Helium Isotopic Signature of a Plate Boundary Suture in an Active Arc-Continent Collision

We report new noble gas signatures of groundwaters, hot springs, and bedrock samples from two major fault systems that form the plate boundary suture between the Eurasian Plate and the Philippines Sea Plate. In the south, the Eurasian Plate dips east beneath the Philippine Sea Plate, and the boundary between the arc and accretionary prism is marked by the Chihshang Fault, which also dips east beneath the arc. The fault is well-defined by seismicity and extends to at least 25 km, close to the crust- mantle boundary. This fault could be an important conduit for the passage of mantle helium along the suture. In the north, as the subduction direction flips from the east-dipping Eurasian Plate to the north-dipping Philippine Sea Plate, the plate boundary fault is represented by a more complex system of east- and west-dipping structures. Low helium isotopes in this area evidence the weaker mantle signal, both before and after a M-w 6.4 earthquake. It is possible that as the subduction direction flips from south to north along strike, the tortuosity of the fault-related conduits increases, reducing the flow and limiting the release of mantle-derived gases.