SHALLOW STRUCTURAL AND RADAR CHARACTERISTICS OF THE COUGAR BUTTES ANTICLINE, NORTHERN SAN BERNARDINO FOOTHILLS, CA

The Cougar Buttes anticline is in the San Bernardino piedmont a few kilometers east of the Helendale fault and in the westernmost part of the Eastern California shear zone. This active fold consists of Neogene sedimentary rocks capped by Quaternary alluvium. Resistant petrocalcic soils help maintain the geomorphic expression of the fold, that is asymmetric with its steeper limb dipping into the San Bernardino foothills and its shallower limb inclined to the north toward Lucerne Valley. The hinge-line of the fold is about horizontal along a trend of 280°. The rigid petrocalcic horizon is closely to widely jointed with mostly steeply dipping to vertical joints. Collectively the joint trends are widely dispersed but locally there are consistently orthogonal sets parallel and normal to the fold axis in strike. Many joints are planar and dilational and cut directly through both matrix and large limestone clasts. The densest class of dilational joints occurs in the hinge zone of the anticline and may continue for many meters. Calcareous mineralization fills these veins. Rare hinge-parallel veins contain dip-parallel slickenlines on polished fault surfaces. The gently north-dipping limb of the fold forms a long pediment, and there are a number of whaleback ridges on this surface created by minor folds mimicking the larger structure. The hinges of the ridges are commonly cracked by axis-parallel dilational veins. As this pediment has never been deeply buried, joint formation likely tracked stresses during evolution and growth of the anticline. Some of the dispersion in joint and fracture orientation may also be related to thermal cracking of the petrocalcic pediment layer. We collected 100 MHz GPR transects across the crest of the fold and along washes that expose medial parts of the fold to determine if a fault is coincident with the structure. In both the high and low transect sets, disrupted lateral reflectors in the Neogene stratigraphy and Quaternary cover suggest that a south-dipping reverse(?) fault is present. Geometry of structures imply that the Cougar Buttes anticline is not a simple fault-tip fold but it is likely that the fold and fault are co-genetic. Dilational joints and veins throughout the carapace of the fold suggest that the petrocalcic horizon has acted like a brittle sheet over a rheologically less brittle core.