Journal of Structural Geology最新文献

The ideal development of layer-bound, polygonal faults occurs in an isotropic stress field. However, some populations of layer-bound faults appear to have originated as ‘polygonal’ faults, but display atypical plan-view fault geometries that suggest development under anisotropic stress conditions. High-resolution 2D/3D seismic data in the southern Qiongdongnan Basin displays three tiers of layer-bound faults with distinct strike variations and complex structural patterns that depart from the geometries of isotropic polygonal fault cells. The lower fault tier displays preferred orientations along E-W, NE-SW, NW-SE directions; the middle fault tier shows a primary E-W orientation and a secondary N-S trend; the shallow tier features strong N-S and E-W orientations. Fault patterns in map view include orthogonal, quasi-polygonal and circumferential geometries, among which the orthogonal pattern is dominant in the shallow fault tier. Several factors are inferred to cause these variable geometrical and structural patterns in layer-bound faults, they are: pre-existing tectonic faults, an uplifted region, local slopes, contourite depressions, and large contourite-related channels. The orthogonal fault patterns have longer E-W orientation parallel to the contourite channels and shorter N-S trends perpendicular to the thalwegs. The presence of submarine channels is suggested to locally deflect the stress orientation, which in turn impacts fault patterns.

Fracture stratigraphy study for a subsurface sedimentary rock sequence can be a challenging and demanding task. Typically, the data obtained from seismic and well-logs are heavily impacted by resolution issues and are difficult to interconnect. In this work, we document and extract fracture properties (orientation, density, intensity, etc.) from a layered carbonate sequence for fracture stratigraphy characterization. High-resolution subsurface Ground Penetrating Radar (GPR) data images, coupled with drone and previously documented well-log profiles, were analyzed to achieve the structural characterization task. The studied outcrop is localized in the Potiguar Basin (Brazil), where the Cretaceous Jandaíra Formation carbonates are exposed for hundreds of meters. The sequence is subdivided into an upper packstone/grainstone portion and a lower wackestone bed package. We documented the higher fracture intensity/density in the lower bed package portion, highlighting that depositional texture and intra-bed stylolites control the higher fracture distribution in the sequence. Finally, a 3D conceptual model describing the overall results is presented. This model summarizes and shows the innovative fracture stratigraphy method based on the GPR data analysis.

The South China Sea (SCS) opened due to the extension of a compressional setting of the paleo-Pacific subduction. The pre-existing structure significantly influences the geometry of rift basins and the kinematic evolution of the rifting. However, structural evidence of the paleo-Pacific subduction in the northeastern SCS remains enigmatic. The deformation front, serving as the structural evidence of paleo-subduction, is associated with accretionary style deformation and would be reactivated during the subsequent extension phase. In this study, we use a multi-channel seismic profile to investigate the pre-existing structure related to the paleo-Pacific subduction in the northeastern SCS, emphasizing the influence of pre-existing structure on the rift evolution. The seismic profile reveals imbricate reflections in the lower crust. These reflections are interpreted as the deformation front of the Paleo-Pacific subduction. Notably, the deformation front is hyperextended in the Chaoshan Depression. The result of the stretching factors indicates that the ductile lower crust experienced preferential thinning during the rifting beneath the Chaoshan Depression. In the northern part of the profile, a transparent reflection zone was identified and interpreted as a magmatic arc related to Mesozoic subduction. One major achievement of our study is these seismic reflections provide the structural evidence for Paleo-Pacific subduction and reveal that the northeastern SCS has experienced crustal shortening and imbrication through a series of dipping thrusts. Subsequently, we discuss the role of pre-existing structures in lower crustal thinning and continental rifting. We propose the deformation front of the Mesozoic subduction, as a pre-existing weakness, facilitated the kinematic evolution of the rifting in the northeastern SCS during the Cenozoic.

Intraplate strike slip deformation structures play a crucial role in understanding how the earthquake are triggered, and respond to long-term deformation in plate interiors. One of the examples for intraplate structure is the Lake Salt Fault Zone (LSFZ) in Türkiye, located at the Central Anatolia region which has hosted a few moderate magnitude earthquakes. The LSFZ extends in NW-SE direction along the eastern border of the Lake Salt basin. In the western and central sections, it exhibits a rather linear trace, and it marks the west-northwestern boundary of the Cappadocian plateau. Along its strike, two big cities, namely, Aksaray and Niğde, and some significant eruption centers (the Hasan, Keçibuyduran and Melendiz Mountain stratovolcanoes) are located, and there is a 12 km right lateral offset. LSFZ has four main segments, namely, Karacaören, Keçikalesi, Obruk and Büyükkaraoğlan fault segments, and they have hosted two moderate-sized (Mw = 5.1 to 5.2) earthquakes (on September 20, 2020 and February 25, 2023) at the localities approximately 5 km ENE and SSW of Obruk Town (Niğde). Their focal mechanisms revealed that the LSFZ exhibits dominantly dextral strike-slip faulting with normal component. The vertical and horizontal displacement rates along the LSFZ are 0.14 mm/yr and 4.6 mm/yr, respectively. The recurrence interval of earthquakes of Mw ≥ 6.7 on the LSFZ is more than one thousand years, owing to the low slip rate. We propose that the LSFZ is in a seismic gap having potential to host a large earthquake.

