TY - JOUR T1 - A Dual-Level Magmatic System Beneath the East Pacific Rise, 9°N JF - Geophysical Research Letters Y1 - 2022 A1 - Dunn, Robert A. KW - East Pacific Rise KW - seismic anisotropy KW - seismic tomography AB - Data from three co-located wide-angle ocean-bottom seismograph experiments were used to make a new two-dimensional tomographic image of the East Pacific Rise at 9°N. The upper half of the crustal magmatic system has a different seismic structure than the lower half, presumably corresponding with differences in melt storage and crustal cooling. The upper region is narrow (∼4.5 km wide at 2–3 km depth bsf) and estimated to contain higher melt fractions; the lower region broadens with depth and contains lower melt fractions. The shape of the system suggests steeply-dipping isotherms and relatively stronger hydrothermal circulation in the upper gabbroic section of the crust, with moderately flattening isotherms and less circulation in the lower gabbroic section. VL - 49 UR - https://onlinelibrary.wiley.com/doi/abs/10.1029/2022GL097732 N1 - _eprint: https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2022GL097732 ER - TY - JOUR T1 - Uppermost crustal structure across the eastern Lau spreading center from P-to-S converted waves JF - Marine Geophysical Research Y1 - 2020 A1 - Lata, Charu A1 - Dunn, Robert A. KW - Backarc spreading center KW - oceanic crust KW - S-waves KW - subduction zone AB - P and S wave data from the L-SCAN active-source wide-angle reflection/refraction experiment are modelled to investigate upper crustal structure in the Lau backarc basin. A combination of ray tracing and finite difference numerical wavefield simulation is used to identify P and P-to-S converted seismic phases. The phases primarily arise from two shallow interfaces, one at \textasciitilde 80 m depth or less, and the other at 500–650 m depth. The shallower interface is deeper than the sediment base, is observed across the study area, and is interpreted as a ‘layer 2Aa’ boundary, proposed to result from a rapid change in crack density. The deeper interface is interpreted as the layer 2A–2B boundary, corresponding to a transition from lavas to sheeted dykes. Layer 2A, on average, is 150 m thicker in crust that formed at the spreading center when spreading was located near the arc (< 50 km away), as compared to when spreading was located farther away from the arc (> 70 km away). Layer 2A thickness and Vp/Vs values indicate that a thicker and more porous lava layer, dominated by basalts to basalt-andesites, cap near-arc crust, while a thinner and less-porous, mostly basaltic, volcanic layer caps the far-arc crust. These results are consistent with the waning influence of slab-derived volatiles on crustal formation as seafloor spreading moves away from the active arc. VL - 41 UR - https://doi.org/10.1007/s11001-020-09419-5 ER - TY - JOUR T1 - Seismic imaging of magma sills beneath an ultramafic-hosted hydrothermal system JF - Geology Y1 - 2017 A1 - Canales, J. Pablo A1 - Dunn, Robert A. A1 - Arai, Ryuta A1 - Sohn, Robert A. VL - 45 UR - https://pubs.geoscienceworld.org/geology/article/45/5/451-454/207914 ER - TY - JOUR T1 - Three-Dimensional Seismic Structure of the Mid-Atlantic Ridge: An Investigation of Tectonic, Magmatic, and Hydrothermal Processes in the Rainbow Area JF - Journal of Geophysical Research: Solid Earth Y1 - 2017 A1 - Dunn, Robert A. A1 - Arai, Ryuta A1 - Eason, Deborah E. A1 - Canales, J. Pablo A1 - Sohn, Robert A. VL - 122 UR - http://doi.wiley.com/10.1002/2017JB015051 ER - TY - JOUR T1 - Low frequency baleen whale calls detected on ocean-bottom seismometers in the Lau basin, southwest Pacific Ocean JF - The Journal of the Acoustical Society of America Y1 - 2015 A1 - Brodie, Dana C. A1 - Dunn, Robert A. VL - 137 UR - http://asa.scitation.org/doi/10.1121/1.4904556 ER - TY - JOUR T1 - Seismic evidence of effects of water on melt transport in the Lau back-arc mantle JF - Nature Y1 - 2015 A1 - Wei, S. Shawn A1 - Wiens, Douglas A. A1 - Zha, Yang A1 - Plank, Terry A1 - Webb, Spahr C. A1 - Blackman, Donna K. A1 - Dunn, Robert A. A1 - Conder, James