MODES OF DEFORMATION DURING 1.45-1.35 GA INTRACONTINENTAL TECTONISM IN THE SOUTHERN ROCKY MOUNTAIN REGION: IMPLICATIONS FOR THE THERMAL AND MECHANICAL STRUCTURE OF THE CRUST

Geologic structures formed during regional 1.45-1.35 Ga intracontinental tectonism in the southwestern U.S. run the gamut from discrete pseudotachylyte-bearing seismogenic faults, through narrow mylonitic shear zones, to broad areas of pervasive flow fabrics. The spatial distribution of these structures defines two distinct rheologic domains that closely correspond to thermal domains defined by ⁴⁰Ar/³⁹Ar thermochronologic data: a domain of brittle-ductile deformation characterized by discrete faults and shear zones in northern and central Colorado (T >350°C), and a domain of pervasive crustal flow in south-central Colorado and northern New Mexico (T >500°C). Sparse constraints on ca. 1.4 Ga pressures support the interpretation that these thermal/rheologic domains represent middle and deeper crustal levels, respectively, that are currently exposed at the surface due to differential post-Mesoproterozoic exhumation. Shaw and others (2005) and Jones and others (2010) suggested that such a weak layer in the middle or lower crust would be unable to support significant topographic gradients leading to development of a broad intracontinental plateau. This model has several important impications: (1) a high-standing plateau with thick crust would have promoted regional denudation and crustal thinning, (2) some detritus eroded from a plateau would have been deposited in the broadly contemporaneous Belt basin (ca. 1.47-1.40 Ga), (3) a plateau implies a geodynamic cause, for example convergent margin or mantle upwelling, (4) large volumes of melt residuum in the lower crust related to ca. 1.45-1.35 Ma plutonism would affact the evolution of the plateau. Analysis of regional structural, thermochronologic, geochronologic, metamorphic and tectonic data suggests that many features of ca. 1.4 Ga intracontinental tectonism are consistent with the plareau model.