Study of Multilayered Divergent Wrench-Fault Systems by Using Sandbox Models

LIN, CHING-WEEI, MELVIN FRIEDMAN, and JOHN M. LOGAN, Texas A&M University, College Station, TX

Sandbox models are used to evaluate the effects of mechanical layering (structural stratigraphy) on the deformation response (structural styles) of sediments during basement-controlled divergent wrench faulting. Three divergent boundary conditions (5.8 cm strike-slip motion with 0.6 cm, 1.2 cm, or 1.8 cm dip-slip) are generated by moving two adjacent plates along a 45 degrees-dipping precut (basement fault). Dry sand and silicone putty (overlying the basement fault) simulate the brittle and ductile behavior of lithified rocks, respectively. Four combinations of sand and silicone putty (5 cm thick sand alone, 5 cm thick sand overlying a 0.64 cm, 1.28 cm, or 1.92 cm thick silicone putty layer) are used to investigate the effects of structural stratigraphy on the deformation response.

Similar to the behavior of simple and convergent models, these divergent models show that with increasing putty thickness (1) the effect of dip-slip motion decreases, (2) there is a better development of Riedel shears and splay faults, (3) the angle between Riedel shear and basement fault increases, and (4) the Riedel shears do not propagate to the basement/putty interface so that the corresponding negative flower structures or grabens become indistinct.

In detail, for all models with minor amounts of dip-slip motion along the basement fault, a basic sequence of faults develops in map view from en echelon Riedel shears (R1) and splay faults to "secondary shears" and/or P-shears. These are similar to sequences seen in previously described simple and convergent wrench models. The sand-only models with large dip-slip motion develop Y-shears (parallel to the strike of the basement fault) with significant vertical motion. With increasing displacement these are followed by en echelon Riedel shears that bound several spoon-shaped depressions within the overall shear zone. These depressions are developed adjacent to the major Y-shear, are en echelon, and have their deeper portions consistent with the sense of shear along Y.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)