Bulletin of the Geological Society of Malaysia, Volume 31, July 1992, pp. 51 – 65
Geology Department, University of Malaya, 59100 Kuala Lumpur
Abstract: A major regional trend in the geomorphology, geology and hydrocarbon fields of NW Borneo is aligned NE-SW and parallel to the Mulu shear zone, a tectonic feature which has imposed a cleavage only·on the lower part of the massive Melinau Limestone in N. Sarawak. The cleavage dies out at higher stratigraphic levels towards the NW. The Mulu shear zone can be traced northwards to Brunei where it forms a narrow belt of intensely sheared rocks in the Temburong Formation as well as causing overconsolidation in younger, softer sediments. Evidence of other shear zones which are possibly the displaced continuation of the Mulu shear zone or major shear zones parallel to it, occur in central and SW Sarawak between Miri and Bintulu, in the Tatau and Kuching areas. In west and central Sabah, NE-trending faults are prevalent, and at least one fault between circular sedimentary basins has associated dioritic plugs, indicating a deep structural weakness.
In addition to cleavage, other manifestations of the shear zone are a hardening of Tertiary sediments which spall on exposure, ptygmatic folds associated with quartz veins and the imposition of schistosity with the development of sericite. The various manifestations probably represent differing reactions to shear according to depth.
Offshore, particularly in Sabah, seismic sections across the main NE regional trend show splayed fault patterns which converge at depth to what is presumed to be a major tectonic fracture zone in basement.
Age relationships and genetic connotations of the Mulu Shear Zone and its associated structures are discussed.