Gee
Jeffrey S.
Gee
Jeffrey S.
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ArticlePaleointensity applications to timing and extent of eruptive activity, 9°–10°N East Pacific Rise(American Geophysical Union, 2006-06-08) Bowles, Julie A. ; Gee, Jeffrey S. ; Kent, Dennis V. ; Perfit, Michael R. ; Soule, Samuel A. ; Fornari, Daniel J.Placing accurate age constraints on near-axis lava flows has become increasingly important given the structural and volcanic complexity of the neovolcanic zone at fast spreading ridges. Geomagnetic paleointensity of submarine basaltic glass (SBG) holds promise for placing quantitative age constraints on near-axis flows. In one of the first extensive tests of paleointensity as a dating tool or temporal marker we present the results of over 550 successful SBG paleointensity estimates from 189 near-axis (<4 km) sites at the East Pacific Rise, 9°–10°N. Paleointensities range from 6 to 53 μT and spatially correspond to the pattern expected from known temporal variations in the geomagnetic field. Samples within and adjacent to the axial summit trough (AST) have values approximately equal to or slightly higher than the present-day. Samples out to 1–3 km from the AST have values higher than the present-day, and samples farther off axis have values lower than the present-day. The on-axis samples (<500 m from the AST) provide a test case for using models of paleofield variation for the past few hundred years as an absolute dating technique. Results from samples collected near a well-documented eruption in 1991–1992 suggest there may be a small negative bias in the paleointensity estimates, limiting resolution of the dating technique. Possible explanations for such a bias include local field anomalies produced by preexisting magnetic terrain; anomalously high magnetic unblocking temperatures, leading to a small cooling rate bias; and/or the possibility of a chemical remanence produced by in situ alteration of samples likely to have complicated thermal histories. Paleointensity remains useful in approximating age differences in young flows, and a clear along-axis paleointensity contrast near 9°50′N is suggestive of a ∼150–200 year age difference. Paleointensity values of off-axis samples are generally consistent with rough age interpretations based on side scan data. Furthermore, spatial patterns in the paleointensity suggest extensive off-axis flow emplacement may occur infrequently, with recurrence intervals of 10–20 kyr. Results of a stochastic model of lava emplacement show that this can be achieved with a single distribution of flows, with flow size linked to time between eruptions.
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ArticleDrilling 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, Olivia 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, XixiExpeditions 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.
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ArticleSignificance of short‐wavelength magnetic anomaly low along the East Pacific Rise axis, 9°50′N(American Geophysical Union, 2023-05-16) Berrios‐Rivera, Natalia ; Gee, Jeffrey S. ; Parnell‐Turner, Ross ; Maher, Sarah ; Wu, Jyun‐Nai ; Fornari, Daniel ; Tivey, Maurice ; Marjanović, Milena ; Barreyre, Thibaut ; McDermott, JillMagnetic anomaly variations near mid‐ocean ridge spreading centers are sensitive to a variety of crustal accretionary processes as well as geomagnetic field variations when the crust forms. We collected near‐bottom vector magnetic anomaly data during a series of 21 autonomous underwater vehicle Sentry dives near 9°50′N on the East Pacific Rise (EPR) covering ∼26 km along‐axis. These data document the 2–3 km wide axial anomaly high that is commonly observed at fast‐spreading ridges but also reveal the presence of a superimposed ∼800 m full wavelength anomaly low. The anomaly low is continuous for ≥13 km along axis and may extend over the entire survey region. A more detailed survey of hydrothermal vents near 9°50.3′N reveals ∼100 m diameter magnetic lows, which are misaligned relative to active vents and therefore cannot explain the continuous axial low. The axial magnetization low persists in magnetic inversions with variable extrusive source thickness, indicating that to the extent to which layer 2A constitutes the sole magnetic source, variations in its thickness alone cannot account for the axial low. Lava accumulation models illustrate that high geomagnetic intensity over the past ∼2.5 kyr, and decreasing intensity over the past ∼900 years, are both consistent with the broad axial anomaly high and the superimposed shorter wavelength low. The continuity of this axial low, and similar features elsewhere on the EPR suggests, that either crustal accretionary processes responsible for this anomaly are common among fast‐spread ridges, or that the observed magnetization low may partially reflect global geomagnetic intensity fluctuations.