The South China Sea oceanic lithosphere has been subducting eastward beneath the Huatung Basin/Philippine Sea Plate since the Early Miocene (~18 Ma). The subduction is followed by the oblique collision between the Luzon arc and the subducting Eurasian plate from 6.5 Ma. The North Luzon Trough forearc strata and the Luzon arc are then obducted northwestward as the Coastal Range, eastern Taiwan, in the last 1 Ma. The collision propagates southward and is presently active in the region offshore SE Taiwan. Integrating seismic surveys offshore and a detailed forearc stratigraphy study onshore the Coastal Range, this paper overviews the characteristics of forearc deformation, dynamic sequence stratigraphy, temporal-spatial variations of forearc sedimentation and stratigraphic correlation onshore and offshore forearc sequences in response to the oblique convergent tectonics north of 20°N. Combining onshore and offshore forearc geology together allows us to reconstruct a structural evolution of the North Luzon Trough forearc basin from subduction through collision to obduction, and to discuss the mechanism and processes responsible for developments of the modern-forming forearc Lichi Mélange during the active Taiwan orogeny.
Seismic surveys offshore show that forearc deformation in the subduction zone is primarily caused by increase of rear prism slope and west-vergent thrusting of forearc strata along the prism top since the early forearc sedimentation. East-vergent backthrusting occurs during the late forearc sedimentation and propagates arcward when the volcanic arc collides with the accretionary prism in the collision zone. Bivergent thrusting leads to a development of the forearc Huatung Ridge popup as a bathymetric high which further controls the sedimentation of the younger forearc sequence in the collision zone. In response to the syn-sedimentation deformation, forearc depocenter shifts progressively eastward. Forearc stratigraphy thus changes from two mega-sequences bounded by an unconformity in the subduction zone to three mega-sequences juxtaposed from west to east unconformably in the collision zone. As a consequence, the forearc deformation and stratigraphy in the oblique collision zone off SE Taiwan show a characteristic temporal-spatial pattern that the lowest mega-sequence with the most intensive deformation occurs restrictedly in the west, whiles the middle and the upper mega-sequences with mild deformation crop out in the center and the eastern part of the forearc basin, respectively.
Detailed biostratigraphy study using planktonic foraminifera and calcareous nannoplanktons indicates that the forearc strata onshore the Coastal Range are also composed of three sequences (lower unit S-1: 6.5–5.8 Ma; middle unit S-2: 5.8–3.0 Ma; and upper unit S-3: <3.0–1 Ma). They were previously mapped as lithostratigraphy units of the Lichi Mélange (6.5–3.0 Ma) in the west and the coherent flysch sequences of the Fanshuliao Formation (5.8–3.0 Ma) and the Paliwan Formation (<3.0–1 Ma) in the east. However, the young coherent forearc sequences in the east thrust westward ubiquitously over the old and highly deformed Lichi Mélange in the west along the listric east-dipping Tuluanshan fault during the obduction. Detailed biostratigraphy study reveals that the lower sequence unit S-1 exposes restrictedly in the Lichi Mélange west of the Tuluanshan fault, whereas the middle sequence unit S-2 are either mapped as part of the Lichi Mélange or the coherent Fanshuliao Formation in both sides of the Tuluanshan fault, respectively. The upper sequence unit S-3 exposes exclusively east of middle sequence unit S-2 in the eastern Coastal Range. Stratigraphy and sedimentology study also reveals a temporal-spatial sedimentation variation of the upper sequence unit S-3 owing to deformation of the sequence units S-1 and S-2 together as a Pliocene Forearc Ridge at ~3 Ma, a scenario analog to development of the modern Huatung Ridge at ~1 Ma in active collision zone offshore. Temporal-spatial pattern of forearc sequences onshore the Coastal Range suggests that these three sequence units are juxtaposed from west to east and are bounded by two unconformities analog to what occur today in the collision zone offshore SE Taiwan. The characteristic deformation and an eastward-youngling trend of strata distribution onshore the Coastal Range all indicate Syn-sedimentation deformation during the oblique collision in 6.5–1 Ma. Furthermore, along the N-S orogenic strike, events of the forearc sedimentation and bivergent thrusting occur earlier onshore the obduction zone in the north than the modern collision zone offshore in the south. Across the orogenic strike forearc strata get older and deformation gets intensive from east to west onshore the Coastal Range, a scenario analog to what observed in the oblique convergent region offshore SE Taiwan.
A structure reconstruction reveals that the North Luzon Trough forearc strata have experienced multiple stages of thrust deformation from subduction through collision to obduction. These deformations account for the mechanism and processes to develop the modern-forming highly sheared SSZ-bearing forearc Lichi Mélange tectonically in the western Coastal Range during the last 1 Ma.
