated evaporite diapirs and walls. Following this quiet period, ncompression resumed in the early Miocene, enhancing the formation of gypsum overhangs and allochtonous sheets. The Sivas outcrops expose classic salt tectonics geometries associated with the development of diapirs: halokinetic sedimentary sequences along diapir walls, welds and evaporite sheets or canopies, minibasins, and overturned minibasin wings (overturned edges of minibasins). These exposures are some of the finest field analogues for classical petroleum provinces controlled by salt tectonics such as the Gulf of Mexico and offshore Angola. We illustrate seismic-scale structures and, in nthe vicinity of the evaporite bodies, interesting analogues for drilled structures where seismic data do not provide an image.

3D structural modelling of the southern Zagros fold-and-thrust belt diapiric province

3D modelling of geological structures is a key method to improve the understanding of the geological history of an area, and to serve as a drive for exploration. Geomodelling has been performed on a large 60000 km 2 area of the Zagros fold-and-thrust belt of Iran, to reconcile a vast but heterogeneous dataset. Topography, geological surface data and dips, outcrop surveys, and well and seismic data were integrated into the model. The method was to construct a key surface maximizing the hard data constraints. The Oligo-Miocene Top Asmari layer was chosen, as this formation was regionally deposited before the main Zagros collision phase and because the numerous outcrops allow proper control of the bed geometry in the fold cores. Interpreted seismic data have been integrated to interpolate the surfaces at depth within the synclines. Several conceptual models of fold geometry have been applied to estimate the best way to convert seismic time signal to depth. Several deeper horizons down to Palaeozoic strata were deduced from this key horizon by applying palaeo-thickness maps. During the construction, the 3D interpolated surfaces could be reconverted to time, using a velocity model, and compared with previous seismic interpretations. This exercise obliged us to revise some early interpretations of seismic lines that were badly tied to wells. The 3D modelling therefore clearly improves regional interpretation. In addition, the 3D model is the only tool that allows drawing consistent cross-sections in areas where there are no seismic lines. Emerging Hormuz salt diapirs were added to the model. Dimensions and shapes of the individual diapirs were modelled using a statistical survey on the cropping out Hormuz structures. Modelling reliably demonstrated that the diapirs, when piercing, show a constant mushroom shape whose diameter depends on the stratigraphic depth of observation. This observation allowed us to exemplify relations between the pre-existing diapirs and the anticlines of the area, and to highlight the morphological changes from the inner onshore areas to the coastal and offshore areas. In addition, one of the surprising results of this study was the observation of the increasing diameter of the diapirs at the time of the Zagros collision and folding event, with growth strata and overhangs on the flanks of the diapirs.

The fracture network, a proxy for mesoscale deformation: Constraints on layer parallel shortening history from the Malargüe fold and thrust belt, Argentina

An analysis was performed of the fracture networks in the N-S trending thick-skinned Malargue fold and thrust belt (MFTB). A total of 2000 planar structures including joints and veins were measured in different structural domains ranging from surficial thin-skinned systems detached in the cover to large-scale structures such as basement-cored folds. The investigated stratigraphic section ranges from the Middle Jurassic (Cuyo Group) to the Paleocene (Malargue Group), including sandstones, siltstones, shales, and limestones. Four main fracture sets are identified trending, E-W, NW-SE, NE-SW, and N-S. The abutting relationships provide a reliable chronology between the four fracture sets which are ubiquitously found in the MFTB throughout the various structural domains. Due to this observation, we assume the fracture signal to be regional and developed in response to both large-scale processes and folding. In particular, based on a fold test and the characteristics of data dispersion, the fracture sets I, II, and III exhibit a prefolding origin, while set IV shows a synfolding origin. A regional interpretation of the various fractures is proposed, involving several stages of fracture formation from compaction to folding, including prefolding layer parallel shortening. The fracture signal yields useful insights about the structural history of the MFTB and the spatiotemporal evolution of the foreland tectonic regime since Late Cretaceous times. We then place the various identified fracture sets into the known pattern of geodynamic evolution since the Late Cretaceous.

Kinematics of the SE Canadian Fold and Thrust Belt: Implications for the Thermal and Organic Maturation History

The southern Canadian Foreland Fold and Thrust Belt forms a north-eastward tapering contractional belt comprising Mesoproterozoic intracontinental, Palaeozoic continental margin and Mesozoic to lower Tertiary foredeep sequences that record an upper Cretaceous to Palaeocene contractional history. Shortening is accommodated by a few major thrusts (e.g. Lewis thrust and McDonald thrust), estimated at ∼115–125 km and accompanied by the development of a presumably thick, currently eroded syntectonic foredeep wedge. The contractional features are overprinted by a second deformation phase of Late Eocene-Oligocene extension with the formation of two pronounced halfgraben systems (i.e. Flathead Valley Graben and Rocky Mountain Trench) and bounding listric normal faults that merge at depth with the pre-existing thrust decollements.

Evidence of active shortening along the eastern border of the San Rafael basement block: characterization of the seismic source of the Villa Atuel earthquake (1929), Mendoza province, Argentina

On the 30 May 1929, a massive earthquake occurred in the San Rafael area (southernn Mendoza province) leading to the destruction of the Villa Atuel and Las Malvinas towns.n The region affected by the ground shaking covers a large part of southern South America.n Although no surface breaks have been detected on the surface, several authors have pointedn out active faults that could be related to the event of 1929. Using satellite imagery andn field observations, we investigated two active faults situated on the eastern border ofn the San Rafael Block (SRB) close to or within the epicentral area. The most prominentn faults are the c. 40 km long Las Malvinas and c. 30 kmn long Cerro Negro reverse faults which are located near the epicentral area. Geological andn morphological observations allow us to describe late Pleistocene activity and estimate then long-term slip rates of these faults. Possible ruptures that match our observations andn which are compatible with the cartographic length of these faults would account for an seismic moment magnitude of M 0 = 2.8×10 19 N m and an moment magnitude of M W = 6.9. The morphological signatures ofn these fault segments and the occurrence of the San Rafael earthquake suggests that then southern Mendoza Province is still currently submitted to shortening.