T24 Orogeny: Relating Rigid Plates to Diffuse Lithospheric Deformation I

Sunday, 31 October 2010
8:00 AM - 12:00 PM
Lobby F

MOBILIZING THE CORE OF CENTRAL RANGE IN TAIWAN OROGENY

Paper number: T24-6
Program: Technical Sessions
Time: 9:15 AM – 9:30 AM

Authors

Abstract

The Lishan Fault Zone (LFZ) in Taiwan is a major structural boundary separating the foothills from the central range. The boundary coincides with a zone of high magnetotelluric conductivity and diffusive seismicity. Recent TAIGER magnetotelluric survey (Bertrand et al., 2009) in Central Taiwan around 24oN shows that the Central Range east of the LFZ is largely a region of high resistivity (500-1000 Ohm-m) down to about 40+ km. But under the LFZ a west-dipping conductive zone to at least 20 km depth is clearly shown. In Southern Taiwan, the West Central Range is relatively conductive (<100 Ω-m) above 15 km and less conductive under East Central Range. Seismic tomography clearly shows the Central Range as a region of relatively high velocity at shallow (<15km) depth and relatively low velocity zone (>18 km). In the same region it is nearly aseismic from 1973-1999.9.20 (Wu et al., 1987; 2004); although the Chi-Chi aftershocks occurred in parts of the Central Range east of the Lishan fault, much of it remains quiescent. The seismicity in Central Range returned to Pre-Chi-Chi level in 2007. Noticeably however, south of ~23.5oN, shallow seismic zone has been active and widens southward.

We hypothesize that the high resistivity zone under the Central Range may have resulted from dehydration during rapid exhumation of the greenschist rocks (Vry et al., 2009; Wannamaker, 2009). It is possible that exhumation and dehydration caused by metamorphism are taking place at the LFZ which acts as a structural boundary. The collision of the Philippine Sea plate led to an upthrust of the oceanic plate, the deformation of the lower as well as the upper continental crust to create the Central Range. At the beginning of the current orogeny the mobilized zone was non-exisitent. It developed and broadened to the present width in a few million years. As the southern Central Range is somewhat younger the dehydration process may still be going on at shallow depth, with its accompanying seismicity. We believe that the Lishan Fault Zone is the westward front of propagating metamorphism in the mobile zone.