Using 3D LiDAR Geological Mapping to Improve the Structural Geometry at the Boundary Between Two Geologic Units: A Case Study of the Taoyuan Quadrangle, Taiwan

The geological units of Taiwan are traditionally classified into five units from east to west: the Coastal Range, Backbone Range, Hsuehshan Range (HR), Western Foothills (WF), and Coastal Plain. While the boundaries between these tectonic units are generally associated with major faults, some of these boundaries remain inadequately defined and poorly understood. One notable example is the boundary between the WF and HR, which has historically been identified as the Chuchih Fault. However, biochronological research suggests that the Chuchih Fault does not coincide with the boundary between Paleogene and Neogene strata. Additionally, structural investigations indicate that certain segments of the Chuchih Fault lack the characteristics typically associated with a boundary fault, further complicating its role as a definitive cutoff line between these geological units. Due to limited exposure, there is insufficient field data near the boundary between the WF and HR within the Taoyuan geologic quadrangle area, and the detailed structural geometry remains unclear. In this study, Digital Elevation Modeling (DEM) derived from Light Detection and Ranging (LiDAR) is used to interpret macroscopic geological structures, which are often covered by vegetation. We used the open-access 3D DEM (20-meter resolution) and overlaid it with a 2D high-resolution hillshade image to explore the geology from multiple perspectives in a 3D GIS environment. Faults and folds with wavelengths of several kilometers are determined based on the bedding lineation. In the Taoyuan geological quadrangle, the detailed distribution and thickness of the strata, along with the geometry of folds and faults, are delineated. This analysis reveals the complex geological structures that define the boundary between the HR and WF and illustrates how these structural patterns evolve from the northeast to the southwest within the study area. The fault trace and displacement along the Chuchih Fault have been revised. Several minor faults that may be associated with folding are revealed. Additionally, close to open synclines plunging to the southwest are identified, with anticlines or faults occurring between them. The application of LiDAR DEM for refining geological structural mapping between the WF and HR proves to be a feasible and effective method. The enhanced understanding of structural geometry in the study area indicates that the boundary between the WF and HR is significantly more complex than previously thought. It should not be narrowly defined as a single fault, such as the Chuchih Fault, but rather as a structural zone with intricate fault and fold interactions.