Abstract
Northwest Panay consists of two terranes that form part of the Central Philippine collision zone: Buruanga Peninsula and Antique Range. The Buruanga Peninsula consists of a Jurassic chert-clastic-limestone sequence, typical of oceanic plate stratigraphy of the Palawan Micro-continental Block. The Antique Range is characterized by Antique Ophiolite Complex peridotites and Miocene volcanic and clastic rocks, representing obducted oceanic crust that serves as the oceanic leading edge of the collision with the Philippine Mobile Belt. The Nabas Fault is identified as the boundary between the two terranes. This study employed the gravity method to characterize the Northwest Panay subsurface structure. Results indicate higher Bouguer anomaly values for Buruanga Peninsula than those for Antique Range, separated by a sudden decrease in gravity values toward the east-southeast (ESE) direction. Forward gravity data modeling indicates the presence of an underlying basaltic subducted slab in the Buruanga Peninsula. Furthermore, the Nabas Fault is characterized as an east-dipping thrust structure formed by Buruanga Peninsula basement leading edge subduction beneath Antique Range. Additional geophysical constraints were provided by shallow seismic refraction and electrical resistivity surveys. Results from both methods delineated the shallow subsurface signature of the Nabas Fault buried beneath alluvium deposits. The gravity, seismic refraction and electrical resistivity methods were consistent in identifying the Nabas Fault as the terrane boundary between the Buruanga Peninsula and the Antique Range. The three geophysical methods helped constrain the subsurface configuration in Northwest Panay.
Highlights
Arc-continent collision processes usually result in crustal-growth processes such as arc magmatism, ophiolite emplacement, terrane accretion and suturing (Condie 1997; Dimalanta and Yumul 2004; English and Johnston 2005)
The gravity values significantly decrease to the east as the Buruanga Peninsula neck is approached
East of the Nabas Fault a small peak can be observed in the area where Fragante Formation volcanic rocks outcrop, their values are still lower than those for the Buruanga Peninsula
Summary
Arc-continent collision processes usually result in crustal-growth processes such as arc magmatism, ophiolite emplacement, terrane accretion and suturing (Condie 1997; Dimalanta and Yumul 2004; English and Johnston 2005). The participation of oceanic bathymetric highs (e.g., seamounts, spreading ridge, micro-continental blocks) contributes to the significant geologic features observed in (e.g., Aitchison et al 1995; Byrne and Liu 2002; Brown et al 2006; Yumul et al 2009), the Central Philippines consists of distinct oceanic, continental and ophiolitic terranes (McCabe et al 1985; Zamoras et al 2008). In a collisionrelated tectonic setting such as the Central Philippines, the geophysical signatures may include large positive gravity, resistivity and velocity anomalies for ophiolitic bodies and oceanic bathymetric highs and lower gravity, resistivity and velocity anomalies from structures related to the collision (Hayes and Lewis 1984; Besana et al 1997; Franco and Abbott 1999; Dimalanta and Yumul 2004). In the study area the Nabas Fault at the neck of the Buruanga Peninsula serves as the boundary between the two terranes (McCabe et al 1982; Zamoras et al 2008). No surface manifestation of this tectonic feature was observed since the fault zone is covered by alluvial deposits (Francisco 1956; Zamoras et al 2008)
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