Transpression along the Southern San Andreas Fault, Durmid Hill, California

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Abstract
Transpression is expressed at Durmid Hill in the development and subsequent rotation of en échelon 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° 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.