Journal of Structural Geology最新文献

This research integrated sequential section restoration, detailed seismic attribute analysis, and quantitative fault analysis for key inversion structures along the western Huincul High, a prominent E-W basement discontinuity in the Mesozoic Neuquén Basin. Section restoration of this retroarc foreland basin, indicated that the area inherited a non-uniform basement with a c.a. 30 km wide trough likely related to the Palaeozoic Gondwanan orogeny. This, combined with important Early – Middle Jurassic subsidence (rates of 40mmy⁻¹), provided accommodation for the deposition of the thick Lower – Middle Jurassic Los Molles formation. Restoration also showed the occurrence of two main phases of inversion characterised by distinct styles of accommodation of shortening. Middle to Late Jurassic inversion had a higher degree of horizontal shortening of around 1.18mmy⁻¹, which was accommodated predominantly by newly created shallow thrust faults exhibiting limited vertical displacement. Meanwhile, Early Cretaceous inversion promoted folding and reactivation of normal faults with large, inverted structures attesting up to c.a.1500m of vertical displacement and non-uniform lateral propagation. Seismic attribute analysis highlighted that inversion promoted internal deformation in the hangingwall of the main inverted structures, in the form of a dense network of secondary fractures up to 1 km in length, perpendicular to the strike of the reactivated structures.

With the expanding application of virtual outcrop models (VOMs) within the geosciences, there is a growing need to establish standards for the digital measurement of geological discontinuities exposed therein. Such standards should be tailored towards the complexities of natural outcrops, where geological discontinuities often intersect the outcrop topography and are expressed as 3D traces. Digitizing geological discontinuities expressed as trace data is typically conducted manually via polyline interpolation along the exposed trace, with discontinuity orientation estimated through planar model fitting through the polyline's component nodes. Presently, establishing quality control for such measurements lacks standardization due to the absence of robust benchmarks, with the validity of the resultant orientation data heavily reliant on the experience of the interpreter.

With the aim of bridging this gap, we present the results of the manual digitization and orientation estimation of bedding planes expressed as traces across seven natural outcrops. We use two digitization strategies: one employing a previewed best-fit plane during digitization and another without. The first digitization method is carried out by an expert user who visually filters data according to visual alignment with the intended bedding prior to best fit plane estimation. In contrast, the non-visually aided method mimics acquisition by a novice user, with no a priori data filtering based upon trace geometry with respect to the outcrop. Comparison of the results obtained by these ‘expert’ and ‘novice’ acquisition modes is aimed at building benchmarks and best practices. Specifically, we analyze parameters derived from the digitized traces and their corresponding best-fit planes. We compare these parameters with the deviation of the best-fit plane from the mean orientation of the bedding surface as measured using visually-aided acquisition. Comparing these datasets reveals that visually-aided digitization yields more precise and accurate bedding measurements, characterized by traces with lower vertex collinearity. Notably, comparable results can be achieved in the non-visually assisted dataset by excluding traces with high node collinearity. Consequently, we provide robust benchmarks for trace collinearity and its relationship to best fit plane quality to aid the practical implementation of the results of this study. Furthermore, we supplement quantitative comparative analysis with recommended best practices for 3D trace digitization, such as ensuring high values of coplanarity, maintaining a quasi-constant node-to-node distance relative to the model's resolution, and ensuring a minimum number of nodes to guarantee the robustness of the fitted planar model. Critically, our study highlights the critical role tacit geological knowledge plays in the robustness of 3D trace digitization from virtual outcrop models.

Based on ¹⁰Be dating of offset terraces at two sites, fieldwork, and high-resolution topographic analysis, we obtained the Holocene slip rates along the central section of the Altyn Tagh fault (ATF). At the Alesayi site, the minimum slip rate is confined to 6.4–11.6 mm/yr since 6.0 ± 1.4 ka. At the Suman site, the slip rate is confined to 5.6–9.2 mm/yr since 7.6 ± 0.7 ka. Eventually, the slip rate is tightly bracketed from 6.4 to 9.2 mm/yr, which implies that the microplate model with localized deformation between rigid blocks should be abandoned. Synthesis of late Quaternary slip rates along the fault indicates that the central section, except for restraining bends, maintains a constant slip rate of ∼10 mm/yr. The long elapsed time, ∼400 years, since the most recent earthquake and a significant stress increase resulting from the 2014 Ms7.3 Yutian earthquake led to a high seismic risk along the central fault section.

