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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.

Vanderkluysen, L, Mahoney JJ, Koppers AAP, Beier C, Regelous M, Gee JS, Lonsdale PF.  2014.  Louisville Seamount Chain: Petrogenetic processes and geochemical evolution of the mantle source. Geochemistry Geophysics Geosystems. 15:2380-2400.   10.1002/2014gc005288   AbstractWebsite

The Louisville Seamount Chain is a similar to 4300 km long chain of submarine volcanoes in the southwestern Pacific that spans an age range comparable to that of the Hawaiian-Emperor chain and is commonly thought to represent a hot spot track. Dredging in 2006 recovered igneous rocks from 33 stations on 22 seamounts covering some 49 Myr of the chain's history. All samples are alkalic, similar to previous dredge and drill samples, providing no evidence for a Hawaiian-type tholeiitic shield-volcano stage. Major and trace element variations appear to be predominantly controlled by small but variable extents of fractional crystallization and by partial melting. Isotopic values define only a narrow range, in agreement with a surprising long-term source homogeneity-relative to the length scale of melting-and overlap with proposed fields for the "C" and "FOZO" mantle end-members. Trace element and isotope geochemistry is uncorrelated with either seamount age or lithospheric thickness at the time of volcanism, except for a small number of lavas from the westernmost Louisville Seamounts built on young (<20 Ma old) oceanic crust. The Louisville hot spot has been postulated to be the source of the similar to 120 Ma Ontong Java Plateau, but the Louisville isotopic signature cannot have evolved from a source with isotopic ratios like those measured for Ontong Java Plateau basalts. On the other hand, this signature can be correlated with that of samples dredged from the Danger Islands Troughs of the Manihiki Plateau, which has been interpreted as a rifted fragment of the "Greater" Ontong Java Plateau.

Bowles, JA, Gee JS, Burgess K, Cooper RF.  2011.  Timing of magnetite formation in basaltic glass: Insights from synthetic analogs and relevance for geomagnetic paleointensity analyses. Geochemistry Geophysics Geosystems. 12   10.1029/2010gc003404   AbstractWebsite

Absolute paleointensity estimates from submarine basaltic glass (SBG) typically are of high technical quality and accurately reflect the ambient field when known. SBG contains fine-grained, low-Ti magnetite, in contrast to the high-Ti magnetite in crystalline basalt, which has lead to uncertainty over the origin of the magnetite and its remanence in SBG. Because a thermal remanence is required for accurate paleointensity estimates, the timing and temperature of magnetite formation is crucial. To assess these factors, we generated a suite of synthetic glasses with variable oxygen fugacity, cooling rate, and FeO* content. Magnetic properties varied most strongly with crystallinity; less crystalline specimens are similar to natural SBG and have weaker magnetization, a greater superparamagnetic contribution, and higher unblocking temperatures than more crystalline specimens. Thellier-type paleointensity results recovered the correct field within 1 sigma error with 2 (out of 10) exceptions that likely result from an undetected change in the laboratory field. Unblocking and ordering temperature data demonstrate that low-Ti magnetite is a primary phase, formed when the glass initially quenched. Although prolonged heating at high temperatures (during paleointensity experiments) may result in minor alteration at temperatures <580 degrees C, this does not appear to impact the accuracy of the paleointensity estimate. Young SBG is therefore a suitable material for paleointensity studies.

Mitra, R, Tauxe L, Gee JS.  2011.  Detecting uniaxial single domain grains with a modified IRM technique. Geophysical Journal International. 187:1250-1258.   10.1111/j.1365-246X.2011.05224.x   AbstractWebsite

Mid-ocean ridge basalt (MORB) specimens have often been found to have high ratios of saturation remanence to saturation magnetization (M(rs)/M(s)). This has been attributed either to dominant cubic anisotropy or to insufficient saturating field leading to overestimation of M(rs)/M(s) of a dominantly uniaxial single domain (USD) assemblage. To resolve this debate, we develop an independent technique to detect USD assemblages. The experimental protocol involves subjecting the specimen to bidirectional impulse fields at each step. The experiment is similar to the conventional isothermal remanent magnetization (IRM) acquisition experiment but the field is applied twice, in antiparallel directions. We define a new parameter, IRAT, as the ratio of the remanences at each field step and show it to have characteristic behaviour for the two assemblages; IRAT similar to 1 at all field steps for USD and <1 with a strong field dependence for multi-axial single domain (MSD) grains. We verified the theoretical predictions experimentally with representative USD and MSD specimens. Experiments with MORBs gave low IRATs for specimens having high M(rs)/M(s). This argues for a dominant MSD assemblage in the MORBs, possibly cubic in nature. Although undersaturation of the samples can indeed be a contributing factor to the exceptionally high M(rs)/M(s), this study shows that the nature of the assemblage cannot be dominantly USD.

