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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).

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.

Rifting along southeastern Eurasia in the Late Cenozoic led to the formation of a magma-poor rifted margin facing the South China Sea to the southeast and the Philippine Sea to the east. Further rifting along the outer part of the margin during the Middle to Late Miocene was accompanied by an extensive episode of intraplate flood volcanism that formed the Penghu Archipelago. Previous geophysical studies in the area of the Strait have focused primarily on the shallow structures of the rift basins and the depth to the Moho. In this study, we present the results of a joint Chinese and Taiwanese 3D ambient noise tomography study from which we calculate the regional-scale 3D S- wave structure of the Taiwan Strait. The S-wave model shows a thinning of the crust beneath the rift basins where, locally, thin high-velocity layers suggest the presence of intrusive bodies. The rift basins and, along the west coast of Taiwan, the foreland basin are imaged as c. 5 to 10 km thick low velocity zones that extend eastward beneath the Taiwan mountain belt. In the upper 10 km of the crust, the basaltic rocks of the Penghu Archipelago are imaged as a high velocity zone that, with depth, becomes a relatively low velocity zone. We interpret this low velocity zone in the lower crust and upper mantle beneath the Penghu Archipelago to image a thermal anomaly related to the still cooling magma feeding system and the melt reservoir area that fed the flood basalts at the surface.

Rifting along southeastern Eurasia in the Late Cenozoic led to the formation of a magma-poor rifted margin facing the South China Sea to the southeast and the Philippine Sea to the east. Further rifting along the outer part of the margin during the middle to late Miocene was accompanied by an extensive episode of intraplate flood volcanism that formed the Penghu Archipelago. Previous geophysical studies in the area of the strait have focused primarily on the shallow structures of the rift basins and the depth to the Moho. In this study we present the regional-scale 3-D S wave structure of the Taiwan Strait that is derived from a joint Chinese and Taiwanese 3-D ambient noise tomography study. The S wave model shows a thinning of the crust beneath the rift basins where, locally, thin high-velocity layers suggest the presence of intrusive bodies. The rift basin and the foreland basin along the west coast of Taiwan are imaged as low-velocity zones with thicknesses between 5 and 10 km and extending eastward beneath the Taiwan mountain belt. In the upper 10 km of the crust, the basaltic rocks of the Penghu Archipelago are imaged as a high-velocity zone that, with depth, becomes a relatively low-velocity zone. We interpret this low-velocity zone in the lower crust and upper mantle beneath the Penghu Archipelago to image a thermal anomaly related to the still cooling magma feeding system and the melt reservoir area that fed the flood basalts at the surface. ©2016. American Geophysical Union. All Rights Reserved., H.K-C. is supported by Ministry of Science and Technology (grant MOST104-2628-M-008-005-MY3). D. Brown acknowledges funding by MINECO grant-GL2013-43877-P. Q. Li and Z. Ye acknowledge funding by Sinoprobe-02-03. S wave tomography model presented in all figures can be obtained from the lead author (Hao Kuo-Chen, kuochen@ncu.edu.tw).

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 Eurasian margin, from the shelf in the north to the slope and continent-ocean transition in the south and the offshore, is currently involved in the collision. Taiwan, then, can 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 location and pattern of its seismicity, in its topography, and in its structural architecture, among other things. The continental margin of the Eurasian Plate that is currently involved in the Taiwan orogeny is thought to have evolved from a sub-continental subduction system in the Late Cretaceous to a rifting margin by the Early Eocene and, during the late Early Oligocene, to sea-floor spreading and the formation of the South China Sea, followed by localized extension in the Middle Miocene and, finally, collision with the Luzon Arc by the Early Miocene. Imaging features of the margin’s structure in the Taiwan orogen is possible with seismic tomography, which shows, for example, that there are notable changes in velocity that can be directly attributed to structures in the basement. For example, there is a marked increase in Vp beneath the Hsuehshan Range which can be interpreted to be related to the uplift of higher velocity basement rocks by basin inversion. This is accompanied by significant seismicity that reaches a depth of more than 30 km’s, and by surface uplift to form the highest topography in Taiwan. Furthermore, beginning at 8 km depth, but becoming especially prominent at 12 km and 16 km depth, there is an embayment of relatively high Vp that can be interpreted as the onshore projection of a basement high that occurs between the Mesozoic basement shelf break and the Taihsi Basin. Southward, there is a notable northeast-southwest-oriented increase in seismicity across the on land projection of the Mesozoic basement shelf break, with seismicity predominantly located beneath the Alishan Ranges, the highest topography in this part of the mountain belt. The topography also shows a pronounced re-entrant that coincides with the orientation and onshore projection of the Mesozoic basement shelf break that extends northeastward across the Western Foothills and into the Hsuehshan Range. The shallow structure of south-central Taiwan contains a number of features, such as changes in structural grain, or basement involvement in the deformation, that can be attributed to the presence of pre-existing structures in the Eurasian continental margin as it enters into the deformation of the Taiwan fold and thrust belt.

