Mechanical Stratigraphy Research Articles

Quantitative Prediction of Natural Fractures in Shale Oil Reservoirs

Natural fractures are the key factors controlling the enrichment of shale oil. It is of great significance to clarify the distribution of natural fractures to guide the selection of sweet spots for shale oil. Taking the Qing-1 Member shale oil reservoir in the northern Songliao Basin, China as an example, a new method considering the factors affecting fracture distribution was proposed to quantitatively predict the structural fractures. And the effect of natural fractures on shale oil enrichment was discussed. Firstly, the types and characteristics of fractures in shale oil reservoirs are characterized by using core and outcrop data. Combined with the experimental analysis, the influences of fault, mechanical stratigraphy, mineral composition and content, TOC, and overpressure on fracture intensity were clarified. Then, the number and density of fractures are quantitatively predicted according to the power-law distribution of fault length. Next, geomechanical simulation and fracture prediction were carried out on the model which was established with comprehensive consideration of the influencing factors of fracture distribution. Finally, the fracture distribution is evaluated comprehensively based on above prediction. The prediction results in this work are consistent with the core measurements.

The role of tectonic inheritance in the development of a fold-thrust belt and superimposed rift: An example from the São Francisco basin, eastern Brazil

The São Francisco intracratonic basin, eastern Brazil, hosts multiple sedimentary successions younger than 1.8Ga that witnessed major plate reorganizations since its generation. During the Brasiliano/Pan-African Gondwana amalgamation, in the Ediacaran-Cambrian boundary, the western half of the basin was involved in the Brasília foreland fold-thrust belt, which propagated eastwards deforming its Proterozoic-Paleozoic strata. Lately, as the South Atlantic rift evolved giving rise to the Brazilian continental margin, the basin was affected by extensional tectonism leading to the development of the Cretaceous Abaeté half-graben. During this episode, elements related to a prominent structure of the Brasília foreland fold-thrust belt, the João Pinheiro fault, were reactivated controlling the graben architecture. Aiming to unravel the evolutionary history of the fault zone since its nucleation until its Cretaceous extensional reactivation, we conducted a structural investigation based on field, well-tied 2D seismic data, and paleostress analysis. Our study revealed that the fault represents an emergent ramp of the basal detachment of the Brasilia foreland fold-thrust belt. Formed under EW-oriented shortening, the structure exhibits a gentle recces-salient geometry in map view, which reflects variations in the detachment depth influenced by pre-orogenic rift structures. Trains of NS-trending chevron folds are abundant and their high symmetry and dominance are expressions of the control exerted by mechanical stratigraphy and low-friction basal detachment. The superposed Abaeté half-graben developed under an extensional regime with a NE-trending maximum extension. Under this condition, a stratigraphic interface along the João Pinheiro fault zone was reactivated giving rise to the graben border fault. NS- to NW-oriented preexisting bedding planes were activated as small normal faults. Although newly-formed faults are present, reactivation is the most effective mechanism of extension accommodation. Therefore, this study provides insights on the influence of tectonic inheritance on the architecture, deformation pattern, strain/stress partitioning and evolution of both extensional and compressional intracontinental systems.

The role of mechanical stratigraphy on CO2 migration along faults – examples from Entrada Sandstone, Humbug Flats, Utah, USA

Bleaching patterns along a faulted succession provides new insight into fluid migration pathways controlled by the fault zone and mechanical stratigraphy. In order to investigate this effect, we logged stratigraphic layers in Humbug Flats, Utah, USA both in host rocks and faulted rocks, where a 40 m offset normal fault cuts through the Entrada Sandstone. We measured petrophysical and mechanical properties using TinyPerm II, Schmidt Hammer and laboratory core testing for selected layers. Bleaching has been identified as a proxy for fluid migration parallel to bedding, which fades out with distance from faults. A new observation is that fracturing of bed interfaces contributes to horizontal flow out from the fault zone. Longer-distance comprehensive bleaching dominates the most permeable beds (units), hosting porosity-controlled deformation band associated to fault zones. Fractures prevail in low permeability and stiffer units, where isolated fracture parallel bleaching haloes are observed within fault damage and process zones. Strong correlation is observed between permeability/porosity and stiffness/strength properties for the tested samples of different layers along the stratigraphic logs. For the studied fault, fractures provide important conduits for vertical flow across low permeability and stiff layers, whereas the more porous layers provide bedding-parallel flow paths out of the fault zone.

