Chia-Yu Lu

Oblique convergence, indentation and rotation tectonics in the Taiwan Mountain Belt: Insights from experimental modelling

Abstract Experimental modelling allows one to describe the development and kinematics of structures in mountain belts formed during oblique convergence. In the collision geometry of the Taiwan mountain belt, the Chinese continental margin is oriented about N60°E, whereas the N16°E Philippine Sea plate backstop is moving toward the Eurasian plate in the N55°W direction. Sand☐ experiments have been conducted to simulate the Cenozoic arc-continent collision in Taiwan with a foreland-dipping backstop. The kinematics of deformation comprises a combination of compression and rotation which locally result in a partitioning between thrusting and wrenching. The experimental modelling shows that the faults or shear zones are rotated around the indentation point of the backstop by transcurrent and bookshelf faulting. An overall clockwise rotation (or right-lateral movements) occurs to the north of the indentation point and anticlockwise rotation occurs to the south of the indentation point. Oblique indentation and rotation structures can be demonstrated by (1) the crescent shape of the thrust wedge, (2) pull-apart structures induced by strike-slip tectonics, and (3) bookshelf-type strike-slip faulting resulting from the block rotation around the indentation point.

Kinematic analyses of the Hsüehshan Range, Taiwan: A large-scale pop-up structure

The Hsuehshan Range, exposed in the northern and central Taiwan slate belt, is a fault-bounded structural high cored by biotite grade slates and metasandstones. Syntectonic overgrowths in pyrite pressure shadows indicate that much of the eastern Hsuehshan Range experienced coaxial strain histories and that finite strain magnitudes generally increase toward the hinterland. Near the eastern boundary of the Hsuehshan Range, however, pressure shadows record noncoaxial strain histories consistent with a top-to-the-east sense of shear along a steep NW dipping shear zone. This noncoaxiality is attributed to SE directed backthrusting on the Lishan fault, which separates the higher-grade, Eo-Oligocene rocks of the Hsuehshan Range from the lower-grade Miocene rocks of the Backbone Range. Because it is bounded to the east by the SE-vergent Lishan fault and to the west by a series of NW-vergent thrusts (e.g., the Chuchih fault), the Hsuehshan Range is envisaged as a pop-up structure. Strain magnitudes measured from pressure shadows in the coaxial part of the range are consistently lower than those predicted by steady state wedge models that assume all deformation is accommodated by penetrative strain. Departure from the model predictions is attributed primarily to strain localization along discrete fault surfaces (e.g., the Lishan fault). The Hsuehshan Range tapers in width to the south; thus the pop-up may be buried or die out to the south where the collision is younger.

The Arc–Continent Collision in Taiwan

We present a new compilation of magnetic, geologic, GPS and seismic data and propose that the geometry and kinematics of the Taiwan arc–continent collision are dominated by the partial subduction of a continental margin promontory and associated fracture zone. A prominent magnetic high in the pre-collision zone southwest of Taiwan serves as proxy for the edge of the continental crust of normal thickness (i.e., ~30 km). The high ends abruptly in central Taiwan, suggesting truncation by a NW-striking fault zone interpreted as a rift-related transfer zone. The NW-striking fault zone correlates with a steeply dipping, crustal-scale cluster of earthquakes recognized as the Sanyi-Puli seismic zone, indicating reactivation of the transfer zone. The truncated anomaly and transfer zone define a triangular-shaped continental margin promontory partially subducted beneath Taiwan. Island-wide GPS data show: (1) divergent flow around the promontory; (2) significant shortening between the promontory and the arc in the central part of Taiwan; and (3) significant lateral extrusion of the orogen south of the promontory–arc collision. Collision of the relatively rigid promontory resulted in a tripartite division of the fold and thrust belt as it developed from about 2 Ma to present and the development of a recess or syntaxis in the central part of the Central Range. The syntaxis correlates with the highest topography, thickest crust and highest values of P-wave attenuation, suggesting that it may be an area of high rates of rock uplift and exhumation, consistent with other orogenic syntaxes recognized around the world. This interpretation is also consistent with geomorphic parameters from the central part of the Central Range that indicate high rates of uplift and with the absence of seismicity in this area. Although detailed thermochronologic and kinematic data are limited in central Taiwan, the available data suggest a recent increase in rates of exhumation consistent with the interpretation that the promontory collided relatively recently (~2 Ma).

