T33E-01
Deep Seismic Imaging of Crustal Structures in the Northern South China Sea and the Western Philippine Sea Between TAICRUST and TAIGER
Several deep seismic surveys, by using portable ocean-bottom seismometers (MicrOBS) and small air-gun shots, in the northern South China Sea (SCS) and the western Philippine Sea were conducted after 1995 TAICRUST experiment. In 2007, 30 MicrOBS data along three NW-SE lines were acquired in the continental slope of the northern SCS. The results show that west of 117 E along the continental slope, transitional crust consisted of mainly the extended and thinned continental crust was imaged. However, east of 117 E, both extended continent northwest of the continent-oceanic boundary (COB) and thick (13 km) oceanic crust between the COB and the Manila Trench were found. We observed that thickness of the igneous body (7-7.5 km/s) in the thick oceanic crust is from about 1 km at COB southeastward increasing to 7 km below the Manila Trench. Therefore, the transitional crust along the continental slope of the northern SCS may have not resulted only from volcanism or magmatism of post seafloor spreading in the South China Sea but also due to subduction of Eurasian Plate beneath the Philippine Sea Plate. Tectonic setting of the transitional crust with subduction and volcanism appeared in the northern SCS is similar to those in the Central Asian orogenic belt and the Moroccan Atlantic Margin. In 2006, 24 MicrOBS stations along one E-W and two S-N lines were retrieved in the western Philippine Sea. OBS imaging indicated that the crustal thickness of the Huatung Basin decreases northward from 10-11 km to 7 km whereas the crustal thickness of the western Philippine Basin increases northward from 7 km to 9 km. The thick oceanic crust in both regions is associated with a high P-wave velocity of 6.6-7.5 km/s and a low Poisson's ratio of 0.28 imaged in the middle and lower crust. However, the difference of the crustal thickness resulted from the formation of Gagua Ridge between the Huatung Basin and the western Philippine Basin. We found that three old fracture zones, one below the summit of the Gagua Ridge and the others beneath two edges of the Gagua Ridge, are observed from a large lateral variation of P-wave velocity of about 5.5-6.4 km/s and a low Poisson's ratio of 0.29 in the upper crust. Therefore, Gagua Ridge had been generated by compression, shearing and uplifting (or transpression) due to the eastward convergence of the Eurasia Plate and the northwestward convergence of the Philippine Sea Plate. 2009 TAIGER experiment, by providing strong air-guns and more OBS deployment, will enable us to enhance the imaging of the lower crustal and the upper mantle covering from the northern SCS, Manila subduction zone and the western Philippine Sea.
T33E-02
Spatial Variation of the Tectonic Stress Field along the Ryukyu-Taiwan-Luzon Convergent Margin
We apply a recently developed damped stress inversion method to a large dataset consisting of high-quality focal mechanism solutions from global and regional moment tensor catalogs to invert the detailed crustal stress field along the western convergent margin of the Philippine Sea plate (PSP), namely the Ryukyu- Taiwan-Luzon system. The transition from oblique subduction to regional collision is best characterized by significant variations in the direction of the maximum compressional axis (£m1). In the Ryukyu subduction zone, £m1 is generally consistent with the relative plate motion between PSP and Eurasia plate, except the southernmost segment near the Gagua Ridge where £m1 turns to almost N-S. The azimuth of extensional axis (£m3) is mainly perpendicular to the local strike of the Ryukyu Trench in the outer-rise region, and shows a NNW-SSE direction in the Okinawa Trough. This observation is consistent with the local extensional processes reported previously. A clear stress boundary trending NW-SE is identified in northeast Taiwan separating the stress regime associated with the Ryukyu subduction system from the collision system in Taiwan. For most part of Taiwan, £m1 agrees with the plate convergent direction, rather than shows a fan- shape pattern as earlier suggested. It is interesting to note that £m1 exhibits prominent clockwise and counterclockwise rotations to the north of the Lukang Magnetization High (LMH) and to the south of the Paikang High (PH), respectively. Such patterns suggest that not only the PH but also the LMH may play important roles in dominating the tectonic stress field of Taiwan. Counterclockwise rotation of the £m1 axis appears to extend to the south along the Luzon Arc until ~20¢XN. No significant deviation from the predicted plate convergent direction can be found farther south, marking the 20¢XN line as the incipient point of the stress regime associated with the Luzon arc-Taiwan collision.
