Publications

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2017
2016
Harding, AJ, Arnulf AF, Blackman DK.  2016.  Velocity structure near IODP Hole U1309D, Atlantis Massif, from waveform inversion of streamer data and borehole measurements. Geochemistry Geophysics Geosystems. 17:1990-2014.   10.1002/2016gc006312   AbstractWebsite

Seismic full waveform inversion (FWI) is a promising method for determining the detailed velocity structure of the igneous oceanic crust, especially for locations such as the Mid-Atlantic Ridge with significant lateral heterogeneity and seafloor topography. We examine the accuracy of FWI by inverting, after downward continuation to datum just above the seafloor, a multichannel seismic (MCS) profile from Atlantis Massif oceanic core complex at 30 degrees N that passes close to Integrated Ocean Drilling Program (IODP) Hole U1309D and comparing the results against borehole measurements and existing on-bottom refraction data. The comparisons include the results of IODP Expedition 340T, which extended the sonic logging and vertical seismic profiling to the bottom of the borehole at 1400 m below seafloor. Compared to travel time tomography, the refinement in velocity and velocity gradient produced by FWI significantly improves the overall match to the borehole measurements, and allows the multilevel pattern of deformation and alteration of the detachment footwall seen in Hole U1309D to be extrapolated across the Central Dome. Prestack depth migration of the profile using the FWI velocities reveals the top and edges of the high-velocity, gabbroic core of the massif. It also indicates that the comparatively uniform gabbroic rocks drilled at Hole U1309D extend to approximate to 2.5 km below seafloor but overlie an extended, approximate to 2 km thick, mantle transition zone.

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.

Menke, W, Zha Y, Webb SC, Blackman DK.  2015.  Seismic anisotropy indicates ridge-parallel asthenospheric flow beneath the Eastern Lau Spreading Center. Journal of Geophysical Research-Solid Earth. 120:976-992.   10.1002/2014jb011154   AbstractWebsite

Seismic anisotropy beneath the Eastern Lau Spreading Center (ELSC) is investigated using both Rayleigh waves and shear waves, using data from the 2009-2010 ELSC ocean bottom seismograph experiment. Phase velocities of Rayleigh waves determined by ambient noise cross correlation are inverted for azimuthally anisotropic phase velocity maps. Splitting of S waves from five intermediate and deep focus earthquakes was determined by waveform analysis. Taken together, Rayleigh wave and S wave data indicate that significant (similar to 2%) anisotropy extends to at least 300km depth. Both data sets indicate a fast direction aligned within a few degrees of the N10 degrees E striking ELSC and somewhat oblique to the N25 degrees E strike of the neighboring volcanic arc. We therefore describe the fast direction as spreading perpendicular, not convergence perpendicular and interpret it as due to ridge-parallel flow of the asthenosphere. However, the region arcward (east) of the ELSC has the stronger anisotropy, suggesting that the strongest flow gradients may occur in the region between the spreading center and the arc, in contrast to being centered beneath the ELSC. Fluids released from the underlying plate may produce anisotropic hydrous materials, but more importantly lower the viscosity, thus enhancing along-strike flow. Both could contribute to an along-strike fast direction signature. Seafloor spreading diminishes south of the seismic array, ceasing altogether south of latitude 25 degrees S (500km south of the array center), a region dominated by much slower tectonic extension, suggesting that asthenosphere is inflowing from the north to accommodate the increase in asthenospheric volume associated with the seafloor spreading.

Wei, SS, Wiens DA, Zha Y, Plank T, Webb SC, Blackman DK, Dunn RA, Conder JA.  2015.  Seismic evidence of effects of water on melt transport in the Lau back-arc mantle. Nature. 518   10.1038/nature14113   AbstractWebsite

Processes of melt generation and transport beneath back-arc spreading centres are controlled by two endmember mechanisms: decompression melting similar to that at mid-ocean ridges and flux melting resembling that beneath arcs'. The Lau Basin, with an abundance of spreading ridges at different distances from the subduction zone, provides an opportunity to distinguish the effects of these two different melting processes on magma production and crust formation. Here we present constraints on the three-dimensional distribution of partial melt inferred from seismic velocities obtained from Rayleigh wave tomography using land and ocean-bottom seismographs. Low seismic velocities beneath the Central Lau Spreading Centre and the northern Eastern Lau Spreading Centre extend deeper and westwards into the back-arc, suggesting that these spreading centres are fed by melting along upwelling zones from the west, and helping to explain geochemical differences with the Valu Fa Ridge to the south(2), which has no distinct deep low-seismic-velocity anomalies. A region of low S-wave velocity, interpreted as resulting from high melt content, is imaged in the mantle wedge beneath the Central Lau Spreading Centre and the northeastern Lau Basin, even where no active spreading centre currently exists. This low-seismic-velocity anomaly becomes weaker with distance southward along the Eastern Lau Spreading Centre and the Valu Fa Ridge, in contrast to the inferred increase in magmatic productivity(1). We propose that the anomaly variations result from changes in the efficiency of melt extraction, with the decrease in melt to the south correlating with increased fractional melting and higher water content in the magma. Water released from the slab may greatly reduce the melt viscosity(3) or increase grain size(4), or both, thereby facilitating melt transport.

