Structural Architecture, Active Geological Structures and Deformation History in the Foothills of Southwestern Taiwan(3/3)

Taiwan is an actively growing and well-monitored orogen that provides opportunities to observe and better understand the mechanisms at converging plate boundaries and the associated seismic hazard. In southern Taiwan, the orogen is considered as a transition between the product from an arc-continent collision, as observed in central Taiwan, and an accretionary wedge associated to a subduction zone, as observed offshore southern Taiwan along the Manila Trench. In parallel, earlier studies conducted offshore southwestern Taiwan and recent seismic tomography imaging of the crust showed that the orientation of the continental shelf edge evolves from NE-SW offshore to nearly E-W beneath southern Taiwan, reinforcing the idea that the passive margin geometry influenced the development of the Taiwan mountain belt. Regarding active deformation, southwestern Taiwan is among the places with the highest strain rates in Taiwan. A wealth of geodetic data shows about ~4.5 cm/yr of westward shortening across the foothills of Tainan, evolving southward to a rapid southwestward tectonic escape at the latitude of Kaohsiung. Yet, the geological structures that accommodate this deformation remain poorly understood. The goal of this proposal is to understand how the Western Foothills of southern Taiwan evolved with time, from the present day to the longer timescale of a few million years. The key questions we aim at answering are as follows: What is the architecture of the Western Foothills of southern Taiwan? What are the active structures that currently accommodate deformation and that would potentially generate large earthquakes? What is the cumulative long-term shortening on the identified faults and folds? How did the foothills evolve at the scale of a few million years? To answer these questions, I propose to construct three balanced geological cross-sections across the foothills of Tainan, from the deformation front to the slate belt. The sections will be supported by surface and subsurface geology, chronological constraints, Late Quaternary ground-surface deformation, tomography, as well as geodesy. The expected outcome of this step is a comprehensive understanding of the geological structures and of the currently active structures in map view and in sections, and if possible, the determination of Late Quaternary shortening or fault-slip rates. In a second step, we will restore the cross-sections to determine the long-term cumulative shortening on the structures identified on the cross-sections. Biostratigraphy and evidence for syntectonic sedimentation are expected to provide time constraints to decipher the deformation history during the Plio-Pleistocene.The expected outcome of this project participates to local seismic hazard assessment in a densely populated area and to longer-term research questions such as how the transition from arc-continent collision to subduction has been operating, how the passive margin geometry influenced the development of the mountain belt, or what is the amount of total shortening across southern Taiwan, from offshore eastern Taiwan to the western deformation front, and implications for plate tectonics reconstructions.