Rocks that were at mid-crustal depths during early Proterozoic time are found in the White Tank Mountains, a mid-Tertiary metamorphic core complex located 65 kilometers west of Phoenix. These rocks allow us to infer how the Proterozoic mid-crust behaved in relation to the upper crust, which is well exposed in the Transition Zone of central Arizona. Proterozoic metamorphics, a foliated Proterozoic tonalite pluton, several Cretaceous-Tertiary granitic plutons, and numerous Tertiary dikes are all present. The metamorphics are interpreted to be mainly sedimentary and volcanic in origin, possibly correlating to the Union Hills and Alder groups. The tonalite pluton was intruded as tabular sheets of varying composition during the formation of the older foliation (S1).
Structures documenting the Proterozoic deformational events can be divided into early foliation development, a gentle folding of this foliation, and the development of a younger foliation along discrete shear zones. The older foliation (S1) strikes northeast and is correlated to the dominant northeast-striking fabric in the Transition Zone. Folding of the S1 foliation and the presence of a younger foliation (S2) in the White Tank Mountains but not in the Transition Zone suggest a difference in behavior between the mid-crust and the upper crust in the Proterozoic.
Several types of remote sensing imagery were used to aid in the geologic mapping of the White Tank Mountains. Imagery from the Thermal Infrared Multispectral Scanner (TIMS) and the NS001 instrument allow a geologist to view the field area in the thermal infrared and in the visible to near-infrared, respectively. Though accurate geologic mapping is not possible with this imagery alone, the imagery is a useful tool when combined with field observations. The TIMS imagery is an effective tool for mapping lithologic units defined by their silicate mineralogy. The NS001 instrument is not as effective for bedrock mapping, but is a very useful tool for mapping Quaternary deposits and surfaces.
Bedrock Geology Project
Proterozoic rock units
Remote Sensing techniques
References and Acknowledgments
Geology and Web pages by Steven Wood and Stephen J. Reynolds, Dept. of Geology, Arizona State University, Tempe, AZ -- July 1998