TWO ASPECTS OF DIKE INTRUSION AT RIFTS AND SPREADING CENTER

Most continental break up events are preceded by massive volcanism that is likely produced by the partial melting of upwelling plumes or plume heads. Dike intrusions radiating thousands of kilometers from the locus of volcanism can produce local weakening of the lithosphere that may permit rifting to continue even as magmatism wanes. The first question addressed here is why do rift-cutting dikes propagate away from the center of magmatism after volcanism has largely ceased.

Rift thermal structures evolve significantly while they are active and this evolution can cause a cessation of localized volcanism. In some large igneous provinces, like the Ethiopian Plateau, there is seismic and petrologic evidence of addition of large amounts of intrusives to the crust. These intrusions will alter the thermal structure of the crust and cause progressive shallowing of the depth of magma chambers. If crustal density increases with depth then magma chambers may become too shallow to allow extrusion of magma. Then sill and dike intrusion may continue well after the peak in volcanism.

Unlike rifts, spreading centers are nearly in thermal steady-state, but dike propagation away from the centers of spreading segments is an essential feature of plate separation. The lithospheric spreading not accommodated by diking occurs via faulting and this results in axial valley relief. No existing models explain both the observed variations in axial valley relief and patterns of faulting. Specifically, standard models require strength in the mantle at the spreading axis to produce axial valleys. Studies of ridge seismicity and seismic moment release at ridges are not consistent with this standard prediction. The second question discussed here is how axial valleys can form without cold, strong mantle lithosphere at the axis. A possible answer relates to the depth range and therefore the amount of magma intruded in dikes. A key feature of a new model is that dikes open in a self-consistent way that requires intrusion well below the base of the axial lithosphere. Faulted valleys form when there is insufficient magma to open dikes in the shallow and deep crust. The model is consistent with observed relationships between axial relief, gabbro layer thickness, and seismicity of slow spreading ridges.