Metamorphism of the deepest exposed arc rocks in the Cretaceous to Paleogene Cascades belt, Washington: Evidence for large-scale vertical motion in a continental arc

P. M. Valley, D. L. Whitney, S. R. Paterson, R. B. Miller, H. Alsleben

Research output: Contribution to journalArticlepeer-review

47 Scopus citations

Abstract

The Swakane Gneiss and the overlying Napeequa Complex in the North Cascade range, Washington, were metamorphosed and deformed during development of a Cretaceous-Paleogene continental arc, and are among the structurally deepest exposed rocks within the Cordilleran arcs of North America. Peak metamorphic conditions in both the Swakane Gneiss and Napeequa Complex were c. 640-750 °C, 9-12 kbar. Clockwise paths and widespread evidence for high-P metamorphism in meta-supracrustal rocks (burial to >40 km) document major vertical tectonic motion during arc construction and unroofing. These and other moderately high-pressure rocks in the North Cascades-Coast Mountains experienced a dramatically different tectonometamorphic history than metamorphic rocks within other Cordilleran arcs. The exhumed arc complexes of the Sierra Nevada and Peninsular Ranges are dominated by relatively low-P metamorphic and plutonic rocks (typically <6 kbar). There is no evidence that the northern Cordillera was thickened to a greater degree than these other belts, suggesting that the greater magnitude of vertical motion in the Cascades may have been related to exhumation mechanisms: Eocene extension in the northern Cordillera vs. erosional unroofing in the central and southern Cordillera.

Original languageEnglish (US)
Pages (from-to)203-220
Number of pages18
JournalJournal of Metamorphic Geology
Volume21
Issue number2
DOIs
StatePublished - Feb 1 2003

Keywords

  • Cascades
  • Continental arc
  • Metamorphism
  • Thermobarometry

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