Louis S. Teng

Geotectonic evolution of late Cenozoic arc-continent collision in Taiwan

Abstract The active collision between the Luzon arc and the Asian continent in the Taiwan area is investigated in terms of plate kinematics and geological records. Regarding plate kinematics, the tectonic evolution of the collision can be reconstructed by superimposing the paleopositions of Luzon arc on the pre-collisional Asian continental margin. Regarding geological records, the collisional history can be interpreted from the stratigraphy of the Coastal Range and the Western Foothills and from the diastrophism of the Central Range of Taiwan. By incorporating geological information into plate kinematics, it appears that the Luzon arc could have begun overriding the Asian continental margin in the late Middle Miocene (about 12 Ma). In the Late Miocene, the impingement of the arc deformed part of the continental margin and might have caused metamorphism of part of the Central Range, but no distinct effects were produced in the sedimentary record. In Mio-Pliocene times (about 5 Ma), the arc changed its direction of motion from north-northwesterly to west-northwesterly and began to override the continental margin rapidly. The accretionary wedge grew increasingly to emerge above sea level and feed continental detritus to the Luzon forearc basin and to induce foreland subsidence on the continental margin. In the early Late Pliocene (about 3 Ma), the collision drastically uplifted the mountain ranges in northern Taiwan, which shed voluminous orogenic sediments into the forearc and foreland basins. As the collision propogated toward the west and the south, the forearc and foreland basins were progressively accreted to the collisional orogen which eventually grew up to its present configuration.

EXTENSIONAL COLLAPSE OF THE NORTHERN TAIWAN MOUNTAIN BELT

As an active collision zone between the Luzon arc and the China continental margin, the Taiwan mountain belt, particularly its south-central part, is undergoing crustal shortening and rapid uplift. The northern part of the orogen is, however, subjected to crustal stretching and rifting as a result of flipping of subduction from the northwest-facing Luzon arc system to the south-facing Ryukyu arc system. Taiwan and its neighboring southern Ryukyu used to be part of the rifted China continental margin before the collision started in late Miocene time. Engineered by the northwest movement of the Philippine Sea plate in the past 5 m.y., the north-south–trending Luzon arc obliquely overrode the northeast-southwest–trending continental margin and tectonized it into a collision orogen that progressively enlarged and migrated from northeast to southwest. Following the southwest-propagating collision, the north-dipping Philippine Sea plate, which was subducting beneath the Ryukyu arc, also extended westward and caused flipping of subduction in the northern part of the collision orogen. In consequence, the orogen lost the compressive support by the colliding plates and became subjected to lithospheric stretching induced by the trench suction. The collision orogen collapsed as a result of crustal thinning, and its central part foundered into a deep submerged basin. A whole spectrum of active tectonic scenarios, ranging from orogenic buildup by collision to orogenic collapse by rifting and subsidence, can be readily observed in the Taiwan-Ryukyu area and provides a vivid example for the process of orogenic collapse.

Structural features off southwestern Taiwan

Abstract Seismic reflection profiles reveal a complex system of faults, mud diapirs, submarine channels, and sedimentary basins in the offshore area southwest of Taiwan. Structural patterns demonstrate the on-going structural transformation from the passive Chinese continental margin to the Taiwan collision orogen. The boundary between the Chinese continental margin and the Taiwan orogen can be clearly delineated by the deformation front of the contractional structures. West of the deformation front, ENE-WSW trending normal faults prevail in the continental margin, which is characterized by a horst-and-graben structural style. East of the deformation front, west-vergent imbricated folds and thrusts sheets abound in the accretionary wedge. The structural grain of the accretionary wedge trends NNW-SSE in the deep offshore area but bends toward NNE-SSW in the shallow near shore area. Mud diapirs emerge through thick sedimentary layers forming NNE-SSW trending anticlines that extend from near shore areas to onland Taiwan. Mud volcanoes have been observed on 3.5 kHz profiles over several diapirs. Structural styles observed across the frontal portion of the submarine Taiwan accretionary wedge are strongly controlled by the tectonic evolution of the arc-continent collision. The structure and location of the deformation front are affected both by the growth of the accretionary wedge and by the geometry of the Chinese continental margin basement.

Slab breakoff as a mechanism for flipping of subduction polarity in Taiwan

We propose a model that involves breakoff of the east-dipping Eurasian slab beneath the Taiwan orogen to account for the flipping of subduction polarity in northern Taiwan. The breakoff was initiated in southern Ryukyu in the early Pliocene and then propagated southwestward into Taiwan, like an opening zipper. Detachment of the Eurasian slab has created a mantle window for the north-dipping Philippine Sea plate to move in laterally, causing a switch in the subduction polarity. Slab breakoff not only provides a testable model for the lithospheric structure of Taiwan, but also accounts for the high heat flow, rapid uplift, synorogenic extension, deep-focus earthquakes, and north-diminishing crustal contraction in the Taiwan mountain belt.

