Collisional Deformation in the Philippines: Origins af Anticlines for Hydrocarbon Exploration

Emmanuel G. Ramos

Manila Observatory

Abstract

Aside from the marginally-deformed geological traps (reefs, arenite beds, etc.), the genesis of four types of anticlines arising from certain tectonic conditions in the Philippines can be used as targets for hydrocarbon exploration. The four types of structures are the anticlines found in the 1^st order anticlinorium, those in the 2^nd order anticlinorium, the pop-up structures along transpressional faults, and the fault-drag anticlines created along the Philippine Fault. The 1^st order anticlinorium is created by the collision of Palawan into the mobile zone, creating a series of large anticlines that formed the islands and ridges from the west Panay-Mindoro collisional mountain ranges, to the now deforming SE Bohol orogeny. The 2^nd order anticlinorium is created as the Marinduque Island maintains a rigid position relative to the mobile zone, colliding with the crust to its east and creating anticlines that warp in an apparent E-W convergence. Pop-up structures are created in transpressional faults where folds are created parallel to and adjacent to the fault. Many of these structures were formed through sinistral faulting in various sites within the mobile zone, from Mindoro to the present alignment of the Philippine Fault. The fault-drag anticlines (or antiforms) are created as the western block deforms from the transcurrent stresses of the Philippine Fault (and its former alignments). Owing to the diminishing intensity of sinistral stresses into the opposing (western) block, these fault-drag anticlines appear to plunge to the south. The continuing development of these faults while transcurrent motion was occurring made the axes of the anticlines bend in west-facing concaves. The northern end of these fault-drag anticlines (or antiforms) is also diagnostic since their axes all appear to be fault-truncated. Recognition of these four types of anticlines can serve as structural guides for hydrocarbon exploration within the Philippine mobile zone.