Gravity variations along the Southeast Bohol Ophiolite Complex (SEBOC), Central Philippines: Implications on Ophiolite Emplacement

Abstract

The basement complex of Bohol Island consists of the Southeast Bohol Ophiolite Complex (SEBOC), Cansiwang Melange and Alicia Schist. The SEBOC is a complete, but dismembered ophiolite with outcrops generally trending northeastsouthwest and dipping north-west. The harzburgite units of the SEBOC are almost always observed to be thrusted onto the Cansiwang Melange, which in turn is thrusted onto the Alicia Schist. Bouguer gravity values on Bohol range from about +60 mGal in the west to +120 mGal in the east, in the region to the north-east of the SEBOC outcrops. Based on the present distribution of the SEBOC units and their thrust fault relationship with the Cansiwang Melange and Alicia Schist, it is proposed that the SEBOC was emplaced by onramping towards the south-eastward direction. However, the orientation of the Bouguer highs suggests that the thrusting direction of the ophiolite units is towards the south-west and not towards the south-east.

Geochemical and Geophysical Characteristics of the Balud Ophiolitic Complex (BOC), Masbate Island, Philippines: Implications for its Generation, Evolution and Emplacement

“…Slightly lower anomalies (15 mGals) were observed over the Panguiranan Chert. The low gravity anomaly of the BOC is comparable to Bouguer anomalies of other dismembered ophiolites such as the Marum ophiolite complex in Papua New Guinea, East Halmahera ophiolite in Indonesia, and the South East Bohol, Cebu and Antique ophiolite complexes in Central Philippines (e.g., Milsom 1984;Milsom et al 1996;Barretto et al 2000;. It also suggests that the ophiolite is not deeply rooted in the mantle.…”

“…Previous works on Central Philippine ophiolites and ophiolitic sequences show these discrete crust-mantle slices to be of supra-subduction affinity [e.g., Tacloban, Samar, Malitbog, Southeast Bohol, Cebu] (e.g., Barretto et al 2000;Faustino et al 2003;Tamayo et al 2004;Suerte et al 2005;. Apart from the Oligocene Amnay Ophiolite in Mindoro which has the ocean island basalts (OIB) or enriched mid-oceanic ridge basalts (EMORB) component in addition to its transitional island arc tholeiite -N-MORB geochemistry (Jumawan et al 1998;Perez et al 2013), the general observation is that ophiolites in the central Philippines were generated with variable degrees of subduction influences (e.g., Faustino et al 2006;Dimalanta et al 2009;Yumul et al 2013).…”

“…Based on geophysical observations, most ophiolites in the region are allochthonous and shallow-rooted. Gravity and magnetic investigations show that the SEBOC, Sibuyan and Tacloban ophiolites were emplaced as thin slivers of oceanic crust (e.g., Barretto et al 2000;Dimalanta et al , 2009. Their occurrence along the peripheries of islands as highly-dismembered and often poorly-preserved units suggests complex emplacement histories consistent with emplacement along subduction zones.…”

“…Florendo (1987) suggests the existence of an Early Cretaceous subduction in the Central Philippines followed by back-arc spreading to the east. Recent isotopic ages and geochemical data suggest that the ophiolite and ophiolitic units in the eastern and central Philippines were formed from a single oceanic basin, called the proto-Philippine Sea Plate (e.g., Florendo 1987;Maac and Ylade 1988;Barretto et al 2000; A left lateral strike-slip fault traverses the archipelago. The study area is highlighted in a box.…”

“…(c) General comparison of lithology, age and geochemical characteristics of the ophiolites in the Central Philippines. Numbered references are as follows: (1) Rangin et al (1991), (2) Tamayo et al (2001), (3) Barretto et al (2000), (4) Faustino et al (2006), (5) Wolcke and Scholz (1988), (6) Hashimoto (1981), ( Suerte et al 2005;. The delineation of the Balud Ophiolitic Complex (BOC) in southwest Masbate, Central Philippines gives additional information that hopefully would contribute to the reconstruction and understanding of the evolution of the Philippines and this part of Southeast Asia.…”