Porosity development in central Alborz Upper Jurassic deposits (N-Iran): sequence stratigraphy, diagenesis and mechanical stratigraphy

Porosity development in central Alborz Upper Jurassic deposits (N-Iran): sequence stratigraphy, diagenesis and mechanical stratigraphy

The fold-thrust belt stress cycle: Superposition of normal, strike-slip, and thrust faulting deformation regimes

Mesoscale deformation in fold-thrust belts and foreland areas occurs during the interaction of evolving stress regimes and mechanical stratigraphy, leading to a progressive deformation sequence. Tensile, hybrid, shear, compactive shear, and compactive failure may all occur in a thrust faulting stress and deformation regime, often contemporaneously in adjacent layers. Mesostructural studies, however, typically identify structural suites that appear inconsistent with a thrust faulting stress regime, such as vertical opening-mode fractures, normal faults, and strike-slip faults. We present a conceptual model that considers the progressive sequence associated with development and eventual waning of fold-thrust belt activity. The general sequence consists of five stages: (1) burial deformation in a normal faulting stress regime, (2) early horizontal contractional deformation in a strike-slip stress regime, (3) horizontal contractional deformation in a thrust faulting stress regime, (4) late horizontal contractional deformation and initial relaxation in a strike-slip stress regime, and (5) horizontal extensional deformation in a normal faulting stress regime. This sequence can be recognized in horizontal or subhorizontal strata in fold-thrust belts and foreland areas, and in more structurally complex structural positions. This model explains common suites of mesostructures, and can be used to (i) predict mesostructural suites in different structural positions, (ii) explain apparently anomalous, but inherited, structures, and (iii) infer paleostress history.

Mechanical zonation of rock properties and the development of fluid migration pathways: implications for enhanced geothermal systems in sedimentary-hosted geothermal reservoirs

Oil and gas operations in sedimentary basins have revealed the occurrence of significant temperature anomalies at depth, raising the possibility of major geothermal resource potential in the sedimentary sequences. The efficient development of such a resource may require enhancement by hydraulic stimulation. However, effective stimulation relies on an initial assessment of in situ mechanical properties and a model of the rock response. Here, we examine the distribution of mechanical properties (unconfined compressive strength, UCS; ultrasonic velocity-derived Poisson ratio, ν; and, scratch toughness, Ks) along the cored interval of a sedimentary formation with a known low-to-medium temperature geothermal anomaly in the Permian Basin, U.S. Our results reveal the presence of mechanical stratigraphy along the core, demonstrated by the alternation of distinct soft–hard (i.e.,less stiff-to-stiff) mechanical zone couplets composed of: (1) mechanically softer 0.17-m-thick Zone-A and 0.18-m-thick Zone-C with mean values of UCS = 110 MPa, ν = 0.25, Ks = 1.89 MPa·√m; and (2) mechanically harder 0.41-m-thick Zone-B and 0.15-m-thick Zone-D which show mean values of UCS = 166 MPa, ν = 0.22, and Ks = 2.87 MPa·√m. Although X-ray diffraction analyses of the samples suggest that the entire rock matrix is dominated by dolomite, the harder zones show an abundance of quartz cement (> 30%) and relatively lower phyllosilicate mineral content (< 2%) than the softer zones. Further, we observe that the mechanically harder zones have the greatest occurrences and thicknesses of hydrothermal alterations (anhydrite veins and nodules), indicating that the rock had experienced hydrothermal fluid circulation (basinal brines) in the past. We infer that the mechanical stratigraphy most likely influenced the spatial clustering of fractures that facilitated hydrothermal fluid migration in the past, and provides insight that is relevant for the exploitation of geothermal energy resources in sedimentary basins. We suggest that the harder zones or formation intervals with higher ratios of the hard zones relative to soft zones represent viable targets for hydraulic stimulation of a sedimentary-hosted geothermal reservoir, both for the emplacement of new fractures and the linkage of pre-existing fractures to allow efficient fluid circulation. Our findings in this study provide insight that is relevant for understanding the complexity of pre-existing mechanical heterogeneity in sedimentary-hosted geothermal reservoir targets in other places.