2014
Zha, Y, Webb SC, Wei SS, Wiens DA, Blackman DK, Menke W, Dunn RA, Conder JA.  2014.  Seismological imaging of ridge-arc interaction beneath the Eastern Lau Spreading Center from OBS ambient noise tomography. Earth and Planetary Science Letters. 408:194-206.   10.1016/j.epsl.2014.10.019   AbstractWebsite

The Lau Basin displays large along-strike variations in ridge characters with the changing proximity of the adjacent subduction zone. The mechanism governing these changes is not well understood but one hypotheses relates them to interaction between the arc and back-arc magmatic systems. We present a 3D seismic velocity model of the shallow mantle beneath the Eastern Lau back-arc Spreading Center (ELSC) and the adjacent Tofua volcanic arc obtained from ambient noise tomography of ocean bottom seismograph data. Our seismic images reveal an asymmetric upper mantle low velocity zone (LVZ) beneath the ELSC. Two major trends are present as the ridge-to-arc distance increases: (1) the LVZ becomes increasingly offset from the ridge to the north, where crust is thinner and the ridge less magmatically active; (2) the LVZ becomes increasingly connected to a sub-arc low velocity zone to the south. The separation of the ridge and arc low velocity zones is spatially coincident with the abrupt transition in crustal composition and ridge morphology. Our results present the first mantle imaging confirmation of a direct connection between crustal properties and uppermost mantle processes at ELSC, and support the prediction that as ELSC migrates away from the arc, a changing mantle wedge flow pattern leads to the separation of the arc and ridge melting regions. Slab-derived water is cutoff from the ridge, resulting in abrupt changes in crustal lava composition and crustal porosity. The larger offset between mantle melt supply and the ridge along the northern ELSC may reduce melt extraction efficiency along the ridge, further decreasing the melt budget and leading to the observed flat and faulted ridge morphology, thinner crust and the lack of an axial melt lens. (C) 2014 Elsevier B.V. All rights reserved.

Marcuson, R, Blackman DK, Harmon N.  2014.  Seismic anisotropy predicted for 2-D plate-driven flow in the Lau back–arc basin. Physics of the Earth and Planetary Interiors. 233:88-94.   10.1016/j.pepi.2014.06.007   AbstractWebsite

The Lau back–arc basin and spreading centers exhibit significant along-strike variations in plate motion rates and distance between the spreading centers and the Tonga trench. The mantle processes are expected to be complex in this region and seismic anisotropy offers a key constraint. Linked geodynamic and mineral alignment models of the mantle wedge predict the scale of anisotropic variation and underlying assumptions can be tested by comparison to observed anisotropy. Three cross-basin transects allow us to isolate the effects of trench-spreading center distance as well as rates of convergence and spreading. Lattice preferred orientations (LPO) corresponding to each transect’s 2-D flow field are calculated for peridotite polycrystals. Predicted P-wave anisotropy of several percent and shear-wave split times up to 3.5 s are determined, with measurable differences predicted between transects. Variation in the 2-D estimates of flow driven by each transect’s plate kinematics will induce along-strike flow in the mantle wedge. We find that the rate of such flow would likely not alter the subduction/spreading induced LPO sufficiently to explain the reported trench/arc parallel fast seismic directions in the Lau Basin; additional geodynamic factors must play a role.

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.

Blackman, DK, Appelgate B, German CR, Thurber AR, Henig AS.  2012.  Axial morphology along the Southern Chile Rise. Marine Geology. 315:58-63.   10.1016/j.margeo.2012.06.001   AbstractWebsite

Morphology of four spreading segments on the southern Chile Rise is described based on multi-beam bathy-metric data collected along the axial zones. The distribution of axial volcanoes, the character of rift valley scarps, and the average depths vary between Segment 1 in the south, terminating at the Chile Triple junction, and Segment 4 in the north, which are separated by three intervening transform faults. Despite this general variability, there is a consistent pattern of clockwise rotation of the southern-most axial volcanic ridge within each of Segments 2, 3, and 4, relative to the overall trend of the rift valley. A combination of local ridge-transform intersection stresses and regional tectonics may influence spreading axis evolution in this sense. (C) 2012 Elsevier B.V. All rights reserved.

