Taiwan is the result of the most active arc-continent collision in the world (uplift of 3cm/yr and convergence of up to 8 cm/yr). The Eurasian plate and the South China Sea have been moving to the SE wrt the Philippine Sea plate, with oceanic lithosphere of the southeast China continental margin subducting beneath the Luzon arc, which sits atop the Philippine Sea plate. Around 4 Ma ago, the margin of Eurasia entered the subduction zone, resulting in collisional orogeny that formed Taiwan. Taiwan consists of parallel belts representing 7 different tectonostratigraphic provinces. The collision is occurring at an oblique angle, with the northern margin colliding first, so that mountains are forming in the North and propagating south. This oblique collision allows spatial changes to be viewed as temporal evolution.
Arc accretion is arguably the most important mode of continent formation throughout the post-Archean Earth (perhaps even before). Taiwan offers the possibility of studying these processes in the world's most active arc collision zone with a temporal view of how such collisions progress, due to the oblique nature of the collision as mentioned above. The PIs put forward two end member hypotheses to be tested concerning what is happening to continental lithosphere during the collision: 1) It is subducted, with crust-mantle detachment causing crust to accrete while deeper lithosphere is subducted; 2) It doesn't subduct, but jams up in the subduction zone, leading to continuous deformation resulting in thickening of the lithosphere.
The two end-member scenarios for what is happening to continental lithosphere during the collision provide special opportunities to investigate some of the most fundamental questions of mountain building:
- Does continental subduction play a controlling role in arc-continent collision? Is the Eurasian continental lithosphere subducting beneath Taiwan? - One of the fundamental issues in orogeny is mass balance. How do factors such as erosion, crustal thickening, mantle flux, etc. interact quantitatively? - How did the orogen evolve through time? - How does surface kinematics relate to deep structure? - How does anisotropy vary in space, laterally and vertically? Do the S-splitting directions change with depth? Is the deformation of the orogen vertically coherent from surface to upper mantle?
To answer these questions the PIs will carry out an integrated geophysical imaging, earthquake recording and geodynamic research program to study the Taiwan orogeny. By combining detailed 2-D studies along transects and 3-D images for the whole region, the orogen and its evolution can be characterized. The data acquisition program includes broadband regional seismic and teleseismic recording, onshore-offshore and land refraction-reflection seismic transects, magnetotelluric sounding, petrophysics and gravity modeling. The geometry of the plate interactions, the mode of crustal deformation, and the material properties will provide a new quantitative basis for geodynamic modeling. The various datasets will be integrated into a geodynamic model which can be used to test conceptual models for mountain building.
The project will involve collaboration from six US institutions as well as collaboration and support from Taiwanese and Japanese groups. There is also the distinct possibility of the participation of scientists from mainland China. The educational impact of this project will be large, with more than 25 students from the participating institutions, US and foreign. In addition, the "Texas Teacher in the Field" program would involve one high school teacher in recording earthquakes offshore Taiwan, developing related teaching modules for Texas high school teachers.
We conducted a detailed subsurface seismic and electromagnetic imaging on land and marine profiling and seismic recording at using the prime US research vessel R/V Langseth. Our purpose is to scientifically test current hypothesis of how the Taiwan mountain range was built and contribute to the studies of orogeny (mountain building) in general. In the following we shall describe briefly the reasons for conducting this research, what was done and what we have discovered? The location of Taiwan, the plate boundaries and the main geological characteristics are shown in Figure 1. Taiwan is located in the Pacific region, but its tectonics is actually a direct result of the interactions of the Philippine Sea (rather than the Pacific) and the Eurasian plates. The details and extensive references can be found in a paper by the PI and colleagues. For many years our understanding of mountain building (or orogeny) was gained chiefly through observations at the surface, i.e., from geology, geochemistry, geomorphology etc. Geophysical methods, especially seismology, that aim to image the seismic velocity structures at depth had successfully mapped the major internal global structures of the Earth, but the spatial resolution of the methods, due to the sparse recording station distribution, had been insufficient for detailed interpretation of orogeny. In the last two decades however the development of powerful and compact instruments flourished. Significant numbers of seismographs could be deployed to areas of interest. The spatial resolution of images was improved so much that by combining the age and rock data at the surface and the seismic velocities and electrical resistivity structures at depth we have added important new clues. Under the NSF-supported research we (1) established seismic networks on land for (a) enhanced earthquake recording for (b) monitoring our specially designed explosions and for (c) recording air gun source on land from our research ship offshore when conducting marine surveys around the island; (2) obtained electromagnetic data for conductivity on land and (3) conducted marine seismics to determine crustal deformation. We also deployed seismic stations on the ocean floor to record earthquakes and air gun shots over some of them. The TAIGER (TAiwan Integrated GEodynamics Research) experiments (Figure 2) were conducted between 2006 and 2009 in cooperation with our Taiwan colleagues. With additional funding from the National Science Council and other organizations in Taiwan to our Taiwan counterparts we were able to exceed our planned data gathering. The data form a good basis for testing geological hypotheses, a necessary step for us to advance beyond our current level of understanding. Modern tools of seismic tomography, precise location of earthquakes, detecting fault motion from buried faults as well as measuring electrical conductivity help us to detect the geometry of structures, how faults fault move, what are the temperature and pressure and whether there is a lot of water beneath the mountains. TAIGER results allow us to clearly define the plates, the plate boundaries and the plate motions that lead to collision and orogeny. The following summary describes the main conclusions, but we emphasize that scientific investigation is ongoing and we are looking forward to convert out raw data into "observations" and compare them to our expectations from our hypotheses and elucidate our understanding further. One of the important contributions of the TAIGER networks is to add ocean bottom seismic data offshore of eastern Taiwan for mapping the offshore area. Also, by widening the lateral coverage it allows seismic rays from distant earthquakes to sample better the deeper region under Taiwan, to a depth of about 200 km (Figure 3). Below about 80 km in the upper mantle, the steeply east-dipping high velocity anomalies from southern to central Taiwan are clear, but only the extreme southern part is associated with seismicity. (It is generally known that the subduction zone is relatively cool when it descends from the cool exterior into the hotter upper mantle and therefore is seismically active and has a relatively high velocity.) Under central Taiwan the seismicity disappears. Central Taiwan is widely known to be a result of collision of the PSP and EUP. Here the two plates are "sutured" together across the Longitudinal Valley. On the west side we have crystalline rocks that is a part of the EUP and on the east side the volcanic and sedimentary materials of the PSP. In addition to the thickening of crust and under the Central Range, the low Vp/Vs ratio of around 1.5 at a mid-crustal depth of 25 km in the Central Range (Figure 4) indicates that current temperatures could exceed 700OC, consistent with the low seismicity there. The remarkable thickening of the crust under the Central Range, its rapid uplift without significant seismicity, its deep exhumation and its thermal state contribute to make it the core of orogenic activities on Taiwan Island. TAIGER fufills its original plan well.
