Magmatic accretion of the upper oceanic crust

Abstract

The focus of this work is the temporal and spatial evolution of upper oceanic crust accreted at the southern Juan de Fuca Ridge (JdFR), an intermediate rate spreading center. The area of concentrated study is along the steep escarpment forming the south-facing wall of the ridge-perpendicular West Blanco Depression (WBD) and exposes crust formed between 1.2 and 1.7 Ma. Data sets utilized include GLORIA, SeaMarc II and high resolution (110 kHz) side scan sonar imagery, multibeam bathymetric maps, and an extensive suite of rock samples, photographs and video coverage obtained with the French submersible Nautile along the scarp face. Detailed sections of lava flow morphologies are constructed from 14 dives, and analysis includes interpretation of petrographic and geochemical data within the context of that reconstruction.In the model proposed, episodes of crustal building are linked to variations in mantle melting conditions and magma resupply with shallow mixing episodes. The upper crustal section accreted in 2-3 major on-axis extrusive stages involving normal (N-type) mid-ocean ridge basalts (MORBs). Flow types associated with each stage imply an overall decline in effusion rate following a vigorous initial output. This magmatic accretionary process was interrupted at irregular intervals by subsidence and block faulting that resulted in a collage of crustal blocks recognizable as texturally distinct domains within the Deep-Tow sonar imagery.Superimposed, time-dependent mantle diapiric flow beneath the north-westward migrating JdFR can explain a seamount chain located on the Juan de Fuca plate and the consistent, seamount spacing. Geochemistry of rocks exposed on the scarp is accounted for by batch-melting of a depleted spinel peridotite source (N-type) followed by fractional melting of an amphibolite-veined mantle residue (M-type). With ridge migration, a seamount forms near-axis while N- and M-type MORB sills are injected into the lower section of the upper crust after which massive M-type diabases are intruded at the base. Most recently, off-axis volcanism associated with Parks Plateau emplaced enriched MORBs into the upper crust.