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Description of Problem

The Colorado Rockies are characterized by some of the highest elevations in the continental United States. There is no consensus on when the mountains in the region attained these elevations or on the mechanisms responsible for uplift. Some studies suggest that the Rockies reached their current elevations during Early Tertiary contraction with most subsequent incision the product of climate change (e.g. Gregory and Chase, 1994); others infer that the region experienced regional uplift as recently as the last few million years, possibly attributable to mantle upwelling associated with the northward propagation of the Rio Grande Rift (e.g. McMillan et al., 2006).  One of the few locations in the Rockies with evidence of significant Late Tertiary faulting is the Gore Range and adjacent Blue River Valley in central Colorado, representing the northernmost significant manifestation of the Rio Grande Rift. 

The objective of this investigation is to use apatite (U-Th)/He thermochronometry to determine the detailed timing, rates, and magnitude of cooling and unroofing of the Gore Range. Deciphering the unroofing history of the Gore Range will further bear on the timing and causes of exhumation and uplift in the broader Rocky Mountain region, as well as the timing and mechanisms of basin development during the evolution of the Rio Grande Rift.  This project is the topic of Rachel Landman’s MSc thesis at the University of Colorado, Boulder.  Rachel intends to complete her degree in Fall 2010.  Much of Rachel’s salary and tuition have been funded by this PRF grant.  We describe two phases of Rachel’s research below. 

Research Efforts

Rachel’s first research phase, also described in last year’s PRF report, included: 1) significant background research on the problem, 2) compilation of a geological and topographic map in ArcGIS showing major features of the study area and sample locations, l3) acquisition of mineral separates from Chuck Naeser at the USGS for which co-existing AFT data were previously published (Naeser et al., 2002), 4) initial picking of Naeser’s mineral separates for high-quality, inclusion-free apatites for (U-Th)/He analysis, 5) several week-long field efforts to better understand the geology of the study area and to collect additional samples for analytical work, with the primary targets being low elevation samples along the Blue River fault and two vertical transects, 6) mineral separation from many of these newly acquired samples for subsequent analysis, 7) set-up of the lithium metatungstate method of mineral separation by density, leading to a significantly more efficient mineral separation procedure, and 8) preparation of an initial suite of apatite crystals for (U-Th)/He analysis.  These efforts promoted interaction with researchers at the USGS, including Chuck Naeser, Karl Kellogg, and Jonathan Caine, all of whom have active or prior research projects in the Gore Range.

Rachel is partway through her second research phase, primarily focused on data interpretation and synthesis.  This work includes: 1) thermal history interpretation of apatite (U-Th)/He data for 15 samples from the Gore Range, both from her newly collected samples and Naeser’s mineral separates, 2) interpretation of the significance of these results for elevated mid-Tertiary geotherms and Late Tertiary unroofing of the region, 3) acquisition of new funding and expansion of her work during summer 2010 to Gore Canyon and the northern Gore Range to follow-up on her intriguing initial results in the southern Gores, 4) submission of an abstract (Landman and Flowers, 2010) to present the new results at the GSA National Meeting in October 2010, and 5) writing and defense of her MSc thesis in Fall 2010.  We ultimately plan to publish the results of this study in a peer-reviewed journal.

Results and Implications

New apatite (U-Th)/He data for 15 samples from the southern Gore Range are younger eastward toward the Blue River fault.  These results suggest two separate cooling and unroofing episodes in Oligocene and Miocene time. Eocene dates are preserved in the western Gore Range at elevations >3600 m, while Oligocene dates occur at lower elevations.  Miocene dates are yielded by eastern Gore samples from an elevation range of 4000-3000 m. The youngest date of 6.9 Ma occurs at the eastern edge of the range, implying ~2 km of unroofing in the easternmost Gore Range since the late Miocene.

A cross fault separates the southern from the northern Gores. The rolling hills north of the cross fault are characterized by a maximum relief of just 200 m, vertical displacement on the Blue River fault of up to 800 m, and retain a partial cover of Mesozoic sedimentary and mid-Tertiary volcanic rocks. We infer that the entire range underwent late Oligocene unroofing. The geomorphic contrast between the northern and southern Gores is likely due to the restriction of significant displacement along the Blue River fault to the southern Gores, inducing Miocene unroofing, creating significant relief, and stripping any remaining sedimentary cover from this area. This two-phase unroofing history is similar to that inferred for other rift basins to the south (e.g., Chapin and Cather, 1994), implying the broadly synchronous onset and evolution of a >600 km segment of the Rio Grande Rift (Landman and Flowers, 2010).

Acquisition of Additional Funds

Rachel has obtained additional funding from four other sources to help support her research and expand her project.  This would not have been possible without the PRF funding that allowed this investigation to be initiated.  First, Rachel was awarded a $3900 Amherst College Fellowship in 2009 to help cover fieldwork and analytical expenses.  Second, in that same year, Rachel obtained $1000 from the undergraduate mentoring program in the CU Dept of Geological Sciences to support Ryan Nell, an undergraduate geology major.  Ryan assisted Rachel in both field and lab work over the summer, and is now carrying out a senior thesis project in my group.  Third, Rachel was awarded a $3200 GSA student research grant to fund her expanded field efforts in summer 2010.  Finally, Rachel obtained a $7500 ExxonMobil Geoscience Grant to pay her summer salary in 2010 to expand her research project as described above.