Scaled sandbox models have successfully simulated the geometries and progressive evolution of pull-apart basins developed in a weak sedimentary cover above right-stepping (releasing) dextral strike-slip fault systems in rigid basement. Synkinematic basin infill was added progressively as the models were deformed. Vertical and horizontal sectioning of the completed models has allowed the full three-dimensional architecture of the pull-apart system to be analyzed. We present three representative end-member experiments: 30° underlapping releasing sidestep, 90° sharp nonoverlapping releasing sidestep, and 150° overlapping releasing sidestep. The pull-apart basin geometries are typically sigmoidal to rhombic grabens, the geometries of which are dependent upon the architecture of the underlying basement fault systems. Underlapping releasing sidestepping faults (offset angles of 30-75°) typically form elongate rhomboidal grabens; 90° releasing offset sidesteps produce shorter, squat, rhomboid pull-apart basins; and overlapping releasing sidesteps (115-150°) produce box-like grabens with highly kinked basin sidewalls. The pull-apart basins are bounded by terraced oblique-slip extensional sidewall fault systems that link the laterally offset principal displacement zones (PDZ) of the main dextral strike-slip faults. The sidewall faults show changes in kinematics from dominantly dip-slip extension in

©Copyright 1997. The American Association of Petroleum Geologists. All rights reserved.

¹Manuscript received June 24, 1996; revised manuscript received November 20, 1996; final acceptance June 10, 1997.

²Fault Dynamics Project, Geology Department, Royal Holloway, University of London, Egham, Surrey TW20 OEX, United Kingdom.

Supported by the Fault Dynamics Project [sponsored by ARCO British Limited, PETROBRAS U.K. Ltd., BP Exploration, Conoco (U.K.) Limited, Mobil North Sea Limited, and Sun Oil Britain]. We also acknowledge additional funding from Sun Oil Britain. Landsat Thematic Mapper data courtesy of RTZ Mining and Exploration Limited, South American Division. Jurrian Reijs is thanked for field data relating to the Salina del Fraile pull-apart basin. Chris Elders is thanked for reviewing earlier versions of this manuscript, and J. Crowell, C. Morley, and K. Biddle are thanked for constructive reviews of the manuscript. The contents of this paper have been