McCLAY, K. R., and M. WHITE, Fault Dynamics Project, Geology Department, Royal Holloway, University of London, Egham, Surrey, UK

ABSTRACT: 3D Evolution Of Rift Structures

The 3D evolution of rift systems have been modelled using scaled sandbox analogs. This paper presents models of orthogonal, oblique and offset rift systems and compares the results with natural rift systems. Particular attention was focused upon the development of transfer systems and the switching of fault polarity along strike. The orientation of the zone of rifting was varied systematically with respect to the extension direction, from orthogonal rifting where alpha = 90 degrees (alpha = angle between the rift axis and the extension direction) to alpha = 30 degrees. The zone of rifting was also offset in order to simulate extension in a segmented rift system. Completed models were sectioned both vertically and horizontally and the structures were then digitized and displayed using 3D graphics workstation. The analogue models are characterized by asymmetric half graben structures bounded by planar faults parallel to the margins of the zone of rifting. Intra-rift deformation is characterized by rotational 'soft' domino faults initially perpendicular to the extension direction but at high extension rotate into parallelism with the rift margin faults for alpha <90 degrees. Displacement transfer between pairs of extensional faults or groups of extensional faults is accomplished by relay ramps. Changes in polarity of the extensional fault systems within the rift zones are achieved by zones of interlocking conjugate faults. Geometric and kinematic models for extensional fault geometries and their progressive evolution in 3D rift systems are presented and compared wit examples from the east African Rift system and extended terranes from SW USA.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.