3-DIMENSIONAL MODELS OF DEFORMATION NEAR STRIKE-SLIP FAULTS

We use three-dimensional elastic models to help guide the kinematic in terpretation of crustal deformation associated with strike-slip faults . Deformation of the brittle upper crust in the vicinity of strike-sli p fault systems is modeled with the assumption that upper crustal defo rmation is driven by the relative plate motion in the upper mantle. Th e driving motion is represented by displacement that is specified on t he bottom of a 15-km-thick elastic upper crust everywhere except in a zone of finite width in the vicinity of the faults, which we term the ''shear zone.'' Stress-free basal boundary conditions are specified wi thin the shear zone. The basal driving displacement is either pure str ike slip or strike slip with a small oblique component, and the geomet ry of the fault system includes a single fault, several parallel fault s, and overlapping en echelon faults. We examine the variations in def ormation due to changes in the width of the shear zone and due to chan ges in the shear strength of the faults. In models with weak faults th e width of the shear zone has a considerable effect on the surficial e xtent and amplitude of the vertical and horizontal deformation and on the amount of rotation around horizontal and vertical axes. Strong fau lt models have more localized deformation at the tip of the faults, an d the deformation is partly distributed outside the fault zone. The di mensions of large basins along strike-slip faults, such as the Rukwa a nd Dead Sea basins, and the absence of uplift around pull-apart basins fit models with weak faults better than models with strong faults. Ou r models also suggest that the length-to-width ratio of pull-apart bas ins depends on the width of the shear zone and the shear strength of t he faults and is not constant as previously suggested. We show that pu re strike-slip motion can produce tectonic features, such as elongate half grabens along a single fault, rotated blocks at the ends of paral lel faults, or extension perpendicular to overlapping en echelon fault s, which can be misinterpreted to indicate a regional component of ext ension. Zones of subsidence or uplift can become wider than expected f or transform plate boundaries when a minor component of oblique motion is added to a system of parallel strike-slip faults.