Understanding how fluids flow to form halo-bearing veins is essential to assess the fundamental processes involved in fracture propagation and the formation of hydrothermal ore deposits. Haloes may mimic damage zones during fracture propagation, contributing to the identification of scaling relations between halo width and fracture displacement. In this work, we examine geometry, kinematics and mineral composition of well-exposed halo-bearing fault-vein network field samples. We studied a total of 18 veins from Iron-Oxide Copper Gold (IOCG) deposits in the Chilean Atacama Desert and from the Chinese Cathaysia tectonic block. Vein length and width and halo width were measured directly at the outcrop and later under optical microscope. We established a scaling relation, over five orders of magnitude, between halo width (HW) and vein width (VW) of the form ${\log }_{10}HW=1.07*{\log }_{10}VW+1.04$ which suggests that the majority of analyzed haloes were formed as a result of crack tip process zone damage. Such ratios and scaling relationships, apart from elucidating the physical mechanisms driving halo/damage zone formation, have potential implications for a more reliable estimation of the nature and size of ore grade variations away from high-grade mineralized veins to the relatively lower-grade surrounding wall rock volumes.

We document the structural setting, microstructure, and mineralogy of a crack-seal-type dilational jog that developed in the stepover between two faults cutting through a phacoid of massive serpentinite, itself embedded within a serpentinite shear zone at the base of the Dun Mountain Ophiolite, New Zealand. Our outcrop and microstructural measurements allow us to constrain the boundary conditions for a numerical model (part 2) that quantitatively explores the relationships between stress, fault slip, and incremental cracking in block-in-matrix shear zones. The dilational jog is c. 3 cm wide and contains hundreds of crack-seal bands, each c. 20–30 μm wide. Internally, the jog comprises two mineralogically distinct crack-seal domains: serpentine-only domains and serpentine-andradite garnet domains. Additionally, individual crack-seal bands have a double-layer structure: in serpentine-only domains each band comprises a thin (<2 μm) layer of chrysotile and a thicker layer (c. 25 μm) of polygonal serpentine/lizardite, whereas each band in serpentine + andradite domains comprises a thinner (c. 5 μm) layer of microcrystalline andradite and a thicker layer (c. 15 μm) of polygonal serpentine/lizardite. Micro-CT analysis shows that the serpentine + andradite domains have conic or ellipsoidal shapes with long axes subparallel to the inferred jog opening direction, and that andradite is smeared along micro-transform surfaces inside the jog. Our conceptual microstructural model invokes jog formation during progressive serpentinization of the host rock. Incremental crack opening along the jog-wall rock interface promotes relatively rapid initial precipitation of chrysotile or andradite at high fluid:rock ratios. As cracks fill and pressure re-equilibrates, relatively slow growth of polygonal serpentine/lizardite is favoured until the cracks are sealed and the cycle repeats. Our observations suggest that the precipitation of andradite (instead of chrysotile) was controlled both by structural boundaries within the jog (e.g., micro-transform surfaces) and by local element transport (e.g., Ca from serpentinizing clinopyroxene grains in the host rocks) to patches of the crack wall.

Field structural observations from the Himachal Himalaya, in the northwest of the mountain belt, challenge existing tectonic models and raise questions as to their validity. Microstructures and geochronological data reveal two discrete episodes of Barrovian metamorphism, the earliest during the Eocene-Oligocene transition, before an early period of recumbent folding. This metamorphic event occurred in association with a km-scale extensional ductile shear zone that is itself now recumbently folded on the km-scale, with an axial plane pressure solution cleavage. The Eocene-Oligocene gneiss complex is thus exposed in its core. The second episode of Barrovian metamorphism occurred in association with another regional-scale extensional shear zone during the Oligo-Miocene transition, thus synchronous to the South Tibetan Detachment System. This transects the recumbent fold stack. Microstructures show that the Main Central Thrust was initiated after the second phase of extension, and the associated second episode of Barrovian metamorphism had ceased operating. Further, the previously unrecognized km-scale Phojal Back-fold affects all of the above structures. Confusion caused by the misidentification of this structure led to the tectonic-wedge model, but this hypothesis can be invalidated by the structural evidence presented here. Our data support an alternative hypothesis that requires tectonic mode-switches in association with a succession of accretion events as India indents into Eurasia.

The mesoscale deformation structures in eastern Taiwan are considered to have recorded progressive deformation during rapid convergence and uplift in response to arc-continent collision. However, detailed deformation mechanisms and kinematic history of faulting remained poorly known. The Chimei Fault in eastern Taiwan thrusts the igneous forearc basement over the orogen-derived turbidites, and its outcrops provide opportunities to understand deformation mechanisms of the fault rocks across a bi-material fault during the arc-continent collision. To unravel the structural and mechanical architecture of the Chimei Fault, we performed field observations, paleostress analysis, and fold analysis. The Chimei Fault shows a fault core surrounded by damage zones. The width of the damage zones across the fault core is asymmetric, with the footwall turbidites exhibiting wider damage zone with higher fracture intensity than the hanging wall andesitic complex. Our paleostress analysis reveals that the mechanically stronger hanging wall can accommodate larger differential stress than the weaker footwall. Different deformation styles in the footwall damage zones, including pinch-and-swell structures, boudins, and postdating fractures, suggesting progressive deformation while sediment lithification in response to the activities of the Chimei Fault.