Cenozoic tectonics of the China continental margin: insights from Taiwan

Abstract The continental margin to the east and south of China comprises an active margin in the East China Sea, a collision mountain belt in Taiwan, and a passive margin in the South China Sea. These three segments were generally regarded as separate tectonic entities and their interrelations have long been the subject of debate. Here we synthesize available information to outline the tectonic and geological background of the China margin, examine the link between Taiwan and the neighbouring China margins, and thereby establish a Cenozoic evolutionary model. The China margin is floored with a pre-Cenozoic continental basement covered with an up to 10-km-thick pile of Cenozoic sedimentary strata. The continental basement has been invariably stretched and moulded into a series of northeast-trending horsts and grabens. Except in the Okinawa Trough of the East China Sea, the Cenozoic sedimentary cover typically exhibits a two-tier tectonostratigraphic structure, with narrow Palaeogene rift basins draped by a blanket-like Neogene-Quaternary sequence. The two-tier structure prevails in the entire inner part of the China margin, including the Taiwan Strait off western Taiwan. In the outer China margin, however, the two-tier structure persists only in the South China Sea, and is in stark contrast with the collisional orogen of Taiwan and the Ryukyu arc of the East China Sea. By untangling the contractional deformation of the northern Taiwan mountain belt, it has been possible to reconstruct a precollisional tectonostratigraphic section with a distinctive two-tier structure shown by a Palaeogene half-graben covered with a Miocene drape sequence. When put together with Palaeogene rift basins of the Taiwan Strait, it becomes clear that the precollisional continental margin of Taiwan resembles that of the South China Sea, characterized by two lines of Palaeogene rift basins. Hence before the collision started in Late Miocene times, Taiwan was part of the passive South China margin that extended northward to the southern Ryukyu area. Ever since the end of the Cretaceous, the China continental margin has been dominated by extensional tectonics, regardless of the presence or absence of subduction zones. In the Early Cenozoic, extensive crustal attenuation resulted in region-wide subsidence and formation of rift basins. Extension in the South China Sea culminated in Late Oligocene times, when part of the outer margin was drifted away by the opening ocean basin. In the East China Sea, the margin remained intact and became separated from the South China Sea margin by a transform fault. From the Miocene onwards, the South China Sea margin has been passively subsiding, sporadically punctuated with basaltic volcanism. In the East China Sea margin, the Okinawa Trough has opened and the Ryukyu volcanic arc thrived. The NE edge of the South China Sea margin was deformed as the Taiwan orogen.

Late Cenozoic sedimentation in California Continental Borderland basins as revealed by seismic facies analysis

The California Continental Borderland began to form during late Miocene time. Only the latest Miocene and younger sediments are ponded in the contemporary basins. Upper Miocene strata are characterized by widespread diatomaceous deposits that conformably drape all of the underlying topographic irregularities. The Pliocene-Quaternary sediments occur as flat-lying ponded clastic sequences that fill the contemporary basins. A seismic facies analysis has defined facies patterns in the Pliocene-Quaternary basin-fill sequences of each basin. In the offshore basins (Patton, Tanner, Santa Cruz, and San Nicolas), fine-grained mass-flow and hemipelagic sediments derived from the local insular sources dominate the basin-fill sequences. Santa Catalina Basin is a transitional basin filled with fine-grained turbidites and hemipelagic sediments derived from both continental and local insular sources. The nearshore basins (Santa Barbara, Santa Monica, and San Pedro Basins and San Diego Trough) are filled by thick sequences of continental sediments that form prominent deep-sea fans. The borderland was dominated by biogenic pelagic sedimentation in the late Miocene and by detrital-sediment-gravity-flow deposition in the Pliocene and Quaternary. Accumulation rates estimated from Pliocene-Quaternary sediment thickness for each basin show that in the offshore basins, as well as in San Diego Trough, rates have remained fairly constant since the Pliocene. In contrast, the nearshore basins exhibit an increasing rate of accumulation versus time that is related to the progradation of continent-derived sediments. Outer- and intermediate-basin accumulation data indicate decreasing contributions with time from offshore highs because of erosion and regional subsidence. Intermediate basins show an increase in mass-wasting deposits (slope facies) with time, which is the result of increasing fine-suspension slope deposition and eventual failure.

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.

Tectonic and environmental control of the clay mineral sedimentation in the late Cenozoic orogen of Taiwan

AbstractMineralogical investigations performed on the Pliocene-Pleistocene mudstones deposited West and East of the Central Range of Taiwan demonstrate the dominant control of tectonics on the clay sedimentation. The oblique collision between the Luzon Arc and the Eurasian margin determined a rapid erosion of the uplifted bedrocks, as well as an increasing compressional deformation migrating from the North to the South. These geodynamic changes are expressed in the sediments by a progressive mineral clay simplification and an upward increase of illite crystallinity. Asian sources are marked by kaolinite and Pacific sources by smectite. The expression of burial diagenesis, climate and depositional conditions is largely obliterated by the extensive tectonic activity, which results in exceptionally rapid uplift, erosion and sedimentation.