Contractional, transcurrent, rotational and extensional tectonics: examples from Northern Taiwan

Abstract Contraction, transcurrent faulting, block rotation and even extension are four essential tectonic mechanisms involved in the progressive deformation of arcuate collision belts. The neotectonic evolution of the Taiwan mountain belt is mainly controlled by the oblique convergence between the Eurasian plate and the Philippine Sea plate as well as the corner shape of the plate boundary. Based on field observations and tectonic analysis, and taking geophysical data and experimental modelling into account, we interpret the curved belt of northern Taiwan in terms of contractional deformation (with compression, thrust-sheet stacking, folding and transcurrent faulting) combined with increasing block rotation, bookshelf-type strike-slip faulting and extension. As a consequence, the formation of the extensional Taipei Basin, the division of conjugate strike-slip faulted domains and the variable nature and distribution of paleostresses should not be interpreted in terms of distinct Plio-Quaternary episodes but should reflect a single, albeit complicated, regional pattern of deformation. Our study demonstrates that in Taiwan, contractional, extensional and transcurrent tectonics as well as rotations combine together and interact within a single complex framework. The crescent-shaped mountain belt develops in response to oblique indentation by an asymmetric wedge indenter. The distribution, nature and relative importance of these deformation modes are a function of the shape of the indenter and the average direction of convergence.

Paleostress analysis as a key to margin extension: The Penghu Islands, South China Sea

Abstract The Penghu Islands belong to the Eurasian passive margin, west of the collision zone of Taiwan. From south to north, the South China Sea basins become narrower and disappear in the Taiwan Strait area. The presence of the Penghu Islands allow description of the tectonic evolution of the margin through the reconstruction of tectonic paleostress orientation based on analysis of fault slip data collected in the field. Brittle structures have been studied on four islands in Neogene basalt flows with thin interbedded sediments. Sets of minor striated faults, tension gashes, dikes and joints enabled us to reconstruct the orientation of late Cenozoic stresses. Two main extensional tectonic events were thus identified: a N-S extension dominated during the Middle-Late Miocene (especially in the northern Penghu Islands), and a more recent NW-SE extension prevailed during the Late Miocene (especially in the southernmost island). The latter extension induced complicated patterns of perpendicular normal fault systems and strike-slip conjugate and transfer fault systems, through permutations between principal stress axes ( σ 1 / σ 2 and σ 2 / σ 3 modes). The ages of these tectonic events are ascertained by the existence of syndepositional normal faults, dike injections and superposed brittle structures. A third, less important compressional tectonic event, is correlated with the Plio-Quaternary collision in adjacent Taiwan (ENE-WSW compression). The comparison between the results of marine studies in the South China Sea basins and our paleostress reconstruction in the Penghu Islands suggests that repeated changes between N-S and NW-SE directions of extension occurred in the past. The counterclockwise change from N-S to NW-SE recorded in the Penghu Islands took place between about 12 and 8 Ma ago, indicating that significant extensional phenomena along the South China margin may postdate the seafloor spreading activity in the central ridge.

Le séisme de Chichi (1999) et sa place dans l'orogène de Taiwan

Abstract The destructive Chichi earthquake occurred in Taiwan on 21 September 1999. This earthquake had exceptional consequences in terms of both the seismic hazard concerns that it raised in the island and the new seismotectonic and geodynamic concepts that arose concerning the Taiwan collision. This earthquake is quite typical of the SE–NW plate convergence. It reactivated a thrust ramp at the front of the belt. The rupture trace shows typical tectonic patterns that control the distribution of the damage associated with fault slip. The importance of geological studies for earthquake mitigation purposes is thus highlighted. Based on two examples (Chelungpu fault for locking and earthquakes, Chihshang fault for creep), the need for a better definition of the seismic cycle in Taiwan is highlighted. The Chichi earthquake suggests that the average shortening velocities are of the same order at the belt front and in the suture zone in the back of the belt. The concept of double convergent boundary is consequently introduced in the lithospheric model of Taiwan.

