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Bimodal Cenozoic Volcanism in Central Sarawak: Hot Spots or Extension?. Nur Iskandar Taib Geology Department, University of Malaya, Kuala Lumpur [email protected] Several Cenozoic volcanic edifices are present in the Upper Rajang Valley, central Sarawak. Early Miocene Bukit Mersing

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Bimodal cenozoic volcanism in central sarawak hot spots or extension

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

Nur Iskandar Taib

Geology Department, University of Malaya,

Kuala Lumpur

[email protected]


  • 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


USUN Rajang Valley, central Sarawak.APAU

LINAU

BALUI

BUKIT

MERSING

HOSE

MTS



REE plot Rajang Valley, central Sarawak.

Bukit Mersing – more enriched than Plio-Pleistocene basalts

Steep REE patterns, no Eu anomaly

OIB-type parent melt, no evidence of continental contamination


Incompatible element (Spider) plot Rajang Valley, central Sarawak.

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


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)


Extension vs. Hot Spots basalts from Semporna of the same age.

  • 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 (?)

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Other explanations basalts from Semporna of the same age.

  • 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)


Cenozoic volcanism in SEA basalts from Semporna of the same age.

Eruption in 1923

So how connected is all this?

Geohazards?


IN MEMORY OF basalts from Semporna of the same age.

CHARLES S. HUTCHISON

Professor Emeritus, University of Malaya

1933-2011

Who pointed me in this direction in the first place


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