NEW STRUCTURAL CONSTRAINTS ON THE LATE-STAGE EXHUMATION OF HIGH-PRESSURE METAMORPHIC ROCKS IN TAIWAN

Capitalizing on the newfound interpretations of the Tailuko and Yuli Belts within the Taiwan Orogen allows for a more comprehensive understanding of exhumation of the metamorphic core. Previous studies of the Tailuko belt suggest it to be dominantly left lateral strike-slip deformed (Ho et al., 2022) whereas the deformation history of the Yuli belt appears to be more complex and dominated by SE-directed underthrusting. However, both units display a late-stage low dipping foliation and associated steeply dipping extensional fractures composed of quartz, calcite, and rarely adularia. An oroclinal feature in the northern portion of the Shoufeng fault, the boundary between the Tailuko and Yuli belts, paired with Ar/Ar ages of adularia found in a northern locality of the Tailuko belt further aids in the contextualization of these veins. The strikes of vein suites found above the tectonically induced orocline are oriented at ~360°; when compared to the ~300° strike of veins below the orocline they coincide with the ~050°/ma dextral curvature of the belts. Additionally, the Ar/Ar dates constrain the absolute age of the late-stage veins and associated stress states to 1.1-2.4 ma. Therefore, we can conclude that the vertical shortening event occurred before the formation of the orocline and also was a widespread change in these belts’ kinematic interactions. Here, we analyze the late-stage extensional veins along the fault using Bingham axial distributions to estimate minimum, intermediate and maximum stress orientations. The results suggest a more complex stress regime within the metamorphic core during its exhumation. The orientation of σ₃ values, the least principal stress and the opening direction of these fractures, cluster along a ~030° axis. However, the interpreted values of σ₁ and σ₂ record a variety of trends and plunges, forming a NW-striking girdle. That is, they suggest both NW shortening, consistent with the strike-slip model as well as vertical shortening, consistent with the observed sub horizontal foliation (Byrne et al., 2023; Chojnacki, UConn MS 2019) and inconsistent with the previously hypothesized entirely left lateral strike slip movement of the fault. This project will entail future research analyzing stress variation, the degree of clustering and also the role of pore pressure along the fault strike.