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 (2 σ ); 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.