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Sempere, JC, Gee J, Naar DF, Hey RN.  1989.  3-Dimensional Inversion of the Magnetic-Field Over the Easter-Nazca Propagating Rift Near 25-Degrees-S, 112-Degrees-25 W. Journal of Geophysical Research-Solid Earth and Planets. 94:17409-17420.   10.1029/JB094iB12p17409   AbstractWebsite

The Easter microplate boundary configuration is being reorganized by rift propagation. A Sea Beam survey of the Easter-Nazca spreading center, which forms the eastern boundary of the microplate, has revealed the presence of a young propagating rift growing northward (Naar and Hey, 1986). The tip of the propagating rift is associated with a high-amplitude positive magnetic anomaly. We have performed a three-dimensional inversion of the magnetic field over the propagating rift tip area. The magnetization solution suggests that the western and eastern pseudofaults strike 014° and 338°, respectively, and converge near the rift tip. These orientations yield a propagation to spreading rate ratio of 1.5, slightly higher than the estimate of Naar and Hey (1986). Using the revised estimate of the full spreading rate along the Easter-Nazca spreading center near 25°S (80 mm/yr) (D. F. Naar and R. N. Hey, unpublished manuscript, 1989), we obtain a propagation rate of 120 mm/yr. Within 27–30 km of the rift tip, the propagating rift curves by about 15° to the east toward the failing rift, probably as a result of the interaction between the two offset spreading centers. As at the Galapagos propagating rift, rift propagation appears to be a very orderly process along the Easter-Nazca spreading center. The magnetization distribution that we obtain exhibits a high at the propagating rift tip. At other large ridge axis discontinuities, similar magnetization highs have been interpreted as being the result of the eruption of highly differentiated basalts enriched in iron. The origin of the high magnetization zone in the case of the Easter-Nazca propagating rift near 25°S may be more complex. Preliminary rock magnetic measurements of basalts recovered in the vicinity of the propagating rift confirm the presence of highly magnetized basalts but suggest that the relationship between high magnetization intensities and high Fe content is not straightforward.

Pospichal, JJ, Dehn J, Driscoll N, van Eijden AJM, Farrell J, Fourtanier E, Gamson PD, Gee J, Janecek T, Jenkins GD, Klootwijk CT, Nomura R, Owen RM, Rea DK, Resiwati P, Smit J, Smith GM.  1991.  Cretaceous-Paleogene biomagnetostratigraphy of sites 752-755, Broken Ridge; a synthesis. Proceedings of the Ocean Drilling Program, Scientific Results. 121:721-742.   10.2973/odp.proc.sr.121.181.1991   Abstract

Broken Ridge, in the eastern Indian Ocean, is a shallow-water volcanic platform which formed during the Early to middle Cretaceous at which time it comprised the northern portion of the Kerguelen-Heard Plateau. Rifting during the middle Eocene and subsequent seafloor spreading has moved Broken Ridge about 20°N to its present location. The sedimentary section of Broken Ridge includes Turonian-lower Eocene limestone and chalk with volcanic ash, an interval of detrital sands and gravels associated with middle Eocene rifting and uplift, and a middle-late Oligocene unconformity overlain by a thin section of Neogene-Holocene pelagic calcareous ooze. This paper summarizes the available post-cruise biostratigraphic and magnetostratigraphic data for the Cretaceous- Paleogene section on Broken Ridge. The synthesis of this information permits a more precise interpretation of the timing of events in the history of Broken Ridge, in particular the timing and duration of the middle Eocene rifting event. Paleontologic data support rapid flexural uplift of Broken Ridge in response to mechanical rather than thermal forces. Other highlights of the section include a complete Cretaceous/Tertiary boundary and an opportunity for first-order correlation of Paleogene diatom stratigraphy with that of the calcareous groups.

Blackman, DK, Ildefonse B, John BE, Ohara Y, Miller DJ, Abe N, Abratis M, Andal ES, Andreani M, Awaji S, Beard JS, Brunelli D, Charney AB, Christie DM, Collins J, Delacour AG, Delius H, Drouin M, Einaudi F, Escartin J, Frost BR, Fruh-Green G, Fryer PB, Gee JS, Godard M, Grimes CB, Halfpenny A, Hansen HE, Harris AC, Tamura A, Hayman NW, Hellebrand E, Hirose T, Hirth JG, Ishimaru S, Johnson KTM, Karner GD, Linek M, MacLeod CJ, Maeda J, Mason OU, McCaig AM, Michibayashi K, Morris A, Nakagawa T, Nozaka T, Rosner M, Searle RC, Suhr G, Tominaga M, von der Handt A, Yamasaki T, Zhao X.  2011.  Drilling constraints on lithospheric accretion and evolution at Atlantis Massif, Mid-Atlantic Ridge 30 degrees N. Journal of Geophysical Research-Solid Earth. 116   10.1029/2010jb007931   AbstractWebsite

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 degrees 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 similar to 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.

