RIFT TO DRIFT TRANSITION AT VOLCANIC RIFTS AND RIFTED MARGINS: LESSONS FROM ICELAND

Approximately half of all continental rifts and rifted margins include voluminous synrift magmatic construction. Extension-related subsidence generally results in the burial of basement structures that are fundamental to rifting processes in these environments. Subaerial spreading and deeply glaciated Tertiary crustal sections in Iceland provide a remarkable natural laboratory for the investigation of the rift/drift transition in volcanic rifts and rifted margins near hotspots.

Active spreading in Iceland occurs along discrete, overlapping, volcanic/tectonic rift zones tens of kilometers long and about ten kilometers wide. These include a central volcano with mafic to felsic intrusive and extrusive material, high-T hydrothermal metamorphism and focused subsidence and crustal thickening (to 40 km). Along-strike, fissure swarms are the expression of subsurface dike swarms with thinner crust. As the axial crust spreads laterally it subsides to accommodate upper crustal thickening, resulting in inward-dipping lavas and outward-dipping dikes. The high geothermal gradient results in decoupling within the middle crust and lateral flow that reduces crustal thickness variations (average 25 km) and permits spreading in the rigid upper crust that may not be reflected by lithosphere plate motions. Migration of the Iceland hotspot relative to the North American/Eurasian plate boundary has resulted in large transform offsets in the South Iceland Seismic Zone and Tjörnes Fracture Zone to the S and N, respectively. Propagation of rift zones away from the hotspot has produced propagating rifts, migrating transform faults, and microplates analogous to those found along mid-ocean ridges. Rift propagation causes the rotation of crustal blocks and widespread reactivation of rift-related structures as rift-parallel strike-slip faults. These complexities in the kinematics of spreading in Iceland may explain some enigmatic features of other volcanic extensional terranes.