PUNCTUATED OROGENIC ACTIVITY: EVIDENCE FOR A 100 MA EPISODE OF SEVIER DEFORMATION IN SOUTHWEST UTAH

Deformation associated with the Sevier fold-thrust belt is spatially and temporally extensive, affecting a swath of the Cordilleran interior from western Nevada to Canada during the Middle Jurassic to the Eocene (~165 Ma to ~50 Ma). Timing of deformation in southern Utah was thought to be late Cretaceous to early Paleocene, suggesting gradual west to east migration of the deformation front. Detailed mapping and high-resolution CA-TIMS of zircon from a newly recognized air-fall tuff in the Three Peaks area of southwest Utah supports a period of increased regional orogenic activity at 100 Ma. This is consistent with punctuated (rather than continuous) episodes of orogenic activity in the Sevier fold-thrust belt.

In the Three Peaks area, the Sevier Iron Springs thrust accommodated at least 5.6 km of shortening based on published seismic sections. The Cretaceous Marshall Creek breccia immediately overlies the sub-Cretaceous unconformity at Three Peaks and is closely spatially associated with the trace of the Iron Springs thrust. It consists of large (up to ~1.5 m) angular limestone clasts and interbedded lacustrine carbonate deposits. It is conformably overlain by the Three Peaks Tuff Member of the Iron Springs Formation; a variably thick (7m-17m) dacite, lapilli, air-fall tuff comprised of multiple cooling units. Tuff zircons record a U/Pb crystallization age of 100.18 +/- 0.04 Ma. Rb/Sr isotopic systematics define an “errorchron” date of 160 Ma, suggesting open system behavior, and an age corrected initial 87Sr/86Sr of 0.706. Trace element characteristics are La/Sm=6.7, La/Lu= 130, and a negative Eu anomaly (avg. Eu/Eu* 2.2).

Depositional character of the Marshall Creek breccia is interpreted to record deformation on the Iron Springs thrust. The Three Peaks Tuff Member constrains thrust movement to ~100 Ma. The mid-Cretaceous thus marks an episode of extensive deformation in the Sevier fold-thrust belt, with reported movement on major thrust sheets including the Keystone-Muddy Mountain, Iron Springs, and Willard

thrusts. We suggest that the documented ~100 Ma magmatic flare-up in the Sevier arc included a previously unrecognized volcanic event, partially recorded in the Three Peaks Tuff, and was accompanied by rapid foreland propagation of thrust sheets; this implies that magmatism and deformation are linked processes.