Aseismic deformation within fold-and-thrust belts: example from the Tsengwen River section of southwest Taiwan

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 (Interferometric Synthetic Aperture Radar) observations suggest a velocity discontinuity. Our observations show that active deformation is occurring mainly aseismically and involves the anticline and backthrust. 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.