Uplift, tilting, and underlying structures of coastal northwestern Luzon, Philippines, deduced from marine terraces

Northern Luzon Island of the Philippines, located between the converging Sunda and Philippine Sea Plates, is a seismically active region within a complex tectonic setting. Remnants of Pleistocene marine terraces along the coast of northwestern Luzon have been deformed and provide clues to the active underlying structures. Through remote sensing and field surveys, we mapped and surveyed multiple generations of uplifted Pleistocene marine terraces between the municipalities of Pasuquin and Santo Domingo along the northwestern Luzon coast. We found that wave-cut platforms are the most common type of marine terrace in the area, each consisting of a sedimentary basement (Laoag Formation) and a fluvial or alluvial deposit cover. The uplift of these terraces shows that the area north of Laoag City has experienced westward tilting while the area farther south has been uniformly uplifted. With K-feldspar infrared stimulated luminescence (IRSL) dating of sand samples collected above the abrasion surface of the wave-cut platforms, we infer that the most prominent marine terrace, with a seaward edge 40–50 m above mean sea level, was formed during marine isotope stage (MIS) 5a. This suggests a long-term uplift rate of ∼1 m/kyr along the coast and up to 1.6 m/kyr farther inland. Combining evidence from field surveys and bathymetric and focal mechanism data, we suggest that the large-scale uplift of the coastal area is the result of slip along an east-dipping thrust with a surface trace ∼12 km off the west coast, while a west-dipping branch of the Vigan-Aggao Fault Zone has contributed to tilting, horizontal (sinistral) movement, and localized folding of the coastal area. Through elastic dislocation modeling, we suggest the offshore fault likely has a dip angle lower than 45°, and the difference in uplift patterns between north and south can be attributed to different slip-rate distributions between the two faults. Overall, we estimate west-northwest directed shortening at 1–2 m/kyr across the nearshore and coastal area of northwestern Luzon.