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Decay of an old orogen: Inferences about Appalachian landscape evolution from low-temperature thermochronology

McKeon, Ryan E. and Zeitler, Peter K. and Pazzaglia, Frank J. and Idleman, Bruce D. and Enkelmann, Eva (2014) Decay of an old orogen: Inferences about Appalachian landscape evolution from low-temperature thermochronology. Geological Society of America Bulletin, 126 (1-2). pp. 31-46. ISSN 0016-7606.

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The Appalachian Mountains (eastern United States) are the archetypal old, long-decaying orogen from which major theories for long-term landscape evolution have been derived. However, given the variability of relief and topographic correlation with geologic and tectonic history, it is difficult to describe the orogen as old and uniformly decaying. Long-term and short-term estimates suggest slow and steady erosion at ∼20 m/m.y.; however, intermediate-time-scale data like sediment accumulation rates and river incision suggest unsteadiness, which we assess using apatite (U-Th)/He thermochronology. All cooling ages from the central Appalachian hinterland in Pennsylvania and New Jersey and from the rugged Blue Ridge Mountains of western North Carolina are pre-Cenozoic, which places an upper limit on the volume of sediment that could have been sourced from these regions in connection with the documented large accumulation of Miocene siliciclastics offshore. Interpreting the timing and processes governing landscape evolution in these regions was hindered by complex age relations between neighboring samples and considerable age dispersion within individual samples. Through experiments with physical abrasion using two representative samples from the Blue Ridge Mountains, we find that variable zonation of U and Th in conjunction with radiation damage–induced differences in helium diffusivity is the source of age dispersion. Abraded grains produced a strong correlation between age and effective uranium concentration (eU) that was not observed for untreated grains and is expected as a result of grain-specific accumulation of radiation damage during slow cooling. Cooling histories derived from inverse modeling of the eU-age relationship of the abraded grains suggests that for a period of ∼60 m.y. during the Late Cretaceous, valley floors were exhuming at nearly twice the rate of neighboring ridge tops, generating relief equivalent to the modern landscape. This result illustrates that at least portions of the modern landscape are not a direct erosional remnant of long-dead orogenic processes and suggests that significant modifications of the Appalachian landscape can occur within the framework of slow long-term average erosion rates.

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Additional Information:© 2013 Geological Society of America. Manuscript received 18 October 2012. Revised manuscript received 10 June 2013. Manuscript accepted 12 August 2013. This research was supported by a Geological Society of America Student Research Grant and multiple Palmer Research Grants from the Department of Earth and Environmental Sciences of Lehigh University to R.E. McKeon. Support for the field work and lab analysis leading to the pooled age data set was provided by National Science Foundation grant EAR 9909393 to P.K. Zeitler, F.J. Pazzaglia, and B.D. Idleman. Richard Ketcham is thanked for thoughtful discussions that improved our use of inverse models of abraded grains in HeFTy. Peter Reiners is thanked for hosting R.E. McKeon at the University of Arizona and for comments on an early version of this manuscript. Shari Kelley, James Spotila, and an anonymous reviewer are thanked for their thoughtful and thorough comments that improved the clarity of this manuscript along with editorial handling by Nancy Riggs and Jeffrey Amato. Finally, we thank for their contributions, many previous students at Lehigh University who worked on this project, specifically Tom Becker.
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Geological Society of America Student Research GrantUNSPECIFIED
Lehigh University Department of Earth and Environmental SciencesUNSPECIFIED
NSFEAR 9909393
Issue or Number:1-2
Record Number:CaltechAUTHORS:20140626-104211664
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Official Citation: Ryan E. McKeon, Peter K. Zeitler, Frank J. Pazzaglia, Bruce D. Idleman, and Eva Enkelmann Decay of an old orogen: Inferences about Appalachian landscape evolution from low-temperature thermochronology Geological Society of America Bulletin, January 2014, v. 126, p. 31-46, First published on November 6, 2013, doi:10.1130/B30808.1
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:46529
Deposited By: Ruth Sustaita
Deposited On:26 Jun 2014 19:18
Last Modified:03 Oct 2019 06:45

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