STRAIN PARTITIONING AND HETEROGENEOUS DEFORMATION IN MID-CRUSTAL ROCKS DURING THE PALEOPROTEROZOIC MEDICINE BOW OROGENY: NEW IMPLICATIONS FOR CONTROLS ON LITHOSPHERIC STRENGTH

Strain heterogeneity in the middle crust has been attributed to a "strength beam" at ~10-15 km depth where foliations are subvertical except near syntectonic plutons where fabrics are subhorizontal due to strain softening (Karlstrom and Williams, 1998). Below this "strength beam" the middle crust is inferred to flow as a "relatively inviscid fluid" on geologic time scales, unable to maintain large shear stresses (Wernicke, 1990). These observations agree with theoretical work describing a stratified lithosphere with lithologic type and geothermal gradient controlling the depth of the strong upper-middle crust and weak lower-middle crust (Ranalli and Murphy, 1987). A rare opportunity to study mid-crustal processes exists in the Palmer Canyon block (PCB) of the central Laramie Mountains, southeastern Wyoming, where evidence for the proposed weak layer is absent.

Precambrian rocks exposed in the ~50 km-wide PCB, between the Cheyenne belt and the Laramie Peak shear zone, were deformed at ~27 km in the absence of magmatism during the Medicine Bow orogeny (MBO). The PCB was uplifted ~10 km as a block without rotation providing a large area of exposure from the same crustal depth. While the PCB was deforming and being uplifted en masse, sub-blocks within the PCB were responding differently from each other. Spaced shear and high-strain zones dominate the northern margin of the block while interior and southern portions were folded with contrasting geometries requiring different local transport directions. Additionally, separating these disparately folded areas is a wide zone of rocks that were unaffected by the event.

These observations are interpreted to suggest that shortening within the PCB was accommodated by a "jostling" of large sub-blocks creating local transport directions which are different from the regional movement, similar to basement-involved brittle deformation (e.g., Laramide-style uplifts). Abundant strain partitioning and large unaffected areas imply a strong crust at depths below the inferred strength beam of the model. I propose syntectonic magmatism is the driving force for the presence and location of the weak layer, perhaps by transferring heat up from depth and/or the addition of fluids. Thus, the absence of magmatism in the PCB during the MBO either depressed the weak layer or eliminated it altogether.