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Fault-Bend Folding and Synrift Deposition: An Example from the Central
Sumatra Basin, Indonesia1
John H. Shaw,2 Stephen
C. Hook,2 and Edward P. Sitohang3
We describe an analytical method
for interpreting the geometry and structural history of asymmetric half
grabens in rift basins with extensional fault-bend fold theory. Using seismic
reflection profiles from the Central Sumatra basin and balanced forward
models, we show how local variations in tectonic subsidence relative to
deposition rates yield distinctive patterns of folded synrift strata and
unconformities that record basin history. If the deposition rate exceeds
the local subsidence rate, folded growth strata form upwardly narrowing
kink bands that have been described previously as growth triangles. In
contrast, if the deposition rate is less than or equals the local subsidence
rate, growth strata are folded and truncated at the surface on half-graben
margins. Subsequent increases in deposition rate relative to subsidence
rate form angular unconformities near the half-graben margins. These unconformities
develop without the necessity of erosion and are folded by continuing fault
slip. Strata above and below the unconformities are concordant in the deeper
parts of the half grabens. Thus, angular unconformities on half-graben
margins are helpful for defining sequence boundaries that may reflect changes
in deposition and tectonic subsidence rates. In addition, fault-bend fold
interpretations yield fault geometry and measures of horizontal extension,
both of which control three-dimensional half-graben geometry
1997. The American Association of Petroleum Geologists. All rights reserved.
received April 4, 1995; revised manuscript received June 4, 1996; final
acceptance October 23, 1996.
Exploration and Production Technology Dept., 3901 Briarpark, Houston, Texas
CALTEX Pacific Indonesia, Rumbai, Pekanbaru 28271, Indonesia.
thank Hongbin Xiao and John Suppe for helpful insights into their extensional
fault-bend fold theory, which provided the foundation for this work. Exceptional
reviews by M. Scott Wilkerson and Walter F. Dula, Jr., improved the manuscript.
In addition, discussions with Karsani Aulia, Richard E. Bischke, Peter
A. Brennan, Chris D. Connors, Paul W. Genovese, Tom L. Heidrick, and Elizabeth
A. Lorenzetti provided insights into our structural interpretations and
presentation of the theory. Seismic reflection data were provided by PT
CALTEX Pacific Indonesia.
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