Decollement Layer Research Articles

Unraveling deformation mechanism of the Fukang fold-and-thrust belt: Insight into intracontinental orogenesis of the Bogda Mountain, NW China

Characterized by the superposition of well-developed thick-skinned and thin-skinned structures, the Fukang fold-and-thrust belt is an ideal place for understanding the intracontinental orogenic process of the Bogda Mountain in the Tianshan orogenic belt, China. This study comprehensively examines the geological architecture, structural evolution, and deformation mechanism of the Fukang fold-and-thrust belt using high-resolution seismic reflection data, apatite-fission track analysis, and discrete element simulation. Seismic profile reveals that the Fukang fold-and-thrust belt exhibits superimposed tectonic styles, including a thrust duplex involving Carboniferous strata and an overlapping thrust imbricate structure. Stratigraphic unconformities and thermochronological results from apatite fission track suggest that this belt experienced five main episodes of compressional deformation in the Late Permian, Middle-Late Triassic, Middle-Late Jurassic, Late Cretaceous-Paleocene, and Late Miocene-present. The discrete element simulation results indicate that the Late Permian initial compressional deformation was triggered by reactivation of the pre-existing extensional faults. Subsequent episodic deformation was primarily controlled by thrusting along the Upper Carboniferous and Middle Permian decollement layers. This study highlights the predominant roles of the pre-existing faults and the decollement layers in the formation and evolution of the piedmont fold-and-thrust belt of the Bogda Mountain at distinct stages, serving as critical factors in determining the extent and amplitude of an intracontinental orogenic belt.

Numerical Constraints on Folding and Thrusting in Jiudong Basin: Implication for the Northeastward Growth of Qinghai–Tibetan Plateau

Under regional tectonic shortening in the northern margin of Qinghai–Tibetan Plateau, the fold-and-thrust belts composed of four thrust faults (North Qilian-Shan, North Yumu-Shan, South Heli-Shan, and North Jintanan-Shan) formed from southwest to northeast discontinuously sequentially in the Jiudong Basin area during Late Cenozoic. Meanwhile, the North Qilian-Shan, Yumu-Shan, and Heli-Shan ranges were formed successively, as the Earth's local surface was unequally uplifted. In this study, based on geological and geophysical observations, a simple two-dimensional elastic-plastic numerical finite element method model for a southwest-northeast section in Jiudong Basin is successfully established to simulate the spatiotemporal evolution of the local fold-and-thrust belts. Results show that the computed equivalent plastic strain concentration zones and the four observed thrust faults are consistently correlated in spatial position orientation and time sequence. The simulated upper-surface deformation is congruent with the observed topographic peaks and uplift sequences of the North Qilian-Shan, Yumu-Shan, and Heli-Shan ranges. This study provides a geodynamic basis for understanding the growth mechanism of the northern margin of Qinghai–Tibetan Plateau under tectonic horizontal shortening. Also, we provide a thorough sensitivity analysis for the model parameters of this particular geologic setting. Our sensitivity simulations, considering systematic case variations about the regional geometrical-material parameters, suggest the manifestation of three different possible evolution patterns of fold-and-thrust belts for a wedge above a decollement layer, with wedge plastic deformation migrating from 1) thick to thin end (well-known), 2) thin to thick end, and 3) both ends to middle. Finally, our results suggest that in this region, further growth of mountain ranges is expected to continue in the future.

