Deformation history of the southwestern Taiwan foreland thrust belt: insights from tectono-sedimentary analyses and balanced cross-sections

Abstract

Analysis of the basin-scale and local deformation history and estimates of shortening rates are performed in the southwestern foreland thrust belt of the Plio-Pleistocene Taiwan orogen. Timing of deformational events is constrained using both syntectonic unconformities and other markers of syndepositional tectonic activity. Three cross-sections are investigated and balanced. Intermediate shortening rates are estimated based on balanced and restored cross-sections. The total amount of shortening along studied balanced sections results from superimposed thin-skinned and thick-skinned styles. A shallow aseismic thick décollement is located to depth of 6–8km at the base of thick Late Miocene shales that behaves as ductile layer. The occurrence of a deep seismic décollement to depth of 12–15km is confirmed by seismicity, fault plane solutions of focal mechanisms and a seismic reflection profile providing evidence of reactivated and inverted inherited extensional features beneath the Coastal Plain. Three major tectonic stages are recognized within the southwestern Foothills, which are correlated with local thrusting events. The first stage begins at 5Ma with the onset of the Taiwan collision and ends at 2–1.6Ma. It reflects an initial submarine accretionary stage associated with moderate tectonic activity. The second major stage occurs during Late Pliocene–Early Pleistocene. This “intermediate” collision stage sealed the evolution of the southern foreland thrust wedge from submarine to continental environment in relation with exhumation and denudation of the Central Range. The last stage of deformation begins in the Middle Pleistocene and continues until present. It is associated with high rates of sedimentation and most of the thrusts are activated or reactivated at that time and out-of-sequence thrusting occurs. The collision started synchronously near 5Ma throughout the southwestern Taiwan. Total and intermediate shortening rates are more or less constant through time in studied sections; that may suggest that long-term deformation in the Taiwan foreland thrust belt occurred more or less at steady-state. Since 5Ma, the southward decrease of rates of shortening from 3.6 to 4.8mm/year in northern sections to 2.2mm/year in southern section might be related to both influence of indentation by the Peikang High to the north and southward transition, at the scale of the southwestern Foothills, from collision to subduction setting; the effect of the obliquity of the convergence resulting in regional southward migration of the collision cannot be definitely inferred from this analysis. Our results place new kinematic and time constraints, which must be taken into account in forthcoming geodynamic models of the Taiwan collision.