Undergraduate Honors Theses

Thesis Defended

Spring 2015

Document Type

Thesis

Type of Thesis

Departmental Honors

Department

Geological Sciences

First Advisor

Rebecca Flowers

Second Advisor

Charles Stern

Third Advisor

Suzanne Anderson

Abstract

Despite the prominence of the Rocky Mountains, questions remain about the timing and character of the most recent mountain building event in the Colorado Front Range, the Laramide Orogeny. This history can be investigated using (U-Th)/He thermochronology, a technique sensitive to low temperatures and the uppermost stages of cooling. I collected samples in three areas of the Front Range extending across an ~50 km long east-west transect, and acquired data for four samples from Big Thompson Canyon within the eastern portion of the sampled region. The apatite (U-Th)/He thermochronometer was applied because of its relatively low closure temperature (~70 °C) and potential for furthering our understanding of the effects of radiation damage on the dates acquired from zircon helium (ZHe) analyses. I additionally acquired zircon (U-Th)/He data when it became apparent that most apatite data were compromised by abundant mineral inclusions. 11 single grain apatite analyses for a single sample from Big Thompson Canyon yielded a mean apatite (U-Th)/He date of 66.5 ± 9.62 Ma. 15 single-grain zircon analyses for three samples from Big Thompson Canyon have mean dates ranging from 45.01 ± 7.24 Ma to 64.55 ± 11.13 Ma. The closure temperature of zircon is nominally ~180 ˚C, but may have been lowered in the studied grains as He retentivity decreased due to the accumulation of radiation damage. The similar dates of the zircon samples located at different elevations, as well as the overlap of zircon and apatite He dates, indicate rapid exhumation in the Colorado Front Range during the Late Cretaceous–Early Tertiary Laramide Orogeny.

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