The accreting plate boundary Ardouko?ba Rift (northeast Africa) and the oceanic Rift Valley

The Ardouko?ba Rift, subaerially exposed for ～12 km between the Ghoubbat-al-Kharab and Lake Asal in the French Territory of the Afars and Issas (northeast Africa), has intrinsic features and a regional setting consistent with arguments that it is the site of crustal accretion at approximately the same rate as that found along the rifted Mid-Oceanic Ridge (～2 cm/yr). The ～11 km wide Central Zone of the Ardouko?ba Rift has an internal relief of less than ～300 m and is set between step-like ridges standing up to 800 m above the deepest part of the rift. The lower inward-facing scarps of the Central Zone border a narrow Inner Floor. The Central Zone of the typically ～25–35 km wide oceanic Rift Valley can have a greater and rougher relief and has a width of ～8–16 km, but deep areas with an internal relief of <400 m have a maximum width that is about the same as that of the corresponding area in the Ardouko?ba Rift (～11 km). The width of the Inner Floor of the Ardouko?ba Rift varies from 2 to 5 km; in the oceanic Rift Valley the range is from less than 1 to ～9 km. Equivalence of tectonic and volcanic processes in the two settings has not been demonstrated; but a comparison of a segment of the Rift Valley in the FAMOUS area near 36°50′N in the Atlantic with the Ardouko?ba Rift encourages the tentative use of evidence from the latter to complement arguments about the pattern of vulcanism and scarp formation in the oceanic Rift Valley as a whole. The Inner Floor of the Rift Valley is the main site of horizontal extension without vertical displacements, of normal faulting that involves little or no accumulation of vertical offsets, and of constructional vulcanism, which may be further concentrated along narrow (～1 km wide) fissured zones. The normal faulting that disrupts and constrains more or less orderly growth of the Inner Floor may happen in such a way that the new graben that become new Inner Floors are laterally offset with respect to the middle line of the Rift Valley and to the axis of symmetry of a hypothetical block accounting for the central positive magnetic anomaly.Additional complexities may be introduced by syntectonic and post-tectonic vulcanism, and by normal fault displacement at any one time of young crust along only part of the distance between transform faults. Thus, although opening rate can always be equated in principle with total addition of new crust to the two plates, the assumption is suspect that the concept of spreading rate (rate of addition of crust to one plate or the other) can necessarily be applied precisely to the central part of the Rift Valley. In more general terms, the physical meaning of interpolated spreading rates on the time scale of magnetic anomalies is worth questioning. On evidence from the Rift Valley, the spreading rates need not reflect monotonic additions of new crust, and rocks of the same inferred age from opposite plates may not have the same composition. The problem is highlighted by the apparently symmetrical growth of the North Atlantic over long periods of time.