Dennis Brown

Syn‐to late‐orogenic extension caused by gravitational collapse is a process that has been documented in most orogens. In this paper, we investigate the contemporaneous stress field in Taiwan to examine syn‐orogenic extensional faulting and the possible gravitational collapse modes by which it is taking place. We use a new, large, well‐constrained, clustered earthquake focal mechanism data set from which we calculate the stress field ( σ ₁ , σ ₂ , σ ₃ ), the direction of the maximum compressive horizontal stress ( S _H ), and determine the most likely fault plane orientations and kinematics. Our results show that, in the upper 21 km of the crust, shearing, with lesser compression are the dominant stress regimes in the Western Foothills and Hsuehshan Range, resulting in a complex interaction of strike‐slip (with transpression and transtension) and thrust faulting. Extension is predominately localized in the upper 14 km of the crust in the Central Range (CR), where it is roughly orogen‐parallel. S _H and the best‐fit fault planes in the CR strike roughly NW‐SE, at a high angle to the structural and topographic grain. At depths greater than 21 km, compression and shear, with transpression and minor extension are the dominant stress regimes throughout Taiwan. Along the eastern flank of the mountain belt, mixed compression and shear stress regimes at depths greater than 50 km are related to the Manila and Ryukyu subduction zones. Our new results allow, for the first time, to characterize an ongoing, large‐scale, orogen‐parallel extension in Taiwan related to lateral extrusion with free‐boundary gravitational collapse.

The ML 6.4 Dapu earthquake that struck southwestern Taiwan on 20 January 2025 provides a critical case for understanding the seismogenic mechanisms and fault systems in this tectonically active region. This study applies a deep-learning-based real-time microearthquake monitoring system designed for Taiwan to analyze 3893 aftershocks recorded within 15 days of the mainshock. By incorporating a 3D velocity model and the NonLinLoc earthquake location method, we assess the fault systems associated with the earthquake sequence. Focal mechanisms of 10 M 4.8+ events from the Broadband Array in Taiwan for Seismology catalog are also utilized to interpret fault types. Our results suggest that the Dapu earthquake is linked to an upper crustal fault system, distinct from the surface fault structures. The study further evaluates the potential contribution of both thin-skinned and thick-skinned deformation models for the region. The findings emphasize the significance of real-time seismic monitoring combined with the 3D velocity earthquake location method in enhancing earthquake location accuracy and deepening our understanding of fault systems.

Laboratory measurements show that quartz‐bearing rocks undergo a decrease in P‐wave velocity when approaching the α–β quartz transition, followed by an abrupt increase afterwards. S‐wave velocity changes only slightly. This change can have a signature in crustal geophysical data. Here, we use the AbersHackerMacro to explore how the modal abundance of quartz affects the Vp/Vs ratio and reflectivity across the α–β transition in a number of high‐grade metamorphic rocks. The modelling shows that even with a low modal abundance of quartz, the α–β transition is recognisable in the Vp/Vs ratios of these rocks. α quartz has a strong influence on these Vp/Vs ratios, whereas β quartz does not. There is an increase in the reflection coefficient across the α–β transition with an increase in modal abundance of quartz. Reflection coefficients between different lithologies with less than about 25% quartz can increase or decrease. With greater than 25% quartz, reflection coefficients decrease.

Syn- to late-orogenic extension caused by gravitational collapse is a process that has been documented in most orogens. In this paper, we investigate the contemporaneous stress field in Taiwan to examine syn-orogenic extensional faulting and the possible gravitational collapse modes by which it is taking place. We use a large, well-constrained, clustered earthquake focal mechanism data set from which we calculate the contemporaneous stress field (σ1, σ2, σ3), the direction of the maximum compressive horizontal stress (SH), and determine the most likely fault plane orientations and kinematics. Our results show that, in the upper 21 km of the crust, shearing, with lesser compression are the dominant stress regimes in the Western Foothills and Hsuehshan Range, resulting in a complex interaction of strike-slip (with transpression and transtension) and thrust faulting. Extension is predominately localised in the upper 14 km of the crust in the Central Range, where it is roughly orogen-parallel. SH and the best-fit fault planes in the Central Range strike roughly NW-SE, at a high angle to the structural and topographic grain. At depths greater than 21 km, compression and shear (with transpression and minor extension) are the dominant stress regimes throughout Taiwan. Along the eastern flank of the mountain belt, mixed compression and shear stress regimes at depths greater than 50 km are related to the Manila and Ryukyu subduction zones. Our results suggest that Taiwan is undergoing orogen-parallel extension as a result of lateral extrusion with free-boundary gravitational collapse.

