Structure and Deformation History of the Rapidly Growing Tainan Anticline at the Deformation Front of the Taiwan Mountain Belt

Abstract

This study aims at further documenting the mechanisms of shortening at the front of fold‐and‐thrust belts. We focus on an actively growing anticline located at the deformation front of the Taiwan fold‐and‐thrust belt. Based on a multidisciplinary approach combining mainly subsurface data and geodetic techniques, we show that the Tainan anticline is a pure‐shear fault‐bend fold growing above a 38–45° west dipping back thrust, the Houchiali fault, rooted on a 3.8‐km‐deep detachment. The cumulative shortening is estimated at 2–3 km since 310 ± 50 ka, including ~30–50% of horizontal compaction shortening. The significance of the fold is little in terms of total shortening at the scale of the mountain piedmont, yet the Holocene shortening rate of 10.3 ± 1.0 mm/a accounts for 25% of the present‐day shortening rate across the piedmont. Earthquake scaling relationships applied to the Houchiali fault predict Mw~6 earthquakes that would occur a lot more frequently than indicated from historical earthquake catalogs. Hence, the aseismic slip behavior observed from geodetic measurements since two decades is a representative behavior of the fault at least at the scale of a few centuries. Our results bear out the dominance of pure‐shear folding at the front of fold‐and‐thrust belts and support horizontal compaction as a significant shortening mechanism. In contrast, the back thrust wedge structure and the aseismic slip are peculiar characteristics that likely arise from the combination of low friction and high‐pore pressure related to the thick mudstone formation hosting the wedge and of high syntectonic sedimentation rates. Plain Language Summary The toe of active mountain belts often concentrates human population, fertile land, and economic centers. They are also the location of active faults that expose the population to seismic hazard. This study investigates how compressive tectonic forces are consumed at the front of mountain belts. We focus on an actively growing anticline located at the front of the Taiwanmountain belt. Based on a multidisciplinary approach combining mainly subsurface data and geodetic techniques, we show that the fold grows by thickening of the geological strata due to the westward displacement of a wedge structure defined by a west dipping fault and a 3.8‐km‐deep detachment (i.e., a subhorizontal bedding‐plane fault). The fold has little significance when considering the history of deformation at the scale of the mountain belt; however, it has been a major player since 10,000 years, as it concentrates 25% of the tectonic activity across the piedmont. The lack of destructive earthquakes in historical accounts during the last 300 years shows that the tectonic forces are mainly consumed by slow and nearly steady free slip on the fault, as observed from geodetic methods during the last two decades, rather than by earthquakes.