The Dulit triangle in Sarawak: a most striking example of detachment tectonics

The Dulit triangle is a conspicuous kink in the general NE and ENE structural trendlines of central and north Sarawak.. The 45-km high triangle comprises an ENE-striking base of about 75 km length, and NE and NW sides. Study of radar images and field observations confirm that the disharmonic structures of the triangle comprise open synclines of Neogene Belait Formation and tight folds in upper Oligocene-lower Miocene Nyalau Formation that were bent about vertical axes into NE and SE trending structures. The triangular plan is constrained by deeply reaching, regional strike-slip faults forming a frame about the area. The Dulit triangle developed as result of frame-folding and detachment of post Setap sedimentary strata by the mobile Setap mudstone from the underlying pre-Setap formations. The constricted Dulit triangle also moved en bloc towards west, being extruded laterally along the Dengan and Sekiwa faults.