Cloud-to-ground lightning can result in high-temperature metamorphism of rocks, forming rock fulgurite. Here, we characterize a rock fulgurite on granitic gneiss from Kinmen Island, Taiwan, and demonstrate that high-pressure metamorphic features can be also generated by lightning. With the lightning monitoring system, we detected a lightning event with a peak current of 162 kA and the associated fulgurite. We conduct microanalytical methods on rock fulgurite, including optical microscope, scanning electron microscope with electron backscatter diffraction (EBSD), focused ion beam-transmission electron microscopy, and in-situ synchrotron Laue diffraction analyses. The fulgurite is characterized by an up to 200 μm thick glassy crust overlying host rock for around 10 square meters. Within the glassy crust, typical high-temperature features, such as vesicles, relic mineral fragments, and reduced oxidation-state iron oxides spheres, can be recognized. Below the glassy crust, EBSD analysis documents phase transformation (from monoclinic to triclinic) and planar features (exsolution lamellae) of alkali feldspar (sanidine) grains. Synchrotron Laue diffraction analysis indicates that these planar features are parallel to the (100) plane and preserve residual stress of up to 1.57 GPa, well above the 0.38 GPa recorded in feldspar grains (reference sample from borehole cores) that are not affected by lightning. The findings, including glassy crust, phase transformation and planar features of alkali feldspar grains, and high residual stress, suggest that lightning can result in both high-temperature and high-pressure metamorphic features. Because these assemblages are reminiscent of shock metamorphic textures produced by meteorite impact, we interpret them to be shock-related features. Given that the recognition of shocked minerals (with planar features) and glasses are both parts of the set of diagnostic criteria for meteorite impacts, in the absence of a broader suite of criteria, we recommend caution, since our identification of the Kinmen Island fulgurite indicates lightning can result in low-level shock metamorphic features.

The Taiwan orogen forms an active mountain range that has been evolving since the Late Miocene as a result of the oblique collision between the Luzon Arc, located in the Philippine Sea Plate, and the continental margin of the Eurasian Plate. Due to this configuration, some inherited structures from the continental margin are at high angle to the structural trend of the Taiwan thrust-and-fold belt and are thought to play an important role in present day tectonics. The inherited structures resulted from processes undergone by the Eurasian margin, such as rifting in the Early Eocene, and further local extension in the Middle Miocene. They comprise sub-vertical faults that are presently being reactivated and are actively involved in the evolution of the structure, seismicity and topography of Taiwan, causing transverse zones in its thrust-and-fold belt and foreland.The key objective of this research is to help define the deep structure in southern Taiwan, as well as the location and kinematics of these inherited east-northeast striking faults. To achieve this goal, we undergo a multidisciplinary approach based on the analysis and modelling of gravity (free-air and Bouguer anomaly) and magnetic data. The application of analytical techniques, such as horizontal directional derivatives allows us to identify gradients that can be related to the geometry and minimum horizontal extent of these basement structures in the margin and in Taiwan. Forward modelling of gravity and magnetic data contributes further to provide a better-constrained quantitative approach to their depth as they appear to affect the top of the basement. Finally, these results have been integrated with structural studies and earthquake information in order to improve our understanding of the southern Taiwan and Eurasian continental margin structure.The preliminary results allow us to discuss the limitations of vertical derivatives and residual potential field data when dealing with deep inherited basement structures. The present dataset has proven to be useful to discern the existence of a deformed reactivated basement in the foreland and frontal part of the Taiwan thrust-and-fold belt, and improve our comprehension of its crustal structure. Due to the limitations of potential field data as the depth of the source increases, the resolution diminishes towards the E of the thrust-and-fold belt.This research is part of project PGC2018-094227-B-I00 funded by the Spanish Research Agency of the Ministry of Science and Innovation of Spain. Olivia Lozano acknowledges funding from the same agency through contract PRE2019-091431.

