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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, Canales JP, Harding A.  2009.  Geophysical signatures of oceanic core complexes. Geophysical Journal International. 178:593-613.   10.1111/j.1365-246X.2009.04184.x   AbstractWebsite

P>Oceanic core complexes (OCCs) provide access to intrusive and ultramafic sections of young lithosphere and their structure and evolution contain clues about how the balance between magmatism and faulting controls the style of rifting that may dominate in a portion of a spreading centre for Myr timescales. Initial models of the development of OCCs depended strongly on insights available from continental core complexes and from seafloor mapping. While these frameworks have been useful in guiding a broader scope of studies and determining the extent of OCC formation along slow spreading ridges, as we summarize herein, results from the past decade highlight the need to reassess the hypothesis that reduced magma supply is a driver of long-lived detachment faulting. The aim of this paper is to review the available geophysical constraints on OCC structure and to look at what aspects of current models are constrained or required by the data. We consider sonar data (morphology and backscatter), gravity, magnetics, borehole geophysics and seismic reflection. Additional emphasis is placed on seismic velocity results (refraction) since this is where deviations from normal crustal accretion should be most readily quantified. However, as with gravity and magnetic studies at OCCs, ambiguities are inherent in seismic interpretation, including within some processing/analysis steps. We briefly discuss some of these issues for each data type. Progress in understanding the shallow structure of OCCs (within similar to 1 km of the seafloor) is considerable. Firm constraints on deeper structure, particularly characterization of the transition from dominantly mafic rock (and/or altered ultramafic rock) to dominantly fresh mantle peridotite, are not currently in hand. There is limited information on the structure and composition of the conjugate lithosphere accreted to the opposite plate while an OCC forms, commonly on the inside corner of a ridge-offset intersection. These gaps preclude full testing of current models. However, with the data in hand there are systematic patterns in OCC structure, such as the 1-2 Myr duration of this rifting style within a given ridge segment, the height of the domal cores with respect to surrounding seafloor, the correspondence of gravity highs with OCCs, and the persistence of corrugations that mark relative (palaeo) slip along the exposed detachment capping the domal cores. This compilation of geophysical results at OCCs should be useful to investigators new to the topic but we also target advanced researchers in our presentation and synthesis of findings to date.

Nooner, SL, Sasagawa GS, Blackman DK, Zumberge MA.  2003.  Structure of oceanic core complexes: Constraints from seafloor gravity measurements made at the Atlantis Massif. Geophysical Research Letters. 30   10.1029/2003gl017126   AbstractWebsite

[1] Using the DSV Alvin, the relative seafloor gravimeter ROVDOG was deployed at 18 sites on the Atlantis Massif (located at the ridge-transform intersection of the Mid-Atlantic Ridge and the Atlantis Transform Fault near 30degreesN, 42degreesW). These data along with previously collected shipboard gravity and bathymetry provide constraints on the density structure of this oceanic core complex. A series of quasi 3-D forward models suggests that symmetric east and west-dipping density interfaces bound the core of the massif with dip angles of 16degrees-24degrees in the east and 16degrees-28degrees in the west, creating a wedge with a density of 3150-3250 kg/m(3). The dip angle in the east is steeper than that of the surface slope, suggesting that the detachment fault surface does not coincide with the density boundary. The resulting low-density layer is interpreted as a zone of serpentinization.

