No abstract

As continental rift zones evolve to sea floor spreading, they do so through progressive episodes of lithospheric stretching, heating, and magmatism, yet the actual process of continental breakup is poorly understood. The East African Rift system in northeastern Ethiopia is central to our understanding of this process, as it lies at the transition between continental and oceanic rifting [Ebinger and Casey, 2001]. We are exploring the kinematics and dynamics of continental breakup through the Ethiopia Afar Geoscientific Lithospheric Experiment (EAGLE), which aims to probe the crust and upper mantle structure between the Main Ethiopian (continental) and Afar (ocean spreading) rifts, a region providing an ideal laboratory to examine the process of breakup as it is occurring. EAGLE is a multidisciplinary study centered around the most advanced seismic project yet undertaken in Africa (Figure l). Our study follows the Kenya Rift International Seismic Project [e.g., KRISP Working Group, 1995],and capitalizes on the IRIS/PASSCAL broadband seismic array [Nyblade and Langston, 2002], providing a telescoping view of the East African Rift within this suspected plume province.

A structural model of the Mid-Atlantic Ridge at 37" N is proposed on the basis of travel-time data and synthetic seismograms. At the ridge axis the crust is only 3 km thick and overlies material with an anomalously low 'upper mantle' velocity of 7.2 km s-'. Crustal thickening and the formation of layer 3 and a layer with velocity 7.2-7.3 km s-l takes place within a few kilometres of the axis, producing a 6-7 km thick crust by less than 10 km from the axis. A normal upper mantle velocity of 8.1 km s-l exists within 10 km of the axis. Shear waves propagate across the axis, thus precluding the existence of any sizeable magma chamber at shallow depth.

At present there is a strong conflict between, on the one hand, seismological and thermal models of the Mid-Atlantic Ridge, which indicate that no large crustal magma chamber can exist, and on the other hand petrological models many of which stress the importance of such a chamber. We review the available geophysical and petrological information from the FAMOUS area and 45" N in an attempt to resolve this conflict and demonstrate that a model (the infinite leek) can be constructed which satisfies all the available seismological, thermal, petrographic, major element and traceelement information from these two areas. This mode is as follows: mantle rising from depth begins to melt at about 60 km, and rises in equilibrium with its melt to about 15-25 km below the sea surface. At this level melt segregates and rises rapidly to the base of the crust. Magma injection above this takes place by a process of crack propagation, or by the development of a narrow vertical magma chamber, but no large crustal chamber is present. This model successfully explains the marked petrographic zonation of the floor of the median valley (Hekinian, Moore ck. Bryan).

A detailed comparison between the pre-Cretaceous stratigraphic record in the Williston Basin and the available information about Phanerozoic sea-level changes indicates that most stratigraphic breaks in the basin reflect eustatic control. The basin appears to have been subsiding continuously and steadily about the same centre for most of the Phanerozoic above a thick (55 km) continental crust. Deposition kept up with subsidence, and most of the sedimentary record was laid down close to sea level. Erosional breaks in the stratigraphic record may represent periods of removal of strata during episodes of low sea level. A possible explanation of this essentially steady, nonexponential subsidence record is that a mafic subcrustal body is undergoing transformation to eclogite.Une comparaison dttaillCe entre le registre stratigraphique antC-CrCtacC dans le bassin de Williston et les renseignements disponibles des changements du niveau de la mer au PhanCrozoi'que revtle que la majorit6 des variations brusques de la stratigraphic dans le bassin reflkte une influence eustatique. Le bassin semble s'&tre affaissC avec continuit6 et uniformit6 autour du m&me centre durant la majeure partie du PhanCrozoi'que par-dessus une Cpaisse crofite continentale (55 km). La sCdimentation se maintenait en Cquilibre avec la subsidence, et les matCriaux skdimentaires furent dCposCs en majorit6 prks du niveau de la mer. Les changements stratigraphiques soudains dus a 1'Crosion peuvent correspondre a des pCriodes de disparition des strates durant des episodes lorsque le niveau de la mer Ctait bas. Une interprktation plausible de ce registre d'une subsidence essentiellement uniforme et non-exponentielle est qu'une masse subcrustale est en voie de transformation en Cclogite.[Traduit par le journal]Can. J. Earth Sci. 22, 408-415 (1985)