T33E-03
Coulomb Stress Change due to Seismic Cycle in Western Taiwan Over the Last 270 Years
Vigorous processes of collision and mountain building take place in Taiwan, where the 90 mm/yr of convergence between the Eurasian and the Philippine Sea plates is distributed on a wide deep detachment and active faults. Consequently, Taiwan shows a very high seismic activity, including the well documented 1999 Chi-Chi earthquake on the Chelungpu fault. Here to improve seismic hazard assessment we calculate the Coulomb stress variation on active faults of Taiwan, implied by both coseismic and interseismic deformation since 1736. We pay a special attention to active faults geometry as well as to coseismic slip distribution infer from tectonic and geophysics observations. Our modeling gives results in agreement with current slip distribution infer from geodetic and seismic observations. It suggests that both earthquakes and interseismic loading before the Chi-Chi event increase the Coulomb stress in the upper north-western part of the Chelungpu fault, a region which experienced the highest coseismic slips. More importantly our results reveal an increase of the Coulomb stress in the southern part of the Changhua thrust fault, located west of the Chelungpu fault in an area of high population density.
T33E-04
Deep structure of Taiwan Based on Integrating Crustal Seismic Tomography, Seismicity and Crustal-Scale Balanced Cross Sections
The active Taiwan mountain belt is a product of on-going oblique arc-continent collision between the Eurasian continental margin and the Luzon Island Arc of the Philippine Sea Plate. The obliquity of the collision allows us to view its progression starting from just south of Taiwan and ending with the flipping of subduction polarity in northernmost Taiwan. We seek to obtain a more fine-grained data-rich understanding of the deep structure and progression of this collision, making use of constraints extending from the surface to the upper mantle using new high-resolution seismic tomography (Wu et al. 2007), a comprehensive catalog of relocated earthquakes (Wu et al. 2008) and crustal-scale balanced cross sections based on surface geology and upper-crustal reflection seismic profiles. These datasets offer the opportunity to greatly extend upper-crustal geological observations at depths up to the thickness of the entire lithosphere. In southern Taiwan the tomography consistently shows the Eurasian lower crust and upper mantle bending and subducting eastward at a moderate inclination. However, in central Taiwan the Eurasian slab becomes progressively steeper and eventually overturned. Furthermore the impinging arc and Philippine-sea plate progressively changes shape. We illustrate these changes with a set of serial tomographic/structural sections and with a MOHO contour map for both the Eurasian and Philippine-sea lithosphere. The upper-crustal deformation system of the central Mountains and Coastal Range is substantially but not fully detached from the lower crust and mantle deformation system, with the western fold-and-thrust belt, upper crust of the Central Mountains and the Coastal Range all showing strong evidence being largely detached from the lower crust. In contrast, a possible exception is a prominent east-facing "backfold" in the surface structure of the eastern part of the Central Ranges, which appears to be crustal scale because it overlies the deep bending and overturning of the Eurasian lower crust and MOHO described above. Along strike, the extent of this backfold corresponds to those parts of the mountain belt that underwent collision with the Philippine Sea Plate lithosphere thickened by the Luzon Island Arc. The backfold amplitude quickly diminishes south of Taitung, where the Luzon Island Arc is separated from Taiwan by the Southern Longitudinal trough forearc basin. Across strike, the backfold is characterized by a crenulation cleavage that overprints an earlier slaty cleavage in the backbone slates (Fisher et al. 2007) and associated west-facing structures. Apparently similar crustal-scale back folding is a well-known feature of the southern flank of the Alps as shown by both upper-crustal structure and deep seismic transects.