Mechanical stratigraphy of Mississippian strata using machine learning and seismic-based reservoir characterization and modeling, Anadarko Basin, Oklahoma

The Sooner Trend in the Anadarko (Basin) in Canadian and Kingfisher counties play primarily produces oil and gas from Mississippian strata. The interval consists of interbedded argillaceous mudstones and calcareous siltstones. Such a contrast in rock composition is linked directly to the mechanical stratigraphy of the strata. Brittle (calcareous siltstones) and ductile beds (argillaceous mudstones) are related to the sequence-stratigraphic framework at different scales. We have used seismic and well-log data to estimate and map the geomechanical properties’ distribution and interpret the mechanical stratigraphy of rocks within the Mississippian strata. First, we defined the parasequences that form the main reservoir zones of the Meramecian-Mississippian strata. Once we established the stratigraphic framework, we estimated and compared rock brittleness index (BI) using two independent laboratory-based measurements from the core. The first method, the mineralogical-derived BI, uses mineralogical composition inverted from Fourier-transform infrared spectroscopy analyses, whereas the second method, the mechanical-derived BI, involves measurements of compressional and shear velocities from core plugs. We use the data from core-plug velocity measurements along with well logs and an artificial neural network approach to establish relationships among the geomechanical properties, well logs, and acoustic impedance values. We then applied these relationships to generate 3D geomechanical models constrained to seismic volumes. The resulting grid distributions illustrate the stratigraphic variability of the properties at the parasequence scale. Overall, brittle strata decrease in thickness and abundance basinward as the frequency of interbedded brittle and ductile zones increases and gradually transitions into thin calcite-cemented siltstones and clay-rich mudstones. Analysis of the production performance of selected horizontal wells drilled within the Mississippian strata indicates that the proportion of brittle and ductile rocks along the well path drilled and the drilled area vertical stacking pattern play a significant role in hydrocarbon production for these Mississippian units.

Mechanical stratigraphy in Mesozoic rocks of the San Juan Basin: Integration of stratigraphic and structural terms and concepts

We characterize relationships between stratigraphy and natural fractures in outcrops of Mesozoic strata that rim the San Juan Basin in New Mexico and Colorado. These outcrops expose fluvial and shallow-marine siliciclastic deposits and calcareous mudstones deposited in a distal marine setting. We focus primarily on a regionally extensive fracture set formed during the Eocene to minimize localized tectonic effects on fracture development. Where possible, we supplement our observations with wireline log- or laboratory-derived measurements of rock properties. Our goals are twofold: 1) to illustrate how direct integration of data and concepts from stratigraphy and structural geology can lead to better fracture characterization, and 2) to develop thought processes that will stimulate new exploration and development strategies. Genetic beds form one scale of stratification in the outcrops we describe. For example, sandstone beds can be arranged into coarsening and thickening upward successions that are the depositional record of shoreline progradation. In fluvial settings, cm- to dm-scale sandstone beds can also be part of m-scale single-storey channel complexes that, themselves, can be arranged into amalgamated channel complexes 10s of m thick. In these and other settings, it is important to distinguish between beds and features that can be defined via wireline logs because it is the former (cm- to dm-scale) that are usually the primary control the distribution of natural fractures. The extension fractures we describe are typically bed-bound, with bedding being defined by lithology contrasts and the associated changes in elastic properties. Fracture spacing distributions are typically lognormal with average spacing being less than bed thickness. Although mechanical bedding and depositional bedding are commonly the same, diagenesis can cut across bed boundaries and complicate this relationship, especially where lithologic contrasts are small. Deposits from similar depositional environments which undergo different diagenetic histories can have substantially different mechanical properties and therefore deform differently in response to similar imposed stresses.

