Publications

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2015
Greene, JA, Tominaga M, Blackman DK.  2015.  Geologic implications of seafloor character and carbonate lithification imaged on the domal core of Atlantis Massif. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 121:246-255.   10.1016/j.dsr2.2015.06.020   AbstractWebsite

We document the seafloor character on Atlantis Massif, an ocean core complex located at 30 degrees N on the Mid-Atlantic Ridge, with an emphasis on the distribution of carbonate features. Seafloor imagery, near-bottom backscatter, and bathymetry were analyzed on the Central Dome and the Western Shoulder of the exposed footwall to the detachment, and on the Eastern Block, a hanging wall to the fault. We merged Argo II still images to produce photo-mosaics and evaluated these together with video imagery, acoustic reflectivity, and basic rock composition. The seafloor was classified as unconsolidated sediment, lithified carbonate crust, consolidated carbonate cap, exposed basement, or rubble, and the spatial distribution of each type was assessed. Unconsolidated sediment, exposed basement, and rubble were documented in all three regions studied. Lithified carbonate crust was also present on the Western Shoulder and eastern Central Dome. Consolidated carbonate cap was found on the Eastern Block. The formation of the carbonate rock is interpreted to reflect precipitation and/or sediment cementation via fluids derived from serpentinization. Both processes occur at the nearby Lost City Hydrothermal Field. The newly documented locations of seafloor carbonate lithification therefore mark pathways of past, possibly recent, fluid flux from subsurface water-rock reaction zones and represent an additional constituent of the carbon cycling hosted by oceanic lithosphere. (C) 2015 Elsevier Ltd. All rights reserved.

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

2012
Henig, AS, Blackman DK, Harding AJ, Canales JP, Kent GM.  2012.  Downward continued multichannel seismic refraction analysis of Atlantis Massif oceanic core complex, 30°N, Mid-Atlantic Ridge. Geochemistry Geophysics Geosystems. 13   10.1029/2012gc004059   AbstractWebsite

Detailed seismic refraction results show striking lateral and vertical variability of velocity structure within the Atlantis Massif oceanic core complex (OCC), contrasting notably with its conjugate ridge flank. Multichannel seismic (MCS) data are downward continued using the Synthetic On Bottom Experiment (SOBE) method, providing unprecedented detail in tomographic models of the P-wave velocity structure to subseafloor depths of up to 1.5 km. Velocities can vary up to 3 km/s over several hundred meters and unusually high velocities (similar to 5 km/s) are found immediately beneath the seafloor in key regions. Correlation with in situ and dredged rock samples, video and records from submersible dives, and a 1.415 km drill core, allow us to infer dominant lithologies. A high velocity body(ies) found to shoal near to the seafloor in multiple locations is interpreted as gabbro and is displaced along isochrons within the OCC, indicating a propagating magmatic source as the origin for this pluton(s). The western two-thirds of the Southern Ridge is capped in serpentinite that may extend nearly to the base of our ray coverage. The distribution of inferred serpentinite indicates that the gabbroic pluton(s) was emplaced into a dominantly peridotitic host rock. Presumably the mantle host rock was later altered via seawater penetration along the detachment zone, which controlled development of the OCC. The asymmetric distribution of seismic velocities and morphology of Atlantis Massif are consistent with a detachment fault with a component of dip to the southeast. The lowest velocities observed atop the eastern Central Dome and conjugate crust are most likely volcanics. Here, an updated model of the magmatic and extensional faulting processes at Atlantis Massif is deduced from the seismic results, contributing more generally to understanding the processes controlling the formation of heterogeneous lithosphere at slow-rate spreading centers.

2009
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.

2007
Ildefonse, B, Rona PA, Blackman D.  2007.  Drilling the Crust at Mid-Ocean Ridges An "In Depth" Perspective. Oceanography. 20:66-77. AbstractWebsite
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Ildefonse, B, Blackman DK, John BE, Ohara Y, Miller DJ, MacLeod CJ, IODP Expeditions 304/305 Science Party.  2007.  Oceanic core complexes and crustal accretion at slow-spreading ridges. Geology. 35:623-626.   10.1130/g23531a.1   AbstractWebsite

Oceanic core complexes expose gabbroic rocks on the sealloor via detachment faulting, often associated with serpentinized peridotite. The thickness of these serpentinite units is unknown. Assuming that the steep slopes that typically surround these core complexes provide a cross section through the structure, it has been inferred that serpentinites compose much of the section to depths of at least several hundred meters. However, deep drilling at oceanic core complexes has recovered gabbroic sequences with virtually no serpentinized peridotite. We propose a revised model for oceanic core complex development based on consideration of the rheological differences between gabbro and serpentinized peridotite: emplacement of a large intrusive gabbro body into a predominantly peridotite host is followed by localization of strain around the margins of the pluton, eventually resulting in an uplifted gabbroic core surrounded by deformed serpentinite. Oceanic core complexes may therefore reflect processes associated with relatively enhanced periods of mafic intrusion within overall magma-poor regions of slow- and ultra-slow-spreading ridges.

