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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.  Apatite fission-track data for samples on the eastern margin of the range record cooling through the 130 °C isotherm as recently as 5 Ma, implying up to 4-5 km of unroofing and normal fault displacement since Pliocene time (Naeser et al., 2002).  Evidence for such significant post-5 Ma tectonic activity may indirectly support a component of Late Tertiary surface uplift.

The objective of this PRF-funded investigation is to use apatite (U-Th)/He and ⁴He/³He thermochronometry to determine the detailed timing, rates, and magnitude of cooling and unroofing of the Gore Range.  These methods are sensitive to the cooling of rocks through temperatures as low as 30 °C, significantly lower than the apatite fission-track method, thus providing the opportunity to resolve the post-5 Ma thermal history.  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 PRF funding is supporting Rachel Landman’s MSc thesis at the University of Colorado, Boulder.  Rachel is beginning her second academic year and intends to complete her degree in May 2010.  Rachel’s salary and tuition were fully funded by the PRF grant during the first year of her MSc, in which she completed the majority of her coursework requirements and made significant progress on the first phase of her MSc thesis research.  This first research phase included: 1) significant background research on the problem, 2) the compilation of a geological and topographic map in ArcGIS showing major features of the study area and sample locations, largely thanks to acquiring the geological map base from Karl Kellogg at the USGS, 3) 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, 8) preparation of an initial suite of apatite crystals for (U-Th)/He analysis with results expected by November 2009, and 9) preparation of apatites for irradiation, a necessary step in the acquisition of apatite ⁴He/³He data. 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 being supported by a departmental teaching assistantship this fall, and the funds remaining in the PRF grant will to be used to partially cover her tuition and salary next spring.  Rachel plans to focus on acquiring, compiling and assessing the majority of her thermochronometry dataset by December, with the writing and defense of her MSc thesis occurring in the spring.  This thesis will be made available to the ACS PRF upon its completion.

Rachel successfully obtained additional funds to support her research.  She was awarded a $4000 Amherst College Fellowship to help cover fieldwork and analytical expenses.  In addition, 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.  In the process, Ryan benefited from training in field, lab, and research methods, and Rachel gained mentoring experience.  Ryan is now considering carrying out a senior thesis project in my group.  Thus, this PRF funding contributed significantly to the training and mentoring of multiple students.

References Cited

Gregory, K.M., Chase, C.G., 1994. Tectonic and climatic significance of a late Eocene low-relief geomorphic surface, Colorado. Journal of Geophysical Research 99, 20,141–20,160.

McMillan, M.E., Heller, P.L., Wing, S.L., 2006. History and causes of post-Laramide relief in the Rocky Mountain orogenic plateau. GSA Bulletin 118, 393-405.

Naeser, C.W., Bryant, B., Kunk, M.J., Kellogg, K., Donelick, R.A. & Perry, W.J., Jr., 2002. Tertiary cooling and tectonic history of the White River uplift, Gore Range,         and western Front Range, central Colorado: Evidence from fission-track and 39Ar/40Ar ages.  In Kirkham, R.M., Scott, R.B., Judkins, T.W. (eds.) Late Cenozoic evaporite tectonism and volcanism in west-central Colorado. Geological Society of America Special Paper 366, 31-53.