Geologic evolution of Archean and Paleoproterozoic rocks in the northern Palmer Canyon block, central Laramie Mountains, Albany County, Wyoming

Structural analysis, thermobarometry, and geochronology performed on rocks within the Palmer Canyon block of the central Laramie Mountains combine to suggest that during the Medicine Bow orogeny, ca. 1779–1745, granitic crust was strong at ∼33 km depth; a depth where current flow-law models predict weak continental crust. A strong correlation between pre-existing fabric and subsequent deformation supports the interpretation that widely accepted flow-law models overlook a major control on crustal strength: pre-existing fabric.; Strain within the Palmer Canyon block was heterogeneously distributed during the Medicine Bow orogeny. Deformation in the north is partitioned into discreet shear and high-strain zones. Strain is manifest in other areas in the block as folds of varying styles and includes a large area where the granitic gneiss was not deformed during this event. This strain heterogeneity is interpreted as evidence for relatively strong, granitic crust.; U-Pb ages for sphene growth at ∼1745 Ma, tied directly to the high-strain zones and folding described above, support the interpretation for coeval development of the different strain features during the same deformational event. Pressure-temperature conditions determined by thermobarometry (600 ± 25°C and 9 ± 0.5 kbar) places these rocks at ∼33 km depth during the Medicine Bow orogeny. Models based on quartz flow laws predict granitic rocks at this depth to be weak; however, rocks in the Palmer Canyon block appear to have been behaving strongly during this event.; Changes in structural style and intensity during the Medicine Bow orogeny can be correlated with changes in type and orientation of pre-existing structures and fabrics. This correlation suggests that pre-existing lithologic anisotropy has a strong influence on rock strength, a parameter overlooked by current flow-law models. Because a pre-existing fabric can be expected at all depths in continental crust, the influence of pre-existing fabric and structures should not be ignored by models predicting the strength and behavior of continental crust at these or any depth. Furthermore, the assumption of crustal homogeneity required by flow-law models renders these models inappropriate and insufficient to predict the behavior of the lithosphere in any real tectonic settings.