Transcript GEOSEA 2012 - Nur Iskandar Taib

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
APAU
BUKIT
MERSING
LINAU
BALUI
HOSE
MTS
Geological map of the Hose
Mountains
Not shown are basalts on the
crest of the plateau
Map from Kirk, 1957
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
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
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
– 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 (?)
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)
Cenozoic volcanism
in SEA
Eruption in
1923
So how connected is all
this?
Geohazards?
IN MEMORY OF
CHARLES S. HUTCHISON
Professor Emeritus, University of Malaya
1933-2011
Who pointed me in this direction in the first place