ARCHITECTURE AND DEEP LACCOLITHIC INFLATION OF THE SHORT-LIVED (~15MA) LUZON ARC NEAR TAIWAN

The Luzon Arc is the short-lived ~15Ma and relatively simple product of subduction of South China Sea lithosphere under the Philippine Sea plate, now terminating in the ongoing Taiwan arc-continent collision. Here we document the structural and stratigraphic architecture of the pre-collisional arc offshore of southeast of Taiwan, showing evidence of massive laccolithic intrusion based on our interpretation and structural restoration of reprocessed seismic reflection profiles and bathymetry. The pre-arc and syn-arc seismic stratigraphy is calibrated to extensive geochronologic and stratigraphic studies of the Lanyu and Lutao volcano islands and the recently deformed onshore arc (<1Ma). We show that the Miocene and younger northern Luzon Arc is built on Early Cretaceous Huatung Basin oceanic lithosphere of the Philippine Sea plate, which underwent >100 Ma of pelagic sedimentation at abyssal depths prior to the onset of Eurasian subduction and arc volcanism at ~18-14Ma. This onset is marked in the Huatung basin by seismic facies that thicken arcward, including mass-transport deposits, in contrast with underlying 1-2km thick pelagic facies. The contact between oceanic crust and the pelagic sequence provides a zone of detachment that is the locus of deep massive laccolithic intrusions imaged in our reprocessed seismic reflection profiles. The ~2-3km thick roof stratigraphy, which includes early arc volcanics, has been flexed upward by 2-3km laccolithic inflation that is corroborated by unmetamorphosed blocks of Early Cretaceous abyssal radiolarites and Miocene near-CCD red shales in volcanic breccias on Lanyu Island volcano. Therefore, we proposed that the growth of the arc crust occurred by the emplacement of laccolith intrusions. These laccoliths are among the largest yet reported but show typical sizes and shapes, with estimated magmatic fluxes similar to large volcanoes and plutons. This short-lived (~15Ma) arc has not yet developed a crustal root in contrast with longer lived arcs such as Izu-Bonin-Marianas (~58Ma). The final stage of laccolith emplacement, constrained by overlying growth strata to be ~1.1-0.9Ma, coincides with the subduction of forearc lithosphere, imaged in seismic tomography, which has displaced the mantle wedge near Taiwan, terminating arc magmatism.