Volcanic rifted margins

Martin Menzies
Royal Holloway, University of London

September 15th 2004

When continental break-up and magmatic activity are associated both in time and space, a volcanic rifted margin is produced. >75% of the Atlantic Ocean margins are volcanic rifted margins associated with large igneous provinces/flood basalts (e.g. Greenland-UK, Parana-Etendeka). Other examples exist in the Red Sea-Gulf of Aden, west Australia, East Africa and the margins of the Indian sub-continent.

Volcanic rifted margins comprise three main components.

(1) Large Igneous Province (LIP) represent the pre- to syn-rift stage of continental break-up, in most cases located on the rifted continental margin or straddling the continent-ocean transition. LIPs vary from 1-4 km thick dependent on the extent of erosion, and comprise extrusive and intrusive, mafic and felsic rocks, whose emplacement has transformed the ‘normal’ continental crust.

(2) High Velocity Lower Crust (HVLC) represents a syn-rift stage of continental break-up located seaward of the rifted margin. Thicknesses are estimated to be 15-30 km of juvenile crust, believed to be a deep crustal magmatic system of intrusive or underplated derivatives of basaltic magmas. The HVLC may have been emplaced during deep mantle upwelling or formed as a result of extension. The continent-ocean transition can be very sharp, with the HVLC formed on the seaward side of a rifted and thinned continental margin.

(3) Seaward Dipping Reflector Series (SDRS) represent the syn-rift stage of continental break-up. Sub-aerial & submarine volcano-sedimentary units are located seaward of the HVLC and extend landward to the ocean-continent transition. On the basis of studies of the North Atlantic margins, ‘inner’ and ‘outer’ packages for SDRS have been defined. Inner SDRS were believed to be emplaced during sub-aerial sea-floor spreading or syn-tectonic infilling of rift basins. In contrast the outer SDRS may be submarine sheet flows with associated pillowed flows and hyaloclastites.

Some recent publications:
E. Wolfenden et al (2004). Evolution of the northern Main Ethiopian rift: Birth of a triple junction, submitted to Earth Planet.Sci. Letts., 224, 213-229.

P. Maguire et al (2003). Geophysical project in Ethiopia studies continental breakup. Eos, Tran. Amer. Geophys. Un., 84. 337-340.

M.A. Menzies et al (2002) and 12 papers therein. Volcanic rifted margins.
Geol. Soc. Am

S. Planke et al (2000). Seismic volcano-stratigraphy of large-volume basaltic extrusive complexes on rifted margins. Journal of Geophysical Research 105, B8, 19335-19351.

Several activities relating to VRM that have advanced our understanding in recent years

2003- EAGLE (Ethiopia Afar Geoscientific Lithospheric Experiment)
http://www.le.ac.uk/seis-uk/eagle/

2002 US MARGINS
www.margins.wustl.edu

2000 PENROSE Geological Society of America Special Paper 362. www.geosociety.org/penrose/ www.geosociety.org/bookstore/

1997 VOLCANIC MARGINS International Lithosphere Program
http://www.gfz-potsdam.de/pb4/ilp97/pro_iv2.htm