Blackman Donna K.

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Donna K.

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  • Article
    Lower crustal variability and the crust/mantle transition at the Atlantis Massif oceanic core complex
    (American Geophysical Union, 2010-12-18) Blackman, Donna K. ; Collins, John A.
    Seismic refraction data provide new constraints on the structure of the lower oceanic crust and its variability across the Atlantis Massif oceanic core complex, ∼30°N on the Mid-Atlantic Ridge. A 40 km-long spreading-parallel profile constrains P-wave velocities to depths of up to ∼7 km beneath the seafloor. Two shorter spreading-perpendicular lines provide coverage 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 ∼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.
  • Article
    Downward continued multichannel seismic refraction analysis of Atlantis Massif oceanic core complex, 30°N, Mid-Atlantic Ridge
    (American Geophysical Union, 2012-05-19) Henig, A. S. ; Blackman, Donna K. ; Harding, Alistair J. ; Canales, J. Pablo ; Kent, Graham M.
    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 (~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.
  • Article
    Dynamic accretion beneath a slow-spreading ridge segment: IODP hole 1473A and the Atlantis Bank oceanic core complex
    (American Geophysical Union, 2019-11-07) Dick, Henry J. B. ; MacLeod, Christopher J. ; Blum, Peter ; Abe, Natsue ; Blackman, Donna K. ; Bowles, Julie A. ; Cheadle, Michael J. ; Cho, K. ; Ciazela, Jakub ; Deans, Jeremy ; Edgcomb, Virginia P. ; Ferrando, Carlotta ; France, Lydéric ; Ghosh, Biswajit ; Ildefonse, Benoit ; John, Barbara E. ; Kendrick, Mark A. ; Koepke, Juergen ; Leong, James ; Liu, Chuanzhou ; Ma, Qiang ; Morishita, Tomoaki ; Morris, Antony ; Natland, James H. ; Nozaka, Toshio ; Pluemper, Oliver ; Sanfilippo, Alessio ; Sylvan, Jason B. ; Tivey, Maurice A. ; Tribuzio, Riccardo ; Viegas, G.
    809 deep IODP Hole U1473A at Atlantis Bank, SWIR, is 2.2 km from 1,508‐m Hole 735B and 1.4 from 158‐m Hole 1105A. With mapping, it provides the first 3‐D view of the upper levels of a 660‐km2 lower crustal batholith. It is laterally and vertically zoned, representing a complex interplay of cyclic intrusion, and ongoing deformation, with kilometer‐scale upward and lateral migration of interstial melt. Transform wall dives over the gabbro‐peridotite contact found only evolved gabbro intruded directly into the mantle near the transform. There was no high‐level melt lens, rather the gabbros crystallized at depth, and then emplaced into the zone of diking by diapiric rise of a crystal mush followed by crystal‐plastic deformation and faulting. The residues to mass balance the crust to a parent melt composition lie at depth below the center of the massif—likely near the crust‐mantle boundary. Thus, basalts erupted to the seafloor from >1,550 mbsf. By contrast, the Mid‐Atlantic Ridge lower crust drilled at 23°N and at Atlantis Massif experienced little high‐temperature deformation and limited late‐stage melt transport. They contain primitive cumulates and represent direct intrusion, storage, and crystallization of parental MORB in thinner crust below the dike‐gabbro transition. The strong asymmetric spreading of the SWIR to the south was due to fault capture, with the northern rift valley wall faults cutoff by a detachment fault that extended across most of the zone of intrusion. This caused rapid migration of the plate boundary to the north, while the large majority of the lower crust to spread south unroofing Atlantis Bank and uplifting it into the rift mountains.
  • Preprint
    Axial morphology along the Southern Chile Rise
    ( 2012-06) Blackman, Donna K. ; Appelgate, B. ; German, Christopher R. ; Thurber, Andrew R. ; Henig, A. S.
    Morphology of four spreading segments on the southern Chile Rise is described based on multi-beam bathymetric 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.
