Late Quaternary uplift across northwest Luzon Island, Philippines constrained from emergent coral reef terraces

Abstract

AbstractEmergent coral reef terraces in northwest Luzon Island, Philippines are studied to understand relative sea level (RSL) changes and uplift along the Manila Trench forearc region during the Late Quaternary. Coral reef sequences in Currimao, Badoc, and Badoc Island were mapped and were described based on geomorphological characteristics, elevation patterns, and ages. The lower emergent coral reef terraces, which are dated Holocene, are subdivided into three to possibly four terrace steps (TI–TIV in ascending order) with the highest terrace (TIII–TIV) rising up to ~7–9 m amsl. The well‐preserved staircase morphology and meter‐scale terrace risers possibly imply episodes of abrupt RSL changes that resulted from coseismic uplift during the mid‐Holocene. Around 6.8 kyr BP, ~3 m of rapid uplift (possibly coseismic) is inferred to account for the vertical separation of TI (6.2 kyr BP at 3.5 m amsl) and TII (6.8 kyr BP at 6.6 m amsl) in Currimao site. A subsequent episode of RSL fall occurred at 6 kyr BP, based on dated coral samples, and has emerged the lowest terraces in the study sites. New geomorphic and radiometric data suggest that the mid‐Holocene sea level highstand in northwest Luzon occurred at around 7–6 kyr BP. From the elevation and thorium‐230 age of dated terrace surfaces, we estimated Holocene uplift rates from 0.17 ± 0.12 to 0.82 ± 0.15 mm/yr across the three study sites. The estimated uplift rates of northwest Luzon are relatively lower than the uplift rates reported in similar tectonically active regions of the west Pacific such as Papua New Guinea, Taiwan, New Zealand, and Japan. The geomorphological characteristics and age of emergent coral reef terraces reported in this study offer insights on the preservation of long‐term deformation along the northwest Luzon coastal plain that is bordered by a subduction zone and upper‐plate faults. © 2018 John Wiley & Sons, Ltd.