Abstract

A new approach for restoring sediment volumes onto the sediment source area to estimate uplift timing and magnitude is discussed and used in the context of late Tertiary basin and topographic development in NW Borneo. Sediment volumes for the Baram Deltaic Province were estimated for four time periods (latest Early, Middle and Late Miocene, and Pliocene–Recent), using 2D and 3D seismic horizons, wells and outcrops. Volume restoration onto the palaeo-sediment source area determined exhumation amount at the drainage divide (c. 5 km from 17 Ma to Recent) and provided a reasonable match with other denudation estimation methods (cooling ages and an older, regional sediment restoration study using 1979 vintage sediment isopach maps). Restoration took into account: (1) changing sediment densities during the erosion–deposition cycle; (2) changing area of the sediment source province with time (e.g. changing shoreline location as a result of eustacy, uplift and delta progradation, tectonic shortening); (3) uplift and partial erosion of Baram Deltaic Province sediment. Denudation at the drainage divide for the Middle Miocene, Late Miocene and Pliocene–Recent has proceeded at a similar rate for each period. Initial uplift of central Borneo has been attributed to buoyancy of thinned continental crust that jammed the subduction zone under NW Borneo in the Early Miocene. However, the absence of decay in the erosion rates with time from the Middle Miocene to Recent suggests operation of an additional uplift mechanism that may be related to delamination of mantle lithosphere; slab detachment is the favoured explanation.

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