Transpression is expressed at Durmid Hill in the development and subsequent rotation of en 6chelon folds and ongoing vertical uplift. The transpression-related strain field can be quantified by an analysis of (1) orientation of rotated fold axes, which display an angle of about 20 ø between the San Andreas fault and the fold-axial traces; (2) shortening perpendicular to the fold axes (40%-60%); (3) extension parallel to the fold axes (58%-68%), measured using a boudinaged ash bed; and (4) uplift determined from an antecedent stream (1.1-1.7 mm/yr), the topographic expression of Durmid Hill, and the exposed stratigraphy. The observed deformation may be approximated by a homogeneous, transpressional deformation. This model suggests that an estimated 9%-14% of the total displacement predicted by plate motion models across the San Andreas fault has been accommodated by the Durmid Hill deformation zone adjacent to the fault. This off-fault deformation is not directly coupled to coseismic or interseismic movement on the fault itself. INTRODUCTION Transpression (i.e., a combination of strike-slip shear and shortening perpendicular to the shear zone) has been identified recently in many areas that were thought to have undergone either pure shortening or simple shear deformation. Where shortening is the dominant deformation mode, a component of strike-slip shear is recognized primarily by significant extension parallel to fold axes [e.g., Hatland, 1971; Ratschbacher, 1986; Ratliff et al., 1988]. In strike-slip fault systems transpressional segments can be recognized by uplift and the development of compressional features such as flower structures [e.g., Harding and Lowell, 1979]. The component of shortening is postulated to be dependent on the angle between the strike of the fault and the plate vector determined by relative plate motion calculations. The Durmid Hill, southern California, area offers a unique example of deformation of layered, soft lacustrine sedimentary rocks near a major strike-slip fault, reminiscent of a macroscale clay model [Odonne and Vialon, 1983; Wilcox et al., 1973]. Here these models can be directly compared to deformation in a natural environment. In this study, the strain field of a segment of the southern San Andreas fault (SAF) at Durmid Hill (Figure 1) is quantified and compared with the theoretical strain field of a transpressional shear zone. Shortening perpendicular to fold axes, axial parallel extension, orientation of fold axes, and estimated vertical uplift rates and magnitudes are used as strain indicators. The implications of the observed deformation at Pal:•r number 91TC01443. 0278-7407/91/91TC-01443 $10.00 Durmid Hill for fault displacement studies and the tectonic style in the area are discussed.
Stratigraphy and Crustal StructureThe geology of Durmid Hill was described by Babcock [1969, 1974] and Dibblee [1954]. Two late Cenozoic units are exposed in the area: the Shavers Well Formation (identified as Palm Spring Formation by Dibblee [1954]) and the Borrego Formation. The Shave...