Characterizing the characteristics of natural fractures in shale based on the modified petrophysical model

In order to provide technical support for the shale gas exploration and development in the Lower Silurian Longmaxi Formation of the Sichuan Basin, this paper takes the Longmaxi Formation in the Changning and Fuling Shale Gas Fields as the research object to quantitatively characterize the development characteristics of natural fractures in the Longmaxi Formation shale by means of helium pycnometry, X-ray diffraction (XRD), true density testing and other methods, with the aid of the modified petrophysical model. Then, the development types and genetic mechanisms of natural fractures and their influences on shale gas development are discussed. The following research results are obtained. First, the modified petrophysical model can accurately describe the pore system in the Changning shale with a fitting rate of 0.74. Second, the development of natural fractures in shale is different in various regions. The natural fractures in the Changning Shale Gas Field, controlled by basement thrust faults, decollement layers and internal folds, are locally developed and filled with calcareous, and the average fracture porosity is 0.15%. In the Fuling Shale Gas Field, however, natural fractures, mainly controlled by reverse faults and slippage effect, are commonly more developed and unfilled or semi-filled with siliceous, and the average fracture porosity is 1.30%. Third, under the formation conditions, the opening of natural fractures is different. The natural fractures in the Changning Shale Gas Field are basically closed with weaker flowing ability, while those in the Fuling Shale Gas Field are relatively open with stronger flowing ability. Fourth, the occurrence mode of shale gas is influenced by natural fractures, and it is internally dominated by free gas. The initial gas production of shale gas wells is higher. In conclusion, (1) the regression coefficient is introduced to calculate the actual total organic matter content, which promotes the modified petrophysical model to describe matrix pores and fracture pores more accurately; (2) the development of natural fractures in the shale producing pay of the Sichuan Basin is relatively beneficial to shale gas enrichment and exploitation, but the flowing ability of the natural fractures will be weakened under the original formation conditions.

Stratigraphy, Geometry, and pattern of Imbricated zones, NW Zagros Fold and Thrust Belt in Iraqi Kurdistan Region

The High Folded and Imbricated zones are the formalized name represent the part of the Tectonic sub-division of Iraq and Kurdistan Region of Iraq (KRI). The study of compressional deformation and Stratigraphy are the keys to identify thrust faults structures and define the sequence of thrusting in High Folded and Imbrication zones as a part of Zagros Fold and Thrust Belts. Through this study, we try to describe the compressional thrust faults and stratigraphical unit description. Thrust faults make repetition of the sequences of Jurassic-Triassic Cretaceous sequence over thrust above Cretaceous sequences. In Iran-Iraq border close to Sirwan valley the large thrust faults (Sirwan-Sazan Thrust faults) identify. The fault system makes Triassic formations over-thrust the Valanginian-Albian Balambo Formations. In Balakayati area serious thrust fault exists, which creates twice repetition of Jurassic-Cretaceous formations forming imbrication thrusting system and fault propagation fold. In Bradost-Korek anticlines the Jurassic-Cretaceous formations over-thrust Cenozoic formations. In Kamosk anticline the thrust faults are in the Cretaceous formations. The dimension and the style of the thrust faults are different but generally, most of the decollement layers of the thrusting are the Early Triassic and Middle-Late Jurassic formations. The collection of all these results constrain geological cross-section in Imbricate Zone and restore to obtain the 7.5 km distance of Shortening due to folding and faulting.

Focal Mechanism and Seismogenic Structure of the MS 5.1 Qingbaijiang Earthquake on February 3, 2020, Southwestern China