Klingelhofer, F, Minshull TA, Blackman DK, Harben P, Childers V.  2001.  Crustal structure of Ascension Island from wide-angle seismic data: implications for the formation of near-ridge volcanic islands. Earth and Planetary Science Letters. 190:41-56.   10.1016/s0012-821x(01)00362-4   AbstractWebsite

The study of the internal structure of volcanic islands is important for understanding how such islands form and how the lithosphere deforms beneath them. Studies to date have focused on very large volcanic edifices (e.g., Hawaiian Islands, Marquesas), but less attention has been paid to smaller islands, which are more common. Ascension Island, a 4-km high volcanic edifice with a basal diameter of 60 kin, is located in the equatorial Atlantic (8 degreesS), 90 km west of the Mid-Atlantic Ridge on 7 Ma oceanic lithosphere. We present results of a wide-angle seismic profile crossing the island revealing a crustal thickness of 12-13 kin, an overthickened layer 3 (7 kin thick) and little evidence of lithospheric flexure. Together these results suggest Ascension Island may be older than previously assumed and may have begun forming at an on-axis position around 6-7 Ma. This hypothesis is further supported by the presence of a young 1.4-km high edifice directly adjacent to the Mid-Atlantic Ridge with a volume about 1/7 that of Ascension Island, possibly representing the earliest stages of seamount formation. Excess magmatism appears to be related to the tectonic setting at the ridge-fracture zone intersection. (C) 2001 Elsevier Science B.V. All rights reserved.

Cann, JR, Blackman DK, Smith DK, McAllister E, Janssen B, Mello S, Avgerinos E, Pascoe AR, Escartin J.  1997.  Corrugated slip surfaces formed at ridge-transform intersections on the Mid-Atlantic Ridge. Nature. 385:329-332.   10.1038/385329a0   AbstractWebsite

The strips of ocean crust formed at the inside corners of both transform and non-transform offsets on the Mid-Atlantic Ridge are punctuated by topographic highs-the 'inside-corner highs'(1-3)-where plutonic rocks (including gabbros and peridotites) are frequently found(4,5). Current tectonic models consider the inside-corner highs to be lower-crust and upper-mantle materials that have been exhumed by low-angle detachment faults dipping away from the inside corner to beneath the ridge axis(3,6-8). But much of the evidence for the existence of such faults has hitherto been circumstantial. Here we present sonar images of two ridge-transform intersections on the Mid-Atlantic Ridge (near 30 degrees N), which show that both active and 'fossil' inside-corner highs are capped by planar, dipping surfaces marked by corrugations and striations oriented parallel to the plate spreading direction. Although these surfaces may be the low-angle detachment faults envisaged by the models, they dip at much shallower angles than expected. This could be explained by the lubricating presence of serpentinized peridotite, fragments of which have been dredged from both surfaces. Alternatively, these slip surfaces may instead represent failure surfaces in serpentine-lubricated landslide zones.

Ballard, RD, Uchupi E, Blackman DK, Cheminee JL, Francheteau J, Hekinian R, Schwab WC, Sigurdsson H.  1988.  Geological mapping of the East Pacific Rise axis (10°19' -11°53'N) using the ARGO and ANGUS imaging systems. The Canadian Mineralogist. 26, Part 3:467-486., Ottawa, ON, Canada (CAN): Mineralogical Association of Canada, Ottawa, ON AbstractWebsite
Blackman, DK, Von Herzen RP, Lawver LA.  1987.  Heat flow and tectonics in the western Ross Sea, Antarctica. The Antarctic continental margin: geology and geophysics of the western Ross Sea. 5B( Cooper AK, Davey FJ, Eds.).:179-189., Houston, TX, United States (USA): Circum-Pacific Council for Energy and Mineral Resources Abstract

The western Ross Sea is the site of various tectonic processes: subsidence of Ross Embayment; rapid uplift of the Transantarctic Mountains; and Cenozoic through Recent volcanism. Several plate tectonic reconstructions require relative motion between East and West Antarctica, perhaps by extension and rifting in the Ross Sea. These processes could account for regionally anomalous thermal conditions in the lower crust or upper mantle. Heat flow was measured at two locations in the western Ross Sea. The results, combined with previously published measurements in the Ross Embayment, indicate that there is greater than average heat flow in this area. Although the data coverage is sparse, it is apparent that somewhat anomalous thermal conditions exist in the area and may be related to the observed tectonic structure of the Ross Embayment. (Auth.)