A MAJOR MID-CRETACEOUS SHORTENING EVENT IN THE SOUTHERN SEVIER OROGENIC BELT: CONTINENTAL RECORD OF GLOBAL PLATE REORGANIZATION?

The causes of retroarc shortening at convergent plate margins remain elusive despite decades of study. Long term shortening histories are reasonably extracted from geologic records, but shorter duration events of higher strain rate, which hold the prospect for revealing more direct ties to potential geodynamic causes, are more challenging to reconstruct. Here we describe new geochronologic constraints for a major mid-Cretaceous period of shortening in the southern Sevier fold-thrust belt, and compare the age of this event to potential documented geodynamic causes. The leading edge of the southern Sevier fold-thrust belt trends SSW across southern Nevada into the eastern Mojave Desert of CA, transecting the Muddy Mountains (MM), Spring Mountains (SM), Clark Mountain and Mescal Range (MR), and the New York Mountains (NY). Latest Albian to Cenomanian synorogenic sedimentary deposits and related local volcanic rocks are associated with the Muddy Mountain (MM), Red Spring-Wilson Cliffs-Contact (SM), Keaney-Mullosk Mine (MR), and Sagamore (NY) thrusts, with local names including the Willow Tank Formation and Baseline Sandstone (MM), conglomerate of Brownstone Basin (SM), Lavinia Wash sequence (SM), Delfonte volcanic rocks (MR), and sedimentary rocks of Sagamore Canyon (NY). New U-Pb detrital zircon geochronology provides maximum depositional ages for: sand at the base of the Willow Tank Formation (101.7 +0.4/-0.5 Ma); the conglomerate of Brownstone Basin (3 samples: 102.8 +1.0/-1.2 Ma; 103.3 +1.0/-1.1 Ma; 102.1 +1.7/-0.9 Ma); and sedimentary rocks of Sagamore Canyon (2 samples, 97.3 + 1.2/-0.4 Ma, and 97.2 +1.3/-1.0 Ma). The Sagamore thrust (NY) cuts metarhyolite dated at 98.3 +0.6/-0.4 (base) and 96.8 +1.3/-0.5 (top), which is in turn cut by the Mid Hills monzogranite (90.4 ± 0.8 Ma). Synorogenic conglomerates commonly contain detritus from breached footwall structures and the hanging wall, demonstrating syndeformational deposition. The 103-96 Ma shortening documented here for greater than 100 km along strike, together with other regional evidence for enhanced shortening at this time, may be a Cordilleran record of the recently documented major mid-Cretaceous global plate reorganization related to the termination of subduction in eastern Gondwanaland.