The 3 February 2020 MS 5.1 Qingbaijiang earthquake, southwestern China, is the closest recorded MS ≥ 5.0 event to downtown Chengdu City to date, with an epicentral distance of only 38 km. Here we analyze seismic data from the Sichuan and Chengdu regional seismic networks, and employ a multi-stage location method to relocate the earthquakes that have occurred along the central and northern segments of the Longquanshan fault zone since 2009, including the MS 5.1 Qingbaijiang earthquake sequence, to investigate the seismogenic structure of the region. The relocation results indicate that the seismicity along the central and northern segments of the Longquanshan fault zone has occurred mainly along the eastern branch since 2009, with the hypocentral distribution along a vertical cross-section illustrating a steep, NW-dipping parallel imbricate structure. The terminating depth of the eastern branch is about 12 km. The distribution of the MS 5.1 Qingbaijiang earthquake sequence is along the NE–SW-striking Longquanshan fault zone. The aftershock focal depths are in the 3–6 km range, with the mainshock located at 104.475°E, 30.73°N. Its initial rupture depth of 5.2 km indicates that the earthquake occurred above the shallow decollement layer of the upper crust in this region. The hypocentral distribution along the long axis of the aftershock area highlights that this earthquake sequence occurred along a fault dipping at 56° to the NW. Our surface projection of the inferred fault plane places it near the eastern branch of the Longquanshan fault zone. We infer the MS 5.1 mainshock to be a thrust faulting event based on the focal mechanism solution via the cut-and-paste waveform inversion method, with strike/dip/rake parameters of 22°/36°/91° and 200°/54°/89° obtained for nodal planes I and II, respectively. We identify that the seismogenic fault of the MS 5.1 Qingbaijiang earthquake lies along the eastern branch of the Longquanshan fault zone, and nodal plane II represents the coseismic rupture plane, based on a joint analysis of the event relocation results, mainshock focal mechanism, and regional geological information. Our study provides vital information for assessing the seismic hazard of the Longquanshan fault zone near Chengdu City.

Subsurface Structural Interpretation of Missa Keswal Area, Eastern Potwar, Pakistan

Potwar Basin is although a hydrocarbon prolific basin but shows mixed scenarios regarding the success ratio of the wells. Several wells are producing good but a significant number of wells ended up with a great loss. Missa Keswal area is also a part of the Potwar Basin which was discovered in 1991. The main objective of this research is to find the subsurface structure of the Missa Keswal area with the help of seven seismic lines, 3-D modeling, and the correlation of five wells. Kingdom suite 8.8 is the main software used to delineate the subsurface structure along with some other software. Results indicate that the tectonic framework of the study area is mainly controlled by the Jhelum strike-slip fault and decollement layer i.e., Pre-Cambrian salt. Structural analysis shows that the study area bears NE-SW trending salt cored pop-up anticlinal structure bounded by major thrust fault and back thrust. Patala Formation acts as a source, Lockhart Limestone, Sakesar Limestone, and Chorgali Formation acts as a reservoir while fault surface (often acts a good conduit) and Neogene clays providing a potential sealing mechanism for entrapment.

The role of mechanical stratigraphy in the lateral variations of thrust development along the central Alberta Foothills, Canada

Fold-thrust belts generally exhibit significant variations in structural styles such as differences in thrust geometries and frequencies in imbrication. A natural laboratory of this pattern is preserved in the central Alberta Foothills of the Canadian Rockies, where differences in thrust geometries are represented by the existence vs. non-existence of triangle zones. To seek the factors that make this difference in these regions in terms of structural geometry, stratigraphic thickness variations and mechanical stratigraphy of the sedimentary layers, structural interpretation is conducted based on admissible cross-sections and well log interpretations. In northern region, a backthrust is detached from an incompetent layer (viz. Nomad Unit of the Wapiabi Formation), which gets thinner from the Foothills to the Plains, indicating that it is developed where the shale layers are pinched out where triangle zone is developed. Backthrust is also developed in the southern region, where mechanical strengths of strata (viz. Bearpaw Formation) increase toward the foreland. In the central region, however, only forethrusts are developed along the weak continuous decollement layers (viz. Turner Valley and Brazeau formations), forming an imbricate fan without development of the triangle zone. Incompetent layers such as the top Wapiabi (Nomad), Brazeau (Bearpaw), Coalspur and Paskapoo formations are also pinched out laterally, forming fault glide horizons in different stratigraphic levels in each region. These results indicate that, along the transport direction, triangle zone is developed in relation to the stratigraphic pinch out of the Nomad Unit in the northern region, and is formed associated with the variations in strengths of the layers constituting the Bearpaw Formation in the southern region. It is notable that all the glide horizons are developed along incompetent layers. However, triangle zones are not developed in the areas of continuous stratigraphy of the Nomad Unit, which does not serve as a glide horizon in the central region. This suggests that factors such as stratigraphic thickness changes of incompetent layers and mechanical stratigraphy of the sedimentary layers play an important role in the development of lateral variations in thrust system evolution in terms of triangle zone vs. imbricate fan in the central Alberta Foothills.

