Distribution of strain rates in the Taiwan orogenic wedge

Abstract

To constrain the way Eurasian crust is accreted to the Taiwan orogenic wedge we investigate the present-day 3D seismogenic deformation field using the summation of 1129 seismic moment tensors of events ( M w > 4) covering a period of 11 years (1995 to 2005). Based on the analysis of the principal strain-rate field, including dilatation and maximum shear rates, we distinguish four domains. Domain I comprises the Coastal Plain and the Western Foothills. It is mainly contractional in both the horizontal plane and in cross-section. Domain II comprises the eastern Western Foothills, the Hsuehshan Range and the Backbone Range. It is characterized by the highest contraction rates of 10 − 6 yr − 1 in association with area expansion in cross-section and area contraction in the horizontal plane. Domain III corresponds to the Central Range. It is characterized by area contraction in cross-section and area expansion in the horizontal plane. The maximum contractional axis is typically low and plunges ~ 30°E. Extension is larger, horizontal and strikes parallel to the axis of the mountain range. Domain IV corresponding to the Coastal Range and offshore Luzon Arc shows deformation patterns similar to domain II. This seismogenic strain-rate field, which is found in good agreement with the main features of the geodetic field, supports shortening within a thick wedge whose basal décollement is relatively flat and located in the middle-to-lower crust > 20 km. The east plunges of maximum strain-rate axes below the Central Range argue for the development of top-to-the-east transport of rocks resulting from the extrusion of the whole crust along west-dipping crustal-scale shear zones. The study of seismogenic strain rates argues that the initiation of subduction reversal has already started in the Taiwan collision domain.