Growth normal faulting at the Western edge of the metropolitan Taipei basin since the last glacial maximum, Northern Taiwan

Growth strata analysis is an useful tool in understanding kinematics and the evolution of active faults as well as the close relationship between sedimentation and tectonics. Here we present the Shanchiao Fault as a case study which is an active normal fault responsible for the formation of the 700-m-thick late Quaternary deposits in Taipei Basin at the northern tip of the Taiwan mountain belt. We compiled a sedimentary record, particularly the depositional faciès and their dated ages, at three boreholes (SCF-1, SCF-2 and WK-1, from west to east) along the Wuku Profile that traverses the Shanchiao Fault at its central segment. By incorporating the global sea level change curve, we find that thickness changes of sediments and changes of depositional environments in the Wuku area are in a good agreement with a rapid sea level rise since the Last Glacial Maximum (LGM) of about 23 ka. Combining depositional facies changes and their ages with their thickness, we are able to introduce a simple back-stripping method to reconstruct the evolution of growing strata across the Shanchiao Fault since the LGM. We then estimate the vertical tectonic slip rate since 23 ka, which exhibits 2.2 mm yr¹ between SCF-2 and WK-1 and 1.1 mm yr¹ between SCF-1 and SCF-2. We also obtain the Holocene tectonic subsidence rate of 2.3 mm yr¹ at WK-1 and 0.9 mm yr¹ at SCF-2 since 8.4 ka. We thus conclude that the fault zone consists of a high-angle main fault to the east between SCF-2 and WK-1 and a western lower-angle branch fault between SCF-1 and SCF-2, resembling a tulip structure developed under sinistral transtensional tectonism. We find that a short period of 600-yr time span in 9 - 8.4 ka shows important tectonic subsidence of 7.4 and 3.3 m for the main and branch fault, respectively, consistent with possible earthquake events proposed by previous studies during that time. A correlation between geomorphology and subsurface geology in the Shanchiao Fault zone shows that an array of subtle geomorphic scarps corresponds to the branch fault, however, that the surface trace of the main fault seems to be completely erased by the surface force of erosion and sedimentation. We recommend that analysis in the light of growth-faulting scheme should be conducted in a more systematic way for earthquake geology study in other portions of the Shanchiao Fault within the Taipei Basin, as well as other areas worldwide currently under extensional deformation.