Strike-Slip Faults in the Cagayan Basin: Possible Splays of the Philippine Fault

Raymundo B. Savella

PNOC Exploration Corporation

Introduction

The Cagayan Valley in northern Luzon is a vast expanse of flat lands occasionally marked by rolling hills. On the surface, it appears that the area is generally undeformed compared to other onshore basins like Mindoro or Cebu, which are characterized by a more rugged topography. However, surface and seismic data shows that deformation has affected the basin. Anticlines have been mapped forming elongate, doubly plunging geomorphic features. This is the reason why oil exploration companies flocked to the area. They wanted to test if these anticlines that have been mapped contain hydrocarbons.

Faulting has also affected the basin. Both geologic and geophysical data show the presence of these structures. In fact, the major structures that have been mapped by previous workers using seismic and magnetic data acquired in the Cagayan Basin are strike-slip faults.

If anticlines and strike-slip faults have been identified in the basin, what is its relationship to the Philippine Fault? This major fault system has been mapped along the eastern corridor of Central Luzon. It continues northward and branches out into several splays in the Central Cordillera Ranges before it links with the Manila Trench. No splay/s has been mapped cutting the basin although features that are associated with wrenching have been identified by earlier workers.

This paper proposes that splay/s of the Philippine Fault System are present in the Cagayan Basin as seen on the seismic data.