The thermal history of shallow subduction beneath the southern Sierra Nevada

The modern Sierra Nevada is underlain by some of the coldest continental lithosphere on Earth. Despite voluminous 130-80 Ma arc magmatism, subnormal heat flow has been recognized throughout the Sierra Nevada since the 1970’s. A comprehensive (n=125) thermal history analysis performed via K-feldspar thermochronology in the southern Sierra Nevada and adjacent Mojave Desert from 5-30 km mid-Cretaceous crustal depths reveals T-t historiesdominated by subduction refrigeration between 80-75 Ma and continued subnormal geotherms thereafter. The pattern of cooling as a function of depth and distance from the trench agrees with numerical models of plateau subduction that predict substantial removal of subcontinental mantle lithosphere and lower crust coupled with underplating of trench-derived sediment (Rand Schist). This argues against 10-0 Ma lower crustal delamination of arc cumulates. The shallow subduction signature is at odds with overlap of magmatic arc segments required by most variants of the Baja-BC model where a ~3000x250 km portion of British Columbia (Insular Terrane)thought to be derived from southern and/or Baja California is postulated to have translated north outboard of 100-80 Ma arc magmatism within the Sierra Nevada.

Speaker: Martin Grove, Stanford University