Oceanic spreading centers

Abstract

From fast or superfast to slow and ultraslow spreading rates, the respective roles of asthenosphere and lithosphere in the features of ocean ridges deserve some concluding comments. The faster the spreading rate, the thinner the lithosphere remains over wide areas and the more this thin lithosphere appears autonomous with respect to the underlying asthenosphere. This is illustrated by Toomey et al. (2007) who describe at an oblique alignment of the mantle individual diapirs compared to the ridge alignment along the 9° –10° N EPR and the associated orthogonal transform faults (Figure 4). It appears that this thin lithosphere is decoupled from the soft, underlying asthenosphere and responds by brittle fracturing to the large-scale plate tectonics. In contrast, the progressive thickening of lithospheres from slow to ultraslow spreading ridges is increasingly controlled by the uprising asthenosphere. This is visible in their morphology and their large-scale tectonics. There is some evidence from ophiolites (Nicolas et al., 1999) and slow spreading ridges, based on seismic anisotropy (Li and Detrick, 2003; Nowacki et al., 2012), suggesting that mantle flow is ridge parallel and not more ridge perpendicular. As discussed above, ultraslow ridge morphology and crustal composition respond to variations in mantle upwelling and associated melt delivery. The combined study of ophiolites and geophysical data at oceanic ridges has allowed to comprehend the inner working of ocean spreading processes. The ophiolites allowed us to image deep accreted lithosphere and, indirectly, to investigate accretion processes, whereas the geophysics provide direct images of the accreting ridge systems. These observations showed that these accreting systems are different depending firstly on spreading rates and sometimes displaying significant variations along the same segment of a ridge axis. For example, along the most studied 9° N segment of the EPR, the width of the AML changes significantly and so does the melt-mush distribution along the ridge axis. Along slow spreading ridges, the crust is normally thickest at the center of a segment and thinnest at segment ends. The crustal accretion process at slow spreading centers is further complicated by the presence of ocean core complexes, which are not just present at segment ends, but could be present along a significant part of a segment (Escartin et al., 2008). The ultraslow spreading ridges, a special class of ridges, show extreme variations in seafloor morphology, crustal thickness, and geochemistry; more geophysical studies of ultraslow spreading ridges are required to fully appreciate the diversity of ridge processes on earth.