Geodynamics of synconvergent extension and tectonic mode switching: Constraints from the Sevier-Laramide orogen

Wells, Michael L. and Hoisch, Thomas D. and Cruz-Uribe, Alicia M. and Vervoort, Jeffrey D. (2012) Geodynamics of synconvergent extension and tectonic mode switching: Constraints from the Sevier-Laramide orogen. Tectonics, 31 (TC1002). ISSN 1944-9194

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Abstract

Many orogenic belts experience alternations in shortening and extension (tectonic mode switches) during continuous plate convergence. The geodynamics of such alternations are not well understood. We present a record of Late Cretaceous to Eocene alternations of shortening and extension from the interior of the retroarc Sevier-Laramide orogen of the western United States. We integrate new Lu-Hf garnet geochronometry with revised PT paths utilizing differential thermobarometry combined with isochemical G-minimization plots, and monazite Th-Pb inclusion geochronometry to produce a well-constrained “M” shaped PTt path. Two burial events (86 and 65 Ma) are separated by ∼3 kbar of decompression. The first burial episode is Late Cretaceous, records a 2 kbar pressure increase at ∼515–550 °C and is dated by a Lu-Hf garnet isochron age of 85.5 ± 1.9 Ma (2σ); the second burial episode records ∼1 kbar of pressure increase at ∼585–615 °C, and is dated by radially decreasing Th-Pb ages of monazite inclusions in garnet between ∼65 and 45 Ma. We propose a synconvergent lithospheric delamination cycle, superimposed on a dynamic orogenic wedge, as a viable mechanism. Wedge tapers may evolve from critical to subcritical (amplification), to supercritical (separation), and back to subcritical (re-equilibration) owing to elevation changes resulting from isostatic adjustments during the amplification and separation of Rayleigh-Taylor instabilities, and post-separation thermal and rheological re-equilibration. For the Sevier-Laramide hinterland, the sequence of Late Cretaceous delamination, low-angle subduction, and slab rollback/foundering during continued plate convergence explains the burial-exhumation-burial-exhumation record and the “M-shaped” PTt path.

Item Type:	Article
Publisher’s Statement:	© 2012 by the American Geophysical Union.
ID number or DOI:	10.1029/2011TC002913
Related URLs:	http://onlinelibrary.wiley.com/doi/10.10... http://onlinelibrary.wiley.com/doi/10.10...
Keywords:	Radioisotope geochronology; Pressure-temperature-time paths; Kinematics of crustal and mantle deformation; Continental margins: convergent; Continental tectonics: extensional; amplification; continental-crust; convective instability; Farallon Plate; foreland-basin system; Garnets; geodynamics; hafnium; lead; lu-hf; Lu-Hf garnet; metamorphic core complex; monazite; north-american cordillera; Phosphate minerals; PTt path; rayleigh-taylor instability; separation; Sevier orogen; Steel sheet; synconvergent extension; tectonic mode switching; tectonics; thrust belt; western united-states
Subjects:	Q Science > QE Geology
NAU Depositing Author Academic Status:	Faculty/Staff
Department/Unit:	College of Engineering, Forestry, and Natural Science > School of Earth Sciences and Environmental Sustainability
Date Deposited:	12 Oct 2015 15:46
URI:	http://openknowledge.nau.edu/id/eprint/1369

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