Possible depth limit for underplating by a seamount

The Cazadero metabasite allochthon is located within the central melange belt of the northern California Coast Ranges, U.S.A. It composed mainly of pillowed and massive mafic flows, hyaloclastite and minor pelagic sediments. It is about 3 × 15 km² in extent, and is in fault contact with Late Mesozoic fore-arc flysch deposits of the Great Valley sequence. Three metamorphic zones were mapped in the allochthon, as follows-in order of increasing metamorphic grade: the lawsonite, the pumpellyite and the epidote zone. Ca---Na pyroxenes are ubiquitous; they were used to estimate the P-T path, together with the oxygen isotope geothermometry of Taylor and Coleman. The result shows a bending point in the P-T path for blueschist facies metamorphism of the Cazadero allochthon. At shallow depths, lower grade metamorphism proceeded under predominant pressure-increase from 4 to 7 kbar at nearly constant T of about 150–200 °C, whereas after passing the bending point higher-grade metamorphism occurred with increasing temperature from 200 to 350 °C at nearly constant P of about 7–8 kbar.

The bending point reflects the depth of transfer of underplating materials from the oceanic lithosphere and its overlying sediments to the hanging wall plate. Either seamount volcanics and pelagic sediments, or mixed packages of fragmented oceanic crust with trench-fill turbidites, were subducted beneath the North American plate. They were subjected to early lawsonite- and pumpellyite-zone metamorphism with very low geothermal gradient. We infer that when they reached about 20 km depth, underplating occurred and the metamorphosed rocks became part of the hanging wall. Heat conduction from the overlying hydrated mantle wedge caused a temperature increase at nearly constant depth. The earlier recrystallized seamount volcanics and pelagic sediments were then metamorphosed at a much higher geothermal gradient. The Cazadero allochthon has suffered extremely high P/T subduction zone metamorphism and it preserves very well the record of Cretaceous subduction-underplating processes.