The Yaqui Ridge antiform and detachment fault: Mid‐Cenozoic extensional terrane west of the San Andreas Fault

Abstract

The narrow zone of mid‐Cenozoic detachment terrane in the southwestern United States can be widened and traced across the southern strands of the San Andreas fault zone into the eastern margin of the Peninsular Ranges Batholith. The Yaqui Ridge core complex and detachment fault, in southern Borrego Valley, California, exemplifies the nature of detached terranes in southcentral California. The detachment fault dips 10°–40° to the northeast, and separates a lower core of gneissic Late Cretaceous granodiorite from an unconsolidated, unmetamorphosed megabreccia of Eocene to Miocene age. Foliations in the lower plate generally conform to the strike and dip of the overlying detachment fault, and become less distinct away from the fault. The megabreccia which forms the upper plate is composed of unsorted, fairly well rounded clasts characteristic of batholithic and metasedimentary rocks of the region. Plio‐Pleistocene lacustrine sediments unconformably overlie the megabreccia in some areas. The detachment fault and upper plate are expressed as a series of klippen which parallel Yaqui Ridge along the northeast and southeast flanks. The fault is marked by a typically narrow (∼6–10 cm) band of intensely sheared cataclasite. A chlorite‐breccia zone occurs below the cataclasite, and gradually grades into the less deformed, regionally foliated gneissic granodiorite. On the southeast flank, what appears to be an extension of the cataclasite and detached upper plate is exposed only locally, near the southeast nose of the antiform. Here, the upper plate consists of blocks of pale gray to tan, aphanitic fault gouge. Superimposed upon and cutting through the detachment‐related features is a broad zone of left‐oblique‐slip faulting termed the Yaqui Ridge shear zone. This fault zone trends WNW‐ESE, parallel to Yaqui Ridge, and separates the detachment fault from the ridge, merging with the detachment fault near the eastern margin. Further east, the shear zone widens into a massive zone of gouge >400 m wide and disappears under the sediments of Borrego Valley and the Salton Trough. To the west, the shear zone slices into the eastern margin of the Peninsular Ranges Batholith and extends the marginal plutons in an E–W direction. Four dominant tectonic episodes can be recognized from exposures at and adjacent to Yaqui Ridge: (1) A Late Cretaceous synkinematic cataclasis and metamorphism accompanying and following emplacement of the Yaqui Ridge and related plutons, and forming a pervasive regional NW‐trending foliation and NE‐trending lineation. (2) A subsequent mid‐Cenozoic, shallow, low temperature event of detachment faulting and folding which produces localized brittle cataclasis which overprints the earlier NW‐trending fabrics. (3) The Late Miocene‐Early Pliocene development of Yaqui Ridge shear zone along the northeast margin of Yaqui Ridge, shearing, rotating and overprinting the fabrics formed during the Late Cretaceous plutonism and regional deformation. (4) A Pleistocene episode of high‐angle faulting and folding which produce a series of E‐W‐trending anticlines and synclines in Upper Pliocene to Pleistocene sediments. Linear elements which have formed throughout the several Late Cretaceous to Cenezoic deformations are concordant.