A. VL - 518 UR - http://www.nature.com/articles/nature14113 ER - TY - JOUR T1 - Tracking stress and hydrothermal activity along the Eastern Lau Spreading Center using seismic anisotropy JF - Earth and Planetary Science Letters Y1 - 2015 A1 - Dunn, Robert A. VL - 410 UR - https://linkinghub.elsevier.com/retrieve/pii/S0012821X14007195 ER - TY - JOUR T1 - Crustal construction and magma chamber properties along the Eastern Lau Spreading Center JF - Earth and Planetary Science Letters Y1 - 2013 A1 - Dunn, Robert A. A1 - Martinez, Fernando A1 - Conder, James A. KW - back-arc basin KW - Eastern Lau Spreading Center KW - magma chamber KW - mantle melting KW - mid-ocean ridge KW - seismic tomography AB - The L-SCAN active-source seismic tomography experiment maps the crustal structure and magmatic system along a 120-km-long section of the back-arc Eastern Lau Spreading Center (ELSC), where the ridge undergoes abrupt changes in morphology and composition associated with increasing proximity to the Tofua volcanic arc. Using this dataset, we picked \textasciitilde197,000 P-wave travel times from 57 seismic airgun lines recorded at 83 ocean bottom seismograph stations, and inverted for a 3-D P-wave velocity image. The seismic images reveal a prominent, but narrow, seismic low velocity volume (LVV) located beneath all surveyed ridge segments, consistent with the high temperatures and melt of a crustal magmatic system. Crustal magmatic systems thus underlie spreading axes even where previous seismic reflection surveys did not detect magma lens reflectors, accounting for the heat source of high-temperature hydrothermal vents in these areas. The top of the LVV closely follows the ridge axis and steps across three overlapping spreading centers. As the offset of the overlap increases, the LVV becomes increasingly discontinuous across the ridge limbs. Surprisingly, the LVV is as much as twice as wide, but deeper, in the northern part of the ridge system where the crust is thinner, as compared to the LVV beneath the southern segments, where the crust is relatively thicker. The width of the LVV may be modulated by the degree of deep hydrothermal activity or temporal variations in melt supply, and thus may not correlate directly with the average melt flux as indicated by crustal thickness. Over the past 4Myr, the location of the ridge has swept across different mantle compositional domains, and the crust produced at the ridge formed a zoned pattern. The interpretation is that thick, highly porous, volcanic layers with felsic compositions cap regions of thick crust; thinner volcanic layers of basaltic composition cap regions of thinner crust. The zonal pattern indicates that the influence of slab-derived water on crustal construction has substantially decreased over time. VL - 371-372 UR - https://www.sciencedirect.com/science/article/pii/S0012821X13001891 ER - TY - JOUR T1 - Contrasting crustal production and rapid mantle transitions beneath back-arc ridges JF - Nature Y1 - 2011 A1 - Dunn, Robert A. A1 - Martinez, Fernando KW - seismology AB - The Eastern Lau spreading centre, located on the Lau back-arc basin hydrothermal vent system between Fiji and Tonga, is a focus site of the Ridge 2000 research programme of the US National Science Foundation. The first large-scale seismic study of the area reveals large and abrupt across- and along-strike changes in crustal properties, implying correspondingly large discontinuities in the nature of the mantle supplying melt to the ridge axes. The authors conclude that stable, broad triangular upwelling regions, as inferred for mid-ocean ridges, cannot form near the mantle wedge corner. Instead, the observations imply a dynamic process in which the ridge upwelling zone preferentially captures water-rich low-viscosity mantle when it is near the arc. As the ridge moves away from the arc, a tipping point is reached at which the material is rapidly released, resulting in rapid changes in the character of the crust formed at the ridge. VL - 469 UR - https://www.nature.com/articles/nature09690 N1 - Number: 7329 Publisher: Nature Publishing Group ER -