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Horst, AJ, Varga RJ, Gee JS, Karson JA.  2014.  Diverse magma flow directions during construction of sheeted dike complexes at fast- to superfast-spreading centers. Earth and Planetary Science Letters. 408:119-131.   10.1016/j.epsl.2014.09.022   AbstractWebsite

Dike intrusion is a fundamental process during upper oceanic crustal accretion at fast- to superfast-spreading ridges. Based on the distribution of magma along fast-spreading centers inferred from marine geophysical data, models predict systematic steep flow at magmatically robust segment centers and shallow magma flow toward distal segment ends. Anisotropy of magnetic susceptibility (AMS) fabrics from 48 fully-oriented block samples of dikes from upper oceanic crust exposed at Hess Deep Rift and Pito Deep Rift reveal a wide range of magma flow directions that are not consistent with such simple magma supply models. The AMS is interpreted to arise from distribution anisotropy of titanomagnetite crystals based on weak shape-preferred orientation of opaque oxide and plagioclase crystals generally parallel to AMS maximum eigenvectors. Most dike samples show normal AMS fabrics with maximum eigenvector directions ranging from subvertical to subhorizontal. The distributions of inferred magma flow lineations from maximum eigenvectors show no preferred flow pattern, even after structural correction. We use a Kolmogorov Smirnov test (KS-test) to show that the distribution of bootstrapped flow lineation rakes from Pito Deep are not statistically distinct from Hess Deep, and neither are distinguishable from Oman and Troodos Ophiolite AMS data. Magma flow directions in sheeted dikes from these two seafloor escarpments also do not correlate with available geochemistry in any systematic way as previously predicted. These results indicate distinct compositional sources feed melt that is injected into dikes at fast- to superfast-spreading ridges with no preference for subhorizontal or subvertical magma flow. Collectively, results imply ephemeral melt lenses at different along-axis locations within the continuous axial magma chamber and either direct injection or intermingling of melt from other deeper ridge-centered or off-axis sources. (C) 2014 Elsevier B.V. All rights reserved.

Varga, RJ, Horst AJ, Gee JS, Karson JA.  2008.  Direct evidence from anisotropy of magnetic susceptibility for lateral melt migration at superfast spreading centers. Geochemistry Geophysics Geosystems. 9   10.1029/2008gc002075   AbstractWebsite

Rare, fault-bounded escarpments expose natural cross sections of ocean crust in several areas and provide an unparalleled opportunity to study the end products of tectonic and magmatic processes that operated at depth beneath oceanic spreading centers. We mapped the geologic structure of ocean crust produced at the East Pacific Rise ( EPR) and now exposed along steep cliffs of the Pito Deep Rift near the northern edge of the Easter microplate. The upper oceanic crust in this area is typified by basaltic lavas underlain by a sheeted dike complex comprising northeast striking, moderately to steeply southeast dipping dikes. Paleomagnetic remanence of oriented blocks of dikes collected with both Alvin and Jason II indicate clockwise rotation of similar to 61 degrees related to rotation of the microplate indicating structural coupling between the microplate and crust of the Nazca Plate to the north. The consistent southeast dip of dikes formed as the result of tilting at the EPR shortly after their injection. Anisotropy of magnetic susceptibility of dikes provides well-defined magmatic flow directions that are dominantly dike-parallel and shallowly plunging. Corrected to their original EPR orientation, magma flow is interpreted as near-horizontal and parallel to the ridge axis. These data provide the first direct evidence from sheeted dikes in ocean crust for along-axis magma transport. These results also suggest that lateral transport in dikes is important even at fast spreading ridges where a laterally continuous subaxial magma chamber is present.

Granot, R, Tauxe L, Gee JS, Ron H.  2007.  A view into the Cretaceous geomagnetic field from analysis of gabbros and submarine glasses. Earth and Planetary Science Letters. 256:1-11.   10.1016/j.epsl.2006.12.028   AbstractWebsite

The nature of the geomagnetic field during the Cretaceous normal polarity superchron (CNS) has been a matter of debate for several decades. Numerical geodynamo simulations predict higher intensities, but comparable variability, during times of few reversals than times with frequent reversals. Published geomagnetic paleointensity data from the CNS are highly scattered suggesting that additional studies are required. Here we present new paleointensity results from 18 sites collected from the lower oceanic crust of the Troodos ophiolite, Cyprus (92.1 Ma old). Together with recently published data from the Troodos upper crust we obtain three independent palcointensity time-series. These sequences reveal quasi-cyclic variations of intensities about a mean value of 54 +/- 20 Z Am(2), providing insight into the fluctuating nature of the Cretaceous magnetic field. Our data suggest the CNS field was both weaker and more variable than predicted by geodynamo simulations. The large amplitudes of these variations may explain the wide range of dipole moments previously determined from the CNS. (c) 2007 Elsevier B.V. All rights reserved.

