MCCLAY, KEN R., ARCO Professor of Structural Geology, Royal Holloway University of London, UK
Most strike-slip fault systems in sedimentary basins are segmented or stepped along strike such that they produce regions of transtensional or transpressive deformation. Releasing stepovers or jogs produce transtensional deformation commonly leading to the formation of pull-apart basins whereas restraining stepovers or jogs produce transpressional or contractional deformation leading to the formation of contractional uplifts or "pop-up" structures. Structures within these stepovers or jogs are usually complex and 3-D in character with steeply dipping faults and 3-D rotated fault blocks. Seismic imaging across such strike-slip zones is commonly extremely poor. In many basins, these strike-slip fault systems contain significant hydrocarbon accumulations but their detailed geometry and structural evolution is usually poorly understood. 3-D scaled analog modeling, however, is one way in which we can produce geometric and kinematic models that may lead to a better understanding of the development of pull-apart and "pop-up" structures in strike-slip terranes. This paper describes the results of a series of scaled sandbox analog models designed to investigate the 3-D geometric and kinematic development of pull-apart basins formed at releasing stepovers in strike-slip fault systems and for the development of "pop-up" structures formed at restraining stepovers. Analog models were run both with and without synkinematic sedimentation.
Analog models of releasing stepovers in strike-slip fault systems are characterized by the formation of rhomboidal to elongate pull-apart basins bound by steep transtensional faults parallel to the main principal displacement zones and by steeply-dipping to sub-vertical extensional sidewall faults that link the offset segments of the main strike-slip fault system. The pull-apart basins are generally floored by flat lying or gently dipping fault blocks. The basin floor is commonly cut by a cross-basin fault that links the two main offset segments of the main strike-slip fault system. Within the pull-apart basins the synkinematic sediments are generally flat lying or only gently dipping, and the amount of subsidence is large with respect to the displacement along the main strike-slip fault system.
In contrast, analog models of restraining stepovers produce localized uplifts or pop-up structures. These are rhomboidal in plan view and bound by steep reverse faults that flatten upwards towards the surface of the models. The pop-up structures are distinctly asymmetric with the vergence of the reverse faults changing across the stepover in the main fault system. In models without synkinematic sedimentation, the steep reverse faults within the pop-up flatten to very shallow dips at the surface whereas in models with synkinematic sedimentation the steep reverse faults remain at moderate to high angles within the synkinematic strata.
The results of the analog models are compared and contrasted with natural examples of structures formed at stepovers in strike-slip fault systems from the United States, Indonesia, New Zealand, the Gulf of Aqaba-Dead Sea, and from the Aegean.
AAPG Search and Discovery Article #90912©1999-2000 AAPG International Distinguished Lectures