EVALUATING TILT AT THE GRAYBACK FAULT BLOCK, AZ USING GRAIN SIZE AND GEOCHEMICAL ANALYSES OF THE TEA CUP PLUTON

Normal faulting often produces tilting of crustal blocks between these faults. These tilted crustal sections offer the opportunity to examine tectonic and magmatic processes at a variety of depths as well as to determine the timing of extension and thermal structure of the crust prior to extension. The Grayback fault block in central Arizona has been interpreted to be tilted 90°E, and fission track thermochronology has been used to infer that extension began at 25 Ma and that the pre-extensional geothermal gradient was only 17°C/km (Howard and Foster, 1996). The goal of this study is to assess estimates of both the degree of tilt and the thermal structure using a combination of geologic evidence and (U-Th/He) thermochronology.

The Grayback block is composed of the late Precambrian Ruin Granite, and the late Cretaceous Tea Cup Granodiorite. Major and trace element geochemical data confirms both the felsic nature of the two plutons and the presence of two distinct igneous suites. Geochemical data from within the Cretaceous Tea Cup pluton also reveals spatial variations in the geochemistry of that unit. For example, the easternmost (shallowest) part of the pluton is more mafic than other samples from the pluton. These spatial geochemical patterns may be interpreted as a consequence of tilting of the block and may suggest that further study of the pluton might yield insight into the petrologic evolution of the pluton.

Grain size analyses of the Tea Cup Granodiorite also support that the Grayback block is substantially eastward tilted. Quartz and plagioclase grain sizes were measured and averaged for four samples on an E-W transect, representing increasing depth. Mean quartz area increases from 0.18 mm² (SD=0.4) in the easternmost part of the pluton to 0.34 mm² (SD=0.25) in the westernmost part, suggesting that the westernmost block was deepest at the time of pluton intrusion in the late Cretaceous. (U/Th)/He analyses in progress are expected to yield additional insights into the timing of extension, the thermal structure of the block and degree of tilt.