A method for modeling multiscale geomechanical effects in the stimulated rock volume

I have developed a workflow to efficiently simulate geomechanical effects in the stimulated rock volume (SRV) by including regional geologic structures such as faults and folds as well as high-resolution-oriented mechanical stratigraphy. The motivation is that the local model used for hydraulic fracture analysis should include macroscale 3D geomechanical effects derived from regional tectonic and seismic data. A practical computational strategy is developed to link multiple 3D geomechanical models derived at different scales and their associated stress effects. I apply the workflow to a synthetic reservoir problem composed of a tectonic-scale structural framework model with three embedded mechanical stratigraphic models representing three stimulated vertical wells. I first combine regional stresses solved with 3D finite-element analysis with perturbation stresses from elastic dislocation modeling using elastic superposition concepts. I then apply the macroscale stress effects as unique boundary conditions to an embedded finite-element submodel, a mesoscale stratigraphic model representing the SRV allowing for resolution of variable stress amplification, and stress rotation in geologic sublayers. Finally, I conduct hydraulic-fracture simulations within the SRV models. The simulated hydraulic fractures are controled by the structural position and mechanical stratigraphy. Closest to the back limb of the main structural anticline, hydraulic fractures tend to be height-restricted, and in some realizations, fractures propagate horizontally. Adjacent to the fold and a fault, where differential stresses are elevated, fracture growth is the most unconstrained in height and length. Results suggest that this multiscale approach can be applied to better predict and understand behaviors related to unconventional reservoir stimulation. The workflow could easily be modified for other operational problems and geologic settings.

Macroscopic structural styles in the southeastern Anadarko Basin, southern Oklahoma

The Anadarko Basin in south-central Oklahoma contains a number of oil and gas fields located on anticlinal structures formed during the Pennsylvanian Orogeny. We use new 2-D and 3D seismic and well data to interpret the structural geometry and kinematic evolution of the Carter-Knox, Cruce, Chickasha, and Cement structures. These structures consist of tight faulted-detachment folds within the Pennsylvanian units related to slip along a detachment at the base of the Springer Shale, cored by deeper low amplitude fault-related folds within the pre-Pennsylvanian thick carbonate units which are detached at the base of the Arbuckle Group. Slip on these detachments was derived from the Wichita Uplift, which was a Precambrian-Cambrian failed rift that became inverted during the Pennsylvanian Orogeny. The trend and location of these structures was probably controlled by discontinuities related to pre-existing rift-related basement normal faults.Variations in the structural styles among the major structures are related primarily to the shortening and the influence of late-stage normal/strike-slip faulting. Changes in shortening are manifested in the increasing tightness and relief of the anticlines from the Chickasha-Cement to the Carter-Knox and Cruce structures. Some of the E-W structures were cut by Virgilian age strike-slip and associated normal faults which formed as the maximum compressive stress rotated from NE-SW to ENE-WSW during the Late Pennsylvanian. In summary, the geometry, trend, and evolution of the structures were influenced by the mechanical stratigraphy, pre-existing basement structures, and a rotation in stresses during the final stage of their formation.

Late Miocene Deformation Kinematics Along the NW Zagros Fold‐Thrust Belt, Kurdistan Region of Iraq: Constraints From Apatite (U‐Th)/He Thermochronometry and Balanced Cross Sections

AbstractApatite (U‐Th)/He (AHe) thermochronometric results are integrated with geologic cross sections, structural relationships, and stratigraphic data to reconstruct the growth of the NW Zagros orogenic belt in the Kurdistan region of Iraq. Prolonged exhumation is documented across the belt with deformation advances and retreats from ~14 Ma onward. After in‐sequence propagation of deformation during middle to late Miocene times, preserved growth strata and AHe data show a deformation retreat by latest Miocene time (~5 Ma). In the NW Zagros, the Phanerozoic succession contains two principal décollements in Lower Triassic and middle Miocene units. The Triassic strata are interpreted as the main décollement for a thin‐skinned system that was dominant during most of the Cenozoic. By ~8–5 Ma, the fold‐thrust belt shifted to basement‐involved deformation in association with growth of the mountain front flexure and reactivation of frontal structures. The shift from thin‐skinned to a hybrid thin‐ and thick‐skinned mode of shortening may reflect variations in the mechanical behavior of the upper crust and the presence of inherited basement discontinuities. On the basis of two NE‐SW balanced cross sections spanning the NW Zagros, the estimated total minimum horizontal shortening is ~18.2 km (6%) in the central and ~16 km (7%) in the southern sectors of the Kurdistan region of Iraq. These findings suggest that the evolution of the NW Zagros orogenic belt was likely driven by the mechanical stratigraphy of the sedimentary cover, inherited basement discontinuities, and the dynamic and thermomechanical effects of potential slab breakoff and lithospheric mantle delamination events.

