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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

Wernicke, Brian and Getty, Stephen R. (1997) 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, 109 (9). pp. 1149-1166. ISSN 0016-7606. doi:10.1130/0016-7606(1997)109<1149:ISAGCI>2.3.CO;2.

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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 ≈800–900 MPa, 700–800 °C. Following nearly isothermal decompression and sillimanite-cordierite metamorphism of wall rocks (garnet rims plus matrix) at 300–500 MPa, 650–700 °C, a Sm-Nd mineral isochron including two garnet separates records an initial phase of cooling of the complex at 60 Ma. Slow cooling (≈10 °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 °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^(16)–10^(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.

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Wernicke, Brian0000-0002-7659-8358
Additional Information:© 1997 Geological Society of America. Manuscript received by the Society February 1, 1996; Revised manuscript received December 19, 1996; Manuscript accepted January 22, 1997. Supported by National Science Foundation grants EAR-93-16797 awarded to Wernicke and EAR-93-16283 awarded to D. J. DePaolo. We are grateful for assistance from T. Johnson and B. J. Kriens in sample collection, J. L. Anderson in whole-rock chemical analysis, P. J. Wyllie in interpreting igneous phase relations, M. Choudhury in mineral separations, J. A. Armstrong, P. A. Carpenter, M. A. House, and M. Peters in microprobe and scanning electron microscope microanalysis, J. B. Saleeby in zircon dissolutions, K. V. Hodges and T. Owen in mass spectrometry and data analysis, M. J. Kohn in thermobarometry, and D. J. Stevenson in the physics of gravity currents. S. Getty thanks D. J. DePaolo for the freedom to pursue this topic. Discussions with B. J. Kriens, R. B. Miller, and R. A. Haugerud have stimulated this study. Detailed, constructive reviews provided by D. L. Whitney and Bulletin reviewers R. A. Haugerud, C. A. Hopson, J. M. Mattinson, and J. K. Mortensen greatly clarified the presentation.
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Issue or Number:9
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Official Citation:Brian 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, v. 109, p. 1149-1166, doi:10.1130/0016-7606(1997)109<1149:ISAGCI>2.3.CO;2
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:47426
Deposited By: Tony Diaz
Deposited On:23 Jul 2014 18:26
Last Modified:10 Nov 2021 17:39

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