Horst, AJ, Varga RJ, Gee JS, Karson JA.  2011.  Paleomagnetic constraints on deformation of superfast-spread oceanic crust exposed at Pito Deep Rift. Journal of Geophysical Research-Solid Earth. 116   10.1029/2011jb008268   AbstractWebsite

The uppermost oceanic crust produced at the superfast spreading (similar to 142 km Ma(-1), full-spreading rate) southern East Pacific Rise (EPR) during the Gauss Chron is exposed in a tectonic window along the northeastern wall of the Pito Deep Rift. Paleomagnetic analysis of fully oriented dike (62) and gabbro (5) samples from two adjacent study areas yield bootstrapped mean remanence directions of 38.9 degrees +/- 8.1 degrees, -16.7 degrees +/- 15.6 degrees, n = 23 (Area A) and 30.4 degrees +/- 8.0 degrees, -25.1 degrees +/- 12.9 degrees, n = 44 (Area B), both are significantly distinct from the Geocentric Axial Dipole expected direction at 23 degrees S. Regional tectonics and outcrop-scale structural data combined with bootstrapped remanence directions constrain models that involve a sequence of three rotations that result in dikes restored to subvertical orientations related to (1) inward-tilting of crustal blocks during spreading (Area A = 11 degrees, Area B = 22 degrees), (2) clockwise, vertical-axis rotation of the Easter Microplate (A = 46 degrees, B = 44 degrees), and (3) block tilting at Pito Deep Rift (A = 21 degrees, B = 10 degrees). These data support a structural model for accretion at the southern EPR in which outcrop-scale faulting and block rotation accommodates spreading-related subaxial subsidence that is generally less than that observed in crust generated at a fast spreading rate exposed at Hess Deep Rift. These data also support previous estimates for the clockwise rotation of crust adjacent to the Easter Microplate. Dike sample natural remanent magnetization (NRM) has an arithmetic mean of 5.96 A/m +/- 3.76, which suggests that they significantly contribute to observed magnetic anomalies from fast- to superfast-spread crust.

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.

Bowles, J, Gee JS, Kent DV, Bergmanis E, Sinton J.  2005.  Cooling rate effects on paleointensity estimates in submarine basaltic glass and implications for dating young flows. Geochemistry Geophysics Geosystems. 6   10.1029/2004gc000900   AbstractWebsite

Cooling rate effects on the intensity of thermoremanent magnetization (TRM) have been well documented in ceramics. In that case, laboratory cooling is generally more rapid than the initial cooling, leading to an overestimate of the paleofield by 5-10% in Thellier-type paleointensity experiments. The reverse scenario, however, has never been tested. We examine the effects of cooling rate on paleointensity estimates from rapidly quenched submarine basaltic glass (SBG) samples from 13 sites at 17 degrees 30'-18 degrees 30'S on the East Pacific Rise. Absolute cooling rates determined by relaxation geospeedometry at five of these sites range from similar to 10 to similar to 330 degrees C min(-1) at the glass transition (similar to 650 degrees C). Over the dominant range of remanence blocking temperatures (similar to 200-400 degrees C), the natural cooling rates are approximately equal to or slightly slower than the laboratory cooling rates during the Thellier experiment. These results suggest that while the cooling rate effect might introduce some within-site scatter, it should not result in a systematic bias in paleointensity from SBG. Paleointensity estimates from the 15 sites range from similar to 29 to 59 mu T, with an average standard error of similar to 1 mu T. Comparison with models of geomagnetic field intensity variations at the site indicate the youngest group of samples is very recent (indistinguishable from present-day) and the oldest is at least 500, and probably several thousand, years old. These age estimates are consistent with available radiometric ages and geologic observations.

Varga, RJ, Karson JA, Gee JS.  2004.  Paleomagnetic constraints on deformation models for uppermost oceanic crust exposed at the Hess Deep Rift: Implications for axial processes at the East Pacific Rise. Journal of Geophysical Research-Solid Earth. 109   10.1029/2003jb002486   AbstractWebsite

