GEOLOGICAL SOCIETY OF THE PHILIPPINES
| THE CENTRAL ZAMBOANGA CONVERGENT PLATE BOUNDARY, MINDANAO, PHILIPPINES: IMPLICATIONS ON THE TECTONIC EVOLUTION OF WESTERN MINDANAO |
Cliff L. Querubin1,2, Graciano P. Yumul, Jr.2,3, Arnulfo V. Cabantog1, Jesicca N. Lucero1
The central Zamboanga Peninsula in Mindanao, Philippines represents a structurally complex zone characterized by disparate lithologies with poorly constrained affinities. Previous studies made pertaining to the tectonic evolution of the area were essentially based on dated geologic maps and reports. The mapping of the part of the Zamboanga Peninsula was undertaken in response to the apparent need for an updated geologic database. This study provides a number of findings that have led to a reevaluation of the geology of the particular area.
The main lithologic units comprising the Central Zamboanga Peninsula were identified, segregated, and assigned temporary designations. Of these lithologic units, the most significant are the Dansalan Amphibolites representing the core of Mount Dansalan, the Dansalan Metamorphics fringing the Dansalan Amphibolites and the Tampilisan Melange at the southeast margin of the Dansalan Metamorphics. Field evidence suggests that the Danasalan Amphibiolites are gradational into the Danasalan Metamorphics and that the whole complex represents a domal structure. The contact between the Dansalan Metamorphics and the Tampilisan Melange is a NE-SW trending, SE dipping thrust fault. It is inferred that this incipient convergent plate boundary formed as a consequence of the opening of the SE Sulu Sea Basin. Likewise, the amphibolite/metamorphic complex possibly represents an early rift product from the opening of this same basin. The juxtaposition of the amphibolite/metamorphic complex with nascent proto-Zamboanga arc may have occasioned trench jumping and the formation of the Tampilisan Melange. Following this event is the translation of the NE-SW trending thrust fault into a shear zone through a series of left-lateral strike-slip faulting. Later events include uplift and continued volcanism.
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