T33E-05
Diverse Seismic Imaging Created by the Seismic Explosion Experiment of the TAIGER Project
The TAIGER (TAiwan Integrated GEodynamics Research) project which examines the Taiwan orogeny includes five experiments: natural earthquake recording, man-made explosion recording, Magnetotelluic imaging, marine MCS and sea-land shooting, and deformation evolution modeling. During Feb-Mar 2008, the explosion experiment was carried out. Ten sources with 500~3000kg dynamite were detonated along two transects across northern and southern Taiwan. Over 600 PASSCAL Texans and 40 R-130 instruments record the signals over 100~300 km range. Additional arrays with 100 seismometers were deployed to collect north-south line and fan shoot data for 3D imaging. Furthermore, there are 9 ocean bottom seismometers (OBS) in the Taiwan Strait and two lines with 20 seismometers deployed on the mainland China side. A large volume of qualified data has been created. Except explosion signals, numerous local and regional earthquakes were also recorded even by the Texan instruments. The rich earthquake-explosion dataset now exists at the Institute of Earth Sciences, Academia Sinica operated by the Taiwan Earthquake Center (TEC). Preliminary examination of the data reveal crustal Pg, PmP, Pn and intermediate crustal reflection phases within the transect profiles and in the 3D cross-arrays. These data provide direct seismic imaging of the continental Moho under Taiwan and the sharp Moho root configuration associated with mountain building. Seismic tomography and raytrace methods reveal velocity structure consistent with convergence and vertical exhumation of the Central Ranges.
T33E-06
Joint Local/Teleseismic Tomographic Inversion in Taiwan Using TAIGER and Other Data
Taiwan, one of the most active orogenic belts, is at the intersection of two subduction zones. In southern Taiwan, the South China Sea Slab (SCSS), part of Eurasian Plate (EP), subducts beneath the Luzon arc along the Manila trench. In northern Taiwan, the Philippine Sea Plate (PSP) subducts beneath the Ryukyu arc along the Ryukyu trench. The thin skinned model and lithospheric deformation model have been proposed to explain the formation of orogeny. To distinguish between these two geodynamically possible processes, imaging of the deep structures below Taiwan is necessary. In this study, explosion data, local/regional earthquakes and teleseisms are used to invert the velocity structures of Taiwan from surface to about 150 km. Temporary passive broadband (on land and at the ocean bottom), active sources array datasets of the TAIGER (TAiwan Integrated GEodynamics Research) project and permanent array datasets of the BATS (Broadband Array in Taiwan for Seismology) and CWB (Central Weather Bureau) are used in this study. FMTOMO (fast marching tomography) of Rawlinson et al. (2006) is employed to invert the 3D P-wavespeed beneath Taiwan. The derived velocity perturbations dVp (dVp= Vfinal-Vinital) are clearly related to geology and tectonics. At shallow depth (< 10km), dVp >0 under the Central Range (Pre-Tertiary metamorphic rocks) and dVp < 0 under the Foothills (Pliocene sedimentary). Below a depth about 20 km, the placement of the high and low anomalies is reversed, i.e., dVp>0 under the Foothills and dVp<0 under the Central Range; the low velocity core of the Central Ranges extend down to about 50 km, forming the mountain root. A steeply dipping high velocity zone lies under the thickening 'mountain root' in central Taiwan. In southern Taiwan, the high velocity zone dips eastward coinciding with the Benioff Zone. The geometry of the high velocity zones in the upper mantle are key to understanding the Taiwan orogeny.