2011
Blackman, DK, Ildefonse B, John BE, Ohara Y, Miller DJ, Abe N, Abratis M, Andal ES, Andreani M, Awaji S, Beard JS, Brunelli D, Charney AB, Christie DM, Collins J, Delacour AG, Delius H, Drouin M, Einaudi F, Escartin J, Frost BR, Fruh-Green G, Fryer PB, Gee JS, Godard M, Grimes CB, Halfpenny A, Hansen HE, Harris AC, Tamura A, Hayman NW, Hellebrand E, Hirose T, Hirth JG, Ishimaru S, Johnson KTM, Karner GD, Linek M, MacLeod CJ, Maeda J, Mason OU, McCaig AM, Michibayashi K, Morris A, Nakagawa T, Nozaka T, Rosner M, Searle RC, Suhr G, Tominaga M, von der Handt A, Yamasaki T, Zhao X.  2011.  Drilling constraints on lithospheric accretion and evolution at Atlantis Massif, Mid-Atlantic Ridge 30°N. Journal of Geophysical Research-Solid Earth. 116   10.1029/2010jb007931   AbstractWebsite

Expeditions 304 and 305 of the Integrated Ocean Drilling Program cored and logged a 1.4 km section of the domal core of Atlantis Massif. Postdrilling research results summarized here constrain the structure and lithology of the Central Dome of this oceanic core complex. The dominantly gabbroic sequence recovered contrasts with predrilling predictions; application of the ground truth in subsequent geophysical processing has produced self-consistent models for the Central Dome. The presence of many thin interfingered petrologic units indicates that the intrusions forming the domal core were emplaced over a minimum of 100-220 kyr, and not as a single magma pulse. Isotopic and mineralogical alteration is intense in the upper 100 m but decreases in intensity with depth. Below 800 m, alteration is restricted to narrow zones surrounding faults, veins, igneous contacts, and to an interval of locally intense serpentinization in olivine-rich troctolite. Hydration of the lithosphere occurred over the complete range of temperature conditions from granulite to zeolite facies, but was predominantly in the amphibolite and greenschist range. Deformation of the sequence was remarkably localized, despite paleomagnetic indications that the dome has undergone at least 45 degrees rotation, presumably during unroofing via detachment faulting. Both the deformation pattern and the lithology contrast with what is known from seafloor studies on the adjacent Southern Ridge of the massif. There, the detachment capping the domal core deformed a 100 m thick zone and serpentinized peridotite comprises similar to 70% of recovered samples. We develop a working model of the evolution of Atlantis Massif over the past 2 Myr, outlining several stages that could explain the observed similarities and differences between the Central Dome and the Southern Ridge.

2010
Harmon, N, Blackman DK.  2010.  Effects of plate boundary geometry and kinematics on mantle melting beneath the back-arc spreading centers along the Lau Basin. Earth and Planetary Science Letters. 298:334-346.   10.1016/j.epsl.2010.08.004   AbstractWebsite

