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.

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.

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.