Geophysical constraints on the dynamics of spreading centres from rifting episodes on land

Abstract

Most of the Earth's crust is created along 60,000 km of mid-ocean ridge system. Here, tectonic plates spread apart and, in doing so, gradually build up stress. This stress is released during rifting episodes, when bursts of magmatic activity lead to the injection of vertical sheets of magma -termed dykes -into the crust. Only 2% of the global mid-ocean ridge system is above sea level, so making direct observations of the rifting process is difficult. However, geodetic and seismic observations exist from spreading centres in Afar (East Africa) and Iceland that are exposed at the land surface. Rifting episodes are rare, but the few that have been well observed at these sites have operated with remarkably similar mechanisms. Specifically, magma is supplied to the crust in an intermittent manner, and is stored at multiple positions and depths. It then laterally intrudes in dykes within the brittle upper crust. Depending on the availability of magma, multiple magma centres can interact during one rifting episode. If we are to forecast large eruptions at spreading centres, rifting-cycle models will need to fully incorporate realistic crust and mantle properties, as well as the dynamic transport of magma.

Constraining timescales of focused magmatic accretion and extension in the Afar crust using lava geochronology

“…These results suggest that since B200 ka volcanism in the main rift zone has been primarily focused along the axial graben, demonstrating long-term focused magmatic accretion. The long-term extension rate implied by our age-profile is consistent with the recent rifting activity observed here, which suggests that crustal deformation and extension along the incipient African-Arabian plate boundary occurs via discrete magmatic episodes, focused along the segment axis 3,4 . This model of segment growth is supported by geochemical data for these lavas (ref.…”

“…Observations of the multiple (B14) dyke intrusions 3,[11][12][13][14][15][16][17] and fissure eruptions 18 during this active phase have shown that in the upper part of the crust, the delivery of melts to the DMH rift zone is currently strongly focused along the rift axis. The overall scale and morphology of this B15 Â 60 km magmatic segment is similar to a slow (B20 mm per year) spreading mid-ocean-ridge segment 6 , such as those found along the Mid-Atlantic ridge 19,20 , and the recent episodic rifting activity is similar to that previously observed in Iceland 4,21 . The formation of the DMH segment is broadly constrained by regional K-Ar geochronological studies to have occurred since B1 Ma, following a period of widespread fissural volcanism in Afar between B1-3 Ma (ref.…”

“…1), representing focused zones of dyke intrusion, volcanism and faulting 1 . At crustal levels the magmatic and tectonic process operating at these segments closely resemble those observed at slow-spreading ocean ridges [2][3][4][5] and axial dyke intrusions appears to have a key role in accommodating extension 4 and creating characteristic ridge topography 6 . The development of these narrow zones of faulting and magma intrusion signifies an important stage of rift evolution, where extensional processes and volcanic accretion become localised 1 and final stages of crustal thinning may occur before plate rupture and true oceanic spreading 7 .…”

“…A recent phase of active magmatic rifting (2005-2010) along the Dabbahu-Manda Hararo (DMH) Rift Zone in central Afar (B12.41N, 40.51E; Figs 1 and 2) has demonstrated the critical importance of axial dyke intrusions in accommodating crustal extension across magmatic segments 3,4 . Observations of the multiple (B14) dyke intrusions 3,[11][12][13][14][15][16][17] and fissure eruptions 18 during this active phase have shown that in the upper part of the crust, the delivery of melts to the DMH rift zone is currently strongly focused along the rift axis.…”

Segmented lateral dyke growth in a rifting event at Bárðarbunga volcanic system, Iceland

“…of Ethiopia 1 . Limited geodetic and seismic data have been interpreted in terms of lateral flow of magma with dyke propagation rates initially of up to two to three km per day, and then at a declining rate as magma propagates away from a central feeding source 2,3.4 .…”