Journal of Structural Geology最新文献

We use new (micro-)structural, petrofabric, kinematic, and vorticity data to investigate the origin and deformation conditions of the opposite shear senses (i.e., top-to-the-SSE and -NNW shearing) recorded in the metamorphic rock pile of the Cycladic Massif on Ios Island, Greece. We focused our analyses on the contact between the Cycladic Blueschist Unit and the underlying Cycladic Basement as well as on the upper structural levels of the latter. The opposite shear senses are observed within the same foliation fabric, and they coexist at outcrop scale. Our observations showed that within foliation-parallel veins displaying top-to-the-SSE and top-to-the-NNW shear sense quartz is recrystallized in the grain boundary migration (GBM)/subgrain rotation (SGR) transition and SGR regimes, respectively. Integration of the observations from gneiss/schist samples showed that the SSE-directed shearing possibly commenced at the GBM/SGR transition and continued under SGR. In turn, top-to-the-NNW shearing occurred exclusively within SGR. We suggest that the opposite shear senses were operated asynchronously through foliation reuse under plane strain conditions. Early top-to-the-SSE shearing is associated with increasing simple shear component of deformation towards the contact between the two units, whereas subsequent reversal to top-to-the-NNW shearing is characterized by distributed general shear deformation throughout the study area. Foliation-parallel NNW-directed shearing progressively localized within ductile to brittle normal-sense zones that crosscut the foliation.

Fault zones in porous siliciclastic rocks can be dominated by deformation bands (DBs), which are small-scale tabular structures that usually occur as cluster features. DBs can reduce permeability, contributing to the compartmentalization of oil reservoirs and aquifers. DBs cannot be imaged by seismic methods, but can be imaged by Ground-penetrating radar (GPR). Although DBs and host rocks share the same lithology, GPR imaging is possible because DBs can cause small vertical offsets and reduce the amplitude of the GPR signals. We present an automatic approach for extracting the three-dimensional (3D) architecture of DBs from GPR cubes using multiattribute analysis. We used a 200 MHz GPR cube surveyed on an outcrop of a sandstone formation highly impacted by DBs in the Rio do Peixe Basin, northeastern Brazil. The multiattribute analysis is based on edge evidence and sequential ant-tracking, a combination that can identify narrow zones of attenuated GPR signals. Furthermore, the 3D architecture of DBs was extracted as a geobody using an opacity balancing operator. The geological reliability and limitations of the geobody were demonstrated by comparing slices of the geobody with images of exposed DBs in similar positions, in addition to structural measurements obtained in field and in the geobody.

The Chaochou Fault in southern Taiwan has long been recognized as an active fault, but its exact location is still uncertain. In this study, we focused on the middle part of the Chaochou Fault, where a flight of fluvial terraces was deformed and preserved. High-resolution aerial photos were first used to observe and map these terraces and their vicinity, followed by comprehensive field investigations including RTK-GPS surveys for high-resolution topographic profiles of the deformed terraces and OSL sample collections for obtaining the ages of the terraces. A series of active faults, consisting of Fault A (“FA”), Fault B (“FB”), and Fault Bb (“FBb”), are collectively named the Chaochou Active Fault Zone. The fault zone is recognized and characterized by range-facing scarps, frontal flexural scarps, and discontinuous slopes deformed by FA, FB, and FBb, respectively. The repeated activities of FA were recorded by the incremental fault scarp heights from young to old terraces. Considering the highest fault scarp height of 65.9 ± 6 m observed in T1 and its OSL age of 35.3 ± 4.3 ka, the long-term fault slip rate of FA is about 2 mm/yr.

Models are key for geoscientists working in subsurface fold thrust belts, who want to interpret complex geometries. However, models based on a few landmark outcrop studies dominate interpretation. In these models thrust faults form first as flats along weaker beds and propagate upwards, producing a “hard linked”, fully connected thrust fault structure. The Eisenstadt and De Paor (1987) model challenges the conventional thrust flat-first, reflecting field observations which show that fold thrust outcrops vary remarkably from each other, with a variety of geometric, linkage, and stratigraphic behaviours.

Here we investigate an outcrop of thrusted sediments at St Brides Haven, Pembrokeshire. Structural observations of the outcrop show an imbricated stack, where isolated thrusts have developed within and localised along sandstone layers. The outcrop provides an example of the alternative Eisenstadt and De Paor model of ramps first. But here deformation in the encasing ‘soft’ mudstone layers is accommodate by homogeneous shortening.