Taiwan nos ofrece una oportunidad inestimable para investigar la influencia de las estructuras heredadas del margen continental en el desarrollo de un cinturón de pliegues y cabalgamientos (CPC). En esta zona, el arco de Luzón colisiona oblicuamente con el margen continental euroasiático formando el Orógeno de Taiwan. Para este estudio combinamos la cartografía geológica y los cortes balanceados con la tomografía sísmica (Vp) y los hipocentros de los terremotos. Se definen las rocas pre-Eocenas como el basamento del margen y del CPC, y se utiliza la Vp de 5.2 km/s como representativa de la interfaz entre el basamento y la cobertera sedimentaria. En la zona centro-sur del CPC se producen cambios a lo largo de la orientación de las estructuras y de la estratigrafía, que pueden ser correlacionados con estructuras activas del basamento. Los cambios de las velocidades sísmicas de N a S se interpretan como altos y bajos de basamento y se correlacionan con las áreas donde se producen cambios de la dirección estructural del CPC. Algunos grupos de sismicidad se alinean a lo largo de los bordes de estos bloques de basamento. Interpretamos estas características como indicativos de que el basamento está involucrado en la deformación y que la sismicidad representa la reactivación de fallas de basamento que influencian la estructura del CPC., This research is supported by BES-2014-070122 grant from MINECO within the project with reference number CGL2013-4377-P.

The Eurasian Margin is obliquely colliding with the Luzon Arc to form the Taiwan orogen. This configuration is particularly apparent in south-central part of the island providing a case example to investigate the effects of structural inheritance in the development of the thrust and fold belt. The Eurasian Margin evolved from a pre-Cenozoic continental basement that underwent rifting in the Early Eocene and subsequent sea-floor spreading to form the South China Sea during the late Early Oligocene. The margin underwent localized extension in the Middle Miocene, before the initiation of collision with the Luzon Arc by the Early Miocene. The important along-strike changes in structure and topography of south-central Taiwan thrust and fold belt are evidenced in the detailed geological map and 3 balanced geological cross sections. A 3D tomography model is integrated in this study to help constrain the structure at depth. Major along-strike changes seem to be related to structures oriented at a high angle to the thrust system. These include changes in strike of thrusts and fold traces, the changing elevation of thrusts and stratigraphic contacts, and the growing importance of Middle Miocene sediments within the thrust system that take place from north to south. Horizontal slices of the tomography model illustrate that N-S changes in velocity have the orientation of the inherited structural grain of the Eurasian margin. In particular, the inherited location of the Mesozoic margin’s shelf-slope transition affects the distribution of seismicity and the location of lateral stratigraphic and structural changes. Also, it appears to be associated with the inversion of Eocene- and Miocene-age extensional faults, deeply rooted in the pre-Cenozoic basement that trend oblique to the thrust belt. The inversion of inherited structures affects the uplift of Miocene syn-extensional and syn-tectonic Plio-Pleistocene foreland basin sediments, and of the pre-Cenozoic basement. Section A shows imbricated thrusts merging into a flat, gently west-dipping detachment at the base of PlioPleistocene syn-tectonic sediments in the foothills. While in the area of the Hsuehshan Range, the involvement anduplift of pre-Cenozoic basement is associated to higher topography and to the occurrence of Eocene-Oligocene sediments at the surface. Section B shows thick Miocene stratigraphy and pre-Cenozoic basement rocks involved in the imbricated thrusts. Basement involvement is here also associated with the increased topography observed in the Alishan Range. The Choshui lateral structure that separates sections A and B is associated with a hangingwall culmination wall. It is evidenced in the map by changes in orientation of contacts and fold axis. This lateral structure is also constrained by larger Vp values in the tomography, corresponding to the Alishan basement involvement area. In section C to the south, the imbricated thrusts involve thick Miocene and Plio-Pleistocene sedimentary strata and the westward dipping sole thrust is above the pre-Cenozoic basement. These sedimentary sequences are interpreted to be offscraped above inherited extensional faults that seem to be associated to significant seismic activity at depths of 5-10 km below the sole thrust.

