A geophysical study of the Manila Trench, Luzon, Philippines: 1. Crustal structure, gravity, and regional tectonic evolution

Abstract

The Manila Trench subduction zone is an active convergent plate margin between the South China Sea and the northern Philippines. The trench trends northerly and is associated with a volcanic arc, an east dipping Benioff zone beneath Luzon, and a well‐developed fore arc basin system. The Luzon Trough fore arc basins lie landward of the Manila Trench and contain up to 4.5 km of Cenozoic sediments. Offscraping and accretion of turbidite sediments deposited in the Manila Trench have resulted in the upbuilding and outbuilding of an accretionary prism which forms the seaward margin of the fore arc basin system. The uplifted Zambales ophiolite and its offshore extension form the landward side, of the fore arc basin. Multichannel seismic reflection profiles reveal that folding and thrust faulting of trench strata occur at the base of the trench slope. The major structural décollement at the Manila Trench usually forms near the unconformity separating the hemipelagic sediments from the turbidite sediments. Subduction and perhaps underplating of the basal hemipelagic trench sediments accompany the deformation and accretion of the overlying clastic deposits. Faulting and uplift occur within a narrow zone of the fore arc where seamounts associated with the relict spreading center of the South China Sea basin have presumably been subducted. North of Lingayan Gulf the fore arc is disrupted by active fault systems that trend north and northwest offshore from the Luzon Central Valley and from the southern Cordillera Central. Based on regional geological and tectonic observations, we infer that subduction probably began at the Manila Trench in late Oligocene to middle Miocene time. The long‐term average convergence rate at the Manila Trench is estimated to be between 10 and 20 mm/yr and may be slowing in the north owing to the collision of Taiwan with Eurasia.

Late Mesozoic compressional folds in Dongsha Waters, the northern margin of the South China Sea

“…The thrust faults within the blind folds were usually of minor throws and dipped mostly southeastward, against to the assumed northwestward subduction (Taylor and Hayes, 1983). Furthermore, the folds illustrate well-conserved bedded sequences, unlike those in typical trenchrelated accretionary prisms elsewhere worldwide, e.g., the hanging wall of Manila Trench (Hayes, 1984) and Nankai Trough (Moore et al, 2007), where original sedimentary sequences were tightly folded and/ or highly imbricated to form mélange complex. Thus, the Mesozoic folds over Dongsha Waters feature more like back-thrust tectonics.…”

Winter phytoplankton bloom induced by subsurface upwelling and mixed layer entrainment southwest of Luzon Strait

“…1B) is situated at 118.0°E-121.0°E, 17.0°N-21.0°N along part of Malina trench, which reaches depths of about 5000 m, but the average depth of the SCS is only about 1500 m (Hayes and Lewis, 1984). The area is affected by the northeast monsoon in winter and southwest monsoon in summer (Shaw and Chao, 1994).…”

Occurrence of 1 ka-old corals on an uplifted reef terrace in west Luzon, Philippines: Implications for a prehistoric extreme wave event in the South China Sea region

“…Fault widths were estimated using seismicity and coastal deformation patterns as interpreted from uplifted coral terraces in west Luzon [e.g., (Ramos and Tsutsumi 2010)]. The dip angle of the megathrust-and upper-plate faults was inferred from early geophysical data (e.g., gravity, seismic reflection) across the Manila Trench forearc region (Hayes and Lewis 1984) coupled with focal mechanism solutions (Fig. 1b).…”

“…The geometric characteristics of modeled fault ruptures along the northern Manila Trench forearc region were assumed from seismicity patterns, earthquake focal mechanisms, geophysical data, and coseismic deformation inferred from uplifted coral terraces [e.g., (Ramos and Tsutsumi 2010;Hayes and Lewis 1984)]. Source fault parameters and resulting surface deformation were then established using scaling relations for subduction zone earthquakes, upper-plate faults, and elastic dislocation models (Okada 1985;Murotani et al 2013;Wells and Coppersmith 1994) (Table 2), while also considering the effect of bathymetry and horizontal displacement (Tanioka 1996).…”