High-resolution 3D LiDAR mapping of geologic structures: Implications for thermo-tectonic history in the Taiwan Slate Belt

The thermo-tectonic history of an orogenic belt can be investigated using data on metamorphic grade, thermochronology, and structural geology. However, in the strongly deformed and poorly exposed terrains, field observation and structural correlation present challenges that hinder the construction of large-scale structural frameworks. Previous studies demonstrate that LiDAR-based digital elevation model (LiDAR DEM) reveals geomorphic lineaments caused by interactions between planar geological structures and surface processes. The delineation of these lineaments offers a systematic and comprehensive perspective on regional structures, helping to overcome limitations posed by poor exposure.

In this study, we use 3D mapping of LiDAR DEM and relevant datasets, along with field mapping, to investigate the structural architecture of the strongly deformed and metamorphosed south-central Cenozoic Western Slate Belt in the Central Range of Taiwan. Although the metamorphic grade and low-temperature thermochronologic data are well-established in this region, the structural framework remains unclear, and the relationship between metamorphism and tectonic events is still controversial. Our 3D LiDAR mapping reveals two suites of structural lineaments of interest: bedding (S_b) and metamorphic foliation (S_f). Based on their morphology and field validation, S_b is associated with thick-layered metasandstone and metavolcanic layers, while S_f results from fracturing along an east-dipping, pervasive, and penetrative slaty cleavage. The regional pattern of S_b reveals a previously unmapped, approximately 10 km wavelength, west-facing, tightly folded overturned synform, referred to as the 'Siangyang Synform.' The S_f is axial planar to the Siangyang Synform, suggesting that the cleavage and regional-scale fold developed simultaneously, which is supported by field observations.

This study demonstrates the value of integrating 3D LiDAR mapping and field surveys in strongly deformed metamorphosed terrains. While the orientation and regional patterns of thick-layered, competent rocks are difficult to determine through field surveys alone, they are discernible using the stereo view of LiDAR DEM, revealing macroscopic structural features. By integrating the new structural architecture with published RSCM temperature and geochronologic data, the regional geologic framework shows the peak thermal event postdates or synchronize with the syn-orogenic ductile deformation, highlighting the significance of syn-orogenic undethrusting and metamorphism of the Western Slate Belt during the late-Cenozoic arc-continent collision in Taiwan.