Mid-Tertiary to recent extension in the southern Salton Trough - northern Gulf Extensional Province has resulted in the formation of the Sierra El Mayor core complex. Structural geometries and style of deformation closely resemble those of the turtleback structures in central Death Valley, as opposed to the core complexes in southeastern California and southern Arizona. Displacement along a stacked system of detachment faults was accomplished by brittle deformation and placed unmetamorphosed late-Miocene to Pleistocene marine and non-marine sedimentary rocks on upper amphibolite facies metamorphic tectonites. The metamorphic rocks have been pervasively intruded by igneous rocks of various ages, ranging in composition from tonalites to monzogranites. Mid-miocene (?) hydrothermal metamorphism retrograded the core rocks to greenschist facies, and clasts of these rocks are found in a syntectonic sedimentary breccia indicating that hydrothermal fluids preceded and facilitated faulting. Foliation within metamorphic rocks has been transposed and refolded into a WNW to NW trending antiform. A parallel trending antiform is defined by the dips on the uppermost detachment fault. Penetrative high-angle conjugate normal faults and fractures that are oriented perpendicular to the antiform have pervasively brecciated the rocks within the core. The maximum strain direction, as determined from the orientation of the high-angle faults beneath the upper detachment, is east-west. This is consistent with kinematic indicators that give a WNW sense of movement on the detachment faults. Orientation of the high-angle normal faults above the upper detachment indicates a more NW - directed maximum strain. The structurally lowest detachment zone exposed represents a mid-crustal shear zone upon which mid- and upper-crustal rocks were tilted and cataclastically deformed in response to the opening to the proto-Gulf. Syndepositional marine sediments were deposited directly on the uplifted shear zone. Continued extension along detachment faults tectonically thinned and removed the late-Miocene to Pliocene marine deposits and locally placed Pleistocene redbeds directly against the metamorphic basement. Subsequent high-angle normal faults that cut the detachments are considered to sole into a presently active mid crustal shear zone. Extension associated with the development of Sierra El Mayor core complex is consistent with Mid-Miocene to present Pacific-North America plate motions.