Evidence for submarine weathering from metamorphosed weathering profiles on basaltic rocks, Tananao Metamorphic Complex, Taiwan☆

The metamorphosed paleo-weathering horizon(s) developed on basaltic rocks in the Tananao Metamorphic Complex forms distinct AlFe-rich rocks and occurs as two parallel belts, demonstrating its potential as a stratigraphic correlation unit in this structurally complicated metamorphic terrain. These AlFe-rich rocks have been metamorphosed under amphibolite-/greenschist-facies conditions, leading to the formation of garnet-chloritoid rocks/chloritoid-rich rocks. The chemical changes along three profiles, from metabasite to garnet-chloritoid rock, show prominent K, Rb, Cs and Ba enrichment, yet, also, a large depletion of other elements if it is assumed that Ti, Zr, Nb, Ta and Hf remained relatively immobile. The REEs, however, behaved non-coherently. These chemical characteristics are attributed to pre-metamorphic processes and are shown to be similar to those observed with submarine weathering of basaltic rocks. In contrast, the chloritoid-rich rocks reveal distinct K, Rb, Cs and Ba depletion. It is suggested that this resulted from subaerial weathering processes. K enrichment is not an uncommon feature in Paleozoic/Precambrian paleosols. The present case study demonstrates that submarine weathering should be considered as a probable cause.

Elucidating the geometry of the active Shanchiao Fault in the Taipei metropolis, northern Taiwan, and the reactivation relationship with preexisting orogen structures

The Shanchiao Fault is an active normal fault with documented paleoearthquakes in the Taipei metropolis, Taiwan. While posing direct seismic threat on the multimillion population, its crustal-scale fault plane configuration has not been constrained. This study presents the first attempt to resolve the fault plane dip changes of the Shanchiao Fault within the upper crust by forward modeling late Quaternary deformation. Tectonic subsidence over the last ~23 ka is estimated from vertical displacements of a rapidly formed alluvial fan horizon deformed into a dramatic rollover monocline. A 2-D profile across the Shanchiao Fault is chosen for elastic half-space dislocation modeling, and the results suggest that the fault is listric in the shallow crust with an abrupt change from subvertical ramp (85°–75°) to near-horizontal flat (10°–15°) at 3–4 km depth, consistent with an origin from the inversion of an orogen-related thrust detachment. Given the presence of rift-related fabrics in the underthrust Chinese Continental Margin basement beneath the Taiwanese orogenic wedge, listric ramp-flat-ramp models with a second deeper bend to 60° dip are also tested. Reasonable fits with the geological observations are produced when the lower ramp is located at greater than 8 km depth, which correlates with the hypocentral location of a moderate earthquake in 2004. Joint reactivation of preexisting thrust and rift faults by the Shanchiao Fault is therefore plausible with implications for seismic hazard in the Taipei area.

Energy control by linking individual patterns to self-repeating diffractive optical elements

In general, as diffractive optical elements formed by use of self-repeating patterns possess beneficial characteristics such as scratch resistance, low design effort, ease of fabrication, and natural formation of large panels, an efficient design methodology that was developed with a modified preserving-the-best strategy of genetic algorithms is presented. Both genetic algorithms and simulated annealing are examined by the Markov-chain stochastic process to create the insight needed to use these two heuristic algorithms efficiently. It was found that adding the preserving-the-best strategy to traditional genetic algorithms guarantees the possibility of locating the global optimum. Combining this sufficient and necessary condition for locating a global optimum for genetic algorithms with the built-in chromosome crossover searching mechanism and its neighborhood identification makes this newly developed genetic algorithm an effective method for designing diffractive optical elements. In our study, a prototype was fabricated based on our case study with the modified genetic algorithm. The performance of this prototype was measured and analyzed. Experimental results are shown to agree well with theoretical predictions.

Dykes and Pipes of Remobilized Cornieules within the Cornieule Evaporitic-Complex of the Mont Jovet Area (Northwestern French Alps)

Dykes and pipes of remobilized cornieules outcrop widely within the evaporitic complex of the thrust sole of the Mont Jovet klippe, (North- Western French Alps). The morphology, nature, structures and pattern of these rocks are described. The bodies are not linked to erosive processes; they represent inherited morphologies within the gypsum exhumed by erosion. These upstanding pseudo-cornieules very likely originated from fault-related remobilization of primary cornieules from the tectonic contact. A genetic model is proposed to explain the genesis of these pseudocornieules, to highlight the possible relationships between these pseudo-cornieules and the other documented types of cornieules.