McCLAY, KEN, Arco Professor of Structural Geology, Royal Holloway, University of London, UK
Abstract: 4-D Analysis of Extensional
Fault
Systems in Rift Basins
The 4-D evolution of extensional fault
systems
in sedimentary basins and in particular rift systems has been investigated
using scaled sandbox analog models. Sandbox models have proved to be a
powerful and graphic tool in developing an understanding of the 4-D geometric
and kinematic evolution of extensional
fault
systems. The model results
have been compared with natural examples of
fault
systems at both outcrop
and seismic scales. Many rift basins and passive margins contain major
hydrocarbon accumulations, and an understanding of the geometric and kinematic
evolution of the
fault
systems that control them is vital for successful
exploration and production.
Analog models of rift basins have been constructed using dry, cohesionless, fine-grained quartz sand to simulate the brittle deformation of sedimentary rocks in the upper 10 km of the crust. Extensional deformation in the models was controlled by the orientation and geometry of a zone of stretching at the base of the model, either a rubber sheet or a layer of viscous silicone polymer. Models have been run for orthogonal, oblique, offset, and stepped rift systems. The top surfaces of the models were recorded by time-lapse photography, and completed models were serially sectioned for detailed analysis.
In orthogonal and oblique rift models stretching
of the sandpack above a zone of ductile deformation at the base of the
models produced model rift basins constrained by long, initially segmented
border fault
systems parallel to the underlying zone of basal stretching,
together with sub-basins within the rift formed by domino-style intra-rift
faults. For the orthogonal (90°) and for oblique rift models where
the zone of stretching (rift axis) was up to 15° to the extension direction,
the intra-rift faults were at high angles to the extension direction. For
oblique rift models where the rift axis was 45° or less to the extension
direction the intra-rift, faults were rotated sub-parallel to the rift
axis. Offset and stepped rift models were characterized by highly segmented
border faults and offset sub-basins within the rift without the development
of strike-slip or oblique-slip transfer faults. For the oblique, offset,
and stepped rift models, both the intra-rift and rift border faults are
highly segmented with individual offsets of like-dipping, domino-style,
extensional faults forming characteristic relay ramp structures. Offset,
oppositely dipping extensional faults form interlocking
fault
arrays--transfer
zones. Along-strike displacement transfer within the rift between segmented
and offset sub-basins is accommodated by “soft-linked” accommodation zones
characterized by interlocking arrays of conjugate extensional
fault
systems.
The results of these analog model studies have permitted the construction
of 4-D evolutionary extensional
fault
models that can be applied to natural
fault
systems in sedimentary basins.
The results of the analog models are compared
and contrasted with natural examples of extensional fault
systems from
the Gulf of Suez and Red Sea, Egypt, from the Gulf of Aden, Yemen, from
the North Sea, Indonesia, and Australia. These natural extensional
fault
systems show geometries, segmentation, and offset structures that are extremely
similar to those developed in the analog models.
AAPG Search and Discovery Article #90922©1998-1999 AAPG International Distinguished Lectures