Lawrence, RM, Gee JS, Karson JA.  2002.  Magnetic anisotropy of serpentinized peridotites from the MARK area: Implications for the orientation of mesoscopic structures and major fault zones. Journal of Geophysical Research-Solid Earth. 107   10.1029/2000jb000007   AbstractWebsite

[1] Mantle-derived serpentinized peridotites are exposed both along fracture zones and in areas of extreme tectonic extension at slow to intermediate spreading ridges and may constitute a significant volume of the shallow crust in these environments. Here we examine the potential of magnetic remanence data and structural features in serpentinized peridotites from Ocean Drilling Program (ODP) Site 920 (Mid-Atlantic Ridge south of Kane, MARK) to provide insights into the tectonic processes responsible for the exposure of these deep-seated rocks at the seafloor. Paleomagnetic data from 214 samples from Site 920 document a remarkably consistent inclination (36.1degrees +0.8degrees/ -1.4degrees) that is shallower than either the expected geocentric axial dipole inclination (40.7degrees) or present-day inclination (41.9degrees) at the site. We show that the nearly univectorial remanence in these samples is likely to be a partial thermoremanence, possibly augmented by viscous processes at moderate temperatures. These properties were acquired during cooling from the relatively high temperatures (> 350 degreesC) at which serpentinization occurred. The remanence directions therefore provide some information on the latest stages of uplift of the serpentinite massif. However, interpretation of this tectonic history is complicated by the presence of a pronounced magnetic fabric, which presumably resulted in a deflection of the remanence. We estimate the magnitude and direction of this deflection using a relationship between the anisotropy of magnetic susceptibility and remanence anisotropy. The corrected remanent inclinations (mean 39.5degrees) more closely approximates the time-averaged inclination at the site, indicating that the massif experienced little or no resolvable tilt after serpentinization and cooling to 350 degreesC. Accounting for the anisotropy-related deflection of the remanence also allows us to more accurately restore various structural features within the core to their geographic orientation. After this reorientation the dominant mesoscopic foliation in these rocks, defined by the preferred orientation of orthopyroxene and subparallel serpentine veins, has an average orientation that closely parallels the regional-scale fault zones on the western median valley wall.

Bowers, NE, Cande SC, Gee JS, Hildebrand JA, Parker RL.  2001.  Fluctuations of the paleomagnetic field during chron C5 as recorded in near-bottom marine magnetic anomaly data. Journal of Geophysical Research-Solid Earth. 106:26379-26396.   10.1029/2001jb000278   AbstractWebsite

Near-bottom magnetic data contain information on paleomagnetic field fluctuations during chron C5 as observed in both the North and South Pacific. The North Pacific data include 12 survey lines collected with a spatial separation of up to 120 kin, and the South Pacific data consist of a single long line collected on the west flank of the East Pacific Rise (EPR) at 19 degreesS. The North Pacific magnetic profiles reveal a pattern of linear, short-wavelength (2 to 5 km) anomalies (tiny wiggles) that are highly correlated over the shortest (3.8 km) to longest (120 km) separations in the survey. Magnetic inversions incorporating basement topography show that these anomalies are not caused by the small topographic relief. The character of the near-bottom magnetic profile from anomaly 5 on the west flank of the EPR, formed at a spreading rate more than twice that of the North Pacific, displays a remark-able similarity to the individual and stacked lines from the North Pacific survey area, Over distances corresponding to 1 m.y., 19 lows in the magnetic anomaly profile can be correlated between the North and South Pacific lines. Modeling the lows as due to short polarity events suggests that they may be caused by rapid swings of the magnetic field between normal and reversed polarities with little or no time in the reversed state. Owing to the implausibly high number of reversals required to account for these anomalies and the lack of any time in the reversed state, we conclude that the near-bottom signal is primarily a record of pateointensity fluctuations during chron C5. Spectral analysis of the North Pacific near bottom lines shows that the signal is equivalent to a paleointensity curve with a temporal resolution of 40 to 60 kyr, while measurements of the smallest separations of correlatable dips in the field suggest a temporal resolution of 36 kyr.

