DOES UNDERTHRUSTING CRUST FEED THE 100 MA MAGMATIC FLARE-UPS IN THE SIERRA NEVADA? (Invited Presentation)

The Sierra Nevada arc in California is known for its non-steady state magmatism during the Mesozoic. The Late Cretaceous flare-up peaked at ~100 Ma was the most intense episode of magmatism that contributed to the growth of the arc crust. What drives the flare-up and which sources feed the arc magma are debated. Some existing models argue for fueling the magmatism by partial melting of the underthrusted retro-arc lower crust. Yet, such processes remain conceptional without considering the mass and heat balances associated with the partial melting processes. Here, we use a simple 1-D numerical model and scaling analysis to assess the thermodynamic feasibility of generating arc magma due to the partial melting of the underthrusted lower crust. Results show that the partial melting only contributes to the 10%–30% of the magmatic thickening rate (km/Myr) during the Late Cretaceous flare-up, and the partial melting of a 20-km-thick lower crust generates about 10%–40% of the estimated magma volume in the flare-up. Therefore, we argue partial melting of underthrusted lower crust plays a partial or subsidiary role in driving a magmatic flare-up event. Additional melts from the mantle and/or other crustal sources are needed to achieve the observed magmatic output during the flare-up. The arc root developed by partial melting of the underthrusted crust reduces the time needed to obtain the critical thickness for root foundering, thus influencing the tempo of arc magmatism.