EPISODIC LATE JURASSIC AND MID CRETACEOUS SHORTENING IN THE SOUTHERN SEVIER FOLD-AND-THRUST BELT AND HINTERLAND

Retroarc fold-thrust belts and their metamorphic hinterlands are commonly considered to evolve by quasi-continuous, progressive deformational processes but increasing evidence suggests that strain, and associated metamorphism and magmatism, accumulates during punctuated, episodic events. Here we review evidence for episodic late Middle to Late Jurassic and mid Cretaceous shortening in the Sevier fold-thrust belt and hinterland at the latitude of Las Vegas, culminating in Laramide extension. Jurassic shortening in the fold-thrust belt is evident in zircon (U-Th)/He data from the Wheeler Pass thrust sheet (WPTS) that demonstrates enhanced cooling from 160-140 Ma during and after translation of the sheet over the footwall ramp. In the WPTS of the southern Nopah Range, the tail end of this cooling event is captured in a similar (U-Th)/He data paleodepth transect in addition to cooling of 3 km of structural section from 100 to 85 Ma. Ages of syntectonic conglomerates constrained by detrital zircon geochronology associated with the eastern frontal thrust system range from 103-99 Ma. In the Funeral Mountains of the hinterland, metamorphic rocks record two prograde metamorphic events. Monazite petrochronology from Monarch Canyon documents two periods of prograde metamorphism of Middle Jurassic-Early Cretaceous age (171 to 143, mode at 162 Ma) and mid Cretaceous (105-88 Ma) age, and Late Cretaceous retrograde metamorphism (88-75 Ma) during decompression. Monazite ages and petrography of 11 samples from the Monarch Canyon structural section reveal a transition from Jurassic dates at the shallowest levels to mid and Late Cretaceous dates at the deepest levels, documenting a progressive mid- and Late Cretaceous overprint on Jurassic metamorphic mineral assemblages. Heavy rare earth element trends in monazite together with petrography show that 105 to 88 Ma prograde metamorphism occurred concurrent with prograde garnet crystallization during staurolite breakdown and that 88 to 70 Ma monazite formation was concurrent with garnet breakdown and growth of late staurolite during decompression. The synchronicity between the Jurassic and Cretaceous retroarc shortening events and magmatic flux events in the Sierran arc helps to discriminate driving forces for retroarc shortening.