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The AAPG/Datapages Combined Publications Database
AAPG Bulletin
Abstract
(Begin page 233)
AAPG Bulletin, V.
Analog models of restraining stepovers in strike-slip
fault
systems
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Ken McClay,1 Massimo Bonora2
1Fault
Dynamics Research Group, Geology Department, Royal
Holloway University of London, Egham, Surrey TW20 OEX, United Kingdom; email: [email protected]
2Midland Valley, 14 Park Circus, Glasgow, G3 6AX, Scotland, United Kingdom
AUTHORS
Ken McClay comes from Adelaide, Australia. He has a B.Sc.
(honors) degree from Adelaide University and an M.Sc. degree and Ph.D. in structural
geology from Imperial College, London. He lectured at Goldsmiths College and is now at
Royal Holloway University of London. He has been professor of structural geology since
1991 and is director of the Fault
Dynamics Research Group. He was AAPG distinguished
lecturer in North America 1994-1995 and AAPG International distinguished lecturer
1998-1999. His research involves extension, thrust, strike-slip, and inversion terranes
and their applications to hydrocarbon exploration. He publishes widely, consults, and
gives short courses to industry.
Massimo Bonora comes from Ferrara, Italy. He received his degree
in geological sciences from Ferrara University and his M.Sc. degree in basin evolution and
dynamics from Royal Holloway University of London. Between 1995 and 1998 Massimo worked as
a research assistant in the Fault
Dynamics Research Group at Royal Holloway. Massimo is
now working as a structural geologist within the Latin America team at Midland Valley Ltd.
in Glasgow, Scotland.
ACKNOWLEDGMENTS
The research for this article has been supported by the Fault
Dynamics
Project (sponsored by ARCO British Limited, Petrobrás U.K. Ltd., BP Exploration, Conoco
(U.K.) Limited, Mobil North Sea Limited, and Sun Oil Britain). Ken McClay also gratefully
acknowledges funding from ARCO British Limited and BP Exploration. We thank J. Reijs for
the data for Figure 21. Critical reviews by A. Sylvester, D. Stone,
and J. Sheridan were greatly appreciated. We thank Tim Dooley for many fruitful
discussions and assistance with drafting diagrams. Howard Moore constructed the
deformation apparatus.
Fault
Dynamics Publication No. 74.
ABSTRACT
Scaled sandbox models have successfully simulated the geometries
and progressive evolution of antiformal pop-up structures developed in a weak sedimentary
cover above restraining stepovers in offset sinistral strike-slip fault
systems in rigid
basement. Models were run both with and without synkinematic sedimentation, which was
added incrementally to cover the growing antiformal structures. Vertical and horizontal
sections of the completed models permit the full three-dimensional (3-D) structure of the
pop-ups to be analyzed in detail. Three representative end-member experiments are
described: 30° underlapping restraining stepovers; 90° neutral restraining stepovers;
and 150° overlapping restraining stepovers.
The experimental pop-ups are typically sigmoidal to lozenge-shaped,
antiformal structures having geometries that are dependent on both the stepover angle and
stepover width in the underlying basement faults. Underlapping restraining stepovers
typically form elongate lozenge-shaped pop-ups; 90° neutral restraining stepovers produce
shorter, squat rhomboidal pop-ups; and overlapping restraining stepovers produce sigmoidal
antiformal pop-ups. Trans pop-up cross fault
systems are characteristic at large
displacements on the basement
fault
system. Above the offset principal displacement zones,
the pop-ups are commonly small, narrow, positive flower structures, whereas in the
stepover region, they widen out and become markedly asymmetric. This pop-up asymmetry
switches across the center of the stepover, where the pop-ups are largely symmetical.
Maximum rotations measured within the central highly uplifted region of the pop-ups
increase from 7° counterclockwise for the underlapping (30°) stepovers, to 14°
counterclockwise for the neutral (90°) stepovers, to 16° counterclockwise for the
overlapping (150°) stepovers.
In models where no synkinematic sediments were added during
deformation, the pop-up structures are bound by convex, flattening-upward, oblique-slip
reverse fault
systems that link downward to the offsets in the basement
fault
system. In
contrast, in the (Begin page 234) experiments where synkinematic
sediments were added incrementally during deformation, the pop-ups are formed by
oblique-slip reverse faults that steepen upward into the synkinematic strata with the
formation of
fault
-propagation growth folds.
The analog models are compared with natural examples of pop-up
structures and show strong similarities in structural geometries and stratal
architectures. These models may provide structural templates for seismic interpretation of
complex contractional structures in offset strike-slip fault
systems.
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