The back-arc spreading centers that extend along the Lau Basin exhibit trends in axial morphology, crustal thickness, and geochemistry, which are opposite those typically observed at mid ocean spreading centers We develop 2D numerical models of mantle flow, thermal structure and melting of the Lau back-arc-Tonga subduction system for each of three Lau spreading centers-Valu Fa Ridge, the Eastern Lau, and the Central Lau Our goal is to determine whether along-strike variability in the circulation pattern or hydration within the mantle wedge could explain the trends observed We use present-day plate and subducted slab geometries and velocities to explore a range of mantle potential temperature and water content scenarios and test whether predictions match observations of crustal thickness and water content of the magmas erupted at the spreading ridges Within the range of mantle parameters tested, we find that a potential temperature of 1300 C and source water contents greater than 0 22% wt are required to match observed crustal thickness and lava water contents at Valu Fa. Substantially less water in the sub arc mantle source is required to match the Eastern Lau and Central Lau observations at the same or higher mantle potential temperatures A small background hydration of 0 01% wt water in the mantle wedge is required to match the observations of water in the Central Lau magmas We predict that the arc and back-arc melting regions are interconnected for all potential temperatures and sub arc water contents at the Eastern Lau spreading center and at the Valu Fa Ridge, while at the Central Lau, they are only connected for cases when mantle potential temperature is 1400 C or greater We hypothesize that the longer-lived Eastern Lau and Central Lau rifting and axial volcanism may have dehydrated the mantle wedge and slowed melt production beneath these spreading centers. The Valu Fa Ridge, which is actively propagating into a more hydrated wedge associated with the Tofua arc, experiences enhanced melting relative to the other two spreading centers For the Valu Fa case, we show fast subduction in combination with the proximity of the trench and spreading center results in enhanced upwelling and, therefore, increased crustal production Slower subduction, with a convergence rate of 45 mm/yr, a dry mantle, and a 1350 C mantle potential temperature reduces the difference in crustal thickness between the Central Lau and Valu Fa to within 1 2 km In contrast, for present-day kinematics with a dry mantle and 1350 C mantle potential temperature, our models predict the crustal thickness at Valu Fa to be 3 1 km thicker than Central Lau, much closer to the observed values (c) 2010 Elsevier B V. All rights reserved.

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.

2009
Castelnau, O, Blackman DK, Becker TW.  2009.  Numerical simulations of texture development and associated rheological anisotropy in regions of complex mantle flow. Geophysical Research Letters. 36   10.1029/2009gl038027   AbstractWebsite

The development of Lattice Preferred Orientations (LPO) within olivine aggregates under flow in the upper mantle leads to seismic and rheological (or viscoplastic) anisotropies. We compare predictions from different micromechanical models, applying several commonly used theoretical descriptions to an upwelling flow scenario representing a typical oceanic spreading center. Significant differences are obtained between models in terms of LPO and associated rheology, in particular in regions where the flow direction changes rapidly, with superior predictions for the recently proposed Second-Order approach. This highlights the limitations of ad hoc formulations. LPO-induced rheological anisotropy may have a large effect on actual flow patterns with 1-2 orders of magnitude variation in directional viscosities compared to the isotropic case. Citation: Castelnau, O., D. K. Blackman, and T. W. Becker (2009), Numerical simulations of texture development and associated rheological anisotropy in regions of complex mantle flow, Geophys. Res. Lett., 36, L12304, doi:10.1029/2009GL038027.

Collins, JA, Blackman DK, Harris A, Carlson RL.  2009.  Seismic and drilling constraints on velocity structure and reflectivity near IODP Hole U1309D on the central dome of Atlantis Massif, Mid-Atlantic Ridge 30°N. Geochemistry Geophysics Geosystems. 10   10.1029/2008gc002121   AbstractWebsite

The seismic structure of the upper similar to 1 km of the central dome of Atlantis Massif is investigated in the context of lithologies known from seafloor drilling and physical property measurements obtained within the borehole and on core samples. A new analysis of seafloor refraction data and multichannel reflection data acquired in the immediate vicinity of Integrated Ocean Drilling Program (IODP) Site U1309 was motivated by a discrepancy between initial seismic interpretations, which indicated mantle velocities at shallow depth, and the gabbroic sequence recovered by drilling. A new seismic velocity model is derived that is consistent with the full suite of geological and geophysical data in the central dome area; all of these data show that mafic intrusive rocks dominate the upper portion of the footwall of this oceanic core complex and that laterally extensive zones of ultramafic rocks are not required by the data. The origin of subseafloor reflectivity beneath the central dome was also considered. We find that seafloor scattering complicates the interpretation of multichannel seismic data acquired near Site U1309 but that detectable subsurface impedance contrasts do occur. Downhole variations in alteration may generate reflections observed from the upper kilometer of the central dome.

de Groot-Hedlin, C, Blackman DK, Jenkins CS.  2009.  Effects of variability associated with the Antarctic circumpolar current on sound propagation in the ocean. Geophysical Journal International. 176:478-490.   10.1111/j.1365-246X.2008.04007.x   AbstractWebsite

