Surface uplift in world’s youngest orogen, can crustal thickening explain the uplift in Timor?

Abstract

The island of Timor, SE Asia is the result of Arc-continent collision of the Banda Arc with the Australian Continental margin. Shortening of the Australian plate strata leads to thickening of the crust. Assuming isostasy, this should cause uplift. We hypothesize that the uplift of Timor can be explained by the built-up of a fold-and-thrust belt. During this built up sedimentation of basins which structurally lie on top of this fold-and-thrust belt, was taking place. These syn-orogenic basins, known as the Viqueque formation, are passive recorders of the uplift history. We reconstruct the uplift history by building an age model for the Viqueque formation and using occurrences of depth markers (benthic foraminifera). Combining the age model with depth differences allows us to reconstruct the uplift history. Timor has experienced uplift in two phases: a phase with almost no significant uplift and generally very low uplift rates during the deposition of the lower part of the Viqueque formation, followed by a phase of rapid uplift, up to ~3 mm/yr. The fold-and-thrust belt has been thickened by an average thickening factor of 3.3. If we assume Airy-type isostasy, and that thickening starts around the first deposition of Viqueque strata, we find a theoretical uplift rate of around ~0.4 mm/yr. Homogeneous thickening of the fold-and-thrust belt alone might be enough to explain the low uplift rates in the lower part of the Viqueque formation. But it is not enough to explain the phase of rapid uplift. Another process is likely the cause for the observed uplift. Activation of the Wetar thrust might explain the uplift, but this is poorly constraint. Slab detachment is unlikely the cause because there is no evidence that the slab has broken off. Timor’s position in the Banda-Arc Australia collision zone makes delamination the most likely process. But further research is needed to strengthen this hypothesis.