Magmatism during and after continental break-up

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Abstract
Magmatism accompanying continental break-up is caused primarily by decompression melting of the underlying mantle as it wells up beneath the rift. The amount of melt produced depends mainly on the temperature of the asthenospheric mantle and on the rate of rifting. Break-up above normal mantle generates only small amounts of melt, reaching a maximum of 7 km in fully oceanic crust. If extension lasts for 10 Ma or more, as on many such ‘non-volcanic’ continental margins, then still less melt is generated because the slowly upwelling mantle cools by conductive heat loss. Break-up above abnormally hot mantle surrounding mantle plumes generates much larger melt volumes and ‘volcanic’ continental margins. The largest melt volumes occur when rifting is above a newly initiated mantle plume, with its transient high excess temperatures and flow rates. This creates flood basalt provinces. The resultant basalt may flow distances of over 1000 km, both as surface flows and as dykes and sills intruded laterally in the crust. Only about one-quarter of the melt reaches the surface, the remainder underplating or intruding the lower crust. The excess gravitational potential provided by mantle plumes assists rifting, though plumes do not always cause continental break-up.