Forearc structures and deformation along the Manila Trench

The Manila Trench subduction zone is characterized by varying tectonic structures from north to south. Analyses of new seismic reflection data and bathymetric data indicate distinct morphological and deformational patterns in the forearc region. Differences in the nature of the subducting oceanic lithosphere (i.e. seafloor relief related to seamounts and ridges, sediment supply, reactivated features and faults associated with the South China Sea opening) cause along-strike heterogeneity in the Manila Trench and the Luzon forearc region. The northern segment is classified as an accretionary margin while the southern segment is mainly an erosive margin (with a narrow, steep, and often eroded frontal wedge). This sharp contrast is attributed to abundant sediment supply to the trench in the north and to the highly eroded frontal wedge in the south due to scarce sediment supply. The southern trench segment is prone to submarine slope failures and mass wasting processes. The 17°N latitude boundary also separates the forearc basin into the North Luzon Trough and the West Luzon Trough. Associated with this is the initiation of the Scarborough Seamount Chain (South China Sea extinct spreading ridge) subduction at 16°N latitude. A combination of forearc uplift and submarine mass movements attributed to subduction of bathymetric highs near and south of 17°N latitude produced the Stewart Bank. Seamount and other seafloor spreading features induced complex responses arising to diverse forearc architectures in the southern segment. Seamounts and other seafloor spreading-related features in the subducting slab induce slope steepening and significant vertical movement in the frontal wedge and the forearc region, respectively. Deformation associated with the subduction is overprinted by shearing related to the Philippine Fault Zone splay faults with the frontal wedge shortening associated with the ongoing subduction, further complicating the forearc development in the trench and marine forearc region.