| Summary: | 碩士 === 國立中央大學 === 地球科學學系 === 104 === Manila trench, a convergent plate boundary in the west Pacific Ocean, distributes from offshore SW Taiwan to the north to offshore NW Mindoro to the south. Both ends of the Manila trench terminate as a result of arc-continental collision in different periods. To the north, the Luzon arc collided with the Chinese Continental Margin during Miocene to Pliocene. To the south, the Palawan Continental Block indented into the Philippine Mobile Belt during the Miocene. In this study, we focus on time evolution of arc-continent collision by comparing the slab geometries and upper mantle velocity structures of the two ends.
Using EHB catalog, we sort out earthquakes from 20°N to 22°N and from 12°N to 15°N mapping geometries of subducting slab for the north and south ends of the Manila trench, respectively. A trial-and-error approach is adopted to determine the optimal strike-normal direction to project the earthquakes and ETOPO1 bathymetry into six profiles ranging from subduction to collision. The horizontal distances between reference trench and slab at 200 km depths are also measured. Results show that while there are two bathymetry lows in the north, there is only one in the south, probably due to incipient collision of continents for the former. In terms of geometries of subducting slabs, the southern one is more various than the northern one, with steeping of dip angles initiated at depths and progressively propagating upwards from subduction to collision. The steeping of dip angle can be attributed to the negative buoyancy of slab as attested by the down-dip extension stresses exhibited by intermediate-depth earthquakes.
While the upper mantle velocity structures of the north end near Taiwan are well analyzed by TAIGER project (Taiwan Integrated Geodynamic Research), those of the south end near Mindoro are less well known due to limitation of observations. Therefore, we built broadband stations around NW Mindoro for the investigation. We used DRTs (Differential Residual Times) methods to analyze different first arrival times of teleseismic events, with those of Nasha station as a reference station. Having projected DRTs by lower hemispherical projection, we conclude that there exists steeply dipping aseismic slab beneath central Mindoro, with signatures getting blurred southwards.
|
|---|
