ACTIVE FAULTING IN CENTRAL SULAWESI (EASTERN INDONESIA)
OLIVIER BELLIER 1), MICHEL SEBRIER 1), THIERRY BEAUDOUIN 1), MICHEL VILLENEUVE 2), EKA PUTRANTO 3), IRWAN BAHAR 3), AND INDYO PRATOMO 3)
In the framework of the GEODYSSEA program (an EU-ASEAN program of SE Asia
GPS survey) a subprogram was set up to study the active tectonics of the
Central Sulawesi fault system, in eastern Indonesia. This system consists
of complex distributed left-lateral strike-slip fault zones located within
a triple junction area between three tectonic plates: the Pacific,
Indo-Australian, and Eurasian plates. Seismicity in Central Sulawesi
documents shallow earthquakes located on and around the Central Sulawesi
faults. To characterise the active fault trace geometry and to localise the
high seismic potentiality zones, we analyse SPOT images covering Sulawesi.
This analysis shows, from west to east, the occurrence of two major active
fault zones: The NNW-trending Palu-Koro and the WNW-trending Matano fault
zones. However, deformations are distributed on three fault zones in the
east-central part and SE arm of Sulawesi. The northernmost segments of the
Palu-Koro zone appears geomorphologically more active and bounds the
western part of Palu basin. Study of the Palu-Koro segments evidences late
Quaternary left-lateral geomorphologic feature offsets ranged between 50
and 600 m (streams, alluvial fans, etc.), as well as faceted spurs, shutter
ridges and 300-400 m high triangular and trapezoidal facets. The western
edge of the Palu basin is controlled by a N-trending 2000-2500 m high
escarpments. The significant dip-slip component of the Present-day faulting
is confirmed by Quaternary uplifted marine and alluvial terraces that reach
up to 200 m high. Thus, these geomorphologic characters evidence combined
strike-slip and normal faulting for the Palu-Koro fault Present-day
activity. This transtensional tectonic regime has been confirmed by the
fault slip-vector analyses. We perform an along strike survey of the
Sulawesi faults analysing and dating fault offsets to subsequently
determine fault long-term slip-rate. Datings were done using the U/Th and
ESR methods on the offset marine terraces. In addition, fission track
analysis applied on granodiorites which form the escarpment should
constrain the vertical slip rate. To define precisely the Late Quaternary
seismic history, we excavated seven trenches across the North Palu-Koro
fault zone which exposed paleosoils and colluvial wedges in fault contact
with a poorly weathered alluvial bedrock. This indicates the occurrence of
seismic event during the Late Quaternary, even if the seismic history did
not report any earthquake around the Palu zone.
1) URA-CNRS Geophysique Geodynamique Interne
Bat 509, Univ. Paris-Sud, F. 91405 Orsay Cdx
France
2) URA-CNRS Centre de Sedimentologie-Paleontologie
Univ. de Provence, F. 13331 Marseille
France
3) DGGMR/PPPG - GRDC
Bandung
Indonesia
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