We investigate the geophysical signature within the south-central Taiwan fold and thrust belt of the reactivation of pre-existing structures developed on the Eurasian margin. Seismic tomography (P-wave) and earthquake hypocenters are combined to trace structures mapped on the margin offshore western Taiwan into the fold and thrust belt. The extensional tectonic history of the margin began in the Early Eocene and culminated in the Late Eocene to Early Oligocene with sea-floor spreading and the opening of the South China Sea. Several NE trending basins developed during the rifting of a pre-Cenozoic basement and these were filled with Eocene sediments. Further extension on the outer margin took place during the Middle to Late Miocene, forming basins that are now involved in the Taiwan deformation. Finally, the margin’s transition from the platform to the slope takes place across south-central Taiwan and is oriented at a high angle to the active deformation front. We define the basement as pre-Eocene rocks and use a P-wave velocity (Vp) of 5.2 km/s as a proxy for the interface between them and their younger cover. This Vp interface is characterized by highs and lows that can be interpreted to image basement topography whose possible causes we investigate here. In the Hsuehshan Range there is a pronounced shallowing of the 5.2 km/s surface across the Shuilikeng fault. It is accompanied by an east-dipping cluster of seismicity down to more than 25 km depth, and forming what appears to be a crustal ramp across which the Eocene-age Hsuehshan Basin is being inverted. Westward, the 5.2 km/s interface forms a high called Paikang basement high, the southern flank of which is the on land projection of the Mesozoic basement shelf break. Southward, there is an increase in seismicity and topography that is associated to a NE-SW oriented lateral structure in the fold and thrust belt. South of this lateral structure, beneath the Alishan Range, a shallowing of the 5.2 km/s interface marks a change in the structural grain of the fold and thrust belt. This, together with the increase in predominatedly scattered seismicity that reaches greater than 20 km depth, can be interpreted to indicate basement involvement in the deformation. To the south of the Alishan Range a zone of higher velocity whose orientation is roughly parallel to that of the Mesozoic basement shelf break is interpreted as a basement high. Here, there appears to be SW-dipping clusters of hypocenters associated with the shallowing of the 5.2 km/s surface. We interpret this shallowing of the 5.2 km/s surface to be related to an extensional fault block (or blocks) on the upper slope area of the basement. The seismicity clusters are possibly imaging the reactivation of the extensional faults that bound it.

This article is subject to a CC Attribution 3.0 License., The Alishan area of Taiwan includes a sector of the fold-and-thrust belt that spans the transition from the platform with full thickness of the Eurasian continental margin in the north to the thinning crust of its slope in the south. This part of the fold-and-thrust belt with the highest elevations includes important along-strike changes in structure, stratigraphy, and seismic velocities. Here we present the results of new geological mapping from which we build geological cross sections both across and along the regional structural trend. Fault contour, stratigraphic cut-off, and branch line maps provide 3-D consistency between the cross sections. Minimum shortening is estimated to be 15 km, with displacement overall to the northwest. A P wave velocity model helps constrain the structure at depth by providing insight into the possible rock units that are present there. P wave velocities of > 5.2 km/s point toward the presence of basement rocks in the shallow subsurface throughout much of the south-eastern part of the area, forming a basement culmination. The changes in strike of thrusts and fold axial traces, the changing elevation of thrusts and stratigraphic contacts, and the growing importance of Middle Miocene sediments that take place from north to south are interpreted to be associated with a roughly northeast striking lateral structure coincident with the northern flank of this basement culmination. These transverse structures appear to be associated with the inversion of Eocene- and Miocene-age extensional faults, deeply rooted in the pre Cenozoic basement that were along what was the shelf-slope transition in the Early Oligocene. Inversion causes uplift of the margin sediments and their higher P wave velocity basement during Pliocene-Pleistocene thrusting.