Growing evidence of active deformation in the Malay basin region

Abstract

Very young crustal movements in the Malay basin region point to the possibility of reactivation of regional faults in the basin proper that may compromise their sealing integrity. In addition, active or reactivated faults that are rooted in the pre-Tertiary basement and reach up close to the base of Quaternary seabed sediments in the basin pose obvious hazards to offshore installations, such as production platforms and pipelines. The Malay basin originated in the Late Cretaceous as a major aulacogen on the Malay Dome and developed structurally through modifications by differential extrusion of Indosinian crustal slabs. Initially the extrusion imparted sinistral transtensional wrenching on the axial basement fault along the basin length. In post Mid-Miocene, wrench slip reversal produced transpression, accompanied by general structural inversion. From the Pliocene onward most of the basin area has been considered tectonically quiet on the basis of horizontal stratification, absence of explosive volcanism, absence of earthquake epicentres, and low topographic/ bathymetric relief. However, basement-rooted regional fault zones may intrude into Pliocene-Pleistocene strata and reach as high as 150 metres below the shallow seabed. This suggests reactivation of the faults. Onshore Peninsular Malaysia, small Early Tertiary basins host lacustrine and fluvial-dominated deposits. These basins appear associated with regional fault zones that most probably remained active up to that time. Neogene deposits are apparently missing while the blanket of Quaternary sediments only indicates local disturbances associated with superficial base collapse and gravity sliding. On the other hand, an Early Quaternary pillow-basalt flow near Kuantan on the eastern shore of the Peninsula is traversed by long fractures orientated parallel to faults in the pre-Tertiary basement. The fractures in the basalt are essentially vertical and are evident manifestations of reactivation of the older faults. In Southeast Johor at the edge of the Penyu basin, crustal uplift of 0.5-0.8 m during the past 5000 years is suggested by an abrasion platform that is that much higher compared to the eustatic Holocene sea-level curve of the Peninsula which was established from almost a hundred radiometrically determined bio-shoreline indicators. In the northwest on the shores of Langkawi, a 2500-year old abrasion platform is cut by a long fault zone whose associated secondary structures suggest sinistral displacement. The 26 December 2004 mega-thrust Indian Ocean earthquake is shown by GPS measurements to have displaced the entire Peninsula by several centimetres toward west-southwest. One of the findings of ongoing research in the Langkawi islands is of very recent crustal uplift of 40-50 centimetres that manifests as sea-level notches at elevated positions above present mean sea level.