PALEOSTRESS ANALYSES AND ACTIVE UPLIFT PATTERNS ACROSS AN ARCUATE MOUNTAIN FRONT, HSUEHSHAN RANGE, TAIWAN

The Hsuehshan Range terminates along a northwest-trending boundary north of the Puli Basin in central Taiwan. This mountain front forms the southwest flank of a topographic salient and is approximately coincident with the Sanyi-Puli seismic zone, interpreted to be a reactivated continental-margin fracture zone. Fieldwork along four major rivers across the break reveals two fault systems which were identified based on cross-cutting relationships and the nature of fault-zone material. These fault systems, early-stage and late-stage, are analyzed using the P-T dihedra method (Angelier, Mechler 1977) and the Gauss paleostress method (Zalohar, Vrabec 2007). Both methods of inversion yield similar results: the azimuth of maximum horizontal shortening for early-stage faults is ≈310°, parallel to the current plate convergence vector, and ≈257° for late-stage faults, normal to the topographic break.

Active rock uplift across the topographic break is analyzed using a 40-meter digital elevation model to extract steepness indices (k_s) from the Mei River. Preliminary data reveal a significant increase in steepness that coincides with the increase in topographic slope of the Hsuehshan mountain front. Changes in river steepness are consistent with previous work carried out by Yanites et al. (2010) along the Peikang River, 10 km north of the Mei River. Zones of enhanced uplift appear localized on previously mapped northeast-trending structures, the Tili and Meiyuan faults. However, field observations reveal significant complexities in the regional geology as the latest stages of deformation along the topographic break are oriented on northwest-trending structures which cut across mapped geologic formations. Taken together, steepness indices from the Peikang and Mei Rivers delineate a northwest-trending boundary consistent with the late-stage faults and the major physiographic break in the region. These changes in uplift rates at the mountain front may reflect deformation along deeper crustal structures such as the Sanyi-Puli seismic zone.