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B1-001
10:50
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11:10
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MOUNTAIN BUILDING AND DEFORMATION PARTITIONING
Many orogens on the planet result from plate convergence involving subduction of a continental margin. In such a tectonic context, the lithosphere is strongly deformed building a mountain belt generally constituted by the accretion of various units of continental crust and some remnants of the lost oceanic domain. A complex deformation characterizes crustal shortening and subsequent thickening. It involves a strong partitioning of deformation modes and kinematics, resulting from three main processes (more often combined in a mountain belt). They are: 1) Rheologic layering of the lithosphere (brittle and ductile behavior of crustal layers, structural heritage, décollements) involving specific behaviours during deformation. 2) Subduction kinematics and geometry of continental margins (oblique convergence, shape of indenters). 3) Interaction between Tectonics and surface processes (mass transfer in the orogen).Several questions are addressed through observations from the Taiwan orogen and insights from analog models taking into account surface processes : What is the impact of erosion on exhumation, shape of foliations and stretching lineations ? What is the relationship between deep underplating, induced uplift and flow of crustal material during erosion (evolution of finite strain during wedge growth) ? What is the role played by décollements or weak zones in crustal deformation and what is the impact of structural heritage at different scales (for example, how to account for the early extensional history of a rifted passive continental margin) ? What is responsible for the development of underplating domains evolving through time and what induces cyclicity of basal accretion ? Are synconvergence normal faults an effect of deformation partitioning and erosion ? What is the impact of deformation partitioning, surface processes and stratigraphic layering (mecano-stratigraphy) on folding mechanisms ? What are the relations between deformation and evolution of morphology (for example, relationships between the shape of present day drainage networks and long term partitioning of deformation) ? What is the role of deformation partitioning on the location of major seismogenic faults in active mountain belts ?We propose an original view for the impact of deformation partitioning on the shape and evolution of orogens.
Malavieille Jacques
Mountain building
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B1-002
11:10
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11:30
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MOUNTAIN BELT DYNAMICS, ROCK STRENGTH AND TOPOGRAPHY IN TAIWAN
Recent observations of cleavage patterns, strain histories, and kinematics across the Taiwan mountain belt depict systematic orogen-scale variations with respect to the synorogenic divide and indicate that the pattern of deformation and cleavage development is a predictable consequence of feedbacks between orogen stresses, kinematics, and surface processes. The advection pathway for a collision is defined by the point at which a rock volume is initially accreted and the point where it is exhumed at the surface. In Taiwan, the point of accretion is constrained by the observation that nearly all the sedimentary and metamorphic rocks exposed in the Taiwan Central Range were originally the basement, rift basins, and sedimentary cover of the passive margin of Asia, and the basement and Paleogene sediments have bypassed the detachment level in the foreland and are therefore underplated or accreted to the base of the orogen. Seismic tomography indicates an overall wedge geometry of the collision and limits the maximum possible depth of accretion and burial during mountain-building. Based on these constraints, the advection pathways deepen from west to east across the Western Central Range, with the greatest burial depths experienced by rocks of the Eastern Central Range, which were accreted into the west-facing pro-wedge and subsequently advected beneath the topographic divide before being exhumed to the surface. An exception to the systematic pattern in particle trajectories is the Yuli belt, which has a Neogene history of deep subduction and corner flow before exhuming in the Eastern Central Range during the collision with the Asian continental margin. The result of these systematic W-E variations in orogenic particle trajectories, in conjunction with a N-S gradient in collisional maturity, leads to systematic patterns in metamorphic grade, cleavage intensity, and burial depth across and along the Taiwan Central Range. Whereas conceptual models of feedbacks between surface and deep-Earth processes in mountain belts often focus on climate forcing, we hypothesize that systematic variations in the mechanical properties of rocks that exhume at the surface are themselves an underappreciated control on the evolution of mountain ranges. Thus, crystallinity, fabric development, and consequent variations in rock strength are both a byproduct of the mechanics of the system as well as a control on the surface processes that govern landscape evolution.
Donald Fisher, Roman Dibiase, Eric Kirby, Julia Carr, Yeh En-Chao
cleavage fan, orogenesis, rock advection
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B2-011
11:30
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11:45
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SUBDUCTION OF TRANSITIONAL CRUST IN MANILA TRENCH CAUSING DEEP PLATE-BENDING NORMAL FAULT EARTHQUAKES
Normal fault earthquakes near the trench outer-rise are caused by the down bending of elastic plate at subduction zones. Extensional stress environment is prevalent at the upper half of the down bending elastic plate. As a result, the focal depths of plate-bending normal fault earthquakes are usually shallower than 30 km depth. A notable exception is the 2006 Pingtung offshore earthquake, whose focal depth is 44 km deep. This earthquake is located at the northern end of the Manila trench, where the Eurasia plate is subducted beneath the Philippian Sea plate. The subducted crust at the Manila trench has been categorized as transitional crust, which is made of rifted continental crust. We tested two scenarios of subduction of either transitional or oceanic crust with numerical dynamic models. We found that the weaker viscosity of transitional crust allows stronger bending of subducted Eurasia plate and dissipates the stress within the crust. The extensional stress environment, required for normal fault earthquakes, is pushed to deeper depth below the Moho when transitional crust is subducted. The stress and deformation patterns in the accretionary prism are remarkably different in these two scenarios.
