Structure of the Cougar Buttes Anticline, Northern San Bernardino Mountains, California

The North Frontal thrust system (NFTS) and westernmost part of the Eastern California shear zone (ECSZ) intersect north of the Big Bear area, where the dominant tectonic style is a series of overlapping east striking reverse faults and intervening folds. These structures displace Mesozoic basement rocks in the San Bernardinos and the entire overlying Neogene sedimentary section. A domainal model in which the San Bernardino Mountains are partitioned into a western, thrust-dominated block and an eastern, strike-slip dominated block satisfies observations of the Holocene pattern of seismicity along the NFTS. This deformation zone is also characterized by complex arrays of Quaternary folds and faults. The Helendale fault, the westernmost member of the ECSZ, is a boundary structure between the two San Bernardino tectonic domains. The Cougar Buttes anticline is a few km east of the Helendale fault in the San Bernardino piedmont. This east-trending angular fold is cored by Neogene sedimentary rocks and draped by Quaternary alluvium. The crest of the fold is upheld by a petrocalcic horizon, and has a thin mantle of older Quaternary alluvium (Qvof1 through Qyf2 depending on location). Persistent, steeply dipping orthogonal joint sets occur everywhere in the petrocalcic layer but are densest where one set is parallel to the trend of the fold hinge. Most joints evolved into laminated calcitic veins in which dark, narrow bands of fine rock fragments and lighter bands of carbonate minerals alternate. Very rare slickensides with dip-parallel striations occur on east trending vein surfaces. Both joint sets maintain fairly constant orientations not only across the fold but also well into the present monoclinal alluvial surface. Drainage patterns also reflect the persistent joints. Orientation continuity suggests that the joints originated in a regionally consistent strain field rather than as a local response to fold growth.