TITLE:
Imaging Rock Density Distribution beneath Liwa Fracture Zone in the Southern Part of the Great Sumatran Fault System, Indonesia
AUTHORS:
Djedi S. Widarto, Tedi Yudistira, Jun-Ichi Nishida, Ikuo Katsura, Eddy Z. Gaffar, Susumu Nishimura
KEYWORDS:
Gravity Imaging, Rock Density, Fracture Zone, 2D Modeling, 3D Inversion, Density Contrast
JOURNAL NAME:
International Journal of Geosciences,
Vol.7 No.4,
April
28,
2016
ABSTRACT: We have imaged rock
density distribution beneath Liwa fracture zone in the southern part of the the
Sumatran Fault Zone by modelling and inverting Bouguer gravity data in two-and
three-dimensional environments, respectively.The
purpose of this study is aimed to figure out the subsurface distribution of
rock densities associated with subsurface basement structure representing the
evidence of trans-tensional tectonic product in the SF.In the gravity modeling, to eliminate
distortions to the measured gravity values before modelling and inverting the
data, Bouguer anomalies obtained in field measurements are reduced to the
horizontal plane of z = +800 m as a representation of the average elevation in
Liwa. For the inversion, we used algorithm implementing depth-and minimum
volume weighting parameters in order to obtain a smooth model with better
vertical resolution. The two-dimensional models show clearly surface topography
of the basement rocks and the presence of normal faults. The reduced Bouguer
anomaly of +800 m elevation shows the presence of structural lineaments
extending primarily in a northwest-southeast direction, parallel to Sumatran
Fault Zone and older graben faults showing a negative flower structure. From
the three-dimensional inversion, the model illustrates an increase of density
contrast, lower values being found near the surface and higher values in the
deeper parts. The lower density contrast of 0.15 to 0.3 g/cm3found in the rock groups at depths of
2 km and less is characteristic of relatively homogeneous and poorly compacted
rocks. Rocks with moderate to high density contrast (>1.0 g/cm3)
are recognized at depths of over 2 km. This model suggests a change of basement
morphology as a function of depth, and delineates structural lineaments
extending in northwest-southeast direction. This study supports the previous
thought that Liwa area is underlain by graben structures, formed by
trans-tensional tectonic events. Higher-density Tertiary volcanic breccia and lower-density
Quaternary volcanic products of the Ranau Formation form the basement rocks and
the overlying younger sediments, respectively.