The Gravity Fields of Palawan and New Caledonia: Insights into the Subsurface Geometries of Ophiolites
Milsom, John, Barretto, Jenny, Aguda, Nancy, Bringas, Dennis, Ho, Romeo, Aitchison, Jonathan, Journal of the Geological Society
New gravity data from Palawan, in the western Philippines, emphasize resemblances to New Caledonia. On both islands, ophiolites derived from root zones just offshore rest on basal surfaces that change abruptly in depth by as much as 5 km. The subsurface geometries of these thrust sheets are as important to an understanding of their emplacement histories as are their exposure patterns.
Many aspects of the emplacement and post-emplacement histories of suprasubduction-zone (SSZ) ophiolites are still only poorly understood. However, because of densities that are usually significantly greater than those of continental basements (typically by about 0.2 Mg m3 for the mafic and by more than 0.4 Mg m3 for the ultramafic components), ophiolite geometries can be investigated in three dimensions using gravity measurements. In April 2008, as part of a collaborative study of ophiolites in the northern and western Philippines involving the University of Hong Kong and Philippine government agencies, 120 new gravity stations were established on Palawan in the SW Philippines.
Among the reasons for the Palawan survey was the obvious analogy with New Caledonia, which lies in the Tasman Sea east of Australia and at the northern end of the Norfolk Ridge. The two islands are roughly the same size, are similarly long and narrow (Fig. 1), and are dominated by ophiolites emplaced during arc-microcontinent collision. The ophiolites on New Caledonia are exposed over larger areas (Fig. 1a), but on Palawan they are more complete (see Raschka et al. 1985). In each case, the collision that ended subduction appears to have also ended the oceanic spreading that displaced the microcontinent.
Geotectonic settings. Cretaceous extension at the eastern margin of Gondwanaland culminated in the separation, at about 80 Ma, of New Zealand and the Norfolk Ridge from Australia by the opening of the Tasman Sea (Auzende et al. 2000). This phase ended in the latest Palaeocene (c. 54 Ma), when the ridge collided with the SW-facing 'Loyalty Islands' subduction zone. Ophiolites were emplaced on New Caledonia, spreading ceased in the Tasman Sea and volcanism ceased in the Loyalty Islands. Today, the distance between the former volcanic arc and New Caledonia is about 130 km (Fig. 1a), which is less than most modern arc-trench gaps but comparable with modern fore-arc basins.
Some 20 Ma later, a rather similar chain of events began with the opening of the South China Sea from spreading axes with east-west orientations (Briais et al. 1993). A fragment of southern China that was carried south and east to eventual collision with a NW-facing subduction zone now forms the basement not only of Palawan but also of the Reed Bank and Dangerous Grounds plateaux (Clift et al. 2008). The former volcanic arc is represented by the almost entirely submerged Cagayan Ridge, which is parallel to, and some 160 km SE of, Palawan.
Basement rocks. The oldest rocks on New Caledonia belong to Palaeozoic arc-related terranes, which, together with Jurassic schists and younger arc elements, formed part of the Mesozoic margin of eastern Gondwanaland (Aitchison et al. 1995, 1998). These are all now overlain by sediments of the break-up phase, the rift-drift phase and finally, and with the inclusion of olistostromes, of the pre-collision foredeep. The entire succession forms the lower plate to the ophiolite overthrust.
The oldest rocks on Palawan are Permo-Carboniferous schists (Wolfart et al. 1986). This metamorphic basement is overlain by Cretaceous clastic sediments with some limestones and cherts that are in turn overlain by limestones deposited during the relatively short drift phase. There is a large olistostrome in the extreme north, assigned by Faure & Ishida (1990) to the Late Jurassic. Dating gravity slides, in which sediments are extensively reworked, is notoriously difficult and a later date seems possible.
Ophiolites. The largest …
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Publication information: Article title: The Gravity Fields of Palawan and New Caledonia: Insights into the Subsurface Geometries of Ophiolites. Contributors: Milsom, John - Author, Barretto, Jenny - Author, Aguda, Nancy - Author, Bringas, Dennis - Author, Ho, Romeo - Author, Aitchison, Jonathan - Author. Journal title: Journal of the Geological Society. Volume: 166. Publication date: November 2009. Page number: 985+. © Geological Society Publishing House Jan 2009. Provided by ProQuest LLC. All Rights Reserved.
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