Export 6 results:
Sort by: Author Title Type [ Year  (Desc)]
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, Kendall JM.  2002.  Seismic anisotropy in the upper mantle 2. Predictions for current plate boundary flow models. Geochemistry Geophysics Geosystems. 3   10.1029/2001gc000247   AbstractWebsite

[1] The anisotropic seismic structure due to flow-induced mineral alignment is investigated for a series of models designed to simulate deformation in the upper mantle within a few hundred kilometers of a plate boundary. The orientation distributions of olivine: enstatite aggregates evolve along streamlines of each flow model, based on each grains plastic response to the local stress/strain field. The effective elastic tensor for these textured aggregates provides predictions of P wave anisotropy and shear wave splitting throughout the model space. P and S travel time delay patterns and fast shear wave polarization angles are found to vary significantly with incidence angle for a given model. Comparison of predicted fast P direction for our method versus a finite-strain based estimate shows that agreement is acceptable for much of the model space, but notable differences occur in regions up to several tens of kilometers in size. Two-dimensional models of spreading center flow are presented for slow and fast rates and for several cases in which the ridge migrates over the deeper mantle. The effect of flow in the third dimension is addressed in a few calculations. For one comparison of flow in the mantle wedge at a subduction zone, the introduction of trench parallel flow causes significant changes in the predicted patterns of P wave anisotropy (magnitude, more than orientation) and SKS splitting.

Forsyth, DW, Scheirer DS, Webb SC, Dorman LM, Orcutt JA, Harding AJ, Blackman DK, Morgan JP, Detrick RS, Shen Y, Wolfe CJ, Canales JP, Toomey DR, Sheehan AF, Solomon SC, Wilcock WSD, Team MS.  1998.  Imaging the deep seismic structure beneath a mid-ocean ridge: The MELT experiment. Science. 280:1215-1218.   10.1126/science.280.5367.1215   AbstractWebsite

The Mantle Electromagnetic and Tomography (MELT) Experiment was designed to distinguish between competing models of magma generation beneath mid-ocean ridges. Seismological observations demonstrate that basaltic melt is present beneath the East Pacific Rise spreading center in a broad region several hundred kilometers across and extending to depths greater than 100 kilometers, not just in a narrow region of high melt concentration beneath the spreading center, as predicted by some models. The structure of the ridge system is strongly asymmetric: mantle densities and seismic velocities are lower and seismic anisotropy is stronger to the west of the rise axis.

Blackman, DK, Kendall JM.  1997.  Sensitivity of teleseismic body waves to mineral texture and melt in the mantle beneath a mid-ocean ridge. Philosophical Transactions of the Royal Society of London Series a-Mathematical Physical and Engineering Sciences. 355:217-231. AbstractWebsite

Seismic energy propagating through the mantle beneath an oceanic spreading centre develops a signature due both to the subaxial deformation field and to the presence of melt in the upwelling zone. Deformation of peridotite during mantle flow results in strong preferred orientation of olivine and significant seismic anisotropy in the upper 100 km of the mantle. Linked numerical models of flow, texture development and seismic velocity structure predict that regions of high anisotropy will characterize the subaxial region, particularly at slow-spreading mid-ocean ridges. In addition to mineral texture effects, the presence of basaltic melt can cause travel-time anomalies, the nature of which depend on the geometry, orientation and concentration of the melt. In order to illustrate the resolution of subaxial structure that future seismic experiments can hope to achieve, we investigate the teleseismic signature of a series of spreading centre models in which the mantle viscosity and melt geometry are varied. The P-wave travel times are not very sensitive to the geometry and orientation of melt inclusions, whether distributed in tubules or thin ellipsoidal inclusions. Travel time delays of 0.1-0.4 s are predicted for the melt distribution models tested. The P-wave effects of mineral texture dominate in the combined melt-plus-texture models. Thus, buoyancy-enhanced upwelling at a slow spreading ridge is characterized by 0.7-1.0 s early P-wave arrival times in a narrow axial region, while the models of plate-driven-only flow predicts smaller advances (less than 0.5 s) over a broader region. In general S-wave travel times are more sensitive to the melt and show more obvious differences between melt present as tubules as opposed to thin disks, especially if a preferred disk orientation exists. Mineral texture and the preferred alignment of melt inclusions will both produce shear-wave splitting, our models predict as much as 4 s splitting in some cases.

Blackman, DK, Kendall JM, Dawson PR, Wenk HR, Boyce D, Morgan JP.  1996.  Teleseismic imaging of subaxial flow at mid-ocean ridges: Traveltime effects of anisotropic mineral texture in the mantle. Geophysical Journal International. 127:415-426.   10.1111/j.1365-246X.1996.tb04730.x   AbstractWebsite

Deformation of peridotite caused by mantle how beneath an oceanic spreading centre can result in the development of seismic anisotropy. Traveltime anomalies and shear-wave splitting will develop as seismic energy propagates through such an anisotropic region, thus providing a signature of the deformation field at depth. In this study we investigate the nature of deformation associated with mantle upwelling for two models of flow in the upper 100 km of the mantle. The finite-strain fields of the passive upwelling model versus the buoyancy-enhanced upwelling model are quite different. This suggests that mineral aggregates deform differently in the two models, thus developing seismic signatures that are distinguishable. Numerical estimates of the corresponding mineral textures are made using polycrystal theory for olivine with four operative slip systems. The activation of a slip system is determined for each grain on the basis of the local critical resolved shear stress, The computed grain deformation reflects a balance between stress equilibrium, for the aggregate as a whole, and strain continuity between neighbouring grains within the aggregate. This approach enables a direct link to be made between the model flow fields and the resulting texture development. Given these mineral orientation distributions, elastic parameters are calculated and wavefronts are propagated through the anisotropic structure, Traveltimes for teleseismic body waves are computed using ray theory, and amplitudes are estimated for an across-axis profile extending 100 km from the ridge axis. Relative P-wave residuals of up to 1 s are predicted for the buoyant model with on-axis arrivals being earliest, since near-vertical velocities are fastest beneath the axis. On-axis P-wave arrivals for the passive model are half a second earlier than arrivals 60 km off-axis, and relative delays continue to increase slowly as distance from the ridge increases, S-wave splitting of almost a second is predicted for the buoyant model, whereas less than a half-second of splitting is determined for the passive model.

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