Modeling of thermo-rheological structure of lithosphere under the foreland basin and mountain belt of Taiwan

Abstract

The system of Taiwan mountain belt and the West Taiwan foreland basin is a manifestation of the collision between eastern Asian continental margin and the Philippine volcanic arc. It provides an ideal place for the study of the flexure behavior of lithosphere under the mountain–basin system. The paper presents results of thermal and rheological modeling of the system along Profile B, which extends 200 km in a NW–SE direction from the Taiwan Strait across the foreland basin and mountain range to the Longitudinal Valley. Along the profile, the crustal structure is constrained by wide-angle seismic and gravity data as well as P-wave tomography, while the structure of the foreland basin is constrained by multichannel seismic and drilling. Assuming that a steady-state deformation has proceeded in Taiwan since the collision of the Luzon Arc with the SE Eurasian margin at 6.5 Ma, the present-day thermal and rheological structures of the lithosphere are modeled by finite element analysis. The base of lithosphere is assumed to be isothermal, with a temperature of 1300 °C. The interior heat source consists of radiogenic heating, frictional heating on thrust faults and basal decollement, and body heating by internal friction within thrust sheets. In addition to conduction, the heat convection is carried out by the subduction of the Eurasian margin, the exhumation and erosion in the mountain range, and the sedimentation in the foreland basin. Thermal parameters are carefully selected and updated based on available data on surface heat flow, downhole thermal gradient, and thermal conductivity from Taiwan and nearby regions. The mechanical parameters are determined based on regional geology. The resulting thermal model predicts well the elevated surface heat flow in the mountain range and the depressed surface heat flow in the thrust front. A high-temperature core of >500 °C appears in the bottom of thrust sheets and the uppermost upper crust. Sensitivity analysis indicates that this elevated temperature is mainly due to the frictional heating along thrust faults and basal decollement, and to the heat carried up by the exhumation of the deeper rocks and subsequent erosion. The depth-dependent strength envelopes were computed based on the thermal model and commonly used rheological parameters. Rheological stratifiction of the lithosphere along the profile is depicted. Although the estimation of the thickness of the rheological strata is affected by the uncertainty in lithosphere thickness, the modeling results indicate clearly that the rheological structure of the lithosphere under the mountain range is remarkably different from that in the foreland basin. While the foreland basin has a typical thermal and rheological structure of the rifted continental margin with three brittle layers above three ductile layers, the raised temperature under the mountain range has weakened the lithosphere dramatically. Under compressional stress, the lithosphere beneath the mountain range becomes ductile almost entirely, except a thin (about 6 km in the central part) brittle layer near the surface and perhaps a thin brittle layer in the uppermost mantle. Such a significant weakening of the lithosphere in the mountain range should not be overlooked while discussing the flexure behavior of the lithosphere in the area.