Processes of magmatic and tectonic accretion of oceanic lithosphere at mid-ocean ridges - Constraints from a seismic refraction study at the Mid-Atlantic Ridge near 21.5° N
Mid-oceanic ridges are plate boundaries where new oceanic crust is created. Especially slow-spreading ridges, like the Mid-Atlantic Ridge (MAR), reveal a complex structure denoted by magmatic and tectonic processes. In the working area of this seismic refractions study both types of crustal accretion are present. The northern segment (22.2° N) compensates tectonically the tensional stresses caused by the plate tectonic movements of the African and the Northern American plates. So called detachment faults or oceanic core complexes (OCCs) develop during that tectonic phase. In the meantime the southern segment (21.5° N) is a magmatically robust segment. The peculiarity of this segment is that it growths south- and northwards along the ridge axis, starting at about 5 m.y. ago. Ridge propagation was strong enough to break through a stable small offset transform fault. During propagation the transform migrated southwards, leaving behind a V-shaped structure the so called inner and outer pseudofaults. From five seismic refraction and wide-angle profiles, ridge-parallel and ridge-crossing, the seismic velocity structure was observed. The results show a strong crustal variation. The ridge-crossing profiles illustrate the temporal evolution of the crustal accretion within the magmatic robust segment. Past magmatic activities can be reconstructed. The different morphological and geological features of the area required different inversion and modelling procedures. A broad variety of methods for interpretation of the collected geophysical data were applied to gain a subsurface image and to allow a geological reconstruction. First arrival seismic tomography, joint refraction and reflection tomography, and joint seismic and gravimetric tomography were used. Along the northern profile tomography for the near offset travel time arrivals was used, yielding the shallow part of the subsurface. Joint forward modelling of seismic travel times and gravimetric data made it possible to resolve the structure at greater depth. The southern and hence magmatically dominated ridge segment shows crustal thickening along the ridge axis from 4 km at the segment ends to about 8 km in the segment centre whereas the crust in the northern basin thins more than beneath the southern ridge tip. Layer 2 is rather constant and the main thickening is taken by layer 3. The seismic velocities in the ridge tip tend to be lower, which could be caused by strong fracturing and partial alteration. In the seismic velocity models crustal thinning has been observed also with increasing distance to the spreading axis. The latter suggests intensified magmatic activity with focussed melt supply in the segment centre leading to an upwelling of the seafloor and an hourglass shaped bathymetry with a small axial valley at the segment centre that widens towards its ends. Melts are transported laterally at crustal levels towards the segment ends, preferable towards the southern ridge tip, while the larger part remains at the segment centre. The northern segment has a much larger variation of the crustal thickness across the ridge axis. Tectonically dominated crust thins extremely to approximately 40% of average oceanic crust at the western ridge flank near 22°19’. Partly the upper crust is completely missing and high seismic velocities of 7 km/s are reached already a few hundred metres below the seafloor. The asymmetric crustal accretion is also reflected in the seismic velocities that reach a level of normal oceanic young crust on the eastern ridge crest. This long lived detachment fault shifted the plate boundary towards the west. However, it does not expose mantle material in its central surface. This can be caused at least by two factors: 1) during the tectonic phase the area is magmatically starved but still magmatic accretion occurs. 2) The detachment fault is a steep normal fault, marked by higher seismicity, near the ridge axis and is rotated based on the “rolling-hinge” model to a shallow low-angle fault caused by the slip and the tensional stresses. If the fault is rotated from an optimum angle a new fault will be generated and this fault block (rider or rafted block) stays on the surface of the detachment fault. A petrologic survey detected serpentinised mantle at the steep southern wall of the core complex facing towards the southern segment end. This suggests a three-dimensional structure of the core complex with a detachment fault rooted in an intrusive zone in the mid-segment setting, exposing gabbroic rocks, and a detachment fault rooted near crust-mantle boundary zone towards the segment end unroofing mantle rocks. The uplift of the massif can not be only explained by flexural rotation caused by the tension of the plate tectonic processes. There has to be an additional force. This could be a result of lower dense serpentinised mantle. The density difference will be compensated by an uplift to reach the isostatic equilibrium
