SOME PROBLEMS AND PROGRESS IN UNDERSTANDING THE EVOLUTION OF METAMORPHIC CORE COMPLEXES (MCC'S) OF THE NORTHERN BASIN AND RANGE

After years of study, unresolved questions about metamorphic core complexes (MCCs) continue to present challenges in terms of our understanding of the tectonic history and driving mechanisms for continental extension. In the northern Basin and Range (NBR), unresolved questions include: How thick was the crust after Mesozoic shortening? Did thickening drive syn- and post-shortening extensional collapse? If estimates of high P in metamorphic rocks in the Snake Range (SR), Ruby Mts (RM) and Albion-Raft River-Grouse Creek Mts (ARG) (suggesting burial up to ~ 30 km) are evidence for thick crust, how exactly were the MCCs buried and then exhumed and exposed? Did high strain mylonitic fabrics preserved in MCCs form during early extensional collapse or during later regional extension? How can we resolve their ages? To answer these questions, we take a “whole-crust” approach and compare the history of surface deposition adjacent to MCCs to the magmatic histories of these regions and to the t-T history of deep crust as read from inherited or xenocrystic zircons in magmas.

Our results show that extension occurred long after the end of shortening and that the time lag between shortening and extension increases southward, tracking the migration of the Eocene-Oligocene magmatic front. The Cenozoic unconformity, developed across the NBR (beneath mostly volcanic rocks) provides regional evidence that rocks currently exposed in MCC’s were nowhere at the surface. Minimal tectonic activity/erosion took place after volcanism, prior to BR faulting ca. 17 Ma. Study of xenocrystic zircons in igneous rocks suggest that the cratonal basement rocks underlying mid-crustal MCC rocks were near zircon solidus T's through the end of the Mesozoic and early Cenozoic, with no evidence for major thermal or tectonic events until Eocene-Oligocene magmatism, which drove partial melting and flow in the crust. P-T conditions and P-T paths of metamorphic rocks, including new work in the SR using residual pressures of quartz included in garnet, provide difficulties with the above conclusions as they require the existence of crustal penetrating structures to bury and subsequently exhume rocks from 30 km depths. These structures are not obvious in the field and are hard to reconcile with the above observations/conclusions.