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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.

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.

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.