Joaquina Alvarez-Marron

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.

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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.

The active Taiwan mountain belt is located in a complex geodynamic setting that involves two subduction processes. To the northeast, the Philippine Sea Plate subducts northward beneath the Eurasian Plate at the Ryukyu Trench, while in the southwestern part, the Eurasian Plate subducts eastward under the Philippine Sea Plate, where it obliquely collides with the Luzon Volcanic Arc. The Taiwan thrust-and-fold belt is created as a result of this ongoing arc-continent collision. Regardless of the predominance of compression in the overall structure of the island, several studies have also reported normal faulting. This study aims to estimate the local and regional stress field using earthquake focal mechanism data to contribute to a better understanding of crustal deformation in the complex tectonic setting of Taiwan. Manually clustered earthquake focal mechanisms are inverted to obtain an estimate of the principal stress (σ1, σ2, σ3) orientations and the stress ratio (σ1-σ2)/(σ1-σ3), from which the direction of the maximum horizontal stress (SH) is calculated. The initial data set contains 11,587 earthquake focal mechanisms compiled from several sources dating between 1990 and 2020. All deep earthquakes in the Ryukyu subduction zone were removed from the data set. The Chi-Chi 1999 and other major earthquakes and aftershocks were also removed as they may reflect a distorted stress field. After preprocessing, a database consisting of 8,510 events with focal depths between 1-144 km and magnitudes ML=0.7-5.9 was used in the inversion. Depth division was performed in a regular 7 km grid up to 28 km depth, all events deeper than 28 km being considered in the same layer. Preliminary results show that, to the southwest, the notable clockwise rotation of SH from SW-NE to a W-E direction and a change in the fault type from strike-slip to reverse to the east coincides with the interaction between the ENE-striking reactivated inherited structures of the Eurasian continental margin and the NNE-striking thrust faults of the foreland thrust-and-fold belt. To the centre-east, results show normal faulting in the upper crust, which changes to reverse faulting with depth, suggesting that there is a stress transition at approximately 14 km. Beneath that depth, there is a general state of compression. Ongoing research aims at integrating these results with those of numerical modelling and with field data in an effort to understand the locus of deformation and the occurrence of extensional tectonics in compressional settings, here and in other mountains belts worldwide. 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. Funding from SERA European Union H2020 INFRAIA-2016-2017 Agreement, 170522 is also acknowledged.  

Various models have been proposed to explain the formation of the Ibero-Armorican Arc (IAA) as either primary or secondary (rotational). Models for secondary arc formation require significant rotation around a vertical axis at the end of the Variscan orogeny. Estimates for the amount of north–south (present coordinates) contraction (horizontal shortening) required for these rotations range from 54 at the core to 91% at the SE tip of the arc, near to the present day Mediterranean coast. These estimates are compared with coeval deformational structures developed in three areas of the orogen: (1) in the autochthonous hinterland in the southern branch of the arc; (2) in the Cantabrian Zone foreland fold–thrust belt in the core of the arc; and (3) in the SE continuation of the southern branch of the IAA in the present day Iberian Chain. From this analysis it follows that the contraction associated with late Variscan deformation is much less than that needed to explain the formation of the IAA from a formerly linear orogen via rotation around a vertical axis. We therefore propose that the geometry of the arc should be re-evaluated. We discuss the palaeomagnetic data purported to support the secondary origin, as well as the role of late Variscan regional strike-slip faults in modifying the geometry of the Iberian and Armorican massifs. Our analysis suggests that the IAA formed mainly from a primary non-rotational margin-controlled curvature, slightly modified by c. 5–20% of superposed contraction during late Carboniferous and/or Alpine times.

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.

Different models have been proposed to explain the formation of the Ibero-Armorican Arc, which require significant vertical axis rotations, at the end of the Variscan orogeny. Estimates of the amount of contraction (horizontal shortening) needed for these rotations range from 54 % to 91 % perpendicularly to the arc. These estimates are compared with coeval deformational structures developed in two areas of the orogen, one in the autochthonous hinterland underlying the Galicia-Trás-os-Montes Zone in the southern branch of the arc, and the other in the Cantabrian Zone foreland in the core of the arc. From this analysis it follows that the late Variscan deformation together with the subsequent Alpine contraction is not sufficient to explain the formation of the Ibero-Armorican Arc as a secondary structure by means of vertical axis rotations. Our analysis suggests this arc is mainly a primary, or non-rotational curve, slightly modified by ca. 10 % of superposed contraction during late Carboniferous and/or Alpine times. Moreover, we propose that the assumptions underlying the interpreted geometry of the arc be re-evaluated, and we discuss the role of late-Variscan regional strike-slip faults in the Iberian and in the Armorican massifs that probably acted consecutively before and during the contraction of the arc.

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.

A high-resolution seismic tomography survey was acquired to obtain a full 3-D P-wave seismic velocity image in the Záncara river basin (eastern Spain). The study area consists of lutites and gypsum from a Neogene sedimentary sequence. A regular and dense grid of 676 shots and 1200 receivers was used to image a 500 m×500 m area of the shallow subsurface. A 240-channel system and a seismic source, consisting of an accelerated weight drop, were used in the acquisition. Half a million travel-time picks were inverted to provide the 3-D seismic velocity distribution up to 120 m depth. The project also targeted the geometry of the underground structure with emphasis on defining the lithological contacts but also the presence of cavities and fault or fractures. An extensive drilling campaign provided uniquely tight constraints on the lithology; these included core samples and wireline geophysical measurements. The analysis of the well log data enabled the accurate definition of the lithological boundaries and provided an estimate of the seismic velocity ranges associated with each lithology. The final joint interpreted image reveals a wedge-shaped structure consisting of four different lithological units. This study features the necessary key elements to test the travel time tomographic inversion approach for the high-resolution characterization of the shallow subsurface. In this methodological validation test, travel-time tomography demonstrated to be a powerful tool with a relatively high capacity for imaging in detail the lithological contrasts of evaporitic sequences located at very shallow depths, when integrated with additional geological and geophysical data.

A high-resolution seismic tomography survey was acquired to obtain a full 3D P-wave seismic velocity image in the Zancara River Basin (east of Spain). The study area consists of lutites and gypsum from a Neogene sedimentary sequence. A regular and dense grid of 676 shots and 1200 receivers was used to image a 500 x 500 m area of the shallow subsurface. A 240-channel system and a seismic source consisting of an accelerated weight drop, were used in the acquisition. Half million traveltime picks were inverted to provide the 3D velocity model that allowed to resolve the structure up to 120 m depth. The project targeted the geometry of the underground structure with emphasis in defining the lithological contacts but also the presence of cavities and fault/fractures. An extensive drilling campaign provided uniquely tight constraints on the lithology; these included core samples and wireline-log geophysical measurements. The analysis of the well-log data enabled the accurate definition of the lithological boundaries and provided an estimate of the seismic velocity ranges associated to each lithology. The final joint interpreted image reveals a wedge shaped structure consisting of four different lithological units. This study features the necessary key elements to test the traveltime tomographic inversion approach in the high-resolution characterization of the shallow subsurface. In this methodological validation test, traveltime tomography demonstrates to be a powerful tool with a relatively high capacity for imaging in detail the lithological contrasts of evaporitic sequences located at very shallow depths, when integrated with additional geological and geophysical data.