STRUCTURE OF THE SANTAQUIN COMPLEX IN THE FOOTWALL OF THE WASATCH FAULT, UTAH, AND ITS INFLUENCE ON WASATCH FAULT DEVELOPMENT

The Wasatch Fault (WF) is a west-dipping normal fault at the base of the Wasatch Mountains with >10km kilometers of throw and the potential to generate M 7.5 earthquakes in an urban area. Near Santaquin, Utah, its footwall is the Santaquin Complex, a 1.7 Ga layered package of amphibolite gneiss, biotite schist, weakly foliated metagranite, augen gneiss, and pegmatite. Foliation and lithologic contacts dip gently to the east. We report initial work from a study to document the structure of the WF and Santaquin Complex and ultimately to assess the influence of pre-existing structure on the WF. A detailed map of a mine entrance wall that crosscuts the WF reveals pods of Santaquin Complex rocks with metamorphic fabric that is preserved but rotated into at least partial parallelism with the WF. Pods are separated by clay-poor pulverized rock and chloritized, low-angle, west-dipping shear zones that parallel the preserved metamorphic fabric. Farther than 60 meters from the WF the fault-rock fabric is absent, and west-dipping faults are rare. However, the Santaquin Complex is riddled with veins, small faults (sub-meter slip), and joints which do not diminish in abundance with distance from the WF and which we interpret to predate the WF. The oldest are thin (1-20mm) quartz and k-spar veins, locally with epidote and/or magnetite, which probably formed near the brittle-ductile transition as the Santaquin Complex was exhumed after peak metamorphism. A small vein set parallels the metamorphic foliation; otherwise their orientations vary widely. The orientations of the joints group in a small set dipping 75° WSW and at random orientations. The small faults occur in a dominant set of orientations dipping moderately E to NE, parallel to the foliation and in a diffuse set with E-W strikes and steep north and south dips. Slickenlines are common and record dip- to oblique-slip displacement. Rare unambiguous shear-sense indicator show more reverse sense of slip than normal. We conclude that preexisting brittle structures in the Santaquin Complex did not influence WF development but that the foliation influenced the orientation of veins and small faults; WF-related deformation extends only minimally into the footwall; and joints and small faults in the Santaquin Complex may have formed during regional Sevier or Laramide deformation.