METAMORPHIC PETROLOGY AND THE “NEVADAPLANO”

The Nevadaplano of the western US Cordillera is an elusive high plateau proposed to have existed in the Late Cretaceous-earliest Tertiary, named after the modern Andean Altiplano.

The Nevadaplano is commonly thought to have been supported by thick (>60 km) crust, as inferred from: 1) Geobarometry of rocks in the footwalls of metamorphic core complexes (MCC’s) indicating Late Cretaceous overburden of >30 km, which, when restored, suggest crustal thicknesses double those of today (~ 30 km); 2) Estimates of total crustal shortening that yield comparably thick crust; 3) Paleoaltimetry data that indicate high elevations (>3 km), inferred to be isostatically compensated by thick crust; 4) The geochemistry and petrogenesis of mid-Cenozoic ignimbrites, which are comparable to those erupted through modern examples of thick-crust. Each of these data sets, however, is challenged by non-unique interpretations and each requires high amounts of subsequent extension.

In a type example from the northern Snake Range MCC, estimates of peak burial based on classical thermobarometry (8+ kbar) are twice those predicted based on the mapped geology and known structures and indicate T gradients of ~20°C/km during peak metamorphism. However, conodont alteration indices and mineral assemblages tied to measured stratigraphic/structural depth in the crust beneath a regional Cenozoic unconformity suggest peak T gradients upwards of 40°C/km. Thus, either MCC’s are cold spots in Cordilleran crust or else geobarometry results within them are unreliable indicators of paleo-depth. Considering that most of the rocks studied subsequently underwent upper greenschist metamorphism and mylonitization during Tertiary extension, and in light of recent studies revealing difficulties with equilibrium assumptions in classical thermobarometry, these enigmatically high pressures stand to be tested.

We carried out studies utilizing two recently developed and independent thermobarometers – modeling residual pressures (i.e. Raman shift) and trace element concentrations (i.e. Ti) in garnet-hosted quartz inclusions – that confirm deep burial and add constraints to the timescales of metamorphism, but which continue to highlight disparities between different approaches to determining the thickness of the crust beneath the Nevadaplano.