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Intracrustal subduction and gravity currents in the deep crust; Sm-Nd, Ar-Ar, and thermobarometric constraints from the Skagit gneiss complex, Washington

Brian P. Wernicke and Stephen R. Getty
Intracrustal subduction and gravity currents in the deep crust; Sm-Nd, Ar-Ar, and thermobarometric constraints from the Skagit gneiss complex, Washington
Geological Society of America Bulletin (September 1997) 109 (9): 1149-1166

Abstract

Isotopic and thermobarometric (pressure-temperature, P-T) data on a gneissic quartz diorite, combined with previous U-Pb ages and P-T data on wall-rock metapelites, define a precise P-T time series for a portion of the Skagit gneiss complex, a deep crustal crystalline complex within a zone of intracontinental collision. Concordant U-Pb and Sm-Nd ages on zircon indicate crystallization at 68 Ma, which preceded metamorphism of nearby pelitic wall rocks (garnet core assemblages) and the pluton (plagioclase-hornblende coronas on garnet) at approximately 800-900 MPa, 700-800 degrees C. Following nearly isothermal decompression and sillimanite-cordierite metamorphism of wall rocks (garnet rims plus matrix) at 300-500 MPa, 650-700 degrees C, a Sm-Nd mineral isochron including two garnet separates records an initial phase of cooling of the complex at 60 Ma. Slow cooling ( approximately 10 degrees C/m.y.) extends from approximately 60 Ma to 50 Ma. Hornblende and biotite Ar-Ar ages of 47 Ma and 45 Ma, respectively, define a second unroofing event; cooling rates during this period were approximately 100 degrees C/m.y. Late Cretaceous deep burial followed by two distinct unroofing episodes is also observed in some Cordilleran metamorphic core complexes and may suggest a common origin. We propose that regional shortening in the Cordilleran interior is accommodated by a process of intracrustal subduction, i.e., the upper crust is forced downward into a crustal asthenosphere, resulting in a cold, buoyant root. Heating and weakening of the root cause the formation of gravity currents, effective viscosity being in the range of 10 (super 16) -10 (super 19) Pa s, spreading outward below nonfluid upper crust. Spreading of the gravity current, which does not necessarily contribute to crustal thinning, is driven by the small density contrast between the root and its mid-crustal surroundings. Hence unroofing is neither a response to overthick crust nor controlled by the orientation of principal stress axes in the upper crust and upper mantle. The root then cools slowly until final unroofing to near-surface levels, which may occur via either extension or shortening with consequent erosion.


ISSN: 0016-7606
EISSN: 1943-2674
Coden: BUGMAF
Serial Title: Geological Society of America Bulletin
Serial Volume: 109
Serial Issue: 9
Title: Intracrustal subduction and gravity currents in the deep crust; Sm-Nd, Ar-Ar, and thermobarometric constraints from the Skagit gneiss complex, Washington
Affiliation: California Institute of Technology, Division of Geological and Planetary Sciences, Pasadena, CA, United States
Pages: 1149-1166
Published: 199709
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 78
Accession Number: 1997-072370
Categories: GeochronologyIgneous and metamorphic petrologySolid-earth geophysics
Document Type: Serial
Bibliographic Level: Analytic
Annotation: Includes two appendices
Illustration Description: illus. incl. 4 tables, geol. sketch map
N48°34'60" - N48°45'00", W121°15'00" - W12°10'00"
Secondary Affiliation: University of California at Berkeley, Berkeley Center for Isotope Geochemistry, Berkeley, CA, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 199724
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