Structures in the active Western Foothills of Southwestern Taiwan: Fault-related folding versus shale tectonics

Abstract Taiwan is an active orogen where a west-verging fold-and-thrust belt deforms a Plio-Pleistocene foreland basin sequence. In southwestern Taiwan, the 3–4 km thick Gutingkeng mudstone, Late Miocene to Early Pleistocene in age, has characteristics similar to mobile shales, such as overpressure conditions and sourcing mud volcanoes. Present uplift rates as fast as 2 cm/year are observed mainly on the footwall of steep thrusts, including the Gutingkeng Fault, which features several mud volcanoes. We integrate surface observations near this fault with regional subsurface data to construct an upper crustal cross section and evaluate the roles of the competing models of fault-related folding and shale tectonics in deforming the region. Field observations indicate steep and well-preserved bedding on the hanging wall and footwall and a wide reverse fault zone with penetrative shearing corresponding to the Gutingkeng Fault. No distinct structure explaining footwall uplift is found in the outcrops. At the cross-section scale, surface geology and subsurface data point to a structural style with fairly narrow anticlines with steep limbs growing above a relatively deep detachment. This geometry is not easily explained using classical fault-related fold models. We infer that folding on the Gutingkeng Fault footwall and in the core of a frontal anticline occurs through layer thickening (pure shear) facilitated by weak mudstone rheology. The increase in pore-fluid pressure under burial and tectonic compression within the fold cores can lead the mudstone to mobilize and undergo plastic flow, causing the inflation of the fold cores. Fold growth, possibly aided by shale tectonics, will have progressively increased the dip of the Gutingkeng thrust, eventually leading to thrust inactivity and uplift mainly occurring on the footwall. Our study area offers a unique opportunity to study an active shale-dominated fold-thrust belt exposed on-land and highlights the importance of combining geodetic observations to investigate ongoing deformations in these settings.