DATING DEFORMATION IN THE BIG THOMPSON CANYON AREA, NORTHERN COLORADO FRONT RANGE: DISCRIMINATING BETWEEN ACCRETIONARY OROGEN MODELS

The 1.8-1.6 Ga rocks of the northern Colorado Front Range were formed through accretionary tectonics either by collision of juvenile island-arcs or by cycles of extension-then-accretion. For the juvenile island-arc model basins between the arcs are expected to be older than the arcs, and long periods of time (>10 Ma) may exist between arc formation, basin formation, and the deformation caused by their accretion. In contrast, the extension-then-accretion model suggests sedimentation within extensional basins must be younger than the earliest arcs and may be deformed during accretion shortly (<10 Ma) after formation. Constraining the timing of sedimentation and deformation in the Big Thompson Canyon area, between Loveland and Estes Park, CO, therefore offers an opportunity to investigate how accretionary orogens operate and which model best applies to the region.

On the flank of Storm Mountain, just north of the Big Thompson Canyon, four main lithologies exist: metasedimentary knotted schist and quartzite, both intruded by granitic pegmatite and tonalitic rocks. The maximum age of deposition of the schist and quartzite protolith has been constrained by LA-ICP-MS of detrital zircons to 1.751 ± 44 Ga, in agreement with previous studies. Two deformation events have been identified in the area. The first deformational foliation has a NE strike and SE dip and is essentially parallel to relict bedding. Intrusion of pegmatite and tonalitic rocks occurred before or during the first deformation as these rocks exhibit folding, tension gashes, and boudinage associated with this first deformational fabric. The second deformation event recorded by a fabric with a NW strike and SW dip, as well as the folding of older features. Zircon from the tonalite constrain the age of intrusion to c. 1.74 Ga. Thus, the closing of the basin, and the first deformation, had to occur at or subsequent to c. 1.74 Ga.

As the minimum age for either deformation event, or the duration between deformations cannot be determined, future work will combine microstructural analysis and electron microprobe dating of monazite in the schist in hopes of better constraining deformation timing. Combining these results with the detrital zircon analysis will build a picture of the accretionary processes following previously described accretionary models.