Tectonic Setting and Timing of Deformation in the Big Thompson Canyon, Northern Colorado Front Range
The 1.8-1.6 Ga rocks of the western United States were formed through accretionary tectonics either by collision of juvenile island-arcs or by cycles of extension-accretion. For the juvenile island-arc model, basins between the arcs are expected to be older than the arcs and long periods of time (>10-20 Myr) may exist between basin formation, arc formation, and the deformation caused by their accretion. In contrast, the extension-accretion model predicts sedimentation within extensional basins must be younger than the earliest arcs and may be deformed by accretion shortly (~10-20 Myr) after formation. The Big Thompson Canyon area, between Loveland and Estes Park, CO, was formed during 1.8-1.6 Ga accretionary orogenesis. Therefore, constraining the timing of sedimentation and deformation in this area offers an opportunity to investigate how accretionary orogens operate and which model best applies to the region. Four main lithologies exist within the central Big Thompson Canyon area. These include metasedimentary knotted schist and quartzite, both intruded by granitic pegmatite and tonalite. The maximum age of deposition of the schist and quartzite protolith has been constrained by Laser Ablation Inductively Coupled Plasma Mass Spectrometry of detrital zircons to 1751±44 Ma, in agreement with previous studies. Two deformation events have affected these rocks. The first deformational foliation has a NE strike and SE iv 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 a foliation, folding, tension gashes, and boudinage associated with this first deformation event. The second deformation event is recorded by all rocks as a foliation with a NW strike and SW dip and folding of older features. Zircon constrains the age of tonalite intrusion to 1742±15 Ma. Microstructural analysis of monazite in a schist indicates that the grains have grown cores and rims parallel to the first deformation. Electron microprobe dating of the monazite give an average core and rim age of 1723.1±4.0 Ma. These ages, with errors, support the timing of the first deformation occurring shortly after the tonalite intrusion. The data collected indicates that there is a ~10-20 Myr time frame between sediment deposition, closing of the basin, intrusive activity, and subsequent deformation in the Big Thompson Canyon area. This ~10-20 Myr constraint fits the proposed duration of events for the extension-accretion model. Published geochemical analysis has shown that the Green Mountain arc to the north and the Salida-Gunnison arc to the south may be related, which could be explained by the arcs forming together and subsequently rifting via slab-rollback. Though, arguments can be made in support of the juvenile island-arc accretion model and further research is necessary, this study concludes that data and previous studies suggest the extension-accretion model best explains the formation of these rocks.