INTERNAL STRUCTURE OF THE SAN ANDREAS FAULT ZONE AT THE A.R. WILSON QUARRY, AROMAS, CALIFORNIA, AS INFERRED FROM 3-D DIGITAL OUTCROP MODELING

Bench-scale structural mapping based on 3-D digital outcrop models shows that gabbro exposed in the Graniterock A.R. Wilson Quarry, which straddles the San Andreas fault and several parallel splays near Aromas, California, consists of relatively intact blocks surrounded by a highly sheared matrix. Between the blocks, which measure 1 to 10 m (or perhaps more) in their long directions, the sheared matrix is characterized by a network of closely spaced and pervasive joints and small-scale faults. As such, the faulted gabbro exposed in the quarry may represent an unusually well exposed outcrop analog for the kinds of brittle structures and block-in-matrix (bimrock) fabrics characteristic of many mélanges. Commercial digital photogrammetry and structural mapping software was used to create thirty-seven 3-D outcrop models within the quarry, of which eight were selected for detailed discontinuity mapping in support of ongoing quarry operations. Between 68 and 372 discontinuity orientations were measured in each model and their poles plotted on equal area nets. The contoured equal area plots consistently show clusters of poles plunging gently to moderately towards the southeast or south-southeast. Some of the plots also show clusters plunging gently towards the southwest and north-northwest. These three clusters persist when all eight data sets are plotted together. Whereas some of the poles coincide with orientations predicted for an idealized shear zone (including joints; R, R', and P shears; and normal, reverse, and strike-slip faults), others appear to be randomly oriented. The most conspicuous disagreement between the measured and predicted ideal orientations is for the strongly developed clusters of ENE striking and steeply dipping discontinuities. These may represent a large population of small faults with reverse displacement, the inherited remnants of older deformation, or excavation induced stress-relief fractures.