Studies of oceanic crust exposed in tectonic windows and in ophiolites have revealed the importance of normal faulting and attendant tilting of upper crustal rock units in the accretion process at oceanic spreading centers. We present paleomagnetic remanence data from 45 fully oriented samples from dikes, gabbros and a small number of basaltic lavas from fast spread crust exposed along the Hess Deep Rift. Over similar to25 km along this escarpment, dikes and dike-subparallel fault zones dip consistently away from the East Pacific Rise (EPR) while lava flows dip toward the ridge. Underlying gabbro is less deformed but contains widely spaced, low-angle fractures, tentatively interpreted as shear zones. As expected from the crustal age (similar to1.07-1.48 Ma), most remanence data indicate reversed polarity magnetization and are compatible with the expected range of secular variation at the site. Overly steep and directionally scattered gabbro remanence and observed low-angle shear structures within this unit are tentatively interpreted as the manifestation of three-dimensional strain along anastomosing shear zones. Although some remanence directions are incompatible with any plausible deformation history, and thus likely reflect orientation errors, the overall data set is consistent with a model involving sequential rotations on (1) outward dipping, EPR-parallel (similar toN-S) normal faults and (2) Hess Deep Rift-parallel (similar toE-W) normal faults Average rotations for these sequential events are 22degrees to the east (defined by the mean dike attitude) and 10degrees to the south (estimated by bathymetry), respectively. This model best explains the remanence data, observed dikes and lava orientations, presence of dike-parallel fault zones, and the observation of steep, little deformed dikes cutting both east dipping dikes and faults. The data support a structural model for spreading at the EPR in which outcrop-scale faulting and rotation is linked to subaxial subsidence and to consequent development of dominantly outward facing normal faults close to the spreading axis. Because these faults form within the neovolcanic zone, they are subject to burial and are expected to have subdued to little surface expression.

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, JS, Cande SC, Hildebrand JA, Donnelly K, Parker RL.  2000.  Geomagnetic intensity variations over the past 780 kyr obtained from near-seafloor magnetic anomalies. Nature. 408:827-832.   10.1038/35048513   AbstractWebsite

Knowledge of past variations in the intensity of the Earth's magnetic field provides an important constraint on models of the geodynamo. A record of absolute palaeointensity for the past 50 kyr has been compiled from archaeomagnetic and volcanic materials, and relative palaeointensities over the past 800 kyr have been obtained from sedimentary sequences. But a long-term record of geomagnetic intensity should also be carried by the thermoremanence of the oceanic crust. Here we show that near-seafloor magnetic anomalies recorded over the southern East Pacific Rise are well correlated with independent estimates of geomagnetic intensity during the past 780 kyr. Moreover, the pattern of absolute palaeointensity of seafloor glass samples from the same area agrees with the well-documented dipole intensity pattern for the past 50 kyr. A comparison of palaeointensities derived from seafloor glass samples with global intensity variations thus allows us to estimate the ages of surficial lava flows in this region. The record of geomagnetic intensity preserved in the oceanic crust should provide a higher-time-resolution record of crustal accretion processes at mid-ocean ridges than has previously been obtainable.

Gee, J, Kent DV.  1999.  Calibration of magnetic granulometric trends in oceanic basalts. Earth and Planetary Science Letters. 170:377-390.   10.1016/s0012-821x(99)00125-9   AbstractWebsite

The validity of magnetic granulometric estimates relies heavily on the ability to distinguish ultrafine particles from coarser grains. For example, populations with dominantly superparamagnetic (SP) or multidomain (MD) grains both are characterized by low remanence and coercivity, and distinguishing these endmembers may provide valuable clues to the origin of magnetization in the intervening stable single domain (SD) size range. The natural grain size variations associated with variable cooling rates in submarine lavas provide a rare opportunity for examining progressive changes in average magnetic grain size, from SP-SD mixtures in submarine basaltic glass to SD-MD mixtures in flow interiors. Based on microanalysis and rock magnetic measurements on pillow basalt samples dredged from the flanks of the Mid-Atlantic Ridge (ages <1 Ma to 70 Ma), a model of preferential dissolution with time of the finest-grained titanomagnetites has recently been suggested as the major process contributing to long-term temporal changes in remanent intensity of mid-ocean ridge basalts. We evaluated the local and long-term temporal trends in effective magnetic grain size predicted by this model using hysteresis data from a large number of submarine basalt samples which span a range of apes from similar to 0 to similar to 122 Ma. Specimens were systematically taken along transects perpendicular to the chilled margin of each sample. The large number of data (similar to 750 loops) and the inferred progressive change in grain size approaching the chilled margin allow recognition of mixing trends between MD and SD grains and between SD and SP grains on a Day-plot. These trends in hysteresis parameters are crucial to resolving the inherent, but frequently overlooked, ambiguity in inferring grain size from hysteresis parameters. We illustrate that two additional rock magnetic tests (warming of a low-temperature isothermal remanence and hysteresis loop shapes) often used to address these ambiguities are inconclusive, requiring some independent knowledge of whether SP or MD grains are likely to be present. Even with a considerably larger data set the substantial intrasample variability in oceanic basalts precludes recognition of any systematic trend in magnetic grain size with age. (C) 1999 Elsevier Science B.V. All rights reserved.

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.  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.