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Decay of an old orogen; inferences about Appalachian landscape evolution from low-temperature thermochronology

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 (November 2013) 126 (1-2): 31-46


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 approximately 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 approximately 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.

ISSN: 0016-7606
EISSN: 1943-2674
Serial Title: Geological Society of America Bulletin
Serial Volume: 126
Serial Issue: 1-2
Title: Decay of an old orogen; inferences about Appalachian landscape evolution from low-temperature thermochronology
Affiliation: Lehigh University, Department of Earth and Environmental Sciences, Bethlehem, PA, United States
Pages: 31-46
Published: 20131106
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 65
Accession Number: 2013-098162
Categories: GeochronologyStructural geology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 3 tables, geol. sketch map
N33°00'00" - N47°30'00", W87°00'00" - W67°00'00"
Secondary Affiliation: University of Cincinnati,
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 201356
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