Presenter: Li, Kueimo
Date: 2016/03/10
Abstract
Analysis of geomorphological, geochronological, geochemical and geophysical features in the segment of the TaiwanLuzon Arc between Taiwan and Luzon (the Bashi Segment) allows the recognition of a double arc structure. The two volcanic chains are separated by 50 km just north of Luzon (18°N), and converge near 20°N. Islets in the western chain are older and largely composed of volcanic rocks of Miocene to Pliocene age.Yang 1996 propose a geodynamic model in which the double arc in the Bashi Strait is the tectonic manifestation of the subduction of the aseismic Scarborough Seamount Chain, the extinct midocean ridge of the South China Sea.
Bautista used hypocentral and focal mechanism data in order to characterize the tectonic configuration of northern Luzon and propose a model for describing the geometry of the subducted slab of the Eurasian plate beneath the northern segment of the Manila Trench. We took into consideration some of the observed bathymetric features and the intraplate deformation pattern in the North Luzon Ridge region in depicting a model that is consistent with most of the observed features and phenomena in the study area. An earthquake catalog covering the period from 1619 to 1997 was used in studying the distribution of strain energy released, while earthquakes covering the instrumental years (1963–1997) were used to analyze vertical distributions of earthquakes in sections.
The new model of the subducted slab of the Eurasian plate beneath the Manila Trench is proposed refinement on Yang 1996. The model suggests the collision and subsequent partial subduction of a buoyant plateau at around 20°N lat. to explain the sharp bend in the trench line, the complicated deformation pattern on the overriding plate fronting the bend and the shallow dip of the subducted slab beneath this zone. A tear in the slab is also inferred to be present as evidenced by the observed gap in strain energy release and the abrupt change in dip from shallow to steep south of 18°N lat. The gap in seismicity and strain energy release (65–300 km depth) at around 17°N lat. may be used to infer the trajectory and location of the subducted extinct midoceanic ridge (MOR).
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