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Marcuson, R, Gee J, Wei E, Driscoll N.  2019.  A 2000 year geomagnetic field record from the Gulf of Papua. Marine Geology. 408:48-66.   10.1016/j.margeo.2018.11.014   AbstractWebsite

A high resolution Holocene record of geomagnetic field direction and intensity in the Gulf of Papua was constructed using paleomagnetic data from eight piston cores collected in the region. The chronology of the cores was based largely on radiocarbon dates from benthic foraminifera. Correlation of seismic reflectors imaged in subbottom data between nearby cores was consistent with the chronostratigraphic framework based on radiocarbon dates and provided additional confidence of the age model. In other regions, where cores sampled different depositional settings associated with the three-dimensional prograding lobes, reflectors could not be confidently correlated between cores. Furthermore, radiocarbon ages from the bottoms of trigger cores and tops of co-located piston cores revealed varying amounts of overpenetration of the piston core below the seafloor. The relative paleointensity, and a general eastward trend in the declination and steepening of the inclination in our new record are generally compatible with the closest ( > 4300 km) existing records. Our record does not agree well with global field models CALS3k and pfm9k, likely reflecting the lack of existing data contributing to field models in the region.

Johnson, CL, Wijbrans JR, Constable CG, Gee J, Staudigel H, Tauxe L, Forjaz VH, Salgueiro M.  1998.  Ar-40/Ar-39 ages and paleomagnetism of Sao Miguel lavas, Azores. Earth and Planetary Science Letters. 160:637-649.   10.1016/s0012-821x(98)00117-4   AbstractWebsite

We present new Ar-40/Ar-39 ages and paleomagnetic data for Sao Miguel island, Azores. Paleomagnetic samples were obtained for 34 flows and one dike; successful mean paleomagnetic directions were obtained for 28 of these 35 sites. Ar-40/Ar-39 age determinations on 12 flows from the Nordeste complex were attempted successfully: ages obtained are between 0.78 Ma and 0.88 Ma, in contrast to published K-Ar ages of 1 Ma to 4 Ma. Our radiometric ages are consistent with the reverse polarity paleomagnetic field directions, and indicate that the entire exposed part of the Nordeste complex is of a late Matuyama age. The duration of volcanism across Sao Miguel is significantly less than previously believed, which has important implications for regional melt generation processes, and temporal sampling of the geomagnetic field. Observed stable isotope and trace element trends across the island can be explained, at least in part, by communication between different magma source regions at depth. The Ar-40/Ar-39 ages indicate that our normal polarity paleomagnetic data sample at least 0.1 Myr (0-0.1 Ma) and up to 0.78 Myr (0-0.78 Ma) of paleosecular variation and our reverse polarity data sample approximately 0.1 Myr (0.78-0.88 Ma) of paleosecular variation. Our results demonstrate that precise radiometric dating of numerous flows sampled is essential to accurate inferences of long-term geomagnetic field behavior. Negative inclination anomalies are observed for both the normal and reverse polarity time-averaged field. Within the data uncertainties, normal and reverse polarity field directions are antipodal, but the reverse polarity field shows a significant deviation from a geocentric axial dipole direction. (C) 1998 Elsevier Science B.V. All rights reserved.

Gee, JS, Webb SC, Ridgway J, Staudigel H, Zumberge MA.  2001.  A deep tow magnetic survey of Middle Valley, Juan de Fuca Ridge. Geochemistry Geophysics Geosystems. 2   10.1029/2001GC000170   AbstractWebsite

We report here results from a deep tow magnetic survey over Middle Valley, Juan de Fuca Ridge. A series of track lines are combined to generate a high-resolution map of the magnetic field anomaly within a 10 x 12 km region surrounding the Bent Hill massive sulfide (BHMS) deposit. A uniformly magnetized body (5 A/m) with a cross section approximating the body inferred from Ocean Drilling Program (ODP) drilling can account for the observed near-bottom magnetic anomaly amplitude. Assuming this magnetization is entirely induced, the average susceptibility (0.11 SI) corresponds to similar to3.5% magnetite + pyrrhotite by volume, consistent with the abundance of these phases observed in drill core samples. However, this uniform magnetization model significantly underestimates the magnetic anomaly measured a few meters above the seafloor by submersible, indicating that the upper portion of the sulfide mound must have a significantly higher magnetization (similar to 10% magnetite + pyrrhotite) than at deeper levels. On a larger scale, the near-bottom magnetic anomaly data show that basement magnetizations are not uniformly near zero, as had been inferred from analysis of the sea surface anomaly pattern. We interpret this heterogeneity as reflecting primarily differences in the degree of hydrothermal alteration. Our results highlight the potential of magnetic anomaly data for characterizing hydrothermal deposits where extensive drill core sampling is not available.

