Interpretation of a Gravity
Profile Across the Sibuyan Ophiolite Complex: Implications on its
Emplacement Mechanism
Estephanie
Gera L. Ramos, Carla B. Dimalanta,
Rodolfo A. Tamayo Jr. and Graciano P. Yumul, Jr.*
National Institute of Geological Sciences, University of the
Philippines, Diliman, Quezon City
*also with the Depatment of Science and Technology, Bicutan, Taguig,
Metro Manila
Geological
investigations carried out in the islands of Tablas, Romblon and
Sibuyan in Central Philippines enabled the characterization of
several lithologic units, most importantly, those crucial in the
delineation of the arc-continent collision between the Philippine
Mobile Belt and the Palawan Continental Block. New formations,
namely, the Sibuyan Ophiolite Complex (SOC) and the Agnoknok Mélange
in addition to previously established rock suites (i.e., Romblon
Metamorphics, Tablas Volcanic Complex, Calatrava Diorite, Binoog
Formation, Anahao Formation, and Peliw Formation) were recognized.
The units of the Sibuyan Ophiolite Complex are concentrated in
Calatrava and Alcantara in Tablas Island and in the central portion
of Sibuyan Island, occupying most of Mt. Guiting-Guiting. The SOC is
thrusted onto the Romblon Metamorphics. The latter makes up the
whole of Romblon Island, the northern and southern areas of Tablas
Island, and the western and eastern coastlines of Sibuyan Island. In
addition, sedimentary suites (Binoog Formation, Anahao Formation,
Banton Volcanics, and Peliw Formation), are widely exposed in Tablas
Island. Metamorphic rocks, ophiolitic rocks, mélanges and deformed
sedimentary sequences have been used as markers in delineating the
location and extent of the Palawan Continental Block – Philippine
Mobile Belt suture zone (e.g. Hamilton, 1979; Rangin et al., 1985;
Sarewitz and Karig, 1986; Faure et al., 1989). The mica-bearing
schist variety of the Romblon Metamorphics has geochemical
signatures similar to continent-derived sediments and thus, as a
whole, the formation might represent a fragment of the Palawan
Microcontinental Block. The Sibuyan Ophiolite Complex, on the other
hand, displays geochemical affinities with subduction-related
oceanic crust – mantle rocks formed possibly along the western (?)
edge of the Philippine Mobile Belt. The lithologic make-up of the
Romblon Island Group suggests that it lies within (?) the suture
zone.
The
gravity method was used to determine the subsurface configuration of
the Sibuyan Ophiolite Complex in order to give light on its
emplacement mechanism and to provide additional constraints on the
Central Philippines collision event. An east-west profile cutting
across the units of the Sibuyan Ophiolite Complex in Sibuyan Island
was investigated. Gravity values were collected along stations,
which are spaced 1 km apart, using a SCINTREX CG-5 gravity meter.
Station locations were carefully selected to minimize terrain
effects. Station locations were identified using a hand-held Global
Positioning System. Elevations were interpolated from topographic
maps. Standard gravity corrections were applied to the collected
data (tide, drift, free-air correction, Bouguer correction). The
resulting Bouguer anomalies were subjected to forward modeling using
the modeling program GM-SysTM. Model constraints were
provided by geologic maps and density values obtained from rock
samples collected from outcrops.
The best
– fit model shows that the Sibuyan Ophiolite Complex is not deeply
rooted in the mantle, with the units of the ophiolite emplaced on
top of the Romblon Metamorphics via east-verging thrust faults. The
direction of structures separating the units of the Sibuyan
Ophiolite may be explained by obduction onto the Palawan
Microcontinental Block during arc-continent collision and then
westward translation due to gravity sliding after isostatic rebound
as is the case in the Oman Ophiolite in the Oman Mountains (Gealey,
1977; Nicolas, 1989). Isostatic rebound is accomplished easily by
the detachment of the subducting oceanic lithosphere from the
continental passive lithosphere (Shervais, 2001). At present,
seismic tomography and earthquake hypocenter studies indicate the
absence of a slab beneath the Central Philippines (Ramos, 2005),
which is consistent with the proposed scenario. This model also
suggests that the basement of the Romblon Island Group consists of
continent-derived materials. |