T33E-07
Structure of Hengchun Peninsula and Adjacent Offshore Areas: Implications for the Development of Taiwan
The Hengchun Peninsula of southernmost Taiwan and the Hengchun ridge offshore to the south are the morphological continuation of Taiwan's Central Range. This observation and the geometric relationship of north to south structural evolution of Taiwan elucidated by many researchers, suggests that the geologic structure of Hengchun Peninsula could provide key insight into the early development of the Central Range. A previous geophysical transect across the peninsula and extending west to the Manila trench and east to the Luzon arc, showed a large accretionary prism, thinning to the west, with a core of relatively high-velocity (5 -6+ km/s) material directly below the peninsula. While this profile provided a useful context for understanding southern Taiwan in general, the velocity structure beneath Hengchun was poorly resolved. Thus, it did not clearly distinguish whether continental margin basement rocks were involved, or whether the core was composed of metamorphosed sedimentary material that had reached maximum depths of 10-20 km in a path through the accretionary prism. To improve the resolution of this key area we model an expanded seismic data set using data from seismographs on the peninsula to supplement the marine data used in the previous analysis. In addition, we compare calculated and observed gravity along this transect and gain insight from geodynamic models. The new result shows a focused zone of higher velocity material (5-6+ km/s) beneath the east edge of the peninsula and 4-5 km/s elsewhere in the core zone. This predominantly lower velocity implies a sedimentary origin for this zone rather than transitional crust of the underthrust Eurasia plate or continental margin material extruded from full collision just to the north. The higher velocity portion of the core region suggests near-vertical particle paths beneath the east edge of the peninsula consistent with exhumation predicted by geodynamic models and nearby apatite fission-track analysis. This observation suggests that processes here are similar to those in the Central Range where the highest grade exhumed rocks are found near the eastern edge of the range. However, the probable lack of continental margin rocks beneath Hengchun Peninsula and the presence of the pre-collision shelf edge 20-50 km north to northwest suggest that a significant amount of additional northwest translation will have to occur before the southern morphological extension becomes geologically similar to the Central Range, cored by Paleozoic metamorphic rocks.
T33E-08
Southern Taiwan – an Evolving "Coastal Range"?
As a part of the TAIGER research, existing tectonic concepts are continuously being reviewed in light of new data and for experimental design. One of the critical areas for understanding the tectonics of Taiwan is southern Taiwan. Seismicity and recent tomographic imaging confirm that the tectonics of Taiwan is controlled by the subduction and collision of two plates: the Philippine Sea plate (PSP) and the Eurasian plate (EUR). In northern and central Taiwan the PSP is in collision with EUR, and at the same time subducts northward under northern Taiwan [Wu et al., 2008]. Before the PSP subducts to sufficient depth, the collision of PSP and EUR produced the Foothills and the Central Range on the EUR and the Coastal Range on the PSP side. For southern Taiwan, with 22.7°N as a rough demarcation, the tectonic interpretation is at variance and still in debate. To the east of southern Taiwan the inactive andesitic volcanic islands of Lutao and Lanhsu mark the top of the Luzon arc, separated from Taiwan by a somewhat deformed fore-arc basin [McIntosh et al., 2005]. To its west the Manila Trench is the western limit of a series of trend-parallel small thrusts on the ocean floor [Lunberg et al., 1997]. As the Trench approaches the continental shelf from the south it gradually loses its bathymetric signature. Southern Taiwan itself is commonly viewed as a part of the accretionary prism, and yet is also considered a continuation of the Central Range, produced by the collision of the Luzon arc and continental shelf. Suppe [1981] and many others had long recognized the central Taiwan orogeny, as a result of the collision of the Luzon Arc with the EUR continental shelf and Central Range, is built from rocks of the continental shelf. Inspection of a map of bathymetry around Taiwan shows that the continental shelf turns noticeably westward offshore of southwestern Taiwan, and that southern Taiwan is situated off the continental shelf. The presence of the Benioff zone under southern Taiwan and the normal faulting earthquakes (M7) west of Hengchun in 2006 indicate that southern Taiwan is a part of the PSP that is moving over the subducting EUR. It appears that southern Taiwan has not yet fully engaged in collision with EUR. This interpretation implies that the high topography of southern Taiwan is not created in the same manner as the Central Range. But how was it created? Where is the boundary between the EUR and PSP on land? If southern Taiwan does move westward as a part of PSP then when it collides with the continental shelf it will become a part of a new coastal range. A reexamination is timely as new sea-land, local and teleseismic tomography, more detailed seismicity, and dense GPS data are becoming available, and when there are chances to enhance experiments early next year for testing ideas. References Lunberg et al., 1997. Tectonophysics 274, 5-23. McIntosh et al., 2005. Tectonophysics, 401,23-54. Suppe, J., 1981. Geol. Soc. China, Mem. 4, 67– 89. Wu et al., 2008. Manuscript submitted to JGR.