New Kinematic Constraints on Tectonic Extrusion in Southwestern Taiwan From a Mud Volcano Offset by a Creeping Fault

Abstract

Southwestern Taiwan consists of a fold‐and‐thrust belt undergoing southwestward tectonic extrusion. Its rapid, possibly predominantly aseismic deformation raises concerns on geological hazards. Across the Coastal Plain, a sharp geodetic deformation gradient coincides with a topographic scarp interpreted as the surface expression of the right‐lateral reverse Chegualin fault. The Gunshuiping mud volcano lies along this fault and displays a dislocated dome‐shaped topography. Using shallow borehole geological data and geochronological techniques, we reconstruct a near‐surface geological model of the faulted units and establish the Holocene evolution of the area. The dome‐shaped topography formed by the accumulation of mud flows and fluvial and aeolian sediments after a transition from shallow marine to continental depositional environments around 4.1 ka ago. Mud‐injection structures within Holocene units close to the fault trace originate from the overpressured Gutingkeng Mudstone based on nannofossil identification. We interpret them as pathways formed by fault‐induced fractures. Dated stratigraphic offsets yield a vertical slip rate of 3.4 ± 0.6 mm/yr since 10 ka and the offset dome‐shaped topography a minimum right‐lateral slip rate of 13.5 ± 2.1 mm/yr since 4.1 ka. The first‐order agreement between the long‐term right‐lateral slip rate and the GNSS‐derived right‐lateral aseismic slip rate of 9.6 ± 1.2 mm/yr is consistent with full creep. However, uncertainties allow a maximum difference of 7.2 mm/yr, indicating possible partial creep and associated seismic hazard. The relatively fast slip rate and structural continuity of the fault, likely extending offshore, give it a significant role in the regional tectonic extrusion.