A role for pre-collision processes in the origin of the aseismic zone of the southern Taiwan Central Range

Abstract:

Although Taiwan is one of the most seismically active areas on Earth, two areas on the island are notable for their lack of earthquakes, the coastal plain of central western Taiwan and an elongate area centered on the eastern flank of the southern Central Range. I focus on the latter area and suggest that pre-collision processes may have contributed to its development. Seismogenic strain offshore southeastern Taiwan reveals that the west side of the Luzon arc hosts a forearc sliver that was initially uplifted during basin inversion then tectonically thinned by pull-apart basin formation as collision initiated. This pre-collision history favors the tectonic insertion of a sliver of oceanic crust at depth in the orogenic suture zone. The presence of oceanic crust in this region is consistent with high seismic velocity observations as well as existing analog models. Strain inversions spanning the aseismic zone broadly show crustal thickening and NW-directed shortening to the east, and crustal thinning and SW-directed stretching to the west. Oceanic crust at depth could contribute to the aseismic zone by acting as a relatively strong body sandwiched between the accreted arc and weaker transitional continental crust to the west. In this scenario the aseismic zone is largely composed of forearc rocks that are not internally deforming by elastic deformation mechanisms. Alternatively the sliver may act as a west-pointing wedge (in profile view) that is promoting weaker transitional continental material to detach, imbricate or delaminate in front of the colliding continental promontory (i.e., the Peikang high) that appears to contribute to the low rates of seismicity in western Taiwan. In any of these latter scenarios it would be reasonable to expect changes in crustal architecture that might promote shallowing of the brittle-plastic transition as is observed around the aseismic zone. The specific mechanism for this shallowing remains unresolved and clearly the presence of a forearc sliver in this region should be considered.