  • Article
    Drilling constraints on lithospheric accretion and evolution at Atlantis Massif, Mid-Atlantic Ridge 30°N
    (American Geophysical Union, 2011-07-19) Blackman, Donna K. ; Ildefonse, Benoit ; John, Barbara E. ; Ohara, Y. ; Miller, D. J. ; Abe, Natsue ; Abratis, M. ; Andal, E. S. ; Andreani, Muriel ; Awaji, S. ; Beard, J. S. ; Brunelli, Daniele ; Charney, A. B. ; Christie, D. M. ; Collins, John A. ; Delacour, A. G. ; Delius, H. ; Drouin, M. ; Einaudi, F. ; Escartin, Javier E. ; Frost, B. R. ; Fruh-Green, Gretchen L. ; Fryer, P. B. ; Gee, Jeffrey S. ; Grimes, C. B. ; Halfpenny, A. ; Hansen, H.-E. ; Harris, Amber C. ; Tamura, A. ; Hayman, Nicholas W. ; Hellebrand, Eric ; Hirose, T. ; Hirth, Greg ; Ishimaru, S. ; Johnson, Kevin T. M. ; Karner, G. D. ; Linek, M. ; MacLeod, Christopher J. ; Maeda, J. ; Mason, O..U. ; McCaig, A. M. ; Michibayashi, K. ; Morris, Antony ; Nakagawa, T. ; Nozaka, Toshio ; Rosner, Martin ; Searle, Roger C. ; Suhr, G. ; Tominaga, Masako ; von der Handt, A. ; Yamasaki, T. ; Zhao, Xixi
    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° 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 ∼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.
  • Article
    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
    (American Geophysical Union, 2009-01-31) Collins, John A. ; Blackman, Donna K. ; Harris, Amber C. ; Carlson, Richard L.
    The seismic structure of the upper ∼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.
  • Article
    Geophysical signatures of oceanic core complexes
    (John Wiley & Sons, 2009-05-05) Blackman, Donna K. ; Canales, J. Pablo ; Harding, Alistair J.
    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 ∼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.
  • Article
    Hydrothermal exploration of the southern Chile Rise: sediment‐hosted venting at the Chile Triple Junction
    (American Geophysical Union, 2022-03-04) German, Christopher R. ; Baumberger, Tamara ; Lilley, Marvin D. ; Lupton, John E. ; Noble, Abigail E. ; Saito, Mak A. ; Thurber, Andrew R. ; Blackman, Donna K.
    We report results from a hydrothermal plume survey along the southernmost Chile Rise from the Guamblin Fracture Zone to the Chile Triple Junction (CTJ) encompassing two segments (93 km cumulative length) of intermediate spreading-rate mid-ocean ridge axis. Our approach used in situ water column sensing (CTD, optical clarity, redox disequilibrium) coupled with sampling for shipboard and shore based geochemical analyses (δ3He, CH4, total dissolvable iron (TDFe) and manganese, (TDMn)) to explore for evidence of seafloor hydrothermal venting. Across the entire survey, the only location at which evidence for submarine venting was detected was at the southernmost limit to the survey. There, the source of a dispersing hydrothermal plume was located at 46°16.5’S, 75°47.9’W, coincident with the CTJ itself. The plume exhibits anomalies in both δ3He and dissolved CH4 but no enrichments in TDFe or TDMn beyond what can be attributed to resuspension of sediments covering the seafloor where the ridge intersects the Chile margin. These results are indicative of sediment-hosted venting at the CTJ.
  • Article
    Ocean system science to inform the exploration of ocean worlds
    (Oceanography Society, 2022-05-23) German, Christopher R. ; Blackman, Donna K. ; Fisher, Andrew T. ; Girguis, Peter R. ; Hand, Kevin P. ; Hoehler, Tori M. ; Huber, Julie A. ; Marshall, John C. ; Pietro, Kathryn R. ; Seewald, Jeffrey S. ; Shock, Everett ; Sotin, Christophe ; Thurnherr, Andreas M. ; Toner, Brandy M.
    Ocean worlds provide fascinating opportunities for future ocean research. They allow us to test our understanding of processes we consider fundamental to Earth’s ocean and simultaneously provide motivation to explore our ocean further and develop new technologies to do so. In parallel, ocean worlds research offers opportunities for ocean scientists to provide meaningful contributions to novel investigations in the coming decades that will search for life beyond Earth. Key to the contributions that oceanographers can make to this field is that studies of all other ocean worlds remain extremely data limited. Here, we describe an approach based on ocean systems science in which theoretical modeling can be used, in concert with targeted laboratory experimentation and direct observations in Earth’s ocean, to predict what processes (including those essential to support life) might be occurring on other ocean worlds. In turn, such an approach would help identify new technologies that might be required for future space missions as well as appropriate analog studies that could be conducted on Earth to develop and validate such technologies. Our approach is both integrative and interdisciplinary and considers multiple domains, from processes active in the subseafloor to those associated with ocean-ice feedbacks.