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

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.