Abstract

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 degrees underlapping releasing sidestep, 90 degrees sharp nonoverlapping releasing sidestep, and 150 degrees 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 degrees ) typically form elongate rhomboidal grabens; 90 degrees releasing offset sidesteps produce shorter, squat, rhomboid pull-apart basins; and overlapping releasing sidesteps (115-150 degrees ) 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 their central sections to oblique slip and strike slip at either end where they merge with the PDZ. The pull-apart basin models are flat-bottomed in the center of the basin and become asymmetric at either end where the sidewall fault systems join the PDZ. Cross-basin fault systems, which are characteristic of all models, cut the floor of the pull-apart basins and link the stepped PDZ. The pull-apart basins evolve progressively from a narrow graben bounded by the oblique-slip link faults to wider rhombic basins flanked by terraced basin sidewall fault systems. The synkinematic strata are generally flat or gently dipping, and deformed only at the basin margins by the terraced sidewall fault systems. The PDZ at each end of the pull-apart basin system are relatively narrow and generally develop in-line horst and graben structures that broaden outward into the pull-apart basin. The analog models are compared with natural examples of pull-apart systems and show many strong similarities in structural geometries and stratal architectures. These analog models may provide useful structural templates for seismic interpretation in petroleum basins formed by or associated with strike-slip fault systems.

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