Analogue modeling of the role of multi-level decollement layers on the geometry of orogenic wedge: an application to the Zagros Fold–Thrust Belt, SW Iran

The presence of evaporate and incompetent formations (i.e., decollement horizons) within the sedimentary sequence of fold-thrust belts can control their structural style and deformation evolution. In the present study, the influence of the decollement layers (e.g., basal and internal decollement layers) on the deformation style of several segments of the Zagros Fold-Thrust Belt (ZFTB), SW Iran (e.g., Fars Arc, Dezful Embayment, and Izeh Zone) was investigated using a series of analogue models of accretionary wedges. The study of seismic profiles to understand the structural evolution of these segments of the belt, where several decollement intervals acted as basal and internal decollements, is complemented by the analogue model results. The experimental results reveal that the thickness of the internal decollement layers influences the creation of fold-dominated or thrust-dominated deformations, respectively. Experimental models and seismic data highlight that incompetent layers act as barrier units against fault propagation (in-sequence and/or out-of-sequence faults) into overlying strata towards southwest by fore-deformation and control the rate of deformation propagation in the ZFTB. The presence of both the basal and internal decollement layers located at different stratigraphic levels is required to form disharmonic decollement folds in the foreland of the ZFTB. In addition, the geometry, spacing, activity, and propagation of faults as well as the topographic height of the critical wedges are directly controlled by low-frictional decollements (Geophys J Int, 165(1):336–356 2006; Geochem Geophys Geosyst, 14:1131–1155 2013). The seismic profiles of the ZFTB showed that in addition to lithological contrasts, the existence and activity of deep-seated and basement faults had a big impact on the structural styles of the region.

Gravity constraints on the underground structural framework and associated volcanism of the Maghrebian allochthonous domain: The Sejnene Numidian flysch, Tunisian Tell

Tectonic framework of the Neogene Numidian flysch is one of the primary subjects in the Tell of northern Tunisia for understanding the late Miocene kinematics around the Mediterranean Basin. Belonging to the extreme North of Tunisia, this domain is marked by a confuse structuring. Much of the Sejnene area is represented by thick Neogene Numidian flysch, which is widely exhibited around the Mediterranean and directly deposited on Mesozoic and Cenozoic strata.Gravity measurements covering the Sejnene area coupled with geological outcropping data were used to constrain the deep structural setting of the Neogene Numidian flysch and its associated volcanic outcrops. Integration of different gravity processed maps, leads us to highlight relationship between outcropping and sealed deformed structures and proposed a tectonic framework model for raised volcanic rock within the thrust sheet system. Maps resulted from gravity processing reveal structural variations marked by several geometries and oriented lineaments. This structuring suggests that the Sejnene study area is marked by two tectonic framework domains: to the North, the areas is mainly characterized by NE- and E- oriented subparallel lineaments corresponding to thrusts, reverse faults and strike slip reverse fault system. Conversely to the South, the same master trends are crossed by others NW- and N- trending transversal and deep-seated lineaments corresponding to strike slip reverse faults.Neogene NW- then N- striking compressional phases have induced inversion of inherited structures that are fossilized by rejuvenation of ancient faults affecting the basement. The final obtained structural setting suggests South-verging evolution of the Numidian domain under low-angle decollement layer to the North and under deep-seated thrusting and strike slip movements to the South. Master strike slip movements induced rising of lithospheric volcanic rocks.Geophysical and geological study of the Sejnene area provides a new insight into tectonic framework of the Numidian flysch and the associated Mogods volcanic rocks. Structuring of this Tellian domain well correlated with orogenic episodes of the western Mediterranean and seems to be related to the Alpine orogeny.

