Geometry of extensional faults developed at slow-spreading centres from pre-stack depth migration of seismic reflection data in the Central Atlantic (Canary Basin)

Abstract

S U M M A R YWe present depth images, from portions of profiles that are close to flow-lines, of Cretaceous oceanic crust in the eastern Central Atlantic. Compared with post-stack time migrations, the images illustrate the improvement resulting from the application of pre-stack depth migration. The images document the scale and geometry of normal faulting in oceanic crust formed over 25 Myr at a half-spreading rate of less than 10 mm yr −1 , and the variation in extensional style with position within the spreading segment. Away from major fault zones (FZs), most faults are subplanar, dip more than 35 • , are associated with moderate basement relief (0.2-1 km relief) and may penetrate to deep crustal levels. These faults could be related to the lifting of the lithosphere out of the median valley to the flanking mountains. Also observed away from FZs are gently dipping to subhorizontal reflections in the upper crust, which resemble detachment faults. In contrast, the inside corner crust is more rugged, with basement highs rising up to 2 km above the intervening basins. This larger-scale topography is associated with a different style of faulting: the depth images reveal gently dipping (<35 • ) faults that are rooted in the deep crust and that project to the ridgeward flank of the dome-shaped large basement highs (1-2 km vertical relief). The faults seem to continue as the ridge-facing flank of these highs and some may extend over the crest of the high to breakaways beyond. In this case, the domal highs that form the exhumed footwall to the faults can be described as oceanic core complexes. These controlling faults are up to 20 km long and have a heave of ∼10 km, sufficient to have accommodated up to 50 per cent extension and to have exhumed deep crustal and perhaps even mantle rocks. We suggest that similar faults can explain the structure and lithologies found at megamullion structures (oceanic core complexes) at inside corners near the present-day spreading ridge.

Seismic structure of an oceanic core complex at the Mid‐Atlantic Ridge, 22°19′N

“…In contrast to the TAG detachment fault, bathymetric data from the 22°19′N core complex suggest that the inactive portion of the detachment fault surface is covered by rafted blocks (Figure 1c). Rafted blocks on top of the OCCs have been described between 13° and 15°N [ Smith et al , 2008] and in the Canary Basin, Central Atlantic [ Reston et al , 2004]. The occurrence of rafted blocks is supported by diabase dykes and basalts sampled from the top, the northern and the eastern slopes of the massif (Figure 2).…”

Madagascar's escape from Africa: A high‐resolution plate reconstruction for the Western Somali Basin and implications for supercontinent dispersal

“…To the SSE in Figure , following a reduction in thickness of the crust demonstrated by the shallowing of the Moho reflector, the top basement gains a weak tectonic fabric. These observations are consistent with a reduction in magma supply and resulting increase in the tectonic extensional component of oceanic spreading [e.g., Reston et al ., ].…”

“…The nature of the crust changes from NNW to SSE along the line. In the NNW, a smooth top basement reflector is imaged at 7.4 s TWTT, which is characteristic of oceanic crust formed by relatively robust magmatic accretion with little tectonic extension [e.g., Reston et al, 2004]. Here an additional reflector can be seen at 8.3 s TWTT (0.9 s below top basement) which delineates an upper and lower crustal layer.…”

Mantle rock exposures at oceanic core complexes along mid-ocean ridges

“…According to our literature analysis at least 172 OCCs have been discovered along mid-ocean ridges so far. This number does not include OCCs located far away from the active ridges, such as those identified near the Canary basin (Ranero & Reston, 1999;Reston et al, 2004), developed at back-arc basins (e.g., Godzilla Megamullion; Ohara et al, 2001Ohara et al, , 2003a or obducted on the continents as a part of an ophiolite (Nicolas et al, 1999;Nuriel et al, 2009;Tremblay et al, 2009;Manatschal et al, 2011;Maffione et al, 2013;Jousselin et al, 2013;Lagabrielle et al, 2015). Almost half (81) of the OCCs discovered are located at the MAR, with a high disparity in the frequency of occurrence between the northern (79) and southern (2) parts.…”