Constraints from sill intrusions and their deeper source magma chambers (seismic high velocity bodies) on the origins of volcanic rifted margins

Abstract:

Volcanic rifted margins are characterized by massive igneous activity originating from the rift margin, characterized by seaward dipping reflectors. These consist of basalt flows and associated magmatic products, from deep magma chambers imaged on seismic data as High Velocity Bodies (HVB) with seismic velocities between 7 and 7.5 km/s. The relationship between rifting and decompression melting have been well quantified, using the HVB’s as constraints on magmatic production to match extension models. Crucial in this approach are the relationship between extension and mantle plumes, with HVB’s generated by mantle plumes often indicative of velocities between 7.5 – 7.8 km/s.

Here I address information that can be obtained from sill complexes in sedimentary basins associated with rifting, representing the earliest phase of magmatism. I use a simple crustal scale hydrostatic model for dikes while incorporating the presence of sills by calculating magmatic overpressures from differences in pressure gradients. It transpires that the presence of sills as observed on seismic reflection and outcrop data, can be predicted. Modelling further suggests that the source of these sill complexes are large magma chambers at or near the Moho, and equate to HVB’s observed on seismic data. Utilizing simple mass balance calculations, the ratio of cumulate thickness (from HVB thickness) and expelled melt (from accumulated sill thicknesses) can be related to MgO content in expelled liquids, primary magma and cumulates. Higher MgO content translates in higher seismic velocities. Thus, HVB velocity can subsequently be used to discriminate between mantle plume, or shallow rift related melting.

The theory is applied to various basins bordering the northern North Atlantic (Vøring Basin, Jameson Land Basin and Rockall Basin) and South Atlantic rifts (Namibia), associated with the Paleocene/Eocene Iceland mantle plume and the Early Cretaceous Tristan da Cunha mantle plume magmatism respectively.