Crustal structure and deformation in the Manila subduction zone

• Main Authors: Leo Armada, 黎歐
• Other Authors: Hsu Shu-Kun
• Format: Others
• Language: en_US
• Published: 2016
• Online Access: http://ndltd.ncl.edu.tw/handle/92127668698998211922

博士 === 國立中央大學 === 國際研究生博士學位學程 === 104 === Crustal structure and deformation in the Manila subduction zone Leo Armada Taiwan International Graduate Program- Earth System Science National Central University ABSTRACT The Manila trench is the active convergent margin where the South China Sea (SCS) basin is being subducted beneath the western portion of Luzon Island (Philippines). The deformation in the study area, as in other subduction zones, is associated with natural hazards like large mega-thrust earthquakes, submarine landslides and tsunamis. These processes pose great risks to populations and infrastructures in surrounding regions. In this regard, it is important to study the present crustal structure and ongoing deformation in the Manila subduction zone. This dissertation presents research results derived from new multi-channel seismic reflection (MCS) data (collected using the Ocean Researcher 5) and high quality bathymetry. The subduction zone is characterized by varying deformation patterns in the subduction interface and the overlying fore-arc region. Analyses of the marine MCS and bathymetric datasets indicate a distinct morphological and deformational boundary near 17°N latitude. 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 SCS opening) cause variations in the deformation observed across this boundary in the Manila trench and fore-arc. The northern segment is classified as an accretionary margin (with a wide frontal wedge with gentle slope) while the southern segment is mainly an erosive margin (with a narrow and steep frontal wedge). Consequently, the steep frontal wedge in the southern segment is prone to submarine slope failures and mass wasting. The boundary also separates the fore-arc basin into the North Luzon Trough (northern fore-arc basin) and the West Luzon Trough (southern fore-arc basin). Abundant sediments in the southern fore-arc basin and less sediments in the northern fore-arc basin imply faster strain loading in the southern segment compared to the northern segment of the subduction mega-thrust. Furthermore, detailed images of the structure indicate that the erosive margin in the southern segment has a history of seamount subduction with associated large submarine slope failures. The inferred ancient submarine mass wasting events may have caused tsunamis in adjacent areas. The data also show that the Scarborough Seamount Chain (extinct mid-ocean ridge) is not yet subducted near 16°N latitude, contrary to previous models. Seamounts in the subducting oceanic lithosphere may induce fracturing of the overlying fore-arc crust as it progresses downward. The fracture networks near the subduction interface may then limit the rupture area of future mega-thrust earthquakes. This will lead to the low probability of great subduction earthquakes (Mw≥9) in the southern segment of the Manila trench, although intermediate sized earthquakes should not be discounted. The upper crustal structure of the southern end of the Manila trench is characterized by compressive deformation. It is also overlapping with strike-slip (shear) motion related to the Philippine Fault Zone. The NW-SE oriented ridge and basin morphologies related to shearing and the steep frontal wedge are conducive to submarine slope failure in this portion of the Manila trench.