TECTONIC EVOLUTION OF THE EASTERN COLORADO FRONT RANGE: A TALE OF TWO SHEAR ZONES

The Idaho Springs-Ralston and Moose Mountain Shear Zones (IRSZ, MMSZ) are neighboring subparallel steeply-dipping km-wide zones of planar foliation in the Proterozoic basement units of the Front Range and Rocky Mountains of Colorado which can be traced from the mountain front a few tens of kilometers west. Both structures are proposed to have played a major role in the tectonic evolution of Colorado and more importantly Laurentia, and have been interpreted as terrane boundary sutures in the 1.8-1.6 Ga amalgamation of the Yavapai province onto the Wyoming craton. Although similar in character, re-examination shows that the structures developed with different deformational processes and histories. The IRSZ deforms an edge of the Boulder Creek Granodiorite (~1.72 Ga) and a complex suite of metasedimentary and metavolcanic rocks, and is both cut by and deforms the northern edge of the Mount Evans Batholith (~1.44 Ga). Structural domain analysis shows that the IRSZ can be modeled as a shear zone that formed along one limb of a km-scale fold in the metamorphic rocks. Shear zone kinematics show a multi-phase, Proterozoic deformational history, but a north-side-up sense of shear is dominant in localized, meter scale, interleaved mylonitic units. There is no marked contrast in metamorphic grade or offset of unit contacts across the zone. The zone was cut by rather than reactivated by small displacement Laramide brittle structures that are no more numerous within the zone than outside it. The MMSZ also flanks a km-scale fold but is mainly confined within the Longs Peak Batholith (~1.42 Ga). Proterozoic mylonite zones have a south-side-up sense of shear, occur in distinct bands less than a meter in thickness which become wider and denser closer to a main 50-100 meter wide mylonite zone. There is a marked contrast in metamorphic grade across its eastern end. Reactivation of the zone during the Laramide orogeny is implied by > 200 m offset of the Paleozoic cover sequence. The key distinctions between the IRSZ and MMSZ, include: 1) size and distribution of mylonitized units; 2) sense of offset; 3) the presence or absence of offset of metamorphic isograds and unit contacts; 4) the extent of mechanical reactivation during the Laramide; and, 5) possibly, the exact age of deformation.