Influence of surface processes and initial topography on lateral fold growth and fold linkage mode

Elongation of randomly distributed fold segments and their potential linkage are important for hydrocarbon exploration because it can greatly influence the morphology of the reservoir and both migration and accumulation of hydrocarbons in antiformal traps. Here we study the effects of surface processes and the presence of a topographic slope on the different linkage modes that can occur, and how these parameters affect the required horizontal offset for perturbations to link. The proposed numerical model represents a sedimentary cover detached over a much weaker basal decollement layer. The upper surface is modified by mass redistribution, which is achieved by a combination of fluvial and hillslope processes. Several series of simulations were performed: (1) without surface processes or regional slope, (2) with regional slope only, (3) with fluvial incision and hillslope processes, and (4) with hillslope processes only. Model results show that the presence of a regional slope reduces the critical distance required for the transition between linkage and no linkage modes, whereas erosion and redeposition of sediments, on the contrary, increase this distance. The location of the saddle point, where fold segments link, and its vertical distance to the crests of the anticlines are different compared to the case without erosion or initial topographic slope, which potentially can affect the morphology of hydrocarbon traps. Moreover, both erosion and redeposition of sediments enhance the fold elongation (growth along the fold axis), once the erosion velocity exceeds the folding velocity. Model results have been compared to the Zagros Fold Belt.

Dynamic constraints on the crustal-scale rheology of the Zagros fold belt, Iran

Thin-skinned fold-and-thrust belts are generally considered as the result of contractional deformation of a sedimentary succession over a weak decollement layer. The resulting surface expression frequently consists of anticlines and synclines spaced in a fairly regular manner. It is thus tempting to use this spacing along with other geological constraints to obtain insights into the dynamics and rheology of the crust on geological time scales. Here we use the Zagros Mountains of Iran as a case study, as it is one of the most spectacular, well-studied thin-skinned fold-and- thrust belts in the world. Both analytical and numerical models are employed to study what con- trols fold spacing and under what conditions folding dominates over thrusting. The models show that if only a single basal decollement layer is present underneath a brittle sedimentary cover, deformation is dominated by thrusting, which is inconsistent with the data of the Zagros fold belt. If we instead take into account additional decollement layers that have been documented in the fi eld, a switch in deformation mode occurs and crustal-scale folding is obtained with the correct spacing and time scales. We show that fold spacing can be used to constrain the friction angle of the crust, which is ~5° the Zagros fold belt. This implies that on geological time scales, the upper crust is signifi cantly weaker than previously thought, possibly due to the effect of fl

Structural styles of Longmen Mountain thrust belt, SW China

Through field geological investigation and seismic interpretation of the Longmen (龙门) Mountain thrust belt, we summarized the following structural styles: thrust belt, fault-related fold (fault bend fold, fault propagation fold, and fault decollement fold), pop-up, triangle zone, duplex, superimposed fold, ductile deformation structures, reverse thrust fault, klippe, decollement structure, etc.. These structural styles have evident distribution characteristics; they had zonation and segmentation in plane. The zonation presents as the thrust nappe tectonic zone to the west of Tongjichang (通济场) fault, fault-related folds between Tongjichang fault and Guankou (关口) fault, and low and mild folds to the east of Guankou fault. The segmentation is evidenced as the scale of reverse thrust faults was minor between Tongjichang No. 1 fault and Tongjichang No. 2 fault. The distance between these two faults became long in the Dayuanbao (大园包) structure, and there developed typical fault propagation fold and pop-up between these two faults. Furthermore, the structures had stratification in profile. The salt layer of T2l provided good conditions for the formation and development of large listric thrust faults; the thrust fault slipped in the salt layer and formed decollement structures and fault-related fold. At the same time, there formed duplex and reverse thrust faults between the two decollement layers.

