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19th International Symposium on Deep Seismic Profiling of the Continents and their Margins, 15th – 20 th March 2020, Perth, Australia, The region of Taiwan is undergoing active, oblique arc-continent colision between the Luzon Arc on the Philippine Sea Plate and the continental margin of Eurasia. The Fold-and-Thrust Belt (FTB) in Taiwan passes southwards into a submarine accretionary wedge at the Manila subduction zone. The aim of this contribution is to examine how an on land FTB changes into a marine accretionary prism in the context of an oblique arc-continent collision. Preliminary results are presented in this poster, This research is part of project PGC2018-094227-B-I00 funded by the Spanish Research Agency from the Ministry of Science Innovation and Universities of Spain. JA is funded by MICINN (Juan de la Cierva fellowship - IJC2018-036074-I).

The February, 2018 Mw 6.4 earthquake in eastern Taiwan caused extensive damage in Hualien city. While damaging earthquakes are common in this region, there are relatively few permanent seismic stations deployed. Two days after the main shock, we deployed 70 temporary seismic stations around Hualien City for 12 days (February 8thto 19th) and station spacing was 1-5 km.During this time, 2,192 aftershocks were recorded from which 580 focal mechanisms were determined. The aftershock sequence extended about 25 km southwestward from the epicenter of the main shock, into the Longitudinal Valley and to depths of between 5-15 km. Earthquake hypocenters indicate that the aftershocks took place along a near vertical to steeply west-dipping plane in the north that becomes more diffuse in the south. Focal mechanisms are predominantly extensional different from the left-lateral strike-slip with thrust component faulting of the main shock. Very few events occurred in the uppermost crust, at depths of less that 5 km and their focal mechanisms are left-lateral strike-slip faulting. From the main shock to the aftershocks, the stress changed rapidly from a NNW-oriented compressional axis (P-axis) to the same direction for the extensional axis (T-axis). This change in the stress field from the main shock to the aftershock sequence may relate to a rapid stress relaxation rate in this complex tectonic environment. This rapid stress relaxation may account for the short recurrence time of large earthquakes in the Hualien area.

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 cut-offs. 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. From these data we interpret the 3D 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 divide the southwest Taiwan fold-and-thrust belt into a number of thrust sheets that form the basis of our description and interpretations. Shortening is variable, ranging from 16 km to 25 km. We show that there are significant along-strike changes in the structure that we suggest have a causal relationship with variations in the geometry of the basal thrust and that these, in turn, are related to basement highs and lows interpreted from the P-wave velocity model. Finally, a number of local- and regional-scale unconformities provide information on the Pliocene through Holocene growth of the fold-and-thrust belt and the change from marine to sub-aerial deformation., This research is supported by the project PROTAI-2 (CGL2013-43877-P).

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. ©2017. American Geophysical Union., D.B., J.A-M., and C.B. acknowledge funding provided by the Spanish Ministerio de Economia y Competividad grant CGL2013-43877-P. H.K-C. acknowledges funding by MOST 104-2628-M-008-005-MY3.

The structure of the south Taiwan fold-and-thrust belt (FTB) 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, serial balanced cross sections, long sections, and a map of the basal thrust that are combined with published borehole data and a 3D P-wave tomography model to constrain the structure at depth. The stratigraphy of the area comprises Eocene syn-rift sediments that are unconformably overlain by Oligocene through Middle Miocene shallow water post-rift sediments. These are unconformably overlain by Late Miocene through Holocene-age syn-orogenic sediments. We define the basement to these as the Mesozoic pre-rift rocks of the Eurasian margin, and assign it a P-wave velocity of > 5.2 km/s in the velocity model. The syn-orogenic sediments thicken and get progressively involved in the thrust sheets toward the south. The south Taiwan FTB can be divided into several thrust sheets. In the northern part of the map area, the structural grain is roughly N-S striking, changing southward to become NE-SW oriented. The westernmost thrust sheets form broad, open synclines cored by Pliocene to Holocene sediments, and tight hanging wall anticlines formed by Miocene rocks. Along its eastern flank, a narrow, complexly folded anticlinorium is developed in Miocene rocks. Restoration of the cross-sections indicates about 15 to 17 km of shortening. The basal thrust deepens from the surface towards the E and SE in a staircase trajectory, with narrow ramps and wide flats, reaching up to 8 km depth in the east before ramping down into the basement. The basal thrust has two pronounced, NE-striking oblique ramps that coincide with sigmoidal changes in shape of structures in the hanging wall. These correlate with a SW-NE trending increase in the P-wave velocity that is interpreted to image a basement high. We interpret this basement high to be an inherited extensional structure of the margin, and that faults associated with it are being reactivated, causing the changes in strike of the basal thrust, as well as in the surface trace of thrust and fold axes.

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 to test the hypothesis of whether or not these regional-scale features of the margin are contributing to along-strike changes in structural style, 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 margins 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 involvement 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 margins shelf and its necking zone.