412 TECTONICS/Seismic Structure At Mid-Ocean Ridges magma-lens reflectors are typically found deeper in the crust, at depths of 2.5–3 km, although shallower reflections are observed in a few places Along the slow-spreading Mid-Atlantic Ridge, evidence for magma lenses has been found along the Reykjanes Ridge Here, an intracrustal reflection at a depth of approximately 2.5 km is observed, which is similar to the depths of magma-lens events observed beneath the intermediate-spreading ridges Seismic data have been collected elsewhere along the MidAtlantic Ridge with little evidence of magma-lens reflections, possibly owing to imaging problems associated with the rough seafloor topography typical of this ridge However, there is evidence from refraction data and seismicity studies that large crustal magma bodies are not common beneath this ridge Microearthquake data show that earthquakes can occur to depths of km beneath the Mid-Atlantic Ridge, indicating that the entire crustal section is sufficiently cool for brittle failure In other areas, slightly reduced velocities within the crust have been identified, indicating warmer temperatures and possibly the presence of small pockets of melt within the crust The prevailing model for magma chambers beneath ridges (Figure 8) incorporates both the geophysical constraints on chamber dimensions described above and geochemical constraints on magma-chamber processes At fast-spreading ridges (Figure 8A), the magma chamber consists of a narrow and thin meltrich magma lens, which overlies a broader crystalmush zone and a surrounding region of hot but solidified rock The dyke-injection events and volcanic eruptions that build the upper crust are assumed to tap the magma lens The lower crust is formed from the crystal residuum within the magma lens and from the broader crystal-mush zone Observations of ophiolites suggest that the injection of sills that tap magma directly from the upper mantle also contributes to the lower crustal section Both seismic and seafloor compliance studies from the 9 –10 N region of the East Pacific Rise indicate melt accumulation at the base of the crust, within either melt-rich sills or a broader partially molten zone At slow-spreading ridges (Figure 8B) a short-lived dyke-like crystal-mush zone without a steady-state magma lens is envisioned At these ridges volcanic eruptions occur and the crystal-mush zone is replenished during periodic magma-injection events from the mantle Observations of seafloor fault exposures of crust created at slow-spreading ridges reveal a heterogeneous crustal section, where altered and deformed lower-crustal and upper-mantle rocks are unconformably overlain by lavas in some locations Crustal accretion at these ridges is inferred to be a highly episodic process, with the internal structure of the crust being strongly disrupted by faulting Moho The base of the crust is marked by the Mohorovicic Discontinuity, or ‘Moho’, where P-wave velocities increase from values typical of lower-crustal rocks (7– 7.5 km s 1) to mantle velocities (more than 8.0 km s 1) The change in P-wave velocity is often sufficiently abrupt that a sub-horizontal Moho reflection is observed in reflection data, from which the base of the crust can be mapped Depths to the Moho derived from seismic-refraction studies provide our best estimates of crustal thickness and are used to study how total crustal production varies in different ridge settings Characteristics of the Moho at Mid-Ocean Ridges Reflection Moho is imaged in much of the data collected at the East Pacific Rise (Figure 9) It can often be traced beneath the region of lower-crustal velocities found at the ridge and occasionally beneath the magma-lens reflection itself Where information on crustal velocities is available, average crustal thicknesses of 6–7 km are measured There is no evidence for thickening away from the ridge crest, indicating that the crust acquires its full thickness within a narrow zone at the axis Unlike at the fast- and intermediate-spreading ridges, at the slow-spreading Mid-Atlantic Ridge the Moho is difficult to identify in seismic-reflection data, possibly owing to poor imaging conditions or a difference in the geological nature of this boundary At this ridge, information on crustal thickness and variations in crustal structure is derived primarily from seismicrefraction studies Average crustal thicknesses are similar to those observed at fast-spreading ridges (6–7 km) However, significant crustal thinning is observed (by 1–4 km) towards transform faults and smaller ridge offsets (Figure 10B) These results are interpreted as reflecting a three-dimensional pattern of mantle upwelling or melt migration to the ridge, resulting in greater crustal production within the central regions of ridge segments away from ridge offsets Variations in crustal thickness within ridge segments are also observed along the East Pacific Rise (Figure 10A) However, in the region with the best data constraints (9 –10 N), the spatial relationships are opposite to those observed at the Mid-Atlantic Ridge Here, the crust is approximately km thinner, not thicker, in the portion of the segment where a range of ridge-crest observations indicate that active