Laramie Peak shear system, central Laramie Mountains, Wyoming, USA: regeneration of the Archean Wyoming province during Palaeoproterozoic accretion

Abstract

The Laramie Peak shear system (LPSS) is a 10 km-thick zone of heterogeneous general shear (non-coaxial) that records significant tectonic regeneration of middle-lower crustal rocks of the Archean Wyoming province. The shear system is related to the 1.78–1.74 Ga Medicine Bow orogeny that involved the collision of an oceanic-arc terrane (Colorado province or Green Mountain block or arc) with the rifted, southern margin of the Wyoming province. The style and character of deformation associated with the LPSS is distinctive: a strong, penetrative (mylonitic) foliation commonly containing a moderately steep, SW-plunging elongation lineation. In mylonitic quartzo-feldspathic gneisses of the Fletcher Park shear zone, shear-sense indicators indicate southside-up, and this interpretation is supported by metamorphic and geochronological studies across the LPSS. We argue that distributed general shear (non-coaxial) involving high-strain zones and multiple folding events yielded a broad, en-masse uplift (Palmer Canyon block) during the late stages of the Medicine Bow orogeny. The LPSS is thus an excellent example of how crystal-plastic strain is distributed in sialic crust during an oceanic arc-continental margin collision. As magmatism (and attendant thermal softening) did not occur in the Wyoming province during its partial subduction beneath the oceanic-arc terranes of the Colorado province, the crystal-plastic strain manifested within the Wyoming province is mechanical in nature and was concurrent with crustal thickening. Strain is localized into discrete shear zones separated by weakly deformed rocks. These high-strain zones are commonly located along contacts between differing rock types and we propose that mechanical and chemical weakening processes may have contributed to strain localization.

Tectonic evolution of a Laramide transverse structural zone: Sweetwater Arch, south central Wyoming

“…The Paleoproterozoic Yavapai and Mazatzal provinces, which underlie the southern part of the Laramide belt, were then sutured onto the Wyoming Province along the east to ENE trending, transpressional Cheyenne zone at ~1.8 to 1.7 Ga [ Karlstrom and Houston , ]. Additional transpressional shear zones, including the Laramie Peak zone, propagated into the southern Wyoming Province [ Resor and Snoke , ], which further experienced a 1.7–1.5 Ga thermal overprint recorded by biotite cooling ages [ Chamberlain et al ., ]. These protracted thermal and deformation events left inherited crustal weaknesses with varying orientations, which partly controlled development of the younger Laramide structures [ Stone , ].…”

P- and S-Wave Receiver Function Images of Crustal Imbrication beneath the Cheyenne Belt in Southeast Wyoming

“…The Laramie Peak shear zone is an ∼10 km thick mylonitic shear zone that formed contemporaneously with the Cheyenne belt and demarcates the Laramie Peak block to the north from the Palmer Canyon block to the south (Fig. 1) (Chamberlain et al, 1993;Resor and Snoke, 2005). Thermobarometric estimates of metamorphic P-T conditions suggest that the Palmer Canyon block was uplifted 10-15 km with respect to the Laramie Peak block at 1.76 Ga (Chamberlain et al, 1993;Patel et al, 1999).…”

“…Thermobarometric estimates of metamorphic P-T conditions suggest that the Palmer Canyon block was uplifted 10-15 km with respect to the Laramie Peak block at 1.76 Ga (Chamberlain et al, 1993;Patel et al, 1999). The Palmer Canyon block uplift is hypothesized to have formed as a result of thrust faulting along with significant internal block deformation; however, the exact fault kinematics, that is, planar or listric, and the relationship between the uplift of the Palmer Canyon block and the formation of the Cheyenne belt is largely unconstrained (Resor and Snoke, 2005).…”