GEODYNAMICS OF SYNCONVERGENT EXTENSION AND TECTONIC MODE SWITCHING: CONSTRAINTS FROM THE SEVIER-LARAMIDE OROGEN

Many orogenic belts experience alternations in shortening and extension during continuous plate convergence. However, such kinematic alternations are not well understood from a geodynamic standpoint. Late Cretaceous to Eocene alternations of shortening and extension (tectonic mode switches) are recorded in the Grouse Creek Mountains of the hinterland of the Sevier-Laramide orogen. We have combined new Lu-Hf garnet geochronometry with existing PT paths determined by differential thermobarometry and monazite Th-Pb inclusion geochronometry to produce a well-constrained PTt path that includes a major episode of synconvergent extension; the PT path is used to test alternative mechanisms for tectonic mode switches. Two burial events (85 and 65 Ma) are separated by ~ 2-3 kbar of decompression and 20 m.y. The first burial episode is Late Cretaceous, records a 3 kbar pressure increase at ~450-525°C and is dated by a Lu-Hf garnet isochron age of 85.5 ±1.9 Ma (2σ); the second burial episode records ~1kbar of pressure increase as temperatures climbed from ~600°C to 630°C, and is dated by Th-Pb ages of monazite inclusions in garnet between ~65 and 45 Ma. Heating continued during the decompression, which, when considered along with a regional association of extension coeval with anatexis, suggests that the exhumation is a response to delamination of mantle lithosphere. 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 PTt path. This is consistent with the interpretation that orogenic wedge tapers may evolve from subcritical to supercritical and back to subcritical owing to elevation changes resulting from isostatic adjustments during the development and foundering of a Rayleigh-Taylor instability and subsequent thermal reequilibration. For example, subsidence, reinforced by convergent flow induced by a developing and downwelling Raleigh-Taylor instability, may promote crustal shortening during development of the convective instability in the interior of a subcritical orogenic wedge. We propose that episodic delamination during continuous subduction provides an alternative mechanism to episodic slab rollback in explaining cyclic tectonic mode switches.