DETAILED STRUCTURAL MAPPING OF THE SIERRA ESTRELLA WILDERNESS AREA, SIERRA ESTRELLA MOUNTAINS, ARIZONA

The Sierra Estrella Mountains are a northwest-trending exposure of Proterozoic igneous and metamorphic rocks located ~22 km southwest of Phoenix, Arizona. Previous fieldwork focused primarily on bedrock descriptions and reconnaissance geologic mapping at a 1:50,000 scale. Twelve Paleoproterozoic to Tertiary map units were defined. Most of the mountain range is composed of high grade gneiss and schist of sedimentary and volcanic origin. The oldest structures are NE trending upright NE and SW-plunging open to isoclinal folds. In the wilderness area, folds plunge predominately SW. These structures are similar to those in other ~1.7-1.6 Ga rocks elsewhere in Arizona. Lineations were only recorded in the central and eastern part of the wilderness area and plunge either east or southwest. Crenulations have been observed but orientations were not recorded previously. Weakly foliated and undeformed granitoid bodies intrude the metamorphic rock and have been interpreted as post-metamorphism. Only the Montezuma Granite has been dated, and shows a 1.38 Ga Rb-Sr age. It has a similar composition and texture to widespread ~1.4 Ga ferroan granites in the southwestern U.S. Moderately to steeply SW dipping mafic dikes cross-cut all Proterozoic rocks in the range and have a similar appearance to ~1.1 Ga diabase dikes in the region. Tertiary aplite and pegmatite dikes occur along major fractures and metamorphic foliation. Some pegmatite dikes contain minor gold and silver while copper is present in the dikes and the adjacent country rock.

While prior work suggests that multiple deformation events occurred in the Proterozoic, the exact deformation history of the Sierra Estrella Mountains remains largely unclear. Detailed structural mapping is being carried out within the Sierra Estrella Wilderness Area as part of a class project to better understand the multiple deformation and igneous events that affected the range. A detailed structural map, structural analysis, and interpretation of the structural history of the area will be presented in the context of the regional geology. Structural controls on magmatism and mineralization will also be interpreted.