GEOMETRIC ANALYSIS OF FOLDED GREYWACKE LAYERS FROM PACHECO PASS, CALIFORNIA

Pacheco Pass is located approximately 177 kilometers southeast of San Francisco, in Central California. Rocks exposed in outcrop along State Highway 152 and the shore of San Luis Reservoir are dominantly a monotonous sequence of well bedded turbidites. These rocks are mapped as a coherent terrain within the Franciscan Complex, an Upper Jurassic to Miocene accretionary prism that represents a classic subduction zone where Pacific ocean crust was subducted beneath North America.

Detailed geometric structural analysis of the Franciscan Complex in Pacheco Pass has shown 4 generations of folding based on style groups. The general fold shapes of these style groups are F1 (isoclinal), F2 (tight), F3 (open), and F4 (kink). In outcrop cross-cutting relationships, such as cleavage or superimposed folding, are not common. Although F2 folds are the most common outcrop-scale folds, the folds of the D2 deformation have greatest variation, from tight to open. This study is analyzing the geometric shape of F2 folds to better define D2 deformation for kinematic analysis, and aid in the identifying of F2 folds, from F1 and F3 folds, in the field where cross-cutting relationships may not be present.

The geometry of profiles of folded greywacke layers from Pacheco Pass, California, are being measured via three methods: 1) visual harmonics analysis of Hudleston (1973); 2) dip isogon analysis of Ramsay (1967); and 3) the use of the Matlab computer program "Fold Profiler" of Lisle et al. (2006). Some 200, <10-centimeter-fold specimens, were cut normal to their fold axes, and their profiles digitally scanned for analysis.

Preliminary results of visual harmonic analysis of 30 fold surfaces indicates a dominance of 2D and 2E (open ellipse to sine) forms, Fourier coefficients of 48 fold surfaces fall in the range of parabolic to sinusoidal; and the dip isogons of 33 folded layers fall in the range of fold class 1B to 1C (i.e. parallel to similar fold geometries). The fold surfaces measured with Fold Profiler plot in a broad band of the 0.5–1.5 aspect ratio to 0.4–0.85 shape parameter, that is, open, close, and tight Parabolas and Semi-Ellipses forms. These data will then be further classified by outer-arc and inner-arc fold boundaries, and by layer composition (such as grain size, chert content) to see how these factors influence fold shapes.