Quantifying long-term glacial denudation with low-temperature thermochronology

Quantifying long-term glacial denudation with low-temperature thermochronology
Densmore, M. S.
2008
University of Michigan, East Lansing
Quantifying the long-term (∼106 yrs) denudation history within an orogen is requisite in understanding the evolution of topography and efficacy of various surface processes. Specifically, glaciation is an important process within alpine environments that significantly alters the landscape. However, few studies have evaluated long-term denudation rates in glaciated settings. This dissertation presents an extensive thermochronometer dataset interpreted with multiple numerical methods to constrain long-term rates of denudation in a glaciated orogen, and determine how glaciers affect the topography. As part of this dissertation, 58 thermochronometer samples were collected within the heavily glaciated Coast Mountains, British Columbia, Canada. Samples were collected over a 60 x 60 km region and span ∼4 km of relief. Analyses yielded 58 apatite (U-Th)/He ages, 4 apatite fission track ages, and 9 zircon (U-Th)/He ages, ranging from 1.1 to 15.4 Ma, 6.9 to 39.2 Ma, and 2.4 to 27.3 Ma, respectively. Cooling ages were interpreted in the context of glacial denudation using a standard linear regression technique as well as 1-, 2- and 3-D numerical models. This approach of multiple techniques allows for robust constraints of denudation within the region as well as an evaluation of each technique with respect to a sophisticated 3-D thermal model. Major results from this study are: (1) an observed pulse of denudation at ∼7 Ma interpreted as the onset of glaciation, (2) glacial denudation rates range from ∼0.2 to 2.2 mm/yr, more than an order of magnitude less than short-term (102 to 103 yrs) estimates and ∼4 times less than observed long-term rates in southern Alaska, (3) exhumation and cooling within the core of the range occurred rapidly during the last ∼7 Myr, perhaps with an increase in the last ∼2 Myr associated with the intensification of glaciation during the Pleistocene, (4) glaciers modify the topography in a nonuniform fashion, incising paleo-fluvial networks as well as reworking the local topography by removing peaks and altering major valley locations. Finally, a sophisticated 3-D thermal model is preferred over 1- or 2-D methods for interpreting large spatial regions with high relief and variable denudation rates.
Ph.D. dissertation
Minerals Data and Information Rescue in Alaska (MDIRA)