Differential Deformation and Hydrocarbon Accumulation in the Superimposed Basins

This article describes the characteristics of the differential deformation in the superimposed basins and its control on hydrocarbon accumulation. The differential deformation of superimposed basin is controlled by polyphase stress field transition, varied deformation medium, and various basin boundaries. The main characteristics of differential deformation are as follows: (1) Polyphase differential deformation controlled by the key tectonic movements and stress field transition. The horse-graben structures are formed at the regional extensional regime at the early stage, the detachment structures and buried hill-drape assemblages are generated associated with the convergent margins at the medium stage, and the fold-thrust belts are originated in the overlying foreland basins at the later stage. (2) Zonal differential deformation controlled by major fault belts that are related to pre-existing basement weak zones. The deformation intensity is changed regularly in various structural zones. (3) Multi-level differential deformation controlled by decollement zones. The deep-level detachment structures are associated to basin-range decoupling, the medium-level detachment structures are related to basement decollement layers, and the shallow-level detachment structures might be having a bearing upon plastic deformation associated with the salt bed or argillites. (4) Segment differential deformation controlled by transfer zones. The structural styles and kinematics might have a great difference in different segments because of the large-scale transfer zones. The local transfer zones might form favorable oil-gas traps. The differential deformation might have a great bearing upon the development of the source rocks, the hydrocarbon accumulation epoch, the trap evolution, the distribution and preservation of the oil-gas pools.

Hydromagmatic eruption during the buildup of a Triassic carbonate platform (Oman Exotics): Eruptive style and associated deformations

Oman Exotics represent remnants of a Triassic carbonate platform, also named Misfah Formation. Several basaltic intrusions or lava flows were coeval with the carbonate platform buildup. We present field observations related to one of these volcanic events, including associated deformation of the host rocks and interaction of volcanism with the sedimentation. These observations allow us to reconstruct the structure of the volcanic conduit and vent and the successive stages of the evolution of the eruption. The initial ascent of magma probably occurred along on a normal fault related to gravity-driven sliding towards the platform edge. Magma was first emplaced in a saucer-shaped sill a few tens of meters below the sea-floor. This intrusion provided a decollement layer, that may have sped up the gravity-driven sliding, opening fractures that brought sea water into contact with the magma, hence triggering a hydromagmatic eruption. Eruption was followed by the collapse of the host limestones that formed a megabreccia infilling the eruptive vent and prohibiting further contact between sea water and magma. Most of the magma was emplaced at depth in two superposed small magmatic bodies (300 × 100 m and 200 × 50 m in sections, respectively) that replaced the host sediments and uplifted the overlying vent. Despite the shallow submarine setting, this eruption produced a vent quite similar to sub-aerial maars developed in soft substrate. This emphasizes that the rheology of the substrate may be more important than the amount of water to control magma–water interaction in hydromagmatic eruptions. In an active carbonate platform like Misfah Formation during Triassic times, the early diagenesis of lime mud determined the mechanical comportment of the sediment at the time of eruption, and especially the way it could be mixed with the magma and/or slide into the vent.

Multi-level decollement zones and detachment deformation of Longmenshan thrust belt, Sichuan Basin, southwest China

As is well known that many decollement layers were developed in the Longmenshan thrust belt, Sichuan Basin, China. Through field investigation, explanation of seismic profiles and analysis of the balanced sections, we can divide the decollement zones into 3 categories: (1) the deep level decollement zones, including the crust-mantle decollement layer, intracrustal decollement layer, and presinian basal decollement layer. The main structural styles of their deformation are the crust-mantle decoupling detachment deformation, the basal ductile shear deformation, etc.; (2) the middle level decollement zones, including the Cambrian-Ordovician decollement layer, the Silurian decollement layer, etc., the main structural styles of their deformation are the isopachous fold, the angular fold, the saddle structure, and the combination styles of them; and (3) the shallow level decollement zones, including the Xujiahe Formation decollement layer of Upper Triassic and the Jurassic decollement layers, the main structural styles of their deformation are the thrust-nappe tectonic, the pop-up, the triangle zone, the duplex, etc. Multi-level decollement zones not only made the Longmenshan thrust belt develop many different deformation styles from deep place to shallow place, but also made some local areas have the superimposition of the tectonic deformation apparently. This study indicates that the multi-level decollement zones have a very important effect on the shaping and evolution of the Longmenshan thrust belt.

