Discussion of a Miocene collisional belt in north Borneo: uplift mechanism and isostatic adjustment quantified by thermochronology

Abstract

Scientific editing by Alex Maltman. John Milsom & Robert Holt write: Knowledge of the gravity field is vital to any study of isostasy. This was acknowledged by ⇓Hutchison et al. (2000) when they noted that the work of ⇓Holt (1998) placed constraints on their geological model for Sabah. However, they went on to ignore these constraints, arguing that the depth of the crustal root beneath the Western Cordillera might have been overestimated due to ‘the model assumption of an excessively thick ophiolitic basement’ and that the presence of thick crust beneath Sabah was ‘in conflict with the outcropping geology’. The Hutchison et al. interpretations of the deep structure of Sabah were illustrated in a series of cross-sections in which some features (notably the topography) were amplified for the sake of clarity. There was, however, no suggestion that any features would be underemphasized, and the depth scales, although described as only ‘rough guides’, indicated normal 5–10 km thicknesses for oceanic crust. Therefore, and in the light of the comments quoted above, the thin crust shown throughout Sabah and the generally somewhat greater thickness of the extended continental crust of the Dangerous Grounds must be regarded as essential elements of the model. The Late Miocene cross-section (⇓Fig. 1), showing the end of orogeny in Sabah, is a guide to the Hutchison et al. view of the present-day situation. Crust is less than 30 km thick beneath the Western Cordillera and less than 20 km thick elsewhere. We contend that these values are incompatible with the gravity field, and that it is no more acceptable to present a geological interpretation that ignores geophysical data than a geophysical interpretation that ignores geology. Some way must be found of reconciling the two. Fig. 1. Sabah in the Late Miocene, from ⇓Hutchison et al. (2000, fig. 6b). …