Measuring penetrative strain is critical for understanding the 3D strain field of structural systems. Here, we investigate ductile strain and kinematics in two Cordilleran structural systems in north-central Idaho: the Salmon River suture zone (SRSZ), which is a west-vergent ductile fold-thrust system that accommodated shortening associated with terrane collision between ∼144 and 105 Ma, and the north-striking, subvertical western Idaho shear zone (WISZ), which accommodated dextral-transpressional shearing between ∼105 and 86 Ma. We collected finite strain data from stretched clasts in three SRSZ thrust sheets, which define 56–87% average thrust-parallel stretching and 35–48% average thrust-normal thinning. Thrust-parallel stretching contributed >27 km of cumulative displacement to the up-dip portion of the fold-thrust system, comparable to the 34 km of total thrust displacement estimated at down-dip levels. In the WISZ, we documented dextral kinematics in lineation-normal planes, and we measured boudinaged and folded granitic dikes to estimate late-stage (∼91-86 Ma) strain, which yielded 65% minimum lineation-parallel stretching and 50% minimum east-west shortening. Subvertical stretching in the WISZ accommodated >9–10 km of exhumation relative to the Idaho batholith to the east. The SRSZ and WISZ both demonstrate the 1st-order importance of ductile stretching for accommodating the large-scale transfer of mass and exhumation in fold-thrust and transpressional systems.

Faults are the primary sources of seismicity worldwide, yet the mechanisms of fault weakening and recovery remain controversial. This study examines the microstructures and nanostructures of fault rock from a seismogenic normal fault within chert-banded dolostones. The fault slip surface exhibits various slip-related structures, including slickenlines, truncated clasts and nanoparticles/fragments. These nanoparticles on the fault slip surface are presented into two forms, single spherulitic nanoparticles (ranging in size from 50 to 300 nm) and agglomerated nanoparticles (ranging from 300 to 500 nm). The principal slip zone is characterized by cataclasites and micron-scale foliations. The cataclasite layer comprises a yellow-greyish matrix, grain-supported, and angular to sub-rounded coarser clasts which are composed primarily of dolomite, with a few clasts of quartz and calcite. The micron-scale foliations are defined by fine-grained fragments ranging from 1 to 20 μm. The microstructural investigations suggest that the single spherulitic nanoparticles may result from thermal decomposition of dolomite along the principal slip surface during fault slip or earthquake. Nano powder lubrication, facilitated by the rolling of single spherulitic nanoparticles, significantly weakens the fault during carbonate fault slip. The DEM simulation results indicate that the shear strength increases exponentially with the increasing volume percent of bonded nanoparticles. The transformation from single spherulitic nanoparticles into agglomerated/bonded nanoparticles through sintering can result in the recovery of frictional strength at the fault plane. The thin foliations in the slip zone are likely the results of laminar grain flow, possibly induced by CO₂ degassing. We inferred that nanoparticles can form through thermal decomposition on fault surfaces, which first facilitate and then inhibit earthquake behavior in thermally unstable rocks such as dolomite. The post-seismic strength recovery can be partly attributed to the formation of agglomerated nanoparticles.

With respect to difficulties seismic and magnetotellurics may have in accurate definition of exact salt diapir geometry, we performed various gravity simulations and calculations to investigate such exploration targets. Our motivation included the fact that not much has been done on caprock impact on the gravity signal. A very significant favorable condition is the high contrast of rock density between salt and basin sedimentary formations, especially carbonates. However, this can be more complicate if caprock formation is present. Therefore we defined approximate bulk density of typical caprock formation based on its usual composition, resulting in density values 2.45–2.70 g/cm3.

We modelled by forward and inverse procedures the gravity signal Gz of salt diapirs with caprock of variable thickness to demonstrate to which extent the salt diapir negative gravity anomaly may be reduced by the impact of caprock formation. In the tested cases the gravity anomaly was reduced by more than 30% depending on respective caprock composition and thickness. Significant contribution to the delineation of salt diapirs themselves, as well as diapirs hidden under caprock, came from the application of horizontal gravity gradients Gzx. We showed the difference of Gzx spikes indicating the edges of density contacts according to the type of gravity survey – land or airborne. It was also proved by calculating the gravity effect of laboratory analogue models of salt deformation and extrusion.

We demonstrated that gravity is still a valuable and relatively cheap tool for in-detail investigations of the salt structures within exploration projects.