A series of small depth charges was detonated along a transect from New Zealand to Antarctica over a period of 3 days in late December of 2006. The hydroacoustic signals were recorded by a hydrophone deployed near the source and at a sparse network of permanent hydrophone stations operated by the International Monitoring System (IMS), at distances up to 9600 km. Our purpose was to determine how well signal characteristics could be predicted by the World Ocean Atlas 2005 (WOA05) climatological database for sources within the Antarctic circumpolar current (ACC). Waveforms were examined in the 1-100 Hz frequency band, and it was found that for clear transmission paths, the shot signals exceeded the noise only at frequencies above 20-30 Hz. Comparisons of signal spectra for recordings near the source and at the IMS stations show that transmission loss is nearly uniform as a function of frequency. Where recorded signal-to-noise ratios are high, observed and predicted traveltimes and signal dispersion agree to within 2 s under the assumption that propagation is adiabatic and follows a geodesic path. The deflection of the transmission path by abrupt spatial variations in sound speed at the northern ACC boundary is predicted to decrease traveltimes to the IMS stations by several seconds, depending on the path. Acoustic velocities within the ACC are predicted to vary monthly, hence the accuracy of source location estimates based only on arrival times at IMS stations depends on the monthly or seasonal database used to predict traveltimes and on whether we account for path deflection. However, estimates of source locations within the ACC that are based only on observed waveforms at IMS hydrophones are highly dependent on the configuration of the IMS network; a set of shots observed only at an IMS station in the Indian Ocean and another in the South Pacific was located to within 10 km in longitude, but was poorly constrained in latitude. Several sets of shots observed only at IMS hydrophones in the Indian Ocean were constrained to within 55 km in latitude but were poorly constrained in longitude.

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.

2008
Castelnau, O, Blackman DK, Lebensohn RA, Castaneda PP.  2008.  Micromechanical modeling of the viscoplastic behavior of olivine. Journal of Geophysical Research-Solid Earth. 113   10.1029/2007jb005444   AbstractWebsite

Efforts to couple mantle flow models with rheological theories of mineral deformation typically ignore the effect of texture development on flow evolution. The fact that there are only three easy slip systems for dislocation glide in olivine crystals leads to strong mechanical interactions between the grains as the deformation proceeds, and subsequent development of large viscoplastic anisotropy in polycrystals exhibiting pronounced Lattice Preferred Orientations. Using full-field simulations for creep in dry polycrystalline olivine at high temperature and low pressure, it is shown that very large stress and strain rate intragranular heterogeneities can build up with deformation, which increase dramatically with the strength of the hard slip system (included for the purpose of enabling general deformations). Compared with earlier nonlinear extensions of the Self-Consistent mean-field theory to simulate polycrystal deformation, the "Second-Order'' method is the only one capable of accurately describing the effect of intraphase stress heterogeneities on the macroscopic flow stress, as well as on the local stress-and strain rate fluctuations in the material. In particular, this approach correctly predicts that olivine polycrystals can deform with only four independent slip systems. The resistance of the fourth system (or accommodation mechanism), which is likely provided by dislocation climb or grain boundary processes as has been observed experimentally, may essentially determine the flow stress of olivine polycrystals. We further show that the "tangent'' model, which had been used extensively in prior geophysical studies of the mantle, departs significantly from the full-field reference solutions.

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.

2007
Blackman, DK.  2007.  Use of mineral physics, with geodynamic modelling and seismology, to investigate flow in the Earth's mantle. Reports on Progress in Physics. 70:659-689.   10.1088/0034-4885/70/5/r01   AbstractWebsite

Seismologists and mineral physicists have known for decades that anisotropy inherent in mantle minerals could provide a means to relate surface seismic measurements to deformation patterns at great depth in the Earth, where direct geologic observations would never be possible. Prior to the past decade, only qualitative relationships or simple symmetry assumptions between mantle flow (deformation), mineral alignment and seismic anisotropy were possible. Recent numerical methods now allow quantitative incorporation of constraints from mineral physics to flow/deformation models and, thereby, direct estimates of the resulting pattern of seismic anisotropy can be made and compared with observed signatures. Numerical methods for simulating microstructural deformation within an aggregate of minerals subjected to an arbitrary stress field make it possible to quantitatively link crystal-scale processes with largescale Earth processes of mantle flow and seismic wave propagation, on regional (100s of kilometres) and even global scales. Such linked numerical investigations provide a rich field for exploring inter-dependences of micro and macro processes, as well as a means to determine the extents to which viable seismic experiments could discern between different models of Earth structure and dynamics. The aim of this review is to provide an overview of why and how linked numerical models are useful for exploring processes in the mantle and how they relate to surface tectonics. A brief introduction to the basic concepts of deformation of mantle minerals and the limits of knowledge currently available are designed to serve both the subsequent discussions in this review and as an entry point to more detailed literature for readers interested in pursuing the topic further. The reference list includes both primary sources and pertinent review articles on individual aspects of the combined subjects covered in the review. A series of flow/texturing models illustrate the differences that can arise when different methods or different flow parameters are employed. Representative seismic results illustrate the types of studies done to date and the inferences possible using their anisotropy measurements. Trade-offs involved in the modelling assumptions and seismic data processing methods are touched on. A final example illustrates the effects, relative to a 2D model of mantle flow near a subduction zone, that flow in a third dimension can have on anisotropy patterns.