Depositional and diagenetics controls on the mechanical stratigraphy of aptian laminated limestones of crato formation

Depositional and diagenetics controls on the mechanical stratigraphy of aptian laminated limestones of crato formation

Nature and origin of large-scale and intrasalt deformation within the Wieliczka salt mine, Poland

The Wieliczka salt mine, near Kraków in southern Poland, is a world-famous historical and modern destination for both geoscientists and tourists. Despite numerous publications, there is still a lack of consensus on the nature and origin of the large-scale folds as well as whether the small-scale structures represent tectonic or soft-sediment deformation. In this preliminary work, we offer new ideas on both aspects. At the large scale, we emphasize the mechanical stratigraphy of the layered evaporite sequence, which comprises a thin basal weak layer, a thin strong unit, a thicker weak layer, and a thicker strong unit. We suggest that the inclined to recumbent folds and thrusts formed tectonically due to overthrust shear between the basal detachment and the overriding Carpathian frontal thrust, with different structural styles decoupled by the thick weak layer. At the small scale, we suggest that there was early extension directed toward the east to north-east, followed by contraction vergent toward the north. We infer that there was early, syndepositional gravity gliding down a topographic slope dipping into the coeval Gdów depocenter to the east, and that subsequent north-directed Carpathian shortening generated the small-scale contractional structures as the larger folds were developing

Multiphase, decoupled faulting in the southern German Molasse Basin – evidence from 3-D seismic data

Abstract. We use three-dimensional seismic reflection data from the southern German Molasse Basin to investigate the structural style and evolution of a geometrically decoupled fault network in close proximity to the Alpine deformation front. We recognise two fault arrays that are vertically separated by a clay-rich layer – lower normal faults and upper normal and reverse faults. A frontal thrust fault partially overprints the upper fault array. Analysis of seismic stratigraphy, syn-kinematic strata, throw distribution, and spatial relationships between faults suggest a multiphase fault evolution: (1) initiation of the lower normal faults in the Upper Jurassic carbonate platform during the early Oligocene, (2) development of the upper normal faults in the Cenozoic sediments during the late Oligocene, and (3) reverse reactivation of the upper normal faults and thrusting during the mid-Miocene. These distinct phases document the evolution of the stress field as the Alpine orogen propagated across the foreland. We postulate that interplay between the horizontal compression and vertical stresses due to the syn-sedimentary loading resulted in the intermittent normal faulting. The vertical stress gradients within the flexed foredeep defined the independent development of the upper faults above the lower faults, whereas mechanical behaviour of the clay-rich layer precluded the subsequent linkage of the fault arrays. The thrust fault must have been facilitated by the reverse reactivation of the upper normal faults, as its maximum displacement and extent correlate with the occurrence of these faults. We conclude that the evolving tectonic stresses were the primary mechanism of fault activation, whereas the mechanical stratigraphy and pre-existing structures locally governed the structural style.

Long-wavelength late-Miocene thrusting in the north Alpine foreland: implications for late orogenic processes

Abstract. In this paper, we present new exhumation ages for the imbricated proximal molasse, i.e. Subalpine Molasse, of the northern Central Alps. Based on apatite (U-Th-Sm)/He thermochronometry, we constrain thrust-driven exhumation in the Subalpine Molasse between 12 and 4 Ma. This occurs synchronously to the main deformation in the adjacent Jura fold-and-thrust belt farther north and to the late stage of thrust-related exhumation of the basement massifs (i.e. external crystalline massifs) in the hinterland. Our results agree with other findings along the north Alpine foreland. While site-specific variations in the mechanical stratigraphy of the molasse deposits influence the pattern of thrusting at the local scale, we observe that late-Miocene thrusting is a long-wavelength feature occurring along the north Alpine foreland roughly between Lake Geneva and Salzburg. The extent of this thrusting signal as well as the timing suggests that late-Miocene thrusting in the north Alpine foreland coincides with the geometries and dynamics of the attached Central Alpine slab at depth. Interestingly, this implies that the slab geometry at depth does not coincide with the boundary between the Eastern and Central Alps as observed in the surface geology. Using this observation, we propose that thrusting in the Subalpine Molasse and consequently also the late stage of thrust-related exhumation of the external crystalline massifs, as well as the main deformation in the Jura fold-and-thrust belt are at least partly linked to changes in slab dynamics.

