PRECAMBRIAN GRANULITE FACIES METAMORPHISM IN THE WET MOUNTAINS OF SOUTHEASTERN COLORADO: IMPLICATIONS FOR LARAMIDE RIGHT-SLIP

The kinematics of the Laramide orogeny has been the subject of controversy, particularly with regards to the magnitude of strike-slip displacement in the Rocky Mountains of New Mexico and Colorado. Some models have suggested that right lateral separation is ~100 km, whereas other models have concluded that it is 20 km or less. A central problem with large displacement has been defining the location of right lateral faults in southern Colorado. We present new metamorphic data that helps to constrain the location of possible right lateral faults in southern Colorado. The Wet Mountains are a Precambrian cored Laramide uplift that exposes a sequence of granulite facies metamorphic rocks in the Mount Tyndall quadrangle. These rocks have been considered to be unique, as no other occurrences of granulite facies rocks have been described in southeastern Colorado. Separating the granulite facies gneiss from other high-grade gneisses in the Wet Mountains is a major structural discontinuity, the Ilse fault zone. The Ilse fault zone is northwest striking subvertical zone of intense brittle deformation that cuts across the entire Wet Mountains, making it a candidate for right lateral displacement. To the northeast of the Ilse fault, adjacent to granulite facies gneisses in the Mount Tyndall area is a sequence of strongly laminated orthogneiss interlayered with lesser amounts of sillimanite rich paragniess, calc-silicate, and amphibolite. The amphibolites contain the assemblage orthopyroxene-clinopyroxene – plagioclase – hornblende +/- quartz suggesting metamorphic conditions in the low-pressure granulite facies. The assemblage orthopyroxene-clinopyroxene-foresterite-grossular-Kfeldspar-termolite-calcite and biotite-sillimanite-Kfeldspar-quartz-plagioclase in calc-silicate and pelitic lithologies, respectively, confirms granulite facies metamorphic conditions. The occurrence of granulites on both sides of the Ilse fault zone combined with small offsets of the contacts of the San Isabel batholith in the southern Wet Mountains limits any transcurrent displacement on the Ilse fault zone to less than 2 km. This suggests that if there are any right lateral faults in this part of southern Colorado, they must lie either to the east or west of the Wet Mountains.