Date of Award

Spring 1-1-2016

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Geological Sciences

First Advisor

Rebecca M. Flowers

Second Advisor

David Budd

Third Advisor

Craig Jones

Fourth Advisor

Robert Anderson

Fifth Advisor

James Hagadorn

Abstract

No consensus exists on the mechanisms that caused surface uplift and high elevations in the southern Rocky Mountains and High Plains regions. The timing of unroofing and uplift is key to differentiating between mechanisms, but also remains debated. Here I show that low-temperature thermochronometric data from Colorado, New Mexico, and Texas support regional Oligocene surface uplift as well as multiple lithospheric modification events during the Cenozoic.

A suite of samples from the southern Sangre de Cristo Range of northern New Mexico allows us to identify distinct signals of Laramide, mid-Tertiary, and middle Miocene cooling, and highlights the significance of climate-driven middle Miocene exhumation in shaping the landscape of the western High Plains. A transect of High Plains surface samples east of this location shows that a previously identified early Oligocene thermal anomaly extended eastward more than 300 km from the Cordilleran front into the Texas Panhandle, and also indicates westward-deepening Oligocene erosion. Additional High Plains samples from mid-Tertiary intrusions reveal substantial erosion on the southern High Plains synchronous with peak ignimbrite volcanism. Together these data suggest a gradient in early Cenozoic lithospheric hydration across the High Plains that corresponds to the elevation gradient, along with additional lithospheric modification after slab roll-back and regional surface uplift during the Oligocene.

Much of the southern High Plains surface is limestone, in which an absence of crystalline apatites limits the extent to which low-temperature thermochronometric techniques can be applied. To address this problem, I investigated the systematics of the conodont apatite (UTh)/ He (CAHe) thermochronometer. CAHe dates from the Illinois Basin indicate that temperatures ≤90 °C caused He loss in conodonts over geologic timescales, and thus imply that the CAHe closure temperature is similar to that of igneous apatite. CAHe dates from limestones in both the Illinois Basin and the southern Sangres are highly dispersed, display negative date-eU correlations, and are older than crystalline AHe dates from the same area. These results suggest that conodonts from limestones are particularly vulnerable to open-system behavior and therefore may be poor targets for thermochronometry. CAHe data for platform elements from shale samples are more promising.

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