We suggest that the prevalence of “hard linked” thrust models is a bias towards conventional models and that promotion of a greater variety of fold thrust structures, geometries and evolution styles is needed to ensure a broader range of interpretations and evolutionary understanding that better reflects reality.

It is easy to draw stress trajectories to investigate the present stress field by interpolating stress orientations determined at control points. However, challenges arise when we deal with the trajectories of paleostresses, because neighboring control points may have the stress orientations of different tectonic phases. We must choose coeval stresses to draw the trajectories. Recent stress inversion techniques can separate stresses from heterogeneous data from fault, dilational fractures, etc. Natural data sets from those structures are often heterogeneous, and age data are usually not enough to classify the stresses by age. As a result, an unsupervised classification problem of the inversion results must be solved to draw the trajectories. Here, we propose a simple and heuristic procedure for this problem. We assume smooth trajectories during each of the phases. The smoothness makes density-based clustering adoptable to solve the problem. The heterogeneity of data sets allows the additional partition of the clusters. As a worked exercise for this technique, the trajectories of minimum horizontal stress orientations were drawn based on the paleostresses determined from the attitudes of felsic dikes and quartz veins formed in mid Cretaceous orogeny in the North Kitakami Terrain, northern Japan. The orogen-parallel and orogen-perpendicular extensional stress fields delineated by the present technique were probably the manifestations, respectively, of the gravitational collapse of the orogen and of regional extensional tectonics in the Far East.

This study provides a comprehensive account of the modes of crustal shortening in the Lesser Himalayan Sequence (LHS) of the Darjeeling-Sikkim Himalaya (DSH). The distributed ductile deformation episodes are integrated with the localized thrusting events in the LHS. The LHS records four major episodes of buckle folding in distributed ductile deformations. From field-based structural correlations, it is demonstrated that multiple orders of third-generation orogen-parallel (F3) and late-stage orogen-perpendicular (F4) folds have resulted in complex interference patterns, varying from plane non-cylindrical (Type 1) to non-plane non-cylindrical (Type 2). Continued N–S shortening in the DSH produced a crustal-scale thrust with ramp-flat geometry, locally known as the Daling Thrust (DT) under the influence of a mechanically weak coal-shale-bearing Gondwana layer, which can be compared with the present-day Main Himalayan Thrust (MHT). The thrust ramp eventually shifted towards the foreland during the India-Asia collision. A fold-duplex model is proposed to explain the potential mechanism of forelandward basal ramp migration. Our model suggests that the southern shallower Main Himalayan Thrust (MHT) flat is susceptible to creeping aseismically due to the influence of coal-shale rheology. In contrast, the mid-crustal ramp and the presently active frontal splay faults (e.g., Main Frontal Thrust, MFT) are potentially seismogenic. These findings have important implications for the interpretation of the MHT seismic cycles.

Pervasive deformation adjacent to salt diapirs can drive significant porosity and permeability reduction in potential reservoir units, yet predicting the intensity and spatial distribution of these subseismic-scale features has remained as a persistent challenge. Previously, Jurassic sandstones adjacent to salt-cored anticlines in the Paradox Basin, eastern Utah, have been used to characterize deformation banding formation. However, these host rocks have undergone significant structural and diagenetic modification following band formation, which adds significant complexity to analyses of this type. Here, we integrate detailed structural transect analysis, petrologic analysis of bands and host rocks, depth-compaction relationships, critical state deformation models, and regional burial evolution models to constrain the timing and conditions of band formation. This analysis demonstrates that bands in this region formed prior to pervasive cementation and are generally expressed as two plastic strain gradients; one characterized by distributed deformation bands across the crest of salt-cored anticlines and a second wherein deformation band density increases near mesoscale normal faults. Comparison of these results with numerical structural evolution models of similar structures indicates that band gradients may have formed as a result of outer-arc extensional stress induced by the rising diapir and are linked with normal faults. These results highlight that the integration of material behaviors with kinematic evolution can constrain models for deformation band formation and may provide a useful workflow for understanding the development of these features in geologically young petroleum systems like those of the Atlantic passive margin.