Radial gravitacional gliding indicated by subsalt relief and salt-related structures: the example of the gulf of lions, western mediterranean

The young Messinian salt basin offshore the Gulf of Lions comprises a shallow decollement layer (maximum of 3.6 km deep) that allows the seismic imaging of the subsalt relief and the correlation between the shape of subsalt relief and gravity-driven structures. The subsalt relief reveals a variable morphology below the salt layer, characterized by both convex and concave shapes, indicating the occurrence of radial gravitational gliding at the scale of the entire Gulf of Lions. Radial gravitational gliding is equally illustrated by the distribution of the Messinian salt layer. The salt isopach map shows overthickened salt mass overlying subsalt relief of concave shape, pointing to a pattern of convergent gliding, and areas of salt thinning (over convex shape of subsalt relief) indicating divergent gliding. Radial gravitational gliding is also reflected by salt-related structures. Families of normal transverse faults striking parallel to the regional dip direction attest the control exerted by components of strike-parallel extension over areas of convex shape of subsalt relief, whereas widespread buckle folds overlying concave shape of subsalt relief indicate the occurrence of a convergent pattern of salt migration and associated translational gliding. The Gulf of Lions provides then an interesting geological setting to focus on gravity tectonics as well as a model to correlate mapped salt-related structures with those predicted by analogue models.

Tectonic shortening and gravitational spreading in the Gulf of Cadiz accretionary wedge: Observations from multi-beam bathymetry and seismic profiling

The Gulf of Cadiz lies astride the complex plate boundary between Africa and Eurasia west of the Betic-Rif mountain belt. We report on the results of recent bathymetric swathmapping and multi-channel seismic surveys carried out here. The seafloor is marked by contrasting morphological provinces, spanning the SW Iberian and NW Moroccan continental margins, abyssal plains and an elongate, arcuate, accretionary wedge. A wide variety of tectonic and gravitational processes appear to have shaped these structures. Active compressional deformation of the wedge is suggested by folding and thrusting of the frontal sedimentary layers as well as basal duplexing in deeper internal units. There is evidence for simultaneous gravitational spreading occurring upslope. The very shallow mean surface and basal slopes of the accretionary wedge (1° each) indicate a very weak decollement layer, geometrically similar to the Mediterranean Ridge accretionary complex. Locally steep slopes (up to 10°) indicate strongly focused, active deformation and potential gravitational instabilities. The unusual surface morphology of the upper accretionary wedge includes “raft-tectonics” type fissures and abundant sub-circular depressions. Dissolution and/or diapiric processes are proposed to be involved in the formation of these depressions.

Main Structural Styles and Deformation Mechanisms in the Northern Sichuan Basin, Southern China

