MECHANISM AND CONSEQUENCE OF FOREARC BACK-THRUSTING IN FORMATION OF LICHI MéLANGE IN COASTAL RANGE, EASTERN TAIWAN

The Coastal Range orogen in eastern Taiwan was resulted from the collision between the Luzon Arc of the Philippine Sea Plate and the Asian continental margin since latest Miocene. The collision is still active today off SE Taiwan where provides the modern analog in understanding the tectonic evolution of the Coastal Range and the mechanism responsible for formation of the Lichi Mélange. Multiple evidence, including geochemistry, sedimentology, stratigraphy study, indicate that two volcanic islands of the Luzon Arc and four remnant forearc basins of the North Luzon Trough were accreted as the Coastal Range in the last 1.5 Ma. The intensively sheared Lichi Mélange represents the lower forearc turbidite sequences (3.5-3.7 Ma) that have experienced two stages of thrusting during the arc-continent collision. During the initial arc-continent collision (~3.5 Ma), the northwest-moving Luzon Arc as a backstop on the upper plate collided the underthrusting Asian continental margin to result in an arcward (eastward) back-thrusting in the western part of the North Luzon Trough forearc basin to form the deformed Lichi Ridge (analog to the modern Huatung Ridge in modern active collision zone off SE Taiwan) along the rear accretionary prism. The Lichi Ridge was especially elevated in areas between volcanic islands to progressively deform the North Luzon Trough into a sausage-like configuration of two pairs of a large, deep remnant forearc basin in the north followed by a small, elevated remnant basin in the south. The large, deep (Shuilien and Taiyuan) remnant basins are characterized by filling of 1.5 km thick Pliocene classic turbidite sequences and ~2 km thick Pleistocene fluxoturbidites, while only 1.5 km thick Pliocene classic turbidites were deposited in the small, elevated (Loho and Taitung) remnant basins in which the Lichi Mélange is also exposed in the basin center. During the advanced arc-continent collision (~1.5 Ma in the north and <0.9 Ma in the south), the deformed Pliocene Lichi Ridge was thrust westward incorporating with some arc crust/mantle blocks of the upper plate materials to become the Lichi Mélange. Finally, the volcanic islands and remnant forearc strata were further thrust westward over the Lichi Mélange along a low-angle decollement to form the Present Coastal Range in eastern Taiwan. The Lichi Mélange beneath the decollement was able be exposed as window structures either by river erosions along the collision suture (the Longitudinal Valley) and in the Loho remnant forearc basin center, or by wave erosion along the coast of the Taitung remnant forearc basin.