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

Abstract

[1] 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 AE 1.9 Ma (2s); 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.

Jurassic Barrovian metamorphism in a western U.S. Cordilleran metamorphic core complex, Funeral Mountains, California

“…Large-magnitude crustal shortening of Cretaceous age is extensively documented in the Sevier orogenic belt; however, this study shows that it extends back into the Late Jurassic, and may have closely followed the initiation of a noncollisional convergent plate margin. The unambiguous result of this study suggests that the record of Late Jurassic crustal thickening may be commonly obscured by the Late Cretaceous metamorphism that is pervasive in the mid-crustal rocks of western U.S. core complexes (e.g., Miller and Gans, 1989;McGrew et al, 2000;Wells et al, 2012).…”

Middle to late Miocene extensional collapse of the North Patagonian Andes (41°30′–42°S)

“…As was recently stated by Wells et al (2012), extension within orogenic belts is a rather common process during orogenesis. Moreover, extensional deformation during plate convergence is observed in elevated hinterlands of ancient and present fold thrust belts all around the world (e.g., the eastern Alps, the Apennines, the Calabria Arc, the Himalayas, the Scandinavian Caledonides, the Sevier-Laramide orogen, among others; for a synthesis see Wells and Hoisch, 2008, and references therein).…”

“…A common, recently modeled mechanism explaining synconvergent extension is slab rollback (Wells et al, 2012;Sternai et al, 2014). Since Orts et al (2015) recently interpreted the migration of magmatic activity north of 41º45´S as a possible case of a shallowing-retreating cycle of the subducted slab, this could constitute an alternative explanation for the extensional event described in this work.…”

“…We envisage anomalously high altitudes at the transition between the eastern domain of the fold and thrust belt and the foreland zone as a possible evidence of an over-steepening of the taper (supercritical state of the thrust wedge). This is one of the mechanisms proposed for explaining synconvergent extensional events (see Wells et al, 2012, and references therein).…”

“…Among the few examples of active hinterland extension, the Cordillera Blanca of Peru in the Andes of South America, probably constitutes one of the most outstanding examples of active large-scale extension within the hinterland in a subduction setting (McNulty and Farber, 2002;Giovanni et al, 2010). Even though there is a large amount of studies focused on this subject, the processes that drive extensional tectonics at the hinterland during convergence remain poorly understood and are still matter of debate (see Wells et al, 2012 for a review).…”