Studies of mountain belts worldwide have shown that the structural, mechanical, and kinematic evolution of their foreland fold-and-thrust belts are strongly influenced by the structure of the continental margins that are involved in the deformation. The area on and around the island of Taiwan provides an unparalleled opportunity to investigate this because the entire profile of the SE margin of the Eurasian plate, from the shelf in the north to the slope and continent-ocean transition in the south and the offshore, is currently involved in a collision with the Luzon arc on the Philippine Sea plate. Taiwan can, then, provide key insights into how such features as rift basins on the shelf, the extensional faults that form the shelf-slope break in the basement, or the structure of the extended crust and morphology of the sedimentary carapace of the slope can be directly reflected in the structural architecture, the location and pattern of seismicity, topography, and the contemporaneous stress and strain fields of a fold-and-thrust belt. For example, east-northeast striking faults that have been mapped on the necking zone of the Eurasian margin can be traced into the island of Taiwan where they are causing important along-strike changes in various aspects of the structural, mechanical, kinematic, and morphological behavior of the fold-and-thrust belt. In particular, across the upper part of the necking zone there is an abrupt north-south change in structure, an increase in the amount of seismicity, an increase in topography, a rotation of the direction of maximum compressive horizontal stress, of the GPS displacement vectors, the compressional strain rate, and the maximum shear strain rate. These changes are interpreted to be caused by east-northeast striking, dextral strike-slip faulting in the basement that is taking place as a result of the reactivation of pre-existing faults along the upper part of the necking zone. The abrupt southeastward increase in topography across the upper part of the necking zone is the surface expression of the basal thrust of the fold-and-thrust belt ramping down into the basement, with maximum elevations reached in the basement-involved thrust sheets, suggesting a causal link between basement involvement in the thrusting and high topography. On the shelf, the roughly northeast-oriented Hsuehshan Trough is inverting along almost north-south striking basin bounding faults that penetrate into the middle crust and have well-clustered, deep seismicity. There are no substantial differences in the contemporaneous stress and strain field. There is, however, a clear relationship between basement involvement in the thrusting and the development of high topography in the Hsuehshan Range. Only the upper part of the slope is involved in the fold-and-thrust belt in southernmost Taiwan. In this area, there is a reduction of the amount of seismicity and lower topography. The largest part of the corresponding thrust wedge developed in the lower slope is offshore. This work is funded by the Spanish Ministerio de Ciencia, Innovación y Universidades grant PGC2018-094227-B-I00.

The southwest Taiwan fold‐and‐thrust belt is forming on the outer shelf and slope of the Eurasian continental margin. It comprises a roughly N‐S striking, west verging imbricate thrust system that has been developing since the Late Miocene. Here we present the results of new surface geological mapping from which we construct balanced and restored cross sections and along‐strike sections. From these we compile maps of the basal thrust, thrust branch lines and, where possible, stratigraphic cutoffs. To interpret the structure in the subsurface and beneath the basal thrust, we use a P wave velocity of 5.2 km/s as a proxy for the top of the Mesozoic basement. We divide the southwest Taiwan fold‐and‐thrust belt into a number of thrust sheets that form the basis of our description and interpretations. From these data we interpret the 3‐D structure of the fold‐and‐thrust belt and the influence that the structure and morphology of the continental margin is having on its development. We show that there is a significant along‐strike change in the structure. This change takes place across a transverse zone that is composed of a suite of structures at the surface. We suggest that this transverse zone has a causal relationship with variations in the geometry of the basal thrust which in turn is related to (possibly fault bounded) basement highs and lows that are inherited from the continental margin.

Studies of mountain belts worldwide show that along‐strike changes are common in their foreland fold‐and‐thrust belts. These are typically caused by processes related to fault reactivation and/or fault focusing along changes in sedimentary sequences. The study of active orogens, like Taiwan, can also provide insights into how these processes influence transient features such as seismicity and topography. In this paper, we trace regional‐scale features from the Eurasian continental margin in the Taiwan Strait into the south central Taiwan fold‐and‐thrust belt. We then present newly mapped surface geology, P wave velocity maps and sections, seismicity, and topography data to test the hypothesis of whether or not these regional‐scale features of the margin are contributing to along‐strike changes in structural style, and the distribution of seismicity and topography in this part of the Taiwan fold‐and‐thrust belt. These data show that the most important along‐strike change takes place at the eastward prolongation of the upper part of the margin necking zone, where there is a causal link between fault reactivation, involvement of basement in the thrusting, concentration of seismicity, and the formation of high topography. On the area correlated with the necking zone, the strike‐slip reactivation of east northeast striking extensional faults is causing sigmoidal offset of structures and topography along two main zones. Here basement is not involved in the thrusting; there is weak focusing of seismicity and localized development of topography. We also show that there are important differences in structure, seismicity, and topography between the margin shelf and its necking zone.