Structure and geological history of the Lepanto-Cervantes releasing bend on the Abra river fault, Luzon Central Cordillera, Philippines

Abstract

The Luzon Central Cordillera corresponds to the uplifted and tectonized magmatic arc associated with the Manila Trench subduction zone. It is cut into N-S trending strips by the horsetail-like overprint of the Philippine strike-slip fault. The median branch, herein named the Abra River Fault, splits away to the west from the Dalton Fault and runs north-northwestward for 240 km through the range. Trending N150E in its southern part, it undergoes a sinistral bend in the Lepanto-Cervantes area, and then strikes N-S, makes a second bend and runs north-northeast up to the northwestern tip of the island. In the bend area, the compartment east of the fault has subsided, forming a narrow strip along the southern Lepanto segment and a wider trough parallel to the fault which stretches 25 km from Cervantes. The basement of the strip is position dependent, corresponding either to the Oligo-Miocene arc succession (volcanoclastic sediments and dioritic intrusive), or to the pre-Late Eocene ophiolitic basement of the arc. In the Lepanto area, two volcanic formations are preserved: the oldest one, the Malaya Formation is latest Miocene in age (6.1 ± 0.3 and 5.7 ± 0.28 Ma by K/Ar) and represents the infill of the trough, the youngest, the Mount Pudso volcanics, is Holocene in age (0.5 Ma by K/Ar). The Cervantes trough contains a thick (reaching 1700 m) volcanoclastic sequence dated as latest Miocene-Early Pliocene (3.7 ± 0.18−6.2 ± 0.28 Ma by K/Ar (the Malaya Formation). On the western and southern sides, the Abra River Fault separates the very deformed infill from the strongly uplifted pre-Late Miocene basement. On the east side, the basement, whose elevation increases gradually eastward, is disconformably overlain by the Mio-Pliocene sequence. To the north, the trough is slightly deformed. The structural analysis of the Lepanto-Cervantes strip, together with the local and regional geological and geochronological data, allow us to propose a three-stage history. The first stage during the early Late Miocene, is the major left-lateral wrench displacement of the Abra River Fault and the appearance of the bend under a transpressive regime. The second stage from latest Miocene to Early Pliocene, and marked by strong volcanic activity, comes with and/or immediately follows the subsidence of the Lepanto-Cervantes strip. The last stage, starting in the Late Pliocene, is characterized by the E-W foreshortening of the Cervantes Trough, and the beginning of its erosion.