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The AAPG/Datapages Combined Publications Database
AAPG Bulletin
Abstract
AAPG Bulletin, V.
1Manuscript received September 22, 1997; revised manuscript
received September 8, 1998; final acceptance October 3, 1998.
2Department of Petroleum Geoscience, University of Brunei
Darussalem, Bandar Seri Begawan, 2028, Negara Brunei Darussalem; e-mail:
[email protected]
ABSTRACT
Seismic reflection data across six boundary
faults from east Africa
have revealed information about the way
boundary
faults develop with respect
to the synkinematic basin fill. Previous models of basin fill assume that
lateral fault propagation and deposition occur simultaneously, which gives
rise to along-axis onlap of the basin fill onto the prerift basement (propagating
basin model). Seismic reflection data from
boundary
faults in east Africa
show only rare evidence for along-axis broadening of the basins. Instead,
the basin margins appear to be fixed from a very early stage in the basin
history (nonpropagating basin model). Depocenters shift not by gradual
lateral propagation, but by abrupt changes in location. Such observations
indicate that
boundary
fault propagation to near-maximum length occurs
rapidly during the very early stages of rifting, probably by linkage of
numerous small faults. Displacement then builds (following the b-value
earthquake model) along the fault during basin development, out of phase
with the main propagation phase. Mature fault systems may have experienced
several phases of jumps in fault length or the location of fault activity.
Major
boundary
faults exhibit three basic patterns of evolution; these
patterns of evolution can have many variants and include (1) simple displacement,
where maximum displacement is approximately at the center of the fault
and decreases gradually toward the fault tips, (2) variable along-strike
displacement that incorporates linkage of in-line or en-echelon, relatively
large-displacement faults of similar age leaving transverse anticlines
and synclines, and (3) asymmetric propagation where one tip of a fault
remains more or less fixed while the other tip propagates a relatively
long distance. Asymmetric propagation can occur smoothly and gradually
or, alternatively, there can be a jump in the location of the fault, and
a new fault of different age can form along-strike from the older fault.
Eventually the new fault propagates and links with the older fault segment,
reactivating the older fault to some extent. Addition ally, long-lived
faults may be composites of fault types 1-3.
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