Receiver Function Imaging of Deep Penetration of the Eurasian Slab in the Mantle Transition Zone beneath Taiwan
Date Issued
2010
Date
2010
Author(s)
Liu, Yun-Shan
Abstract
Taiwan is located in the convergent plate boundary zone where the Philippine Sea plate has obliquely collided on the Asian continental margin. Two subduction zone systems of opposite polarity are juxtaposed under the central Taiwan: to the northeast the Philippine Sea plate (PSP) is obliquely underthrusting beneath the Eurasian plate (EP), and to the south the EP is subducting eastward beneath the PSP. To the further south of 22oN, it transforms to an ocean-ocean subduction zone where the South China Sea (SCS) oceanic lithosphere, part of the EP, is subducting eastward along the Manila trench beneath the PSP. One of the key questions closely related to the dynamic evolution of the Taiwan subduction-collision zone is whether the subduction of the aseismic Eurasian lithosphere takes place beneath the central Taiwan. If it does, to what depth has the subducted slab penetrated? Numerous tectonic models have been proposed to either support or go against the existence of the Eurasian lithosphere beneath central Taiwan. Because of the lack of deep earthquakes and high-resolution tomographic images of deep structure beneath Taiwan, the debate still remains unresolved.
It has been known that a sequence of phase transformations of olivine, the major mineral constituent in the upper mantle, primarily cause seismic velocity and density jumps at depths near 410 and 660 km. A decrease in temperature induced by a subducted slab would shoal and depress the boundaries of the phase transformations at the 410- and 660-km seismic discontinuity, respectively; thus increasing the thickness of the mantle transition zone (MTZ). Likewise, the water would raise and broaden the phase transition zone near 410 km depth.
In order to decipher the long-term debate on the depth extent of slab penetration beneath Taiwan, we conduct a receiver function analysis to map the lateral variation of topography on the 410- and 660-km discontinuities and further investigate the cold temperature and enriched water linked to the distribution of the subducted EP and SCS slab in the MTZ. The data are obtained from teleseismic P wave coda recorded by the broadband stations across Taiwan and adjacent offshore islands during 1996-2008. A common conversion point (CCP) stacking technique is employed to enhance the signals of Pds, a P-to-S conversion at the depth d. The differential (P410s-P) and (P660s-P) travel time residuals provide a means to delineate the phase boundaries associated with the 410- and 660-km discontinuities.
Overall the MTZ beneath Taiwan region is thicker than the global average of 250 km, so as the temperature is lower. In the south of Taiwan, the eastwardly-subducted EP is found to extend northward to 23.5°N, where the slab penetrates at least to 410 km depth but does not reach the base of the upper mantle. In the south of 22oN, the stacked receiver functions to the east of 121oE reveal the earlier P410s arrivals with broad and multi-peak pulses, whereas the P660s phase to the west of 121oE clearly arrives relatively late. While the water retained atop the 410-km discontinuity could result in the much elevated and broadening phase transition zone, analogous to those observed in the receiver functions, the subducted SCS slab may have carried hydrous minerals to great depths and released water into the overlying mantle wedge through the breakdown of these minerals. In addition, the remnants of cold subducted materials accumulated at the bottom of the MTZ may lower the temperature which causes the depressed 660-km discontinuity beneath the Taiwan and Taiwan Strait region.
Subjects
receiver function
Pds conversion
410-km discontinuity
660-km discontinuity
mantle transition zone
South China Sea oceanic lithosphere
Eurasian plate
Philippine Sea plate
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