Bimodal Cenozoic Volcanism in Central Sarawak: Hot Spots or Extension?

1. Bimodal Cenozoic Volcanism in Central Sarawak: Hot Spots or Extension? Nur Iskandar Taib Geology Department, University of Malaya, Kuala Lumpur iskandartaib@um.edu.my

2. Several Cenozoic volcanic edifices are present in the Upper Rajang Valley, central Sarawak. • Early Miocene Bukit Mersing • High K Basalt • Erupted conformably within the Bawang Member of the Belaga Formation of the Rajang Group • Plio-Pleistocene – Usun Apau, Hose Mountains, Linau-Balui, Niewenhuis Mountains, several others in Kalimantan. • Bimodal – Early dacite followed by later basalt. At Usun Apau, basalts are about 2-2.5 m.a., dacites about 4 m.a. • Linau-Balui erupted on river terraces, thought to be Recent but are a lot older. • Rajang Group – Eocene, in this area deep-water sediments, an accretionary wedge deformed by the Sarawak orogeny (end of Eocene). The basement under the Rajang is enigmatic. • Map from Kirk, 1957

3. USUNAPAU LINAU BALUI BUKIT MERSING HOSE MTS

4. Geological map of the Hose Mountains • Not shown are basalts on the crest of the plateau • Map from Kirk, 1957

6. REE plot Bukit Mersing – more enriched than Plio-Pleistocene basalts Steep REE patterns, no Eu anomaly OIB-type parent melt, no evidence of continental contamination

8. Incompatible element (Spider) plot Bukit Mersing – more enriched than Plio-Pleistocene basalts in incompatible elements also No significant Nb (and other HFSE) depletion Also indicates OIB-type parent melt with no arc/subduction signature

9. Plio-pleistocene basalts have similar isotopic signature to basalts from Semporna of the same age. Components/contaminants ruled out – I-MORB, Sulu and Celebes Sea crust. Also EM1, HIMU and DMM mantle components. EM2 as a mantle component remains a possibility. Macpherson et al. (2010) interpretation for Semporna basalt – REE parent melt (as represented by SCS seamounts) contaminated with Archean domains within the basement. If true the basement underneath the Rajang Valley is continental in nature. After MacPherson et al. (2010)

10. Extension vs. Hot Spots • Both have been used as explanations for the presence of volcanics. In some cases, actual evidence exists, in others not. Both produce “OIB”s, so the presence of OIBs is ambivalent. The two are interrelated – hot spots beget triple junctions and extension, but not all extension is due to hot spots. Bimodal volcanism is usually associated with rifts, but continental hot spots may also produce the stratified magma chambers that give rise to bimodal volcanism. • Evidence of hot spots: • Tracks (usually seen on oceanic hot spots) • High heat flow and volcanism • A swell or bulge on the earth’s surface • Seismic tomography • Evidence of extension: • High heat flow and volcanism • Rift valleys, grabens, linear lakes, other linear depressions and associated sedimentary basins • Normal faulting, seismicity (?) X X X X don't have yet X X X X X

11. Other explanations • Plio-Pleistocene volcanism occurs where the crust is thin (or weak) and mantle-derived melts can make it to the surface (MacPherson et al., 2010). • Example: Hainan and seamounts in the South China Sea – thinning of crust due to opening of South China Sea • Compatible with model of Cenozoic Vietnamese volcanism, attributed to extension of crust where regional strike-slip faults intersect (e.g. Nguyen et al., 1996)

12. Cenozoic volcanism in SEA Eruption in 1923 So how connected is all this? Geohazards?

13. IN MEMORY OF CHARLES S. HUTCHISON Professor Emeritus, University of Malaya 1933-2011 Who pointed me in this direction in the first place