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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.

Johnson, PH, Kent DV, Tivey MA, Gee JS, Largon RL, Embley RW.  1997.  Conference on the magnetization of the oceanic crust steers future research. Eos Trans. AGU. 78:199-202.: AGU   10.1029/97eo00133   AbstractWebsite

Because marine magnetic anomalies arise from the combination of seafloor spreading and geomagnetic polarity reversals, they delineate a history of global plate motions and geomagnetic field behavior. Thirty years ago, interpretation of sea surface magnetometer profiles led to the plate tectonics revolution. Recent developments in high resolution magnetic studies are similarly changing our view of the structure and evolution of oceanic crust and beginning to answer basic questions concerning geomagnetic field behavior.In response to these developments, the Conference on the Magnetization of Oceanic Crust was held September 21–24,1996, on Orcas Island in Washington State. Forty-seven scientists representing 20 institutions in seven countries attended the conference, which was funded by the National Science Foundation, the Ridge Interdisciplinary Global Experiment (RIDGE), and the United States Science Advisory Committee (USSAC).

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.

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Kelley, DS, Karson JA, Blackman DK, Fruh-Green GL, Butterfield DA, Lilley MD, Olson EJ, Schrenk MO, Roe KK, Lebon GT, Rivizzigno P, * SPAT, Gee JS *.  2001.  An off-axis hydrothermal vent field near the Mid-Atlantic Ridge at 30 degrees N. Nature. 412:145-149.   10.1038/35084000   AbstractWebsite

Evidence is growing that hydrothermal venting occurs not only along mid-ocean ridges but also on old regions of the oceanic crust away from spreading centres. Here we report the discovery of an extensive hydrothermal field at 30 degrees N near the eastern intersection of the Mid-Atlantic Ridge and the Atlantis fracture zone. The vent field-named 'Lost City'-is distinctly different from all other known sea-floor hydrothermal fields in that it is located on 1.5-Myr-old crust, nearly 15 km from the spreading axis, and may be driven by the heat of exothermic serpentinization reactions between sea water and mantle rocks. It is located on a dome-like massif and is dominated by steep-sided carbonate chimneys, rather than the sulphide structures typical of 'black smoker' hydrothermal fields. We found that vent fluids are relatively cool (40-75 degrees C) and alkaline (pH 9.0-9.8), supporting dense microbial communities that include anaerobic thermophiles. Because the geological characteristics of the Atlantis massif are similar to numerous areas of old crust along the Mid-Atlantic, Indian and Arctic ridges, these results indicate that a much larger portion of the oceanic crust may support hydrothermal activity and microbial life than previously thought.

Kent, DV, Kjarsgaard BA, Gee JS, Muttoni G, Heaman LM.  2015.  Tracking the Late Jurassic apparent (or true) polar shift in U-Pb-dated kimberlites from cratonic North America (Superior Province of Canada). Geochemistry Geophysics Geosystems. 16:983-994.   10.1002/2015gc005734   AbstractWebsite

Different versions of a composite apparent polar wander (APW) path of variably selected global poles assembled and averaged in North American coordinates using plate reconstructions show either a smooth progression or a large (approximate to 30 degrees) gap in mean paleopoles in the Late Jurassic, between about 160 and 145 Ma. In an effort to further examine this issue, we sampled accessible outcrops/subcrops of kimberlites associated with high-precision U-Pb perovskite ages in the Timiskaming area of Ontario, Canada. The 154.91.1 Ma Peddie kimberlite yields a stable normal polarity magnetization that is coaxial within less than 5 degrees of the reverse polarity magnetization of the 157.51.2 Ma Triple B kimberlite. The combined approximate to 156 Ma Triple B and Peddie pole (75.5 degrees N, 189.5 degrees E, A95=2.8 degrees) lies about midway between igneous poles from North America nearest in age (169 Ma Moat volcanics and the 146 Ma Ithaca kimberlites), showing that the polar motion was at a relatively steady yet rapid (approximate to 1.5 degrees/Myr) pace. A similar large rapid polar swing has been recognized in the Middle to Late Jurassic APW path for Adria-Africa and Iran-Eurasia, suggesting a major mass redistribution. One possibility is that slab breakoff and subduction reversal along the western margin of the Americas triggered an episode of true polar wander.

Kent, DV, Gee J.  1996.  Magnetic alteration of zero-age oceanic basalt. Geology. 24:703-706.   10.1130/0091-7613(1996)024<0703:maozao>2.3.co;2   AbstractWebsite

The youngest sampled submarine lava flow, which erupted June 1993 on the Juan de Fuca Ridge, provides the basis for a tight constraint on the initial or zero-age magnetization state of MORE, Detailed profiles of magnetic hysteresis parameters, Curie temperatures, and unblocking temperatures of NRM with respect to the chilled margin of a pillow fragment show evidence of significant oxidation, which preferentially affected the finest grain-size fraction and principal remanence carrier of the titanomagnetite magnetic mineralogy. The oxidation must have occurred during or immediately after initial cooling, implying that MORE is already appreciably magnetically altered before aging. Nevertheless, successful results of Thellier paleointensity experiments on the basalt sample lend support to the idea that crustal magnetization represented by MORE preserves a record of geomagnetic intensity variations that may be reflected in small-scale magnetic anomalies.

Kent, DV, Gee J.  1994.  Grain Size-Dependent Alteration and the Magnetization of Oceanic Basalts. Science. 265:1561-1563.   10.1126/science.265.5178.1561   AbstractWebsite

Unblocking temperatures of natural remanent magnetization were found to extend well above the dominant Curie points in samples of oceanic basalts from the axis of the East Pacific Rise. This phenomenon is attributed to the natural presence in the basalts of three related magnetic phases: an abundant fine-grained and preferentially oxidized titanomagnetite that carries most of the natural remanent magnetism, a few coarser and less oxidized grains of titanomagnetite that account for most of the high-field magnetic properties, and a small contribution to both the natural remanent magnetism and high-field magnetic properties from magnetite that may be due to the disproportionation of the oxidized titanomagnetite under sea-floor conditions. This model is consistent with evidence from the Central Anomaly magnetic high that the original magnetization acquired by oceanic basalts upon cooling is rapidly altered and accounts for the lack of sensitivity of bulk rock magnetic parameters to the degree of alteration of the remanence carrier in oceanic basalts.

Klootwijk, CT, Gee JS, Peirce JW, Smith GM, McFadden PL.  1992.  An Early India-Asia Contact - Paleomagnetic Constraints from Ninetyeast Ridge, ODP Leg 121. Geology. 20:395-398.   10.1130/0091-7613(1992)020<0395:aeiacp>2.3.co;2   AbstractWebsite

New paleomagnetic results from sedimentary rock and basement of the Ninetyeast