2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 4
Presentation Time: 8:45 AM


ALLEN, Joseph L.1, SHAW, Colin A.2, FERRI, Felicity E.1, GANAK, Megan E.1, GRAVES, Abbey S.2, JOHNSON, Anthony G.1, LYMAN, Shawn J.2 and OFSA, Jessica L.1, (1)Geology and Physical Sciences, Concord University, Athens, WV 24712, (2)Department of Geology, Univ. Wisconsin, Eau Claire, 105 Garfield Ave, Eau Claire, WI 54702, allenj@concord.edu

We report the discovery of a diverse suite of tectonites that include mutually overprinting brittle and plastic fault rocks within Paleoproterozoic basement along the southern margin of the Laramide White River uplift in west-central Colorado. The deformation system, herein referred to as the Grizzly Creek shear zone (GCSZ), includes mylonite, ultramylonite, mm- to cm-scale pseudotachylyte veins (pst), mylonitized pseudotachylyte (mpt), cataclasite, and younger brittle faults hosted by gneisses and two sheared granitoids. The GCSZ strikes W to SW and dips north (30–60º) above subvertically foliated biotite-muscovite gneisses and a porphyritic granodiorite. In steep canyon exposures above Grizzly Creek, the GCSZ consists of a 10–15-m-thick mylonite, overlain by >300 m of gneisses cut by pst fault veins (n >75). Cumulative vein thickness measured across the shear zone indicates that >0.25% of the host rock was frictionally melted during multiple episodes of pst generation. The upper 120 m of the pst zone changes character 4 km westward along strike, where a strongly foliated granitoid that hosts pst, mpt, and thin mylonites is in pst-bearing fault contact with underlying gneisses. Mineral lineations on mylonitic foliation planes plunge N-NE, and diverse kinematic indicators, including plastically deformed pst injection veins, indicate top-to-S displacement. The GCSZ is truncated by the Cambrian-Precambrian nonconformity, where pst and mpt is included in a weathered paleoregolith below the Sawatch Formation, documenting a Proterozoic origin for the shear zone.

We interpret the GCSZ to record cyclic seismogenic faulting and plastic flow at the brittle-plastic transition during ~N-S compression. The system may be temporally correlative with Mesoproterozoic intracontinental transpression along the NE-striking Colorado mineral belt shear zone system, which incorporates a similar suite of tectonites 55 km to the SE. The GCSZ was brittly reactivated in the Phanerozoic and is cut by a S-vergent reverse fault with ~200 m of stratigraphic offset. On a regional scale, the GCSZ is along strike of a prominent E-W bend in the Grand Hogback monocline to the west and is coincident with the Laramide flexural hinge of the White River monocline, suggesting a Precambrian ancestry for these structures.