Geology Reference
In-Depth Information
Fig. 14.10 The K
2
O versus
SiO
2
for the calc, HK-calc, K
and EK (highly evolved K-
series) rocks of
Ringgit
-
Besser. The
boundaries are drawn
according to the suggestion of
Peccerillo and Taylor (1976)
[after Edwards et al. 1994]
7
6
5
4
3
2
HK calc-alkaline
1
calc-alkaline
42
44
46
48
50
52
54
56
58
SiO wt%
2
EK series
K series
HK cale series
cale series
Archipelago. The age of these complexes decreases from Sumatra (late Palaeozoic)
to eastern end of the system in the Banda arc (late Tertiary). According to Audley-
Charles (1975), the present arc became a single entity in the mid-Tertiary. The pre-
mid-Tertiary geological evolution of the arc west of Sumbawa was distinct from
that to the east. McElhinny et al. (1974) thought that the pre-mid-Tertiary evolution
of the eastern Sunda and Banda arcs was related to that of the neighbouring
Australian continent while the western Sunda arc might be unrelated (McElhinny
et al. 1974). Figure
14.10
shows the presence of calc-series, highly potassic calc-
series, shoshonites and leucititic rocks from Ringgit
Beser complex in east Java.
Occurrence of these rocks are related to their melting from a Benioff zone and the
leucititic rocks coming from a deeper source.
This evolutionary pattern re
-
ects the tectonic environment along Sunda arc. The
crust is relatively thick (
25 km) in Sumatra and the presence of Palaeozoic
granites and pelitic schists (Katili 1973) suggests a mature continental crust. The
young eastern Sunda and Banda arcs (Tertiary) present many immature features on
oceanic crust (Purdy et al. 1977). The 20 km thick crust beneath Java and Bali, has
a velocity structure intermediate between that of continental and oceanic and the
oldest exposed rocks are of Mesozoic age (Katili 1973).
The Sunda arc presents a regular spatial distribution of earthquake epicentres
(Fitch and Molnar 1970). According to Fitch (1970) shallow focus earthquakes
(<70 km) are con
*
ned to a less than 200 km wide narrow zone along the entire
length of the arc, located between the trench and the volcanic arc. He also described
the presence of intermediate and deep focus earthquakes (>70 km) occurring in a
parallel zone further from the trench. Hatherton and Dickinson (1969) discussed
andesitic volcanism and seismicity in Indonesia, the lesser Antilles and other island
arcs, and Cawthorn (1977) showed a systematic correlation between the depth of
the Earthquake foci with respect to the potash content of the orogenic magmas (also
see Beswick 1976).
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