Neotectonics of the Marikina Valley fault system (MVFS) and tectonic framework of structures in northern and central Luzon, Philippines

Abstract

Recognition of neotectonic features along the Marikina Valley fault system (MVFS) in central Luzon, Philippines indicates a dominantly dextral strike-slip motion during its most recent activity believed to be Late Pleistocene to Holocene in age. Variations in the ratios of vertical to horizontal displacements for the segments imply a dominantly dextral motion of the West Marikina Valley fault (WMVF) and oblique dextral motion for the East Marikina Valley fault (EMVF). The displacement data further suggest that rupturing along the EMVF involved multiple segments and occurred separately from the events along the WMVF segments. Estimated earthquake magnitudes for the WMVF and EMVF based on single-event offsets fall within the range M 7.3–7.7. The vertical slip component in the northern part of the Marikina Valley is associated with the development of a basin between the EMVF and WMVF while the large vertical component in the southernmost segment of the EMVF (Talim) is attributed to volcanism-related extension. Lateral advection of the block bounded by the MVFS and the Philippine fault zone (PFZ), rather than pure shear resulting from an assumed east–west compression, best explains the observed kinematics of the MVFS. This is the result of compression during the westward drift of the Philippine Sea Plate and northern Luzon and occurs through slip along the WMVF and EMVF at rates of 5–7 mm/yr.

Introduction

The 135-km long dextral Marikina Valley fault system (MVFS) or Valley fault system (PHIVOLCS, 1999) is a major fault transecting the eastern portion of Metropolitan Manila and belongs to a system of faults and subduction zones that accommodates oblique convergence between the Philippine Sea Plate (PSP) and the Eurasian Plate (Fig. 1). The over 1600 km-long Philippine fault zone (PFZ) is an active sinistral structure that extends from Luzon to Mindanao. The 1990 Luzon earthquake was centered along one of its splays, the Digdig fault. The PFZ accommodates much of the relative movement of the two plates between the trench systems and may act to decouple the northwestward movement of the PSP from the southeastward movement of the Eurasian plate. An alternative view is that the PFZ accommodates the boundary-parallel component of the overall plate convergence as a trench-linked strike-slip fault related to the Manila trench and/or the Philippine trench (Fitch, 1972, Karig, 1983, Yeats et al., 1997). The known and predicted slips of the PFZ and most of its splays are consistent with this west-northwest to northwest motion of the PSP. The Digdig fault and the PFZ are predominantly sinistral strike-slip faults (although there is a considerable reverse component across the Digdig fault). Except for its northeastern portion, the predominantly dextral MVFS strikes parallel to the N–S-trending portion of the Digdig fault and to the PFZ directly east of the MVFS. Current GPS data cannot resolve MVFS' kinematics given the large uncertainty involved, and Thibault (1999) suggested that resolvable movement across the MVFS is minor. Nevertheless, a dextral sense of strike-slip is indicated by the longer-term, near-field slip data presented in this paper. How slip is accommodated among the faults is important to understanding the nature of the plate boundary; this paper will in part address this issue.

The MVFS branches southward from the PFZ (Fig. 2) and bounds the Marikina Valley, the northern part of which is interpreted as a pull-apart basin. We have not investigated the nature of the junction of the MVFS and PFZ in detail due to logistical constraints. The southern part of the East Marikina Valley fault (EMVF) lies on the western side of the Southern Sierra Madre, whereas the West Marikina Valley fault (WMVF) bounds the west side of the valley (Fig. 2). The MVFS appears to terminate to the south against another tectonic feature, the Macolod Corridor (Fig. 2), which is a zone of volcanoes related to northeast-trending extensional structures that define a rift. The rifting front is located south of Tagaytay Highlands. However, the rifting front does not coincide with any known faults, so its location is only approximate. The trace of the WMVF farther south of the ridge, if it exists, has been buried by young eruptive materials from Taal Volcano.

This study will contribute to understanding the style, nature and distribution in space and time of some of the deformation resulting from plate interactions in the Philippines. Knowledge of these effects will also contribute to the understanding of plate kinematics and geometry of subducting plates and their boundaries. Mapping of active fault traces and associated morphologic features will contribute in evaluating the role the MVFS plays in the plate boundary. In addition, more reasonable assessments of the nature, timing and size of future surface ruptures along the MVFS can be made, which are of critical importance to estimating seismic hazard and risk for Metro Manila.