Active transfer faulting in the NW Sub-Himalaya (India) observed by space-borne topographic analyses

The results of surface morphology and InSAR analyses carried out along a NNE-SSW oriented (~7 km-long) transfer fault are presented. This structure actively accommodates differential motion across two blocks in the western Sub-Himalayan belt of India. Segmentation is most pronounced in this sector of the accretionary wedge, where minimal climatic variations and a three-fold sub-division (Kangra Renetrant-Nahan Salient-Dehradun Reentrant) are well recognized. Segmentation is here clearly elucidated by variation in surface topography, geologic structure and recorded seismicity over the last few decades. However, the transition from one segment to another is poorly documented and understood. This article primarily documents surface topography and drainage anomalies within the transition zone between the Kangra Reentrant and the Nahan Salient through the study of space imageries spanning nearly six-decades (i.e. early 1960's to present). Geomorphic observations and InSAR analyses demonstrate that the segmentation is largely governed by lateral variation in geological structure, deformation patterns and mechanical stratigraphy and accommodated by transverse movement along the transfer fault. It should be noted that transfer faults commonly represent a privileged pathway for the major drainage and therefore surface manifestations are rarely preserved, e.g. Yamuna Tear, Ganga Tear etc. A rare instance of preserved surface topography associated with the recent activity along the Sabilpur transfer fault at the transition between the Kangra Renetrant and the Nahan Salient is documented here. Our investigations demonstrate that the Sabilpur Active Fault (SAF) accommodates differential tectonic convergence between the two blocks predominantly by left-lateral strike-slip kinematics with a component of uplift related to thrusting at HFT.

Natural Fractures in Carbonate Basement Reservoirs of the Jizhong Sub-Basin, Bohai Bay Basin, China: Key Aspects Favoring Oil Production

Analysis of natural fractures is essential for understanding the heterogeneity of basement reservoirs with carbonate rocks since natural fractures significantly control key attributes such as porosity and permeability. Based on the observations and analyses of outcrops, cores, borehole image logs, and thin sections from the Mesoproterozoic to Lower Paleozoic in the Jizhong Sub-Basin, natural fractures are found to be abundant in genetic types (tectonic, pressure-solution, and dissolution) in these reservoirs. Tectonic fractures are dominant in such reservoirs, and lithology, mechanical stratigraphy, and faults are major influencing factors for the development of fractures. Dolostones with higher dolomite content are more likely to have tectonic fractures than limestones with higher calcite content. Most tectonic fractures are developed inside mechanical units and terminate at the unit interface at nearly perpendicular or high angles. Also, where a thinner mechanical unit is observed, tectonic fractures are more frequent with a small height. Furthermore, the dominant direction of tectonic fractures is sub-parallel to the fault direction or oblique at a small angle. In addition, integrating diverse characteristics of opening-mode fractures and well-testing data with oil production shows that, in perforated intervals where dolostone and limestone are interstratified or dolostone is the main lithologic composition, fractures are developed well, and the oil production is higher. Moreover, fractures with a larger dip angle have bigger apertures and contribute more to oil production. Collectively, this investigation provides a future reference for understanding the importance of natural fractures and their impact on oil production in the carbonate basement reservoirs.

The early Palaeogene transition from thin-skinned to thick-skinned shortening in the Potosí uplift, Sierra Madre Oriental, northeastern Mexico

ABSTRACT The central Sierra Madre Oriental in Mexico developed primarily in the Late Cretaceous to early Palaeogene as a thin-skinned fold belt above a décollement hosted in Jurassic evaporite. In some regions shortening transitioned to sub-décollement structural levels, resulting in thick-skinned uplifts such as the Potosí uplift in Nuevo León. Thick-skinned deformation in the Potosí uplift involved folding of sub-décollement strata into an NNW-trending anticlinorium, cleavage development, thrust and conjugate strike-slip faulting, and extension fractures associated with barite mineralization. These structures consistently record subhorizontal shortening, directed ~52–65° in the southern uplift, ~69–72° in the northern uplift. Palaeocene to mid-Eocene zircon (U-Th)/He cooling dates record the timing of exhumation associated with thick-skinned uplift and suggest a continuation of shortening rather than a separate tectonic event. Zircon (U-Th)/He dates across the southern Potosí uplift and the Aramberri uplift, ~50 km to the south, range from ~66–53 Ma, whereas dates in the northern part of the Potosí uplift range from ~49–44 Ma. We attribute the transition to thick-skinned shortening to the mechanical strengthening of a planar décollement as rheologically weak evaporite was evacuated beneath synclinal keels of detachment folds. Along-strike differences in timing of exhumation and shortening directions may relate to differences in mechanical stratigraphy. Thicker intervals of evaporite in the northern uplift allowed thin-skinned shortening to continue while the southern uplift transitioned to thick-skinned shortening as the evaporite décollement was exhausted. As a result, stress-strain trajectories in the northern uplift refracted clockwise during continued deformation. Our findings provide new insight into the structural evolution of the Potosí uplift and may provide a framework for studying other thick-skinned uplifts in the orogen, and more generally orogenic belts that record a transition in deformation styles during progressive shortening.

