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

Abstract

This paper presents the first field, geochemical and geophysical information on the recently recognized Early Cretaceous Balud Ophiolitic Complex (BOC) in the island of Masbate in the Central Philippines. Mapping of the western limb of the island revealed that only the upper crustal section of the BOC is exposed in this area. Geochemically, the pillow basalts are characterized by transitional mid-oceanic ridge basalt-island arc tholeiitic compositions. Gravity surveys yielded low Bouguer anomaly values that are consistent with the highly dismembered nature of the BOC. Short wavelength, high amplitude magnetic anomalies registered across the study area are attributed to shallow magnetic sources. This is taken to support the model that the ophiolitic complex occurs as thin crustal slivers that are not deeply-rooted in the mantle. Comparing BOC with other ophiolites in the Central Philippines, such as those in the islands of Sibuyan, Leyte and Bohol, suggests the possibility of a common or contiguous source for similarly-aged and geochemically composed crust-mantle sequences in the region.

Surface Rupture Kinematics of the 2020 Mw6.6 Masbate (Philippines) Earthquake determined from Optical and Radar Data

“…The Late Oligocene Nabangig Formation is the oldest lithologic unit underlying the area of interest and is composed of a clastic sequence with recrystallized limestone. The Middle Miocene Lanang Formation lies unconformably over the Nabangig Formation, and consists of highly deformed sedimentary rocks ranging from conglomerates to mudstones (Manalo et al, 2015). Majority of the study area is underlain by the Miocene-Pliocene Buyag Formation.…”

Bouguer Anomaly of Central Cebu, Philippines

“…Constraints on the subsurface geometries of upper mantle-crust sequences (called 'ophiolites') such as the Southeast Bohol Ophiolite Complex, the Balud Ophiolitic Complex, the Zambales Ophiolite Complex, Sibuyan Ophiolite Complex, and the Palawan Ophiolite have also been provided by gravity data (e.g. Barretto et al, 2000;Dimalanta et al, 2009;Manalo et al, 2015a;Milsom et al, 2009;Salapare et al, 2015). Estimates of crustal thicknesses in parts of the Philippines or for the whole Philippine island arc system have also been obtained from gravity anomalies (e.g.…”