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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van Wijk, JW, Blackman DK.  2007.  Development of en echelon magmatic segments along oblique spreading ridges. Geology. 35:599-602.   10.1130/g23294a.1   AbstractWebsite

En echelon magmatic segments commonly develop along obliquely spreading oceanic ridges. To clarify some of the dynamic aspects of this plate boundary, we performed a series of thermo-mechanical numerical tests. When extension of oceanic lithosphere becomes oblique, deformation within the axial region localizes into distinct upwelling centers. Temperatures are elevated in the upwelling cells, which are shallow mantle features that form the new plate boundary. The predicted features are similar to the axial volcanic ridges documented at Mohns and Reykjanes Ridges, and we conclude that they become the new loci of extensional deformation, upwelling, and magmatic activity. These ridges, suborthogonal to the plate spreading direction, only develop when the axis rift zone is weak. The subsegment length and spacing depend primarily on obliquity and axial width. Predicted crustal thickness along the subsegmented axis varies discernibly; this might explain the morphology and satellite gravity of the flanks of oblique spreading ridges.

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.

2006
Becker, TW, Schulte-Pelkum V, Blackman DK, Kellogg JB, O'Connell RJ.  2006.  Mantle flow under the western United States from shear wave splitting. Earth and Planetary Science Letters. 247:235-251.   10.1016/j.epsl.2006.05.010   AbstractWebsite

We show that SKS splitting in the westernmost United States (polarization of the fastest shear waves and splitting times, including their back-azimuthal dependence) can be explained by a geodynamic model that includes a continuum-mechanics description of plate motions and underlying asthenospheric circulation. Models that include a counterflow at depths of similar to 300 km are preferred, which may indicate a far-field effect of the Farallon slab anomaly sinking underneath the central continental United States. This finding is broadly consistent with earlier suggestions, and we demonstrate that a mechanically coupled system, though with a strong viscosity contrast with depth, is consistent with the data. We explore the depth dependence of predicted anisotropy by means of computing seismogram synthetics and comparing synthetic splits with observations. Some patterns in the data, including null observations, are matched well. Linked models of geodynamic flow and mineral alignment in the mantle provide a means to test the relationship between strain and the saturation of texturing. Lower fabric saturation strains than for global models are preferred by the data, which may reflect the relatively active tectonic setting and thin asthenosphere of the study region. In general, our results show that seismic anisotropy, when interpreted jointly with mineral physics theories, may be used to quantitatively constrain the spatial character of flow, and the degree of force coupling, at depth. (c) 2006 Elsevier B.V. All rights reserved.

Becker, TW, Chevrot S, Schulte-Pelkum V, Blackman DK.  2006.  Statistical properties of seismic anisotropy predicted by upper mantle geodynamic models. Journal of Geophysical Research-Solid Earth. 111   10.1029/2005jb004095   AbstractWebsite

[ 1] We study how numerically predicted seismic anisotropy in the upper mantle is affected by several common assumptions about the rheology of the convecting mantle and deformation-induced lattice preferred orientations ( LPO) of minerals. We also use these global circulation and texturing models to investigate what bias may be introduced by assumptions about the symmetry of the elastic tensor for anisotropic mineral assemblages. Maps of elasticity tensor statistics are computed to evaluate symmetry simplifications commonly employed in seismological and geodynamic models. We show that most of the anisotropy predicted by our convection-LPO models is captured by estimates based on a best fitting hexagonal symmetry tensor derived from the full elastic tensors for the computed olivine: enstatite LPOs. However, the commonly employed elliptical approximation does not hold in general. The orientations of the best fitting hexagonal symmetry axes are generally very close to those predicted for finite strain axes. Correlations between hexagonal anisotropy parameters for P and S waves show simple, bilinear relationships. Such relationships can reduce the number of free parameters for seismic inversions if this information is included a priori. The match between our model predictions and observed patterns of anisotropy supports earlier, more idealized studies that assumed laboratory-derived mineral physics theories and seismic measurements of anisotropy could be applied to study mantle dynamics. The match is evident both in agreement between predicted LPO at selected model sites and that measured in natural samples, and in the global pattern of fast seismic wave propagation directions.