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Low-temperature thermochronologic signature of range-divide migration and breaching in the north Cascades

Thibaud Simon-Labric, Gilles Y. Brocard, Christian Teyssier, Peter A. van der Beek, Peter W. Reiners, David L. Shuster, Kendra E. Murray and Donna L. Whitney
Low-temperature thermochronologic signature of range-divide migration and breaching in the north Cascades
Lithosphere (October 2014) 6 (6): 473-482


Physical and numerical simulations of the development of mountain topography predict that asymmetric distributions of precipitation over a mountain range induce a migration of its drainage divide toward the driest flank in order to equilibrate erosion rates across the divide. Such migration is often inferred from existing asymmetries, but direct evidence for the migration is often lacking. New low-temperature apatite cooling ages from a transect across the northern North Cascades range (Washington, NW USA) and from two elevation profiles in the Skagit River valley record faster denudation on the western, wetter side of the range and lower denudation rates on the lee side of the range. This difference has already been documented further south along another transect across the range; however, in the south, the shift from young cooling ages to older ages occurs across the modern drainage divide. Here, further north, the shift occurs along a range-transverse valley within the Skagit Gorge. It has been proposed that the upper Skagit drainage was once a part of the leeward side of the range but started to drain toward the western side of the range across the Skagit Gorge in Quaternary time. Age-elevation profiles along the former drainage and in the Skagit Gorge restrict the onset of Skagit Gorge incision to the last 2 m.y., in agreement with (super 4) He/ (super 3) He data for the gorge floor. Breaching of the range drainage resulted in its displacement 40 km further east into the dry side of the range. In the 2000-m-deep, V-shaped Skagit Gorge, river stream power is still high, suggesting that incision of the gorge is still ongoing. Several other similar events have occurred along the range during the Pleistocene, supporting the proposed hypothesis that the repeated southward incursions of the Cordilleran ice sheet during this period triggered divide breaching and drainage reorganization by overflow of ice-dammed lakes at the front of the growing ice sheet. Since these events systematically rerouted streams toward the wet side of the range and resulted in leeward migration of the divide, we propose that in fact the Cordilleran ice sheet advance essentially catalyzed the adjustment of the mountain chain topography to the current orographic precipitation pattern.

ISSN: 1941-8264
EISSN: 1947-4253
Serial Title: Lithosphere
Serial Volume: 6
Serial Issue: 6
Title: Low-temperature thermochronologic signature of range-divide migration and breaching in the north Cascades
Affiliation: University of Lausanne, Institute of Earth Surface Dynamics, Lausanne, Switzerland
Pages: 473-482
Published: 20141010
Text Language: English
Publisher: Geological Society of America, Boulder, CO, United States
References: 67
Accession Number: 2014-097226
Categories: Quaternary geology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 1 table, sketch maps
N48°39'59" - N48°39'59", W121°15'05" - W121°15'05"
Secondary Affiliation: University of Pennsylvania, USA, United StatesUniversity of Minnesota, USA, United StatesUniversite Grenoble-Alpes, FRA, FranceUniversity of Arizona, USA, United StatesUniversity of California, Berkeley, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, 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: 201449
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