Selkin, PA, Gee JS, Tauxe L, Meurer WP, Newell AJ.  2000.  The effect of remanence anisotropy on paleointensity estimates: a case study from the Archean Stillwater Complex. Earth and Planetary Science Letters. 183:403-416.   10.1016/s0012-821x(00)00292-2   AbstractWebsite

Paleomagnetism of Archean rocks potentially provides information about the early development of the Earth and of the geodynamo. Precambrian layered intrusive rocks are good candidates for paleomagnetic studies: such complexes are commonly relatively unaltered and may contain some single-domain magnetite 'armored' by silicate mineral grains. However, layered intrusives often have a strong petrofabric that may result in a strong remanence anisotropy. Magnetic anisotropy can have particularly disastrous consequences for paleointensity experiments if the anisotropy is unrecognized and if its effects remain uncorrected. Here we examine the magnetic anisotropy of an anorthosite sample with a well-developed magmatic foliation. The effect of the sample's remanence fabric on paleointensity determinations is significant: paleointensities estimated by the method of Thellier and Thellier range from 17 to 55 muT for specimens magnetized in a field of 25 muT. We describe a technique based on the remanence anisotropy tensor to correct paleointensity estimates for the effects of magnetic fabric and use it to estimate a paleointensity for the Stillwater Complex (MT, USA) of similar to 32 muT (adjusted for the effects of slow cooling). (C) 2000 Elsevier Science B.V. All rights reserved.

Gee, J, Staudigel H, Natland JH.  1991.  Geology and petrology of Jasper Seamount. Journal of Geophysical Research-Solid Earth and Planets. 96:4083-4105.   10.1029/90jb02364   AbstractWebsite

Fifteen dredges on the summit and upper flanks of Jasper Seamount (122-degrees 44'W; 30-degrees 27'N) recovered a wide variety of lithologies, including pillow lavas, vesicular lapillistones from shallow submarine explosive volcanism, and a range of xenoliths. On the basis of dredge locations, geochemical characteristics, and Ar-40/Ar-39 age data, three distinct phases of volcanism can be distinguished, a shield-building tholeiitic/transitional phase (Flank Transitional Series, FTS), followed by a flank alkalic series (FAS), and a late-stage Summit Alkalic Series (SAS). All three series consist exclusively of differentiated (Mg# = 54 to 21; Mg# = Mg2+/(Mg2+ + Fe2+)) compositions. The FTS represents a low-pressure differentiation trend from tholeiitic/transitional basalts to quartz-normative residual liquids and probably accounts for more than 90% of the volume of Jasper. Ar-40/Ar-39 age data, the dominant reversed polarity of Jasper, and a plausible duration (< 1 m.y.) for shield construction suggest FTS volcanism began about 11 Ma and ended about 10 Ma. FTS lavas probably erupted from a NW trending, hotspot track-parallel rift system. The intermediate alkalinity FAS lavas, which probably comprise 3-8% of the volume of Jasper, erupted from 8.7 to 7.5 Ma, possibly after a brief volcanic hiatus or period of reduced eruptive activity. Normative projections suggest the FAS lavas are the product of fractionation or equilibration at elevated pressures. The hawaiites and mugearites of the SAS erupted between 4.8 and 4.1 Ma, after a probable 2.7 m.y. period of volcanic quiescence, and probably constitute < 1% of the seamount volume. A suite of xenoliths incorporated in SAS lavas includes (1) tholeiitic basalt fragments from either the ocean crust or seamount interior, (2) a range of differentiated gabbros largely derived from the ocean crust, (3) residual mantle spinel lherzolites, and (4) pyroxenite and peridotite cumulates. The abundance of crustal gabbro and spinel lherzolite xenoliths in evolved lavas of the SAS suggests that these lavas probably fractionated in a magma chamber at the crust-mantle boundary. The occurrence of orthopyroxene-bearing alkalic cumulate xenoliths in these lavas, however, is enigmatic and may reflect complexities such as magma mixing or the inappropriateness of pressure estimates. The SAS vents of Jasper define a NE-SW volcanic trend which is orthogonal to the FTS rift. The pattern of volcanic activity, including periods of volcanic quiescence, and the general increase in alkalinity, as well as the structural reorganization of magmatic feeder systems of Jasper Seamount, is strikingly similar to the patterns observed on Hawaiian volcanoes. Thus our data from Jasper (690 km3) extend the concepts of structural and petrological evolution of hotspot volcanoes based on Hawaii to moderate-sized seamounts.

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.