Submarine landslides on a carbonate platform slope: forward numerical modelling of mechanical stratigraphy and scenarios of failure precondition

Slope failure and landslides are widespread on the submarine slopes of carbonate platforms. They represent a key component of their stratigraphic evolution and a major geohazard. Several Quaternary slope failures and mass transport complexes were identified on the western slope and margin of the Great Bahama Bank (GBB) platform. This study evaluates several hypotheses for the preconditions associated to these events, in relation with the stratigraphic and environmental history of the platform. The forward stratigraphic simulator Dionisos Flow™ allows the slope stratigraphic evolution to be reconstructed on a 2D platform-to-basin section at high temporal resolution according to the eustatic and environmental history of the last 1.7 Myr. The hydromechanical simulator A2 is applied on this high-resolution stratigraphic grid for computing the mechanical stratigraphy of the section, that is the spatiotemporal pore pressure and stress state distribution. The simulated precondition of the platform margin during glacial lowstands results from the combination of two factors: transient overpressure generation by lateral fluid flow from the emerged platform and the steepening stratigraphic trend of the platform. No significant pore fluid overpressure is generated under high sedimentation rates or as a result of shift in hydrostatic gradient during sea-level falls. Precondition in the lower slope is not achieved in this 2D simulation, with the horizontal stress distribution counteracting the effect of overpressure build-up. It is found that cemented levels in the lower slope succession do not represent a significant preconditioning factor.

Strain localization during burial and exhumation of the continental upper crust: A case study from the Northern Sporades (Pelagonian thrust sheet, Greece)

Abstract Extension is a key process controlling the post-orogenic exhumation of metamorphic rocks in subduction and collision zones. Previous studies have largely focused on the mechanics of localized core-complex style post-orogenic extension and the large-scale effects of various internal (e.g. rheology) and external (e.g. plate motions) parameters on the mode of extension. However, many regions on earth underwent rock exhumation during post-orogenic extension, which is characterized by distributed rather than localized deformation. We explore conditions of distributed deformation as illustrated by the Pelagonian unit in the Aegean subduction system. In particular, we explore the influence of structural inheritance related to the pre-extension shortening and mechanical stratigraphy on the localization of extension on the scale of the upper crust through detailed structural analysis on the islands of Skiathos and Skopelos. Additionally, the time frame of deformation has been established by 40Ar/39Ar dating of key shear zones. Shortening on the islands predominantly took place by ductile top-SW thrusting under low-grade metamorphic conditions, localized in weak calcite marble layers within the Upper Cretaceous and Upper Triassic carbonates at ~55 Ma. We show that the presence of shallow decoupling levels in the upper crust resulted in the formation of thin (several 100 m thick) thrust sheets that are defined for the first time on Skiathos. The Early Paleogene accretion of the Pelagonian upper crust to the upper plate (Eurasia/Rhodopia) was followed by the extensional inversion of the nappe stack. Extension was accommodated by opposite-sense, generally top-NE, ductile to brittle shearing, which localized at inherited heterogeneities such as reverse-sense shear zones and stratigraphic contacts at around 35 Ma, as suggested by our 40Ar/39Ar age spectra. The dense network of such northerly-dipping, inherited weakness zones resulted in a highly distributed pattern of extensional deformation dominated by layer-parallel shearing. We argue that the distribution of crustal heterogeneities substantially influences the style of post-orogenic extension.