Deformation of an alpine ultramafic association in Darvel Bay, East Sabah, Malaysia

Abstract

The island-arc system of the Philippines, which includes the well-known Zambales alpine complex, continues westwards along Palawan island and along the Sulu archipelago into North Borneo. Within this arc, there is a characteristic association of ultramafic rocks, banded amphibolites of gabbroic composition, and a younger flysch sequence characterised by abundant spilites. In the Darvel Bay area of east Sabah (North Borneo), the ultramafic rocks are shown to be concordantly interfoliated with the banded amphibolites. The folding, which is predominantly along an east-west axial direction, has been accompanied by dynamothermal metamorphism characteristically to almandine-amphibolite facies. An increase in metamorphism to hornblende-granulite facies occurs in a "thermal aureole" of the syntectonic ultramafic body in central Darvel Bay. The island-arc system has been subjected to several periods of deformation, the youngest of which was accompanied by greenschist facies metamorphism, which has affected not only the ultramafic-metabasite association, but also the overlying flysch sequence and has converted many of the spilites to amphibolites. Structural data indicate that ultramafic intrusions accompanied both the older and the younger periods of deformation. Subsequent intense faulting has further obscured the earlier tectonic history of the ultramafic association.