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Abstract: Three-Dimensional Fault Systems in Rifts

Kenneth R. McClay

The 3-D geometries and kinematics of extensional fault systems in rift basins have been the focus of considerable recent debate. In particular, the symmetry and asymmetry of rift systems and the transfer of extensional fault displacements along strike have attracted considerable attention during exploration for hydrocarbons in rift systems. New insights as to the geometric and kinematic evolution of rift systems have been obtained by scaled laboratory models where the 4-D evolution of extensional fault systems (3 spatial dimensions and time) have been simulated using sand and clay analogs. Results of the kinematic and geometric modeling of orthogonal, oblique, and offset rift systems are presented and compared with structures found in natural rift systems. Particular attention was foc sed on the progressive evolution of 3-D fault patterns, the development of fault transfer systems, accommodation zones, fault propagation, and the switching of rift graben polarities along strike. Homogeneous sandpacks were used to simulate isotropic pre- and synrift stratal geometries. During extension, the accommodation space was infilled with syn-extension strata up to a constant horizontal base level. Wet clay models were used to simulate more competent sedimentary strata, but in these models no syn-extension strata were added. In both types of models, the orientation of the zone of rifting was varied systematically with respect to the extension direction, from orthogonal rifting where ^agr = 90° (^agr = angle between the rift axis and the extension direction) to ^agr angles as ow as 30°. The zone of rifting was also offset to simulate extension in a segmented rift system. Completed sand models were sectioned both vertically and horizontally, and the structures were then digitized and displayed using a 3-D graphics workstation. The analog models are characterized by asymmetric half-graben structures bounded by planar faults parallel to the margins of the zone of rifting. Intrarift deformation is characterized by rotational "soft" domino faults initially perpendicular to the extension direction, but

at high degrees of extension these rotate into parallelism with the rift margins for ^agr angles less than 90°. Displacement transfer between pairs of extensional faults or groups of extensional faults is accomplished by relay ramps, which, with increased displacement, become breached. A wide variety of breaching geometries are found. Changes in polarity of the extensional fault systems within the rift zones are achieved by accommodation zones of interlocking conjugate faults. Geometric and kinematic models for extensional fault geometries and their progressive evolution in 3-D rift systems are presented and compared with natural outcrop and seismic examples from the Gulf of Aden rift margin, Yemen, the Red Sea, the Gulf of Suez, and the east African rift system and extended terr ins of southwest United States.

AAPG Search and Discovery Article #90985©1994-1995 AAPG Distinguished Lecture