A tectonic study of part of the Philippine fault zone

Abstract

The Philippine Fault Zone which trends n 40°w is widely accepted as a major strike-slip fault comparable to the San Andreas fault. An area in south-eastern Luzon, commonly regarded as part of the main fault zone, has been studied in an effort to date the faulting and to establish its nature and extent. Low-grade metamorphic rocks, of probable pre-Tertiary age, occur only on the north-east side of the Rift; their schistosity and bedding are folded on north-north-westerly axes. Marine early Tertiary formations follow unconformably on both sides of the Rift and locally are folded on east-north-easterly axes. Younger Tertiary terrestrial formations occur only on the south-west side of the Rift. Two main episodes of faulting are recognized, one late Miocene on northerly faults, one Plio-Pleistocene on north-westerly faults. Both these episodes involve considerable dip-slip movements and the two principal Recent faults which have been recognized are also of dip-slip type. Geomorphological evidence shows that the whole area has been subjected to Recent episodic but areally uniform uplift. Direct evidence of strike-slip movements is slight and the structural relations argue against major post-Miocene strike-slip movements. The possibility of earlier strike-slip cannot be excluded and the fault and fold systems show a good correlation with those predicted following the theory of Moody & Hill (1956). Possibly therefore the faults on which the main Miocene and later dip-slip movements occurred were initiated during earlier strike-slip episodes. A pre-Tertiary episode of left-lateral strike-slip in the Rift followed by a Miocene episode of right-lateral strike-slip on conjugate primary faults can be tentatively inferred. The direct evidence of faulting in the Rift, however, relates only to the Plio-Pleistocene activity of the morphogenic phase.