Varga, RJ, Gee JS, Staudigel H, Tauxe L.  1998.  Dike surface lineations as magma flow indicators within the sheeted dike complex of the Troodos Ophiolite, Cyprus. Journal of Geophysical Research-Solid Earth. 103:5241-5256.   10.1029/97jb02717   AbstractWebsite

Mesoscopic flow lineations and anisotropy of magnetic susceptibility (AMS) have been measured for dikes within the Cretaceous-age Troodos ophiolite with the goal of comparing the direction of initial magma now through dike conduits immediately following crack propagation with that of flow of subsequent magma emplaced during later stages of dike growth. Dike margin indicators of flow include cusp axes and elongate vesicles found high in the ophiolite peudostratigraphy and ridge-and-groove structures termed hot slickenlines found throughout the complex. A unique now direction is determined where elongate vesicles near dike margins display imbrication with respect to the margin. Significant changes in vesicle elongation directions across dikes likely indicate either changes in magma flow direction after dike propagation or backflow of magma during the waning stages of intrusion. Surface lineations generally lie subparallel to the direction of flow inferred from AMS determinations on cores within 5 cm of dike margins. Surface lineations also lie subparallel to the long axis (epsilon(1)) of the orientation ellipsoid defined by long axes of groundmass plagioclase phenocrysts measured in sections from AMS cores. Correlation of surface lineations with interior indicators of flow (AMS, plagioclase trachytic texture) indicate that the surface features are good proxies for grain-scale magma flow directions during dike propagation in Troodos dikes. Orientations of surface flow features in the dikes of the Troodos ophiolite indicate an approximately equal mix of subhorizontal to near-vertical magma flow, contradicting the paradigm of primarily vertical flow of magma beneath continuous axial magma chambers at oceanic spreading centers. Our data are consistent with a model of magma emplacement both vertically and horizontally away from isolated magma chambers beneath axial volcanoes spaced along a ridge crest.

Gee, J, Kent DV.  1998.  Magnetic telechemistry and magmatic segmentation on the southern east Pacific rise. Earth and Planetary Science Letters. 164:379-385.   10.1016/s0012-821x(98)00231-3   AbstractWebsite

Results from axial dredges and a profile inversion of magnetic anomaly data along the axis of the East Pacific Rise (EPR) at 13-23 degrees S provide an estimate of the average degree of fractionation for the extrusive layer at this ultrafast-spreading (similar to 145 mm/yr full rate) ridge. We find a high correlation (R = 0.81) between dredge mean FeO* (total iron as FeO) and natural remanence for 34 axial dredges with multiple samples having coincident geochemical and magnetic data. We attribute this good correlation to detailed sampling spanning the full range of cooling-related magnetization changes within a flow and to the young age (0-6 ka) of these axial samples, which effectively minimizes time-dependent magnetization changes due to geomagnetic intensity or alteration. A composite axial magnetic anomaly profile shows large amplitude (up to 400 nT) fluctuations with wavelengths of 50-200 km, which theoretical considerations suggest can reliably be related to the magnetization directly beneath the ship. For much of the southern EPR, seismic data provide independent limits on the axial thickness (259 +/- 55 m) and the pattern of off-axis thickening of the extrusive magnetic source layer. These data also provide evidence for an axial magma lens that effectively eliminates anomaly contributions from deeper magnetic sources. Inversion of the axial magnetic anomaly data utilizing these geophysical constraints yields a magnetization solution which, through use of the regression relating FeO* and natural remanence, may be related to the average degree of differentiation of the extrusive source layer. The magnetic data reveal a pattern of magmatic segmentation that closely parallels the tectonic segmentation of the ridge, suggesting that magma supply may be an important control on the average degree of differentiation of the extrusive layer. (C) 1998 Elsevier Science B.V. All rights reserved.

Juarez, MT, Tauxe L, Gee JS, Pick T.  1998.  The intensity of the Earth's magnetic field over the past 160 million years. Nature. 394:878-881.   10.1038/29746   AbstractWebsite

In contrast to our detailed knowledge of the directional behaviour of the Earth's magnetic field during geological and historical times(1,2), data constraining the past intensity of the field remain relatively scarce. This is mainly due to the difficulty in obtaining reliable palaeointensity measurements, a problem that is intrinsic to the geological materials which record the Earth's magnetic field. Although the palaeointensity database has grown modestly over recent years(3-5), these data are restricted to a few geographical locations and more than one-third of the data record the field over only the past 5 Myr-the most recent database(5) covering the time interval from 5 to 160 Myr contains only about 100 palaeointensity measurements. Here we present 21 new data points from the interval 5-160 Myr obtained from submarine basalt glasses collected from locations throughout the world's oceans. Whereas previous estimates for the average dipole moment were comparable to that of the Earth's present field(6), the new data suggest an average dipole moment of (4.2 +/- 2.3) x 10(22) A m(2), or approximately half the present magnetic-field intensity. This lower average value should provide an important constraint for future efforts to model the convective processes in the Earth's core which have been responsible for generating the magnetic field.