The study investigates tectonic movements in northeast Japan by using long-term (2000–2022) global navigation satellite system (GNSS) and tide gauge measurements. The effect of the 2011 Tohoku-Oki earthquake including the other eight seismic events that occurred within this period is also discussed using GNSS time-series. The result showed the break in GNSS-time series because of occurred earthquakes and pointed out tectonic movements significantly. The GNSS sites located in the central and southern parts of northeast Japan showed that the velocity vectors have strong internal variation and suggest the existence source of alternative deformation because of geological terranes within the region. A least square approach was used, and the trend of sea-level measurements was fitted with the straight line. The obtained results from tide gauge measurements showed a rising trend at almost every site and indicated lithospheric uprising movement because of tectonic activities. This is possible because of the ongoing subduction of the Pacific and Philippine Sea plates beneath the Eurasian and North American plates.

The longer time for recording large earthquakes on a plate boundary fault, the better that understanding of large earthquake rupture behavior and seismic hazard on the fault zone. However, large earthquakes (M ≥ 7) are rarely recorded on the boundary fault with slow slipping rate, such as the Red River fault zone (RRFZ), which is an important plate boundary fault that marks the southwestern boundary of the Yangtze platform or south China block. There have been no large earthquake records on the southern segments (including the segment in Vietnam) of the RRFZ since historical earthquake records began in 886 AD, except the 1652 Midu M 7 earthquake and the 1925 Dali M 7 earthquake on the northern segment. The southern segment of the RRFZ will not have a large earthquake in the future or as a large earthquake seismogenic zone with a long period of recurrence, remains controversial, in part because of the absence of constraints from geological evidence. This controversial seriously restricts the risk assessment of future large earthquakes on the southern segment of the RRFZ. By careful interpretations of high resolution remote sensing images, in combination with a detailed field geological and geomorphic survey, we found a series of fault valleys and bedrock outcrops from Gasha toYaojie and Yuangjiang to Hekou on the southern segment of the RRFZ. Multiple trench excavation and radiocarbon dating sample analyses show that the mid valley trace in the southern segment of the RRFZ is an active fault. Geological and geomorphic evidence from Gasha to Yaojie and Yuanjiang to Hekou indicate that the mid valley trace in the southern segment of the RRFZ exhibits dip slip and dextral strike slip motion characteristics. This result is inconsistent with those of previous studies that the mid valley trace is purely strike slip. Furthermore, trenches opened on the range front trace in the southern segment of the RRFZ in Ejia are found to still be active, differing from previous studies. Thus, the seismic hazard on the southern segment of the RRFZ should be reevaluated.

We study in detail the M_w 5.4 September 6, 2021 Tofalaria earthquake occurred in a mountain area of the Eastern Sayan which is characterized by a low level of seismic activity. An interest in the seismic event is caused, on the one hand, by poor knowledge about stress-strain field of the crust in the considered region, and, on the other hand, by its relation to the NW segment of the ancient Main Sayan fault – a structural boundary between the Sayan-Baikal fold belt and the tectonically stable Siberian platform. Seismic moment tensors and hypocentral depths of the mainshock and its largest aftershock (M_w 4.6) are inverted from intermediate-period surface wave amplitude spectra calculated at the stations located at teleseismic distances. Integral source parameters of the mainshock, characterizing its spatio-temporal development, are also estimated and the fault plane is determined. Epicenters of 31 aftershocks with M ≥ 1.8, occurred up to the end of 2021, are constrained from body waves recorded at regional seismic stations. The obtained results show that the Tofalaria earthquake occurred under the influence of the SW-NE compression, which is observed in Western Mongolia. Focal mechanism of the largest aftershock and the NE elongation of the aftershock epicentral field (22 km) indicate stress redistribution after the mainshock in a local crustal volume, bordered by small-scale faults.

Folds are common structures that provide valuable insights into the direction and amount of shortening and the rheological properties of deformed rocks. Most thin plate folding theory started from M.A. Biot has historically been applied to isotropic materials, but rocks are often anisotropic due to the presence of tectonic foliations, bedding, veins, dykes, etc. Mechanical anisotropy can enhance partitioning of deformation, resulting in low-strain domains and localised high-strain shear domains. Using the Viscoplastic full-field code coupled with the modelling platform Elle (VPFFT-Elle), we investigate the evolving fold geometries, stress field and strain-rate field differences and redistributions resulting from layer-parallel shortening deformation of an isotropic, competent layer embedded in an anisotropic, weaker power-law viscous matrix. We focus on the effect of the orientation of the mechanical anisotropy relative to the competent layer. The simulation results illustrate that the deformation localisation behaviour, and hence fold geometry, depend on (i) the initial orientation of the anisotropy, (ii) the intensity of anisotropy, and (iii) strength of the competent layer, relative to that of the matrix. Variation in the localisation behaviour resulting from different strain-rate distributions lead to two end-member fold geometries: (1) classical Biot-type buckle folding and thickening of the competent layer coupled to the formation of a new axial-planar crenulation cleavage in the matrix, and (2) what we call ‘shear-band folding’ in which sections of the competent layer are offset due to the formation of shear bands in the matrix with opposite sense of shear. This leads to rapid fold amplification. Classical Biot-type buckle folds dominate when the initial anisotropy is parallel or subparallel to the shortening direction, while shear-band folds dominate when the initial anisotropy is normal or at high angle to the shortening direction. Results presented here contribute to our understanding on how mechanical anisotropy controls folding and the rearrangement of the matrix components. Furthermore, the modelled scenarios can serve as a “virtual glossary” to compare real folds in different tectonic settings, providing insights into the possible pre-fold configuration of the folded layer and its anisotropic matrix.