Gee, J, Nakanishi M.  1995.  Magnetic petrology and magnetic properties of western Pacific guyots; implications for seamount paleopoles. Proceedings of the Ocean Drilling Program, Scientific Results. 144:615-630.   10.2973/odp.proc.sr.144.020.1995   Abstract

Despite the importance of seamount paleopoles in reconstructing past tectonic motions of the Pacific Plate, few data exist on the magnetic properties and processes of remanence acquisition in seamounts. We present a basic magnetic characterization and a detailed petrographic and microprobe study of the oxide minerals in mildly to strongly alkalic lavas recovered from five western Pacific guyots sampled during Ocean Drilling Program Leg 144. The Ti-rich chrome spinel compositions and Al- and Mg-enrich- ment in titanomagnetites reflect the alkalic nature of the lavas. The alteration history of these samples is diverse, ranging from low-temperature oxidation to highly oxidizing conditions resulting in an assemblage of magnesioferrite + titanohematite. The natural remanent magnetization (NRM) intensities for all five guyots are quite similar, yielding a combined arithmetic mean NRM intensity of 3.53 A/m, similar to previously reported values from dredged and drilled seamount material. The mean Königsberger ratio (9.8) implies an approximate 10% contribution of induced magnetization. Systematic discrepancies between the observed inclinations and inclinations derived from the magnetic anomaly data for Lo-En, MIT, and Takuyo-Daisan guyots are compatible with a significant bias from viscous and induced magnetization in these Cretaceous guyots.

Ildefonse, B, Blackman DK, John BE, Ohara Y, Miller DJ, MacLeod CJ, Abe N, Abratis M, Andal ES, Andreani M, Awaji S, Beard JS, Brunelli D, Charney AB, Christie DM, Delacour AG, Delius H, Drouin M, Einaudi F, Escartin J, Frost BR, Fryer PB, Gee JS, Godard M, Grimes CB, Halfpenny A, Hansen HE, Harris AC, Hayman NW, Hellebrand E, Hirose T, Hirth JG, Ishimaru S, Johnson KTM, Karner GD, Linek M, Maeda J, Mason OU, McCaig AM, Michibayashi K, Morris A, Nakagawa T, Nozaka T, Rosner M, Searle RC, Suhr G, Tamura A, Tominaga M, von der Handt A, Yamasaki T, Zhao X, Integrated Ocean Drilling Program, Expedition 305 SSP.  2007.  Oceanic core complexes and crustal accretion at slow-spreading ridges. Geology. 35:623-626.   10.1130/G23531A.1   Abstract

Oceanic core complexes expose gabbroic rocks on the sealloor via detachment faulting, often associated with serpentinized peridotite. The thickness of these serpentinite units is unknown. Assuming that the steep slopes that typically surround these core complexes provide a cross section through the structure, it has been inferred that serpentinites compose much of the section to depths of at least several hundred meters. However, deep drilling at oceanic core complexes has recovered gabbroic sequences with virtually no serpentinized peridotite. We propose a revised model for oceanic core complex development based on consideration of the rheological differences between gabbro and serpentinized peridotite: emplacement of a large intrusive gabbro body into a predominantly peridotite host is followed by localization of strain around the margins of the pluton, eventually resulting in an uplifted gabbroic core surrounded by deformed serpentinite. Oceanic core complexes may therefore reflect processes associated with relatively enhanced periods of mafic intrusion within overall magma-poor regions of slow- and ultra-slow-spreading ridges.

Nakanishi, M, Gee JS.  1995.  Paleomagnetic investigations of Leg 144 volcanic rocks: paleolatitudes of the northwestern Pacific guyots. Proceedings of the Ocean Drilling Program Scientific Results. 144:585-604.   10.2973/odp.proc.sr.144.022.1995   Abstract

Paleomagnetic properties of 340 minicore samples from 5 guyots in the northwestern Pacific Ocean were measured on board the JOIDES Resolution and in the paleomagnetic laboratories of the Ocean Research Institute, University of Tokyo, and of Lamont-Doherty Earth Observatory. Stepwise thermal and alternating-field (AF) demagnetizations typically isolate the same characteristic magnetization direction. The shipboard and shore-based paleomagnetic measurements suggest a paleolatitude of ~10°S for Limalok, Wodejebato, MIT, and Takuyo-Daisan guyots. Lo-En Guyot apparently was constructed at a latitude of 31°S. According to the radiometric age of volcanic rocks from MIT Guyot, the reversed polarity of the volcanic basement of MIT Guyot corresponds to Chron M1 or older. A reversed polarity remanence of Wodejebato Guyot was acquired during Chron C33R because radiometric age determinations for Wodejebato Guyot range from 79 to 85 Ma. Our paleolatitude estimates for Lo-En, MIT, and Takuyo-Daisan guyots differ from those derived from previous seamount magnetic anomaly modeling. The sense and magnitudeof this paleolatitude discrepancy are consistent with a significant contribution from viscous and induced magnetizations. We estimated the paleolatitudes of the guyots from the age data and from a previously published absolute-motion model for the Pacific Plate. Differences between these paleolatitude estimates and those derived from paleomagnetic measurements except for Wodejebato Guyot are smaller than 6°.