Previous studies (Drewes, 1974; Lingrey, 1982), as well as this study, included measurements of various foliations and lineations in the penetratively deformed rocks of the eastern Rincon Mountains (see also Davis, 1980). All resulting data were compiled into a Microsoft Excel spreadsheet and plotted on equal-area, lower hemisphere stereonets using GEOrient graphing and statistical analysis software developed by Rod Holcombe. San Pedro detachment fault. Mylonitic foliation and lineation are associated with the San Pedro detachment fault only near the western edge of the Happy Valley 7 ½' Quadrangle. This is interpreted as a consequence of an east-to-west increase in paleodepth and corresponding paleotemperature during initial extensional shearing along the San Pedro detachment fault and its down-dip continuation as a mylonitic shear zone. Foliation and lineation measurements (n=10 and 11, respectively; Fig. 1A) indicate early extension oriented 4° 075°. However, restoration of presumed regional eastward tilting associated with isostatic rebound of the fault footwall during Oligo-Miocene tectonic exhumation (e.g., Spencer, 1984) would result in gentle westward rather than eastward plunge for the lineations at the time of mylonitic shearing.