The crustal shortening of the Andean forearc in northern Chile was accommodated by a combination of both thin- and thick-sinned structural styles. However, fold-related inversion of normal faults appears to be the most important structures in the area. To understand the upper crustal shortening mechanisms that acted during the tectonic uplift of this region, an original structural investigation was carried out, integrated with outcrop and regional-scale observations, balanced cross-sections, and pre-shortening restorations of structures exposed along the Paipote fold-and-thrust belt. On this basis, we presented the first balanced cross-section of this region extending for nearly 27 km is presented. The structural styles consisted of east-directed asymmetrical folds involving Paleozoic to Cenozoic strata. The folds were kinematically related to inverted normal faults and thrust ramps that penetrated downward into the basement. The inverted structures resulted from the reverse reactivation of preexisting Upper Paleozoic to Jurassic west-dipping, basement-rooted normal faults that accommodated the tectonic extension that preceeding the Andean orogenesis. The reverse-reactivation of these extensional structures controlled the development of east-verging anticlines, along which the Mesozoic syn-rift strata were elevated above their regional elevation. Other folds exhibit the typical geometry of fold-related thrust ramps (fault–bend folds and fault–propagation folds). These are proposed to result from the development of low-angle thrusts propagating across precursor normal faults with shortcut trajectories, that detach along Jurassic shales, thus forming complex thin-skinned structures in shallow structural levels. The latter is responsible for accommodating a major crustal shortening (nearly 5 km). The east-directed tectonic transport direction was influenced by the original attitude of precursor extensional faults.

Thrust folds have developed significantly in the meso-Cenozoic rock units of the Errachidia-Boudnib basin. The study areas correspond to the north part of the Pre-African Trough, which is located between the Anti-Atlas and the Atlas systems. It is characterized by faulted anticlines that are indicative of a foreland orogenic context.

The Atlas system's structural evolution underwent several stages, starting with a Triassic extensional event followed by a period of fault reactivation during the Atlas compression. This research focuses on examining the thrust and detachment folds associated with both reverse and strike-slip components. For this purpose, we performed detailed geological mapping and interpretation of folding and fault slip data.

As a result of shortening, field observations reveal that the study area exhibited well-developed thrusting geometry. Preexisting blind faults and multiple decollement levels within favorable formations such as Cenomanian evaporitic marls, Cenomanian-Turonian marls rich in organic matter, and Senonian argillites influence the folding patterns, which are not uniform. The findings demonstrate two key points. First, the most significant folds have formed along major blind thrusts. Subsequently, between these major faults, detachment folds were developed within the Jurassic-Cretaceous strata. Currently, due to erosion, certain thrusts have become visible, including the Ta'bbast thrust fault, Ait Atman (from the first stage), and the Timazguit fault zone (from the second stage). The South Atlas fault largely remains a blind fault. Additionally, the study emphasizes the presence of strike-slip components and en-echelon folding, indicating a transpressional regime during the uplift of the Atlas system.

Gravity-driven sliding of sediments down subaqueous slopes results in mass transport deposits (MTDs) recognised both in outcrop studies and from offshore margins where they may extend for 100's km. While seismic sections may reveal the large-scale geometry of such features, they fail to capture some of the structural and stratigraphic detail necessary for a fuller understanding of the processes involved. Using the late Pleistocene Lisan Formation sediments exposed around the Dead Sea Basin as our case study, we show that interplay between bed-parallel translational slides and associated normal faults may result in stratigraphic repetition through a process we term ‘slide stacking’. This mechanism, where retrogressive slope failure results in slides cutting across earlier normal faults, produces repeated sequences with older over younger stratigraphic relationships more usually attributed to compressional (thrust) deformation. Slide stacking results in a ∼25% attenuation of the upper sequence above the basal shear surface (BSS), which is itself associated with liquefaction and fluidised sediment. The displaced stratigraphy above the BSS is also marked by sedimentary rafts that are broken into blocks by normal faults and become increasingly separated from one another during downslope translation. The hangingwalls of synthetic listric faults form roll-overs that are progressively tightened towards the underlying BSS to create overturned anticlines that apparently verge upslope. The paradoxical situation therefore arises of contractional geometries, such as older over younger stratigraphic repetition across slides, and upslope-verging recumbent anticlines with locally overturned limbs being created during downslope-directed gravity-driven extension. The downslope margin of the slide stack displays earlier normal faults that created scarps where much of the sedimentary buttress, that would otherwise support the toe of the slide, was removed. Consequently, this leads to predominantly superficial and unrestrained downslope slipping, resulting in very localised contractional geometries that do not balance the overall extension, as in classical gravity-failure models. Localised deformation of the sedimentary sequence that unconformably overlies the slide stack indicates that downslope translation continued after the initial rapid slope failure, suggesting that the entire MTD remained inherently unstable. Slide stacking operates at km scales with stratigraphic repetition governed by the throw of earlier normal faults and the amount of downslope translation.