GEOLOGIC CONSTRAINTS ON TILTING OF THE GRAYBACK NORMAL FAULT BLOCK, TORTILLA MTS, ARIZONA

Normal faulting often leads to the rotation and exhumation of large crustal blocks, which can provide complete cross sections that allow the direct examination of a variety of tectonic and magmatic processes. Cross sections also allow for thermochronologic assessments of the paleo-thermal structure of the crust. A critical aspect of these systems is the degree of rotation of the block and the timing of tilt. Many blocks are composed of rocks that lack reliable tilt indicators; therefore the degree of rotation at many of these sites is uncertain.

The Grayback normal fault block in the Tortilla Mts. of central AZ is interpreted to have been tilted as much as 90°E during Miocene extension (Howard and Foster, 1996), which exhumed a 12 km cross-section. These authors indicate a low geothermal gradient (17°C/km) prior to extension. However, tilting of the block may be as little as 45°. The purpose of this study is to evaluate the tilt of the Grayback fault block using structural and petrographic data from Laramide sills and Miocene dikes.

Laramide sills tend to be granitic/pegmatitic in composition while the Miocene dikes are basaltic to andesitic. Miocene dikes have a mean strike and dip of 197°/10°W (n=41). Assuming that these dikes were emplaced vertically as Mode I fractures in an extensional setting, they imply ~80° of eastward tilt. Laramide sills in the field appear to be roughly orthogonal to the Miocene dikes, suggesting they formed horizontally as Mode I fractures in a compression setting. These sills have a mean strike and dip of 023°/59°E (n=16), implying ~60° of eastward tilt, although there is significantly more scatter in these data than the mafic dikes. Petrographic studies of dikes from different paleo-depths also support significant tilting of the block. Dikes from deep paleo-depths exhibit a coarse grained matrix, while samples from shallow paleo-depths have a fine-grained matrix. Phenocrysts in the dikes do not show a grain size trend with depth, suggesting that the phenocrysts formed prior to dike emplacement.

These observations strongly support initial estimates that the Grayback block was tilted ~90° during Miocene extension and represents a nearly continuous 12 km cross section. Geochronologic and thermochronologic work is currently underway to further refine the evolution of this block.