TECTONICALLY UPLIFTED FLUVIAL TERRACES AND ASSOCIATED FEATURES, SAN MANUEL AREA, LUZON, PHILIPPINES

This study arose from borrow source investigations for the San Roque multipurpose dam project, located on the Agno River along the southwest margin of the Central Cordillera in northern Luzon, near the town of San Manuel. The area is undergoing uplift in response to oblique convergence between an eastward dipping subduction zone, located west of Luzon, and a westward dipping subduction zone to the east of the island. The subduction zones are bathymetrically expressed by the Manila Trench and the East Luzon Trench, respectively. Convergence between the two subduction zones is accommodated in part by displacement along the northwest-striking San Manuel fault, which is located 8km southwest of the San Roque site and is a branch of the left-lateral, strike-slip Philippine fault system. Long-term displacement on the San Manuel fault includes a significant component of vertical motion with uplift on the northeast side. The ratio between lateral and vertical displacement may be on the order 3:1.

The Agno River exits the Cordillera into a 20km long by 5km deep topographic embayment in the mountain front that is situated on the up-thrown side of the San Manuel fault. The Agno and adjacent streams have deposited large coalescent fans within the embayment, which have been uplifted by displacement on the San Manuel fault. Episodic uplift and downcutting by the Agno River have created a series of as many as four remnant terraces and associated fluvial features along the northwest side of the embayment, between the San Roque site and San Manuel. Terraces also occur along the mountain front northwest of the embayment where there are reported vertical offsets along the San Manuel fault of 2.5 to 4.5 meters in Holocene terraces and 38 meters in Pleistocene terraces. This paper describes the geomorphic complexes and the relationships between uplift and geomorphic features in the San Manuel-San Roque area. The paper also describes some of the stratigraphic and geophysical characteristics of the geomorphic features that relate to their mode of origin.