Gee, J, Kent DV.  1997.  Magnetization of axial lavas from the southern East Pacific Rise (14 degrees-23 degrees S): Geochemical controls on magnetic properties. Journal of Geophysical Research-Solid Earth. 102:24873-24886.   10.1029/97jb02544   AbstractWebsite

Although the spatial association of iron-rich lavas and high-amplitude magnetic anomalies is well documented, a causal link between enhanced iron content and high remanent magnetization has been difficult to establish. Here we report magnetic data from approximately 250 samples, with 8-16% FeO* (total iron as FeO), from the southern East Pacific Rise (EPR) that provide strong support for the presumed geochemical dependence of remanent intensity. The limited age range (0-6 ka) of axial lavas from this ultrafast spreading ridge (similar to 150 mm/yr full rate) effectively minimizes variations resulting from time dependent chan or low-temperature alteration. Systematic sampling relative to the chilled margin illustrates that substantial grain size-related variations in magnetic properties occur on a centimeter scale. Both microprobe data and Curie temperatures suggest that the average groundmass titanomagnetite composition in the southern EPR samples is approximately constant (modal modified ulvospinel content = 0.67) over a wide range of lava compositions. Saturation magnetization and saturation remanence are highly correlated with FeO* (R = 0.73 and 0.83, respectively), indicating that more iron-rich lavas have higher abundances of otherwise similar titanomagnetite. We show that there is a good correlation between natural remanent magnetization (NRM) and FeO*, provided that sufficient specimens are used to determine the average NRM of a sample (R = 0.63). Because the range of iron contents in mid-ocean ridge basalts is limited, the best fit slope (4.44 A/m per %FeO* in an ambient field of 0.030 mT) should provide reasonable bounds on the equatorial magnetization of submarine lavas (similar to 10 A/m at 8.5% FeO* and similar to 50 A/m at similar to 16% FeO*). Finally, we demonstrate that along-axis variations in NRM closely parallel geochemical changes along the southern EPR. Where magnetization values deviate significantly from those predicted from the range of measured FeO* contents, these discrepancies may reflect additional unrecognized geochemical variability.

Gee, J, Kent DV.  1994.  Variations in Layer 2A Thickness and the Origin of the Central Anomaly Magnetic High. Geophysical Research Letters. 21:297-300.   10.1029/93gl03422   AbstractWebsite

The seismically determined off-axis thickening of the extrusive layer is apparently at odds with the magnetic anomaly high typically associated with the ridge crest. The positive magnetization contrast at the ridge crest is most likely caused by rapid alteration of the extrusive source layer which occurs over spatial scales (2-3 km) comparable to that of the proposed Layer 2A thickening. We present magnetic remanence data from basalts dredged on and near the East Pacific Rise axis at 12-degrees-N which are compatible with a rapid magnetization reduction (approximately 20 k.y. to decay to 1/e). Together with near bottom magnetic profiles from the ultra-fast-spreading East Pacific Rise at 19.5-degrees-S, these data suggest that previous estimates of the time constant of alteration inferred from slow-spreading ridges (0.5 m.y.) may be more than an order of magnitude too high.

Gee, J, Staudigel H, Tauxe L, Pick T, Gallet Y.  1993.  Magnetization of the La Palma Seamount Series - Implications For Seamount Paleoples. Journal of Geophysical Research-Solid Earth. 98:11743-11767.   10.1029/93jb00932   AbstractWebsite

Paleopoles determined from seamount magnetic anomalies constitute the major data source for the Pacific apparent polar wander path, but relatively little is known about the processes of remanence acquisition in seamounts. Since magnetic anomalies reflect both natural remanence (NRM) and the induced field, it is important first to assess whether the NRM is likely to represent an original field direction and second to constrain the magnitude of the induced component. To this end, we present paleomagnetic data from an uplifted, subaerially exposed section through a seamount on La Palma, Canary Islands. The Pliocene Seamount Series of La Palma comprises a >6 km sequence of alkalic extrusives and intrusives which includes all lithologies likely to be volumetrically important in seamounts. The structural tilt of the Seamount Series allows separation of early thermal or chemical remanence from magnetization components acquired after tilting (e.g., viscous remanence). The NRM provides a poor indication of the original magnetization direction, although the characteristic magnetization of many La Palma samples is compatible with the original pretilt direction. Hydrothermal alteration has resulted in the production of Ti-poor magnetite and an increasing contribution of hematite with increasing degree of alteration. More importantly, well-defined magnetization directions which deviate from any reasonable geomagnetic direction at La Palma can be attributed to hydrothermal alteration in a different polarity than prevalent during the original magnetization. Based on a comparison of the magnitude of low-stability components of magnetization and laboratory acquisition of viscous remanence and previous estimates of the induced magnetization, we conclude that viscous and induced magnetization probably account for 15-25% of the total magnetization of seamounts. The resulting paleopole bias is a function of the polarity and paleolatitude of the seamount and ranges from 4-degrees to 16-degrees for Cretaceous seamounts in the Pacific.