The Harold D. Roberts Tunnel is a 23.3-mile water-diversion tunnel that extends through the western part of the Front Range between Grant and Dillon, Colorado. This report concerns the geology of the southeastern segment of the tunnel, which is partly in the Montezuma stock—a Laramide quartz monzonite pluton—but is mainly in Precambrian basement rocks.

The Precambrian metamorphic rocks consist of relatively thick, complexly folded units of amphibolite, migmatite, microcline gneiss, and biotite gneiss. These rocks were invaded by plutons of Boulder Creek Granite and Silver Plume Granite, regarded as syntectonic and late tectonic, respectively, in relation to Precambrian deformation and metamorphism. Most of the basement rocks are distinctly foliated.

Laramide deformation resulted in elevation of the core of the Front Range along northward-trending marginal upthrusts and in development of the Front Range mineral belt, a broad northeast-trending zone of strike-slip faults, veins, and igneous intrusions that extends diagonally across the range. The Montezuma stock was emplaced approximately at the junction of the mineral belt with the western margin of the range relatively late in the deformation. The stock is massive and shows little indication of flow fabric.

Analysis of fault data obtained in the tunnel indicates two complementary sets of strike-slip faults, trending northeast and northwest, apparently related to wrench-fault movement along the mineral belt. Minor faults are randomly oriented but tend to be concentrated in north-trending dip-slip sets approximately parallel to the marginal upthrusts.

The fracture pattern in the Montezuma stock conforms to the regional fault pattern. The major joint sets and fault sets are parallel to one another and probably formed concurrently.

The fracture pattern in the Precambrian rocks is unlike that in the Montezuma stock. Minor faulting was controlled largely by the rock fabric. Joints bear no obvious relation to the faults or rock fabric and are believed to have formed mainly later than the faults. The anomalous rupture pattern in the foliated basement rocks is attributed to their anisotropy and to a probable transition from pseudoductile to brittle behavior as the core of the range was uplifted.

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