Abstract: The Triassic Jialingjiang Formation and Leikoupo Formation are characterized by thick salt layers. Three tectono‐stratigraphic sequences can be identified according to detachment layers of Lower‐Middle Triassic salt beds in the northern Sichuan Basin, i.e. the sub‐salt sequence composed of Sinian to the Lower Triassic Feixianguan Formation, the salt sequence of the Lower Triassic Jialingjiang Formation and Mid‐Triassic Leikoupou Formation, and the supra‐salt sequence composed of continental clastics of the Upper‐Triassic Xujiahe Formation, Jurassic and Cretaceous. A series of specific structural styles, such as intensively deformed belt of basement‐involved imbricated thrust belt, basement‐involved and salt‐detached superimposed deformed belt, buried salt‐related detached belt, duplex, piling triangle zone and pop‐up, developed in the northern Sichuan Basin. The relatively thin salt beds, associated with the structural deformation of the northern Sichuan Basin, might act as a large decollement layer. The deformation mechanisms in the northern Sichuan Basin included regional compression and shortening, plastic flow and detachment, tectonic upwelling and erosion, gravitational sliding and spreading. The source rocks in the northern Sichuan Basin are strata underlying the salt layer, such as the Cambrian, Silurian and Permian. The structural deformation related to the Triassic salt controlled the styles of traps for hydrocarbon. The formation and development of hydrocarbon traps in the northern Sichuan Basin might have a bearing upon the Lower‐Middle Triassic salt sequences which were favorable to the hydrocarbon accumulation and preservation. The salt layers in the Lower‐Middle Triassic formed the main cap rocks and are favorable for the accumulation and preservation of hydrocarbon.

Structural analysis and analogue modeling of the kinematics and dynamics of rockslide avalanches

We present a structural analysis of subaerial natural and analogue rockslide avalanches. Such deposits often have well-developed faults, folds, and hummocks. These structures can be used to determine the kinematics and dynamics of emplacement. Large-scaleterrestrial rockslide avalanches show large runout distances compared to their fall height. Most attempts to explain this phenomenon invoke uidizing mechanisms or lubricating agents to reduce forces opposed to momentum, especially at the base. However, the properties and mechanics of low friction are still poorly understood. Any model for motion and emplacement must integrate geometric, morphologic, and structural features, all crucial in constraining kinematics and dynamics. Here we first examine the morphological and structural features displayed by 13 natural rockslide avalanche deposits; we then use simple and well-constrained analogue models involving the slide of stratified granular material down smooth, curved ramps. These differ from previous analogue models in that we concentrate on observing the structures produced by brittle deformation and use a low-friction sliding surface. Models show that variations in the sliding surface curvature, lateral profile, roughness, and modifications in material cohesion can successfully reproduce the majority of rockslide-avalanche deposit features. After discussing the geometrical and dynamic similarity between experiments and natural examples, we propose a model for structure formation and a fourfold classification based on model and natural deposit morphology and dynamics: hummocky, nonhummocky, dominantly extensional, and dominantly compressional rockslide avalanches. The models require a brittle core and surface that spreads and contracts by adjustment on large numbers of faults that bottom into a low-friction decollement layer. Spreading is accommodated by normal and strike-slip faults, while on deceleration, thrust faulting generates thickening. To be realistic, any physical predictive model must take into account these fundamental kinematic and structural aspects.

Geometric and seismic interpretation of the Perdido fold belt: Northwestern deep-water Gulf of Mexico

We interpret the Perdido fold belt to be composed of detachment folds flanked by megascopic-scale kink bands. The two previous principal fold models for the Perdido fold belt incorporated components of large-scale faulting: an imbricate fault-bend fold model and a high-angle, reverse-faulted detachment fold model. We interpret the Perdido fold belt to be fold dominated. The belt consists of detachment folds overlying a ductile decollement layer. The detachment fold crests are bounded by kink bands, which are narrow zones of angularly folded strata. The low-reflectivity, low–signal-to-noise zones seen in seismic profiles across the Perdido fold belt are poorly imaged sections of the folds that result from steeply dipping bedding in the kink bands. The substantial width of these low-reflectivity zones, their subparallel edges in cross section, and their orientation at a high angle to layering, as well as their organized spacing and conjugate geometry, support a kink-band interpretation. Propagating reverse faults are unlikely in the folds because of the absence of backlimbs overlying the locations of hypothetical faults. Improved seismic imaging efforts have illuminated steeply dipping bedding that is not cut by faulting in many of the low-reflectivity zones in the Perdido fold belt.