Borneo: a stable portion of the Eurasian margin since the Eocene

Abstract

Samples collected from sixteen paleomagnetic sites of Plio-Pleistocene basaltic rocks from southeastern Borneo yield mean direction of D m = 180.4°, I m = −13.8° (α 95 7.8° k = 23.6) and a paleomagnetic pole of λ = 82.7° N, σ = 120.5° E . Samples collected from nine sites, Eocene-Early Miocene basalt, andesite, and shale, yield a mean direction of D m = 182.0°, I m = −2.2° (α 95 = 11.0° k = 22.8) and a mean Eocene-Early Miocene pole of λ = 87.8° N, σ = 173.5° E . These Eocene-Early Miocene directions pass a fold test. Rock magnetic studies suggest that the carrier of remanence in the samples is magnetite. These Eocene to Pleistocene paleomagnetic results from southeastern Borneo suggest the island has been at its present position with respect to Eurasia since Eocene. Our results are consistent with other studies showing no evidence for latitudinal displacement; however, these results do not concur with the previous reports of a large Tertiary counterclockwise rotation. Our data are in general agreement with the rotations predicted by the extrusion tectonic model for southeast Asia and argue against the numerous Cenozoic tectonic reconstructions of the region which require a large counterclockwise rotation of Borneo.