REVISIONS IN THE NARRATIVE PERTAINING TO THE SHOUFENG FAULT IN THE TAIWANESE OROGEN

The movement of the Shoufeng fault, the contact point between the Tailuko and Yuli belts, was previously thought to be entirely left lateral strike slip Ho et al., 2022. A relatively simple interpretation of this proposal predicts subhorizontal NW-SE shortening and NE-SW extension. Recent mapping in the metamorphic rocks (Byrne et al. in revision; Chojnacki, MS 2019), however, reveals the presence of a pervasive subhorizontal foliation, suggesting vertical shortening. Additionally, analysis of fracture orientations from the metamorphic core suggests a new narrative pertaining to stress tensor directions. The majority of veins are Mode I extension fractures striking northwest at roughly 315° and have nearly vertical dips. The vein orientations and their poles were plotted on a stereonet, and Bingham Axial Distributions were calculated. The orientation of sigma 3 values, the least principal stress and the opening direction of these fractures, cluster along a ~030° axis. However, the interpreted sigma values 1 and 2 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 subhorizontal foliation. Upon comparing the principal stress tensors, a dichotomy emerges between the implications of Ho et al., 2022, suggesting one stress regime, and the low-angle cleavage (Byrne, in revision, 2023), implying a different stress regime. This similarity in orientation tensors suggests that there are other aspects of this metamorphic orogen that should be taken into account. For example, extending from ~2.4 to 0.7 Ma, it is hypothesized that an extrusion of subducted blueschist facies under the fault occurred, likely changing the tensor orientation. The curious orientation of foliation and similarity in Sigma values 1 and 2, make it justifiable to say that this fault region is not entirely left lateral strike slip, but also records extrusion of rocks once buried in a subduction zone.