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Blackman, DK, Slagle A, Guerin G, Harding A.  2014.  Geophysical signatures of past and present hydration within a young oceanic core complex. Geophysical Research Letters. 41:1179-1186.   10.1002/2013gl058111   AbstractWebsite

Borehole logging at the Atlantis Massif oceanic core complex provides new information on the relationship between the physical properties and the lithospheric hydration of a slow-spread intrusive crustal section. Integrated Ocean Drilling Program Hole U1309D penetrates 1.4km into the footwall to an exposed detachment fault on the 1.2Ma flank of the mid-Atlantic Ridge, 30 degrees N. Downhole variations in seismic velocity and resistivity show a strong correspondence to the degree of alteration, a recorder of past seawater circulation. Average velocity and resistivity are lower, and alteration is more pervasive above a fault around 750m. Deeper, these properties have higher values except in heavily altered ultramafic zones that are several tens of meters thick. Present circulation inferred from temperature mimics this pattern: advective cooling persists above 750m, but below, conductive cooling dominates except for small excursions within the ultramafic zones. These alteration-related physical property signatures are probably a characteristic of gabbroic cores at oceanic core complexes. Key Points Borehole T indicates shallow present circulation, conductive regime > 750 mbsf Narrow fault zones have seismic, T, resistivity signal indicating localized flow Hydration of gabbroic oceanic core complexes is limited below fault damage zone

Blackman, DK, Collins JA.  2010.  Lower crustal variability and the crust/mantle transition at the Atlantis Massif oceanic core complex. Geophysical Research Letters. 37   10.1029/2010gl045165   AbstractWebsite

Seismic refraction data provide new constraints on the structure of the lower oceanic crust and its variability across the Atlantis Massif oceanic core complex, similar to 30 degrees N on the Mid-Atlantic Ridge. A 40 km-long spreading-parallel profile constrains P-wave velocities to depths of up to similar to 7 km beneath the seafloor. Two shorter spreading-perpendicular lines provide coverage to similar to 2 km depth. The anomalous character of the massif's central dome crust is clear compared to the neighboring rift valley and similar-age crust on the opposite ridge flank. The domal core of the massif, unroofed via detachment faulting, has velocities > 7.0 km/s at depths below similar to 2.5 km sub-seafloor, increasing to 7.5-7.8 km/s over the depth range 4.8-6.8 km. Within the core complex, the Moho does not appear to be sharp as no PmP arrivals are observed. Within the axial valley, velocities do not reach mantle-transition zone values in the uppermost 6 km. We infer that crust there is of normal thickness but that a thinner than average mafic section is present in the central massif. Near IODP Hole U1309D, located on the central dome, there is a low velocity gradient interval at 1-3 km depth with velocities of 6.6-6.8 km/s, that coincides with a 3-5 km wide region where shallower velocities are highest. Given the predominantly gabbroic section recovered from the 1.4 km deep drillhole, this seismic structure suggests that the mafic body extends a few km both laterally and vertically. Citation: Blackman, D. K., and J. A. Collins (2010), Lower crustal variability and the crust/mantle transition at the Atlantis Massif oceanic core complex, Geophys. Res. Lett., 37, L24303, doi:10.1029/2010GL045165.

Blackman, DK, Orcutt JA, Forsyth DW, Kendall JM.  1993.  Seismic anisotropy in the mantle beneath an oceanic spreading centre. Nature. 366:675-677.   10.1038/366675a0   AbstractWebsite

BENEATH an active mid-ocean ridge, the mantle upwells in response to the divergence of the newly formed plates, leading to high temperatures and pressure-release melting below the ridge axis. The width of the upwelling region and the amount of melting depend on mantle rheology1-5, but all models predict a maximum decrease in seismic velocity at the ridge axis. It has also been suggested, however, that the alignment of anisotropic minerals by shear in the upwelling mantle will increase seismic velocity for rays travelling subvertically through the upwelling zone6,7. Here we report the observation of a consistent pattern of anomalously early P-wave arrival times at an array of ocean-bottom seismographs deployed across the axis of the southern Mid-Atlantic Ridge: P-waves from distant earthquakes arrive earlier at stations near the axis than at those further away. Our results are consistent with a model of anisotropy in which the degree of mineral alignment is greatest directly beneath the ridge axis, and significant anisotropy extends tens of kilometres from the axis.