Publications

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2014
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

2008
Blackman, DK, Karner GD, Searle RC.  2008.  Three-dimensional structure of oceanic core complexes: Effects on gravity signature and ridge flank morphology, Mid-Atlantic Ridge, 30°N. Geochemistry Geophysics Geosystems. 9   10.1029/2008gc001951   AbstractWebsite

Our gravity modeling of oceanic core complexes formed at the Mid-Atlantic Ridge near 30 degrees N suggests that their shallow, domal "cores'' could be dominated by mafic intrusive rocks, consistent with recent drilling results at Atlantis Massif. The three-dimensional gravity analysis incorporates additional underway geophysics data in a new compilation and uses a higher-resolution bathymetry model to remove the gravity contribution of seafloor topography. The additional detail is required in order to confidently relate few-kilometer-scale gravity anomalies to specific morphologic/tectonic blocks. Different models of subseafloor core complex structure and density are tested to determine which minimizes the local gravity anomaly. A 3-D core with density 2900 kg/m(3), as measured in the gabbroic section drilled at the central dome, and juxtaposed 3-D hanging wall of fractured basalt, density similar to 2600 kg/m(3), satisfactorily explains most of the Bouguer gravity anomaly at Atlantis Massif. The capping detachment fault terminates or plunges northward beneath the seafloor at the northern limit of the central dome. The southwest shoulder of the massif has lower density, consistent with an upper crustal section similar to 1 km thick, whereas the summit and southeastern shoulder have overall density similar to the central dome. The older core complexes distributed along Atlantis fracture zone are similar in size, depth, and distance of their summit from the transform fault. However, weathering/alteration probably has reduced their density somewhat compared to Atlantis Massif. Bathymetric embayments occur adjacent to the fracture zone in several places on the ridge flanks and are consistently associated with core complexes.