Eh Tan
continent-ocean transition, Manila trench, subduction zone
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B0-011
11:45
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12:00
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DEFORMATION CHARACTERISTICS IN THE TAIWAN ACTIVE COLLISION ZONE AND THEIR GEODYNAMIC MECHANISMS: INSIGHT FROM FEM SIMULATIONS
The Taiwan Island is the product of convergence and collision between the Eurasian plate and the Philippine Sea plate, where the geological structure is complex, seismicity level is high and deformation strong. In order to quantitatively study characteristics of crustal deformation in Taiwan and to deeply understand their geodynamic mechanisms, we calculated the strain rate field in and around Taiwan by using finite element method (FEM), utilizing GPS data from 1995 to 2005 as boundary constraints in simulation. The results show that the calculated velocities and GPS vectors are in good agreement, and calculated orientations of principal stresses are consistent with in-situ stress measurements and focal mechanism solutions, demonstrating the finite element model established in this paper is reasonable. Meanwhile, the computed results show that contractions are predominant in the central part of Taiwan, while extensions exist in the northeastern and southern parts, respectively. The largest deformation is located on the Coastal Range and the adjacent waters in the eastern Taiwan. Also, the calculated slip rates on the Longitudinal Valley Fault (LVF) are 13.81-23.48 mm/yr, and part of the convergence is absorbed by LVF, thus deformation to the west of LVF decays rapidly westwards and northwestwards. In addition, the calculated results imply that the general framework of present-day deformation in Taiwan results from interactions by many factors such as plate collision between the Eurasian plate and the Philippine Sea Plate, geometry of the plate boundaries, faulting and rifting, opening of the Okinawa Trough and retreat of the Ryukyu Trench.
Shoubiao Zhu, Gangxiao Long
Deformation field, Finite element method, Geodynamic mechanism, GPS observation, Taiwan
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B1-012
12:00
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12:15
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THE TULUNGWAN-CHAOCHOU FAULT COMPLEX: AN ACTIVE, CRUSTAL-SCALE FAULT IN AN ARC-CONTINENT COLLISION
The Tulungwan-Chaochou fault complex in the southern Central Range of Taiwan is considered one of the major fault systems in Taiwan. The surface trace of the complex is delineated by one of the most conspicuous topographic lineaments in Taiwan and the faults separate moderately metamorphosed rocks on the east from unmetamorphosed sediments on the west, suggesting a long history of significant displacements. The slip history of the fault and the geometry of the fault at depth remain poorly defined, however. Field mapping of rock fabrics and brittle structures in the hanging wall suggests a regional-scale antiform that verges west-northwest (Huang and Byrne, 2014). Leveling data and uplifted river terraces along the northern, or Tulungwan, segment of the fault complex suggest that it may be active, although no historical large earthquakes have been related to the structure (Huang and Byrne, 2014). More recently, Chen et al. (2018) proposed that the Tulungwan segment projects east, down dip to a cluster of ambient tremors that form a steep, southeast dipping ellipsoidal structure at depths of 15 to 45 km. The tremors occur in the subducted Eurasia crust and the steep dip of the cluster suggests a crustal-scale ramp. This section of the orogen is also characterized by high heat flow, low Vp and Vs values, high Vp/Vs ratios and relatively fast surface and rock uplift rates. The map trace of the Tulungwan fault and the surface projection of the tremors intersect in the area of Meishan hot springs along the western end of the South Cross-Island Highway where Bertrand et al. (2012) document the strongest, crustal-scale resistivity anomaly in Taiwan. Chen et al. (in review) also use isotopic signatures of helium and neon from hot springs and ground waters in the Meishan area to argue for significant mantel contamination, suggesting that the active fault system is deep-seated. This interpretation is consistent with the deep, non-volcanic tremors identified in the same area. Based on these observations and interpretations, we propose that the Tulungwan segment represents an active, crustal-scale fault with significant, but previously under-appreciated, seismic risk.
Tim Byrne, Kate Hui-Hsuan Chen, Ai-Di Chen, Chung Huang
crustal imbrication, Seismology, tremors
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B1-011
12:15
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12:30
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EARTHQUAKE GEOLOGY OF THE ACTIVE SHANCHIAO FAULT, TAIPEI METROPOLIS, AND IMPLICATIONS ON POST-OROGENIC PROCESSES IN NORTHERNMOST TAIWAN
The Taipei Metropolis, home to several million people, is subject to seismic hazard originated from not only distant faults or sources throughout the Taiwan region, but also active faulting directly underneath. The Shanchiao Fault, an east-dipping normal fault outcropping along the western border of the Taipei Basin, is one of the major noetectonic structures in the extensional post-orogenic regime of northern Taiwan. In order to constrain the key but previously unknown or uncertain properties of the fault, geologic, geomorphic, and geodetic data and modeling joined force in our investigation. The surface trace of the Shanchiao Fault, which is mostly hindered by late-Quaternary alluvial deposits, is better described as a rupture zone at least hundreds of meters wide, with only its western branch faults bearing vague topographic signatures. At the near surface level, incessant vertical slips on the fault since the Last Glacial Maximum are deduced from sediment stacking in the rupture zone, which constitutes growth faulting dictated by both tectonic subsidence and eustatic changes. Up to 3 mm/yr of millennial vertical offset since ~23 ka is derived from growth faulting analysis, and contemporary tectonic subsidence is suggested from study of decadal leveling data. Regional GPS data and structure indicated that the rupture zone structure of the Shanchiao Fault is closely related to the sinistral component of the fault as well as the basement-deposit configuration along the western margin of the Taipei Basin. Crustal geometry of the fault, as tentatively constrained by forward modeling of hanging wall deformation documented by a marker horizon, is listric by re-slipping a major mountain-building thrust, and additional involvement of an old rift structure further deep is plausible. Such constraints and knowledge are crucial in earthquake hazard evaluation and mitigation in the Taipei Metropolis, and in understanding the kinematics of transtensional tectonics during mountain collapse in northern Taiwan.
Chih-Tung Chen
earthquake geology, mountain collapse, Shanchiao Fault, structural inversion, Taipei metropolis
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