CONSTRAINTS ON THE TRANSITION FROM SUBDUCTION TO EXHUMATION IN THE NORTHERN CENTRAL RANGE, TAIWAN

We document the geometry and kinematics of foliations in an intact stratigraphic section in the northern Central Range of Taiwan and suggest that it records subduction-related strain overprinted by an exhumation-related fabric. Taiwan is the site of ongoing collision between the Luzon arc on the Philippine Sea Plate (PSP) and the Chinese passive margin on the Eurasian plate. Oblique movement of the PSP to the northwest at ~82 mm/yr and the partial subduction of Eurasian transitional continental lithosphere off the coast of China results in high exhumation rates. Recent work suggests that collision drives non-plane strain with exhumation oriented normal to PSP motion, i.e., SW- or NE-directed flow. The well-preserved stratigraphic section we exploit is interpreted to be a seafloor sequence of intercalated meta-basalts with thin fine-grained sediment interbeds and thicker pebbly phyllite exposed on the Mugua River. The surrounding rocks are highly deformed with three foliations of varying orientations. In this area the youngest of these, S3, is gently dipping with a range of dip directions from NW to N to NE. Our greenstone samples display two foliations, a layer-parallel mylonitic fabric (S1), and an overprinting crenulation cleavage (S2) confined to intercalated phyllite and that has similar orientations to the surrounding S3. Samples of the pebbly phyllite also record a layer-parallel mylonitic fabric (S1) and the siliceous pebbles record an L-S geometry with maximum stretching approximately down dip. Strain fringes around cm-scale pebbles indicate stretching both parallel to and perpendicular to the pebble long axes suggesting either a single flattening deformation or multiple deformations. Importantly, the intersection lineation between S1 and S2 in the intercalated phyllite and greenstone is not normal to the stretching direction indicated by the porphyroblasts and the siliceous pebbles in the pebbly phyllite. This suggests that the crenulation cleavage is not coeval with the mylonitic fabric that this D2 event is the same deformation that produced S3 in surrounding rocks of the northern Central Range. This timing opens the possibility that the strain fringes at high angles to the pebble long axis likewise record D2. If this is incorrect, the fabrics that record exhumation remain cryptic.