Structural architecture of the termination of the Powerline Fault, San Felipe Hills, southern California: A Geometric and Clay Mineral Analysis
Wednesday, May 8th, 2013
Using a variety of field and laboratory techniques we examine the characteristics of the fault fold fabric produced by the termination of the NW-striking dextral strike-slip Powerline fault in the San Felipe Hills, southern California. The area of study lies along the strike of the Clark segment of the San Jacinto fault, a major component of the San Andreas plate boundary fault system. Detailed mapping at a scale of 1:6000 shows that the fault-fold fabric is dominated by a major eastward plunging syncline, and the Alkali Wash oblique-slip and Artesian Trail thrust faults. Poles to bedding attitudes from four sections of the major syncline were plotted on lower hemisphere equal-area stereonets. The four segments were defined on the basis of the changing strike of the axial surface trace of the fold as seen in map view. Analysis of the resulting plots revealed an upright, open, cylindrical syncline that increases in plunge from west to east.
Analysis of the structural characteristics of the Alkali Wash oblique-slip and Artesian Trail thrust faults were achieved through trenching and X-ray diffraction study of gouge collected from fault cores and adjacent damage zones. The EW trending Alkali Wash oblique-slip fault terminates into a major splay of the Powerline fault and extends over several kilometers across the study area. It dips ~86˚ to the north, and in trenches sharply truncates shallower dipping beds in both the hanging wall and footwall blocks. Trenching of the 30˚S dipping Artesian Trail thrust fault revealed a fault core defined by poorly to moderately developed slickenlines on foliated gouge. The slickenlines plunged down the dip of the principal slip surface and were produced by gouging of rigid clasts as the hanging wall moved from south to north along the principal slip surface. Additionally, a well-developed damage zone defined by microbreccia composed of rounded sandstone and conglomerate fragments is evident. XRD analysis of material collected from the fault cores of the Alkali Wash and Artesian Trail faults indicate that gouge is composed primarily of illite/smectite (I/S), with no more than 20% illite in I/S along with traces of quartz, calcite, and kaolinite.