Abstract

Turbidites crop out in large parts of NW Borneo, including Kalimantan and Sarawak. The strata predominantly comprise the Embaluh Group in Kalimantan and lateral equivalents the Rajang Group in Sarawak (collectively known as the Rajang–Embaluh Group). Previous interpretation of these rocks as the product of deep marine deposition and deformation as part of an accretionary prism during Late Cretaceous to Palaeogene subduction implies south-dipping thrusts, northward stratigraphic younging, the existence of a complete arc to trench system, and significant deformation and metamorphism of the turbidites. However new fieldwork described here established consistent southward stratigraphic younging in Kalimantan, no evidence for southward-dipping thrusts and a lack of metamorphism and associated accretionary complex-related deformation features. Furthermore all available palaeontological and radiometric age data indicate that deposition of the bulk of the Rajang–Embaluh Group postdates in-board subduction-related magmatism and thus a complete arc-trench system cannot be demonstrated. Consequently the Rajang–Embaluh Group turbidites are re-interpreted as having formed in either a post-collisional foreland basin or a remnant ocean basin. The lack of a identifiable mountain belt and linked thrust system, and probable oceanic aYnity of crust beneath the Rajang–Embaluh Group basin favour the latter interpretation.

Borneo occupies a pivotal position within southeast Asia. This new model of a remnant oceanic basin setting for extensive turbidite units of Borneo therefore has far-reaching implications for the tectonic evolution of Borneo, the South China Sea and SE Asia. The new model is compared with older, Palaeozoic orogenic belts which also contain thick turbidites sequences.

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