Construction of magmatic rifted margins

Oceanic basins are the ultimate product of rifting. While significant advances have been made in understanding the attenuation of continental lithosphere prior to final break-up, less attention has been paid to the role magma and volcanism plays in the transition from break-up to the formation of new oceanic crust. Using deep seismic images along the Atlantic margins, we show that seaward-dipping reflectors (SDRs) provide evidence for the processes involved.

A key premise is that landward-dipping nonral faults are an integral part of the ascent o fmagma during plate separation, allowing crust both to be stretched and accreted concurrently. We have developed a generic model where buoyant transitional crust is rolled out on either side of an axial horst cored by a domed pluton or magma chamber comprising gabbroic melt fed from upwelling asthenosphere. The axial horst was formed at the point of break-up of the upper crust but its continental origin becomes disguised by multiple intrusions and footwall collapse. The landward-dipping faults defining the horst are a type of low angle detachment which accommodates the construction of new crust by propagating away from the continent as the plates diverge. Melt flows to the inherent low pressure zone beneath the footwalls where space is created for magma to collect as the faults move, a generic process we term “magma-assisted extensional growth”. Below where the faults detach, new gabbroic crust is accreted along the flanks of the magma chamber in dilational shear zones in the mid crust, transferring and deforming gabbro from the footwall to the hanging wall as the shear zones move. Dykes are intermittently sourced from the top of the magma chamber and traverse the horst, leading to periodic eruptions of basalt which collect as sub-aerial lava flows in the hanging walls on either side of it. The lavas become seaward-dipping due to roll-over onto an igneous weld at around 4 km depth where the faults meet the upper edge ofthe magma chamber.

While upper and mid crust is accreted and deforms by simple shear, the underlying lower lithosphere is inherited from the continent and progressively thins by pure shear, making the transitional plate relatively buoyant. The width of the transition depends on the degree of necking and the amount of thickening by intrusions in the lower lithosphere. Eventually, when the mantle lithosphere has thinned to a critical point, the axial horst splits to form an embryonic mid-ocean ridge where normal oceanic crust is formed by seafloor spreading. Heat and extension becomes focussed at the plate boundary and the oceanic crust on either side subsides to oceanic depths as cooling mantle accrets to its base.