2005
van Wijk, JW, Blackman DK.  2005.  Deformation of oceanic lithosphere near slow-spreading ridge discontinuities. Tectonophysics. 407:211-225.   10.1016/j.tecto.2005.08.009   AbstractWebsite

Transform and non-transform discontinuities that offset slow spreading mid-ocean ridges involve complex thermal and mechanical interactions. The truncation of the ridge axis influences the dynamics of spreading and accretion over a certain distance from the segment-end. Likewise, the spreading system is expected to influence the lithospheric plate adjacent to the ridge-end opposite of the discontinuity. Tectonic effects of the truncated ridge are noticeable in for example the contrast between seafloor topography at inside comers and outside comers, along-axis variations in rift valley depth, style of crustal accretion, and ridge segment retreat and lengthening. Along such slow-spreading discontinuities and their fossil traces, oceanic core complexes or mega-mullion structures are rather common extensional tectonic features. In an attempt to understand deforrnation of oceanic lithosphere near ridge offsets, the evolution of discontinuities, and conditions that may favor oceanic core complex formation, a three-dimensional thermo-mechanical model has been developed. The numerical approach allows for a more complete assessment of lithosphere deformation and associated stress fields in inside comers than was possible in previous 3-D models. The initial suite of results reported here focuses on deformation when axial properties do not vary along-strike or with time, showing the extent to which plate boundary geometry alone can influence deformation. We find that non-transform discontinuities are represented by a wide, oblique deformation zone that tends to change orientation with time to become more parallel to the ridge segments. This contrasts with predicted deformation near transform discontinuities, where initial orientation is maintained in time. The boundary between the plates is found to be vertical in the center of the offset and curved at depth in the inside comers near the ridge-transform intersection. Ridge-normal tensile stresses concentrate in line with the ridge tip, extending onto the older plate across the discontinuity, and high stress amplitudes are absent in the inside comers during the magmatic accretionary phase simulated by our models. With the tested rheology and boundary conditions, inside corner formation of oceanic core complexes is predicted to be unlikely during magmatic spreading phases. Additional modeling studies are needed for a full understanding of extensional stress release in relatively young oceanic lithosphere. (c) 2005 Elsevier B.V. All rights reserved.

2000
Blackman, DK, Nishimura CE, Orcutt JA.  2000.  Seismoacoustic recordings of a spreading episode an the Mohns Ridge. Journal of Geophysical Research-Solid Earth. 105:10961-10973.   10.1029/2000jb900011   AbstractWebsite

A period of very active seismicity near 72.7 degrees N, 4 degrees E marks an episode of seafloor spreading on the Mohns Ridge. The earthquakes were recorded from November 1995 to January 1996 by onshore seismic stations and by U.S. Navy hydrophone arrays in the North Atlantic. Both the temporal and spatial histories of the activity suggest that volcanism accompanied the tectonic events. The hydrophone arrays recorded 2-3 orders of magnitude more events than the onshore seismic arrays with up to 1000 events per day observed during the most intense phase of activity. A level of 50-200 events per day was sustained throughout the episode. Initial locations of the events were obtained from the seismic bulletin. Further refinement of the epicenters was possible using P, S (converted to an acoustic phase at the seafloor), and T waves in the hydrophone data, Analysis of arrival time differences between these phases indicates that one main area and two subsidiary areas along the rift were active during the swarm. A few events occurred at a more distant location. The activity tends to concentrate in one area or another for short periods (a few days), but at times it is clear that events occur simultaneously at more than one location. We have not found evidence of steady migration of activity, such as might accompany propagation of a magma-filled dike. We thus infer that despite the 50-70 km length of ridge involved in the spreading episode, rupture and magmatic eruption at the seafloor probably only occurred in a few discrete areas.

1997
Blackman, DK.  1997.  Variation in lithospheric stress along ridge-transform plate boundaries. Geophysical Research Letters. 24:461-464.   10.1029/97gl00122   AbstractWebsite

Three-dimensional numerical models of asthenospheric flow and deformation in the oceanic lithosphere predict variability in the stress field that reflects the geometry of the ridge-transform boundary. A series of 3-D Boundary Element calculations shows how spreading rate, transform offset, and segment length each influence the flow and stress fields that develop during plate-driven asthenospheric flow beneath a ridge-transform boundary. The predicted patterns of stress-supported seafloor relief generally follow those observed: median valleys are predicted at slow-spreading ridges vs no rift valley or, in some cases, a small axial high at fast spreading ridges; nodal deeps occur at ridge-transform intersections for offsets greater than 25 km; longer segments have more along-axis deepening than short segments which do not shoal much in their center. The predicted amplitude of the stress supported topography is up to 20% (across axis) and 40% (along axis) of that observed for an assumed asthenospheric viscosity of 5 x 10(19) Pa s and a weak lithosphere (local compensation).

1993
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.

Phipps Morgan, J, Blackman DK.  1993.  Inversion of combined gravity and bathymetry data for crustal structure: A prescription for downward continuation. Earth and Planetary Science Letters. 119:167-179.   10.1016/0012-821x(93)90014-z   AbstractWebsite

We propose a new and improved recipe for inverting for crustal thickness variations from combined gravity and bathymetry data. The 'new' ingredients with respect to previous studies are the posing of this problem as an inverse problem and the use of a downward continuation filter which can be tailored to find the crustal thickness solution which satisfies a minimum slope or maximal smoothness criteria, or a weighted combination of these desired solution features. We explore several suggestions for determining the 'optimum' smoothing parameters for a given crustal thickness inversion. Then we compare this new method to that used previously in marine gravity studies. We find it to be an 'improved' recipe for dealing with gravity data with observation errors of approximately 3-5 mGal, but not by much. This recipe will be most useful in extracting more geological information from marine gravity data where apparent observation errors are approximately 1 mGal, e.g. from future Global Positioning Satellite navigated studies.