Geologic Structures at the Mahiao, Sambaloran and Malitbog Sectors of the Tongonan Geothermal Field, Leyte, Philippines

Abstract

Recent structuralmapping at the Mahiao, Sambaloran and Malitbog sectors indicate that the Philippine Fault at Tongonan consists of five sub-parallel fault branches trending N30-60W spaced at an average interval of 800 m. Of these branches, only the West and Central Fault Lines have the most prominent geomorphic expressions and display clear and unequivocal left-lateral sense of displacement. Striking perpendicular to the main wrench axis are shorter en-echelon NE-trending faults while the WNWtrending leftlateral faults in Bao and Mahanagdong intersect the main wrench faults at 12 - 30'. The NE- and WNIJ-trending faults are interpreted as conjugate antithetic and synthetic faults, respectively. Bisecting the angle between the wrench axis and the antithetic faults are the few N-S trending tension structures. The trends of the wrench faults. conjugate strike-slip faults and folds of northern Leyte suggest a N40W left-lateral coupling stress. The N-S orientation of the tension structures at Tongonan is consistent with the above stress field ; these structures are believed to be related to a different stress regime possibly caused by plutonic intrusion. Based on the regional strain pattern, it appears that wrenching along the northern Leyte segment of the Philippine Fault did not produce a systematic set of tension fractures.