Seismic observations of Nazca-plate crustal thicknesses providing constraints for a first-order asthenospheric-mantle potential-temperature anomalies assessment

Abstract

The Nazca plate is populated with several magmatic tracks exhibiting complex morphologies responding to time-dependent positioning of spreading centers (East Pacific Rise, Cocos-Nazca). Mantle plumes beneath the Nazca plate are/were located on- or off-ridge (with ridge being spreading center), modulating the resulting hotspot-tracks’ crustal thickness accordingly. We use published seismic observational constraints of Moho depths to study asthenospheric-mantle potential-temperature anomalies (ΔTp) beneath the Nazca plate. We use a simple thermodynamics formulation for adiabatic cooling and decompression melting, and a depth-dependent, static analytical description presenting different options for hydrated-solidus-liquidus curves up to 8 GPa-lithostatic pressure as a function of bulk water content in the peridotite source. The calculated Tp-anomalies are relative to an average Nazca crust (∼6 km thick) formed at the East Pacific Rise with source hydration levels of about 0.01 wt%. As active-upwelling perturbing passive MOR environments, on-ridge plumes added mass (lower crustal intrusion, surface extrusion) to young crust creating OIB hotspot-tracks whose mean crustal thicknesses (≤18 km) are consistent with inferred ΔTp range-values of [-100, 75] °C (Iquique Ridge), [-50, 130] °C (Nazca Ridge), and [-20, 150] °C (Carnegie Ridge south), considering conservative bulk water contents of 0.005–0.08 wt%. The Juan Fernández OIB hotspot track was formed by an off-ridge active-upwelling plume impinging under a 27 Myr-old oceanic lithosphere with ∼7 km-thick pre-existing crust. Currently, the track presents 4-5 km-high isolated volcanic edifices and about 1 km crustal root, totaling about 12–13 km aggregate melt thickness, and suggesting ΔTp range-values of [-20, 160] °C.