Blackman, DK, Karson JA, Kelley DS, Cann JR, Fruh-Green GL, Gee JS, Hurst SD, John BE, Morgan J, Nooner SL, Ross DK, Schroeder TJ, Williams EA.  2002.  Geology of the Atlantis Massif (Mid-Atlantic Ridge, 30 degrees N): Implications for the evolution of an ultramafic oceanic core complex. Marine Geophysical Research. 23:443-469.   10.1023/b:mari.0000018232.14085.75   AbstractWebsite

The oceanic core complex comprising Atlantis Massif was formed within the past 1.5-2 Myr at the intersection of the Mid-Atlantic Ridge, 30degrees N, and the Atlantis Transform Fault. The corrugated, striated central dome prominently displays morphologic and geophysical characteristics representative of an ultramafic core complex exposed via long-lived detachment faulting. Sparse volcanic features on the massif's central dome indicate that minor volcanics have penetrated the inferred footwall, which geophysical data indicates is composed predominantly of variably serpentinized peridotite. In contrast, the hanging wall to the east of the central dome is comprised of volcanic rock. The southern part of the massif has experienced the greatest uplift, shoaling to less than 700 m below sea level, and the coarsely striated surface there extends eastward to the top of the median valley wall. Steep landslide embayments along the south face of the massif expose cross sections through the core complex. Almost all of the submersible and dredge samples from this area are deformed, altered peridotite and lesser gabbro. Intense serpentinization within the south wall has likely contributed to the uplift of the southern ridge and promoted the development of the Lost City Hydrothermal Field near the summit. Differences in the distribution with depth of brittle deformation observed in microstructural analyses of outcrop samples suggest that low-temperature strain, such as would be associated with a major detachment fault, is concentrated within several tens of meters of the domal surface. However, submersible and camera imagery show that deformation is widespread along the southern face of the massif, indicating that a series of faults, rather than a single detachment, accommodated the uplift and evolution of this oceanic core complex.

Koppers, AAP, Yamazaki T, Geldmacher J, Gee JS, Pressling N, Hoshi H, Anderson L, Beier C, Buchs DM, Chen LH, Cohen BE, Deschamps F, Dorais MJ, Ebuna D, Ehmann S, Fitton JG, Fulton PM, Ganbat E, Hamelin C, Hanyu T, Kalnins L, Kell J, Machida S, Mahoney JJ, Moriya K, Nichols ARL, Rausch S, Sano SI, Sylvan JB, Williams R.  2012.  Limited latitudinal mantle plume motion for the Louisville hotspot. Nature Geoscience. 5:911-917.   10.1038/ngeo1638   AbstractWebsite

Hotspots that form above upwelling plumes of hot material from the deep mantle typically leave narrow trails of volcanic seamounts as a tectonic plate moves over their location. These seamount trails are excellent recorders of Earth's deep processes and allow us to untangle ancient mantle plume motions. During ascent it is likely that mantle plumes are pushed away from their vertical upwelling trajectories by mantle convection forces. It has been proposed that a large-scale lateral displacement, termed the mantle wind, existed in the Pacific between about 80 and 50 million years ago, and shifted the Hawaiian mantle plume southwards by about 15 degrees of latitude. Here we use Ar-40/Ar-39 age dating and palaeomagnetic inclination data from four seamounts associated with the Louisville hotspot in the South Pacific Ocean to show that this hotspot has been relatively stable in terms of its location. Specifically, the Louisville hotspot-the southern hemisphere counterpart of Hawai'i-has remained within 3-5 degrees of its present-day latitude of about 51 degrees S between 70 and 50 million years ago. Although we cannot exclude a more significant southward motion before that time, we suggest that the Louisville and Hawaiian hotspots are moving independently, and not as part of a large-scale mantle wind in the Pacific.

Dick, HJB, Natland JH, Alt JC, Bach W, Bideau D, Gee JS, Haggas S, Hertogen JGH, Hirth G, Holm PM, Ildefonse B, Iturrino GJ, John BE, Kelley DS, Kikawa E, Kingdon A, LeRoux PJ, Maeda J, Meyer PS, Miller DJ, Naslund HR, Niu YL, Robinson PT, Snow J, Stephen RA, Trimby PW, Worm HU, Yoshinobu A.  2000.  A long in situ section of the lower ocean crust: results of ODP Leg 176 drilling at the Southwest Indian Ridge. Earth and Planetary Science Letters. 179:31-51.   10.1016/s0012-821x(00)00102-3   AbstractWebsite

Ocean Drilling Program Leg 176 deepened Hole 735B in gabbroic lower ocean crust by 1 km to 1.5 km. The section has the physical properties of seismic layer 3, and a total magnetization sufficient by itself to account for the overlying lineated sea-sur face magnetic anomaly. The rocks from Hole 735B are principally olivine gabbro, with evidence for two principal and many secondary intrusive events. There are innumerable late small ferrogabbro intrusions, often associated with shear zones that cross-cut the olivine gabbros. The ferrogabbros dramatically increase upward in the section. Whereas there are many small patches of ferrogabbro representing late iron- and titanium-rich melt trapped intragranularly in olivine gabbro, most late melt was redistributed prior to complete solidification by compaction and deformation. This, rather than in situ upward differentiation of a large magma body, produced the principal igneous stratigraphy, The computed bulk composition of the hole is too evolved to mass balance mid-ocean ridge basalt back to a primary magma, and there must be a significant mass of missing primitive cumulates. These could lie either below the hole or out of the section. Possibly the gabbros were emplaced by along-axis intrusion of moderately differentiated melts into the near-transform environment. Alteration occurred in three stages. High-temperature granulite- to amphibolite-facies alteration is most important. coinciding with brittle-ductile deformation beneath the ridge. Minor greenschist-facies alteration occurred under largely static conditions, likely during block uplift at the ridge transform intersection. Late post-uplift low-temperature alteration produced locally abundant smectite, often in previously unaltered areas. The most important features of the high- and low-temperature alteration are their respective associations with ductile and cataclastic deformation, and an overall decrease downhole with hydrothermal alteration generally less than or equal to 5% in the bottom kilometer. Hole 735B provides evidence for a strongly heterogeneous lower ocean crust, and for;he inherent Interplay of deformation. alteration and igneous processes at slow-spreading ridges. It is strikingly different from gabbros sampled from fast-spreading ridges and at most well-described ophiolite complexes. We attribute this to the remarkable diversity of tectonic environments where crustal accretion occurs in the oceans and to the low probability of a section of old slow-spread crust found near a major large-offset transform being emplaced on-land compared to sections of young crust from small ocean basins. (C) 20()() Elsevier Science B.V. All rights reserved.

Winterer, EL, Gee JS, Van Waasbergen RJ.  1988.  The source area for Lower Cretaceous clastic sediments of the Galicia Margin : geology and tectonic and erosional history. Proceedings of the Ocean Drilling Program, Scientific Results. 103:697-732.   10.2973/   Abstract

Structural relations shown on seismic-reflection profiles of the Galicia margin indicate that during the Early Creta- ceous rifting of the margin, Galicia Bank and its southern extension (Galicia Ridge) were separated from the mainland of Iberia by a fault-controlled trough, and that the rift-stage sandy turbidite sediments in the fault troughs west of Gali- cia Bank were derived from the erosion of a narrow row of rugged hills that occupied the present area of Galicia Bank and Ridge. The volume and age span of rift-stage clastic sediments, estimated from drill and seismic data, imply rapid erosion of about 1 km of bedrock in the source area, suggesting that the source area was uplifted tectonically during the early stage of rifting. The composition of the sandstone indicates a source area dominated by granitic and schistose rocks, consistent with data from dredge hauls. The abundant fragmental plant debris in the sandstone indicates heavily wooded land, and regional sedimentological and paleobotanical data suggest warm rainy climatic conditions. A combi- nation of erosional lowering (largely compensated by isostatic rebound) and tectonic subsidence reduced most of the source area to sea level by the Aptian, and carbonate banks then flourished over the site of the former landmass.