No abstract

Extension during plate convergence and mountain building is widely recognized, yet the causes of synconvergent extension remain controversial. Here we propose that delamination of lithospheric mantle, aided by decoupling of the crust from the mantle via a reduction in the viscosity of the lower crust through heating, incursion of fl uids, and partial melting, explains many enigmatic yet prevalent aspects of the metamorphic, magmatic, and kinematic history of the Sevier-Laramide orogen of the western United States during the Late Cretaceous. Extension, heating, anatexis, magmatism, and perhaps rock uplift were widespread during a restricted time interval in the Late Cretaceous (75-67 Ma) along the axis of maximum crustal thickening within the Mojave sector of the Sevier orogen, and to a lesser extent within the interior of the Idaho-Utah-Wyoming sector to the north; similar processes may have been active in the Peninsular Range, Sierran, western Mojave, and Salinian segments of the Mesozoic Cordilleran arc. These processes are viewed as predictable consequences of the thermal, rheological, and dynamic state of the overlying crust following delamination of mantle lithosphere beneath isostatically compensated mountain belts. The proposed delamination would have occurred immediately prior to eastward propagation of lowangle subduction of the Farallon plate during the inception of the Laramide orogeny. Following delamination, extension and anatexis of the North American crust were aided locally by egress of slab-derived fl uids from the low-angle Farallon slab. We suggest that lithospheric delamination may have aided in the shallowing of the slab to achieve lowangle subduction geometry. Delamination has been proposed to be common in areas of thickened continental lithosphere in the terminal phase or late in orogenesis. The Late Cretaceous delamination event proposed here for the Sevier-Laramide orogen occurred during protracted plate convergence and was synchronous with, and followed by, continued shortening in the external part of the orogen.

[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.

No abstract

Extensional detachment systems separate hot footwalls from cool hanging walls, but the degree to which this thermal gradient is the product of ductile or brittle deformation or a preserved original transient geotherm is unclear. Oxygen isotope thermometry using recrystallized quartz-muscovite pairs indicates a smooth thermal gradient (140 °C/100 m) across the gently dipping, quartzite-dominated detachment zone that bounds the Raft River core complex in northwest Utah (United States). Hydrogen isotope values of muscovite (δD Ms ~-100‰) and fl uid inclusions in quartz (δD Fluid ~-85‰) indicate the presence of meteoric fl uids during detachment dynamics. Recrystallized grain-shape fabrics and quartz c-axis fabric patterns reveal a large component of coaxial strain (pure shear), consistent with thinning of the detachment section. Therefore, the high thermal gradient preserved in the Raft River detachment refl ects the transient geotherm that developed owing to shearing, thinning, and the potentially prominent role of convective fl ow of surface fl uids.