Coastal Tectonics and Paleo-sea Level of Northwest Luzon, Philippines Constrained from Raised Coral Terraces

Abstract:

Raised Holocene coral terraces in Badoc and Currimao, Ilocos Norte, Philippines are studied to constrain the Holocene coastal tectonics and sea level history of northwest Luzon, Philippines. Situated 80 km along the northern section of the Manila Trench, Currimao and Badoc, Ilocos Norte prove to be interesting sites for uplift studies along tectonically active coasts. In both localities, uplifted coral reef terraces are well preserved and often separated by meter-scale terrace risers while the lower terraces are usually rocky with well-formed notches. Two to three terrace steps were distinctly identified in both localities in which datable coral samples were gathered from the lower terrace steps (T1 and T2). In Currimao, uplifted coral platforms have two (or possibly three) terrace steps: the lower terrace step (T1) measured 3-3.5 m above mean sea level (amsl) while T2 measured 5 m amsl. Three (to possibly four) terrace steps are documented in Badoc in which the lowest terrace (T1) measured 1-2.5 m, followed by T2 which measured 3-3.5 m, and T3 which measured 4-5 m amsl. Age constraints based on ¹⁴C- and ²³⁰Th-dating of fossil corals reveal ~6.4 ky and ~6.8 ky for the lower terrace steps of Currimao and Badoc, respectively while ~1 ky-old corals were obtained from the upper terraces of Badoc Island at 4-5 m amsl. Based on these ages, we infer that the ~6 ky-old terraces correspond to the mid-Holocene marine transgression while the ~1 ky-old terrace is probably related to coseismic and/or aseismic uplift. While emerged coral platforms separated by meter-scale terrace risers suggest abrupt uplift mechanisms, emergence may also be attributed to aseismic uplift related to convergence and seamount subduction along the Manila Trench. Holocene uplift rates estimated in Currimao and Badoc are 0.5 to 0.8 mm/yr and 0.4 to 0.7 mm/yr, respectively and are comparatively lower than the maximum Holocene uplift rates reported in Pangasinan (~1.7 mm/yr) and eastern Mindanao (~1.5 mm/yr).