TRANSPRESSION ALONG THE SOUTHERN SAN-ANDREAS FAULT, DURMID HILL, CALIFORNIA

Transpression is expressed at Durmid Hill in the development and subsequent rotation of en echelon 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-degrees 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.