Aseismic deformation within the fold-and-thrust belt in southwest Taiwan: Example from the Tsengwen River section

We report a rarely observed case of steady aseismic deformation in the context of a fold-and-thrust belt, with a well-documented structural and lithological background. We focus on a 12-km-long section across the foothills of southwestern Taiwan, where about 23 mm/yr of westward compression is observed. From west to east, the surface geological structures include an anticline, a thrust and a backthrust. We determine Holocene uplift rates based on fluvial terraces, invert the interseismic 3D velocity field using existing geodetic datasets, and build a geological cross-section to constrain the possible deep geometry for the structure responsible for the observed surface deformation. Geodetic vertical velocities and Holocene uplift rates show a similar pattern, with rates rapidly increasing eastward, then remaining relatively constant across the fold axis and thrust, and finally sharply decreasing across the backthrust, across which InSAR observations suggest a velocity discontinuity. Our observations show that active deformation is occurring mainly aseismically and involves the anticline (the Wushantou Anticline) and backthrust (the Kouhsiaoli Fault). Our cross-section illustrates a 4-5 km deep wedge with a passive roof thrust corresponding to the backthrust, on the hanging wall of which the anticline is located. A classical fault-bend fold model with a slip rate of 21±2 mm/yr can explain most of the observations, yet local misfit suggests a possible contribution to uplift from pure shear of clayey rocks in the anticline core. Based on published records from a deep well drilled across the fold core and backthrust, clay-rich lithology and elevated fluid content are likely to favor aseismic slip. Without instrumental earthquakes reported on these structures and in the lack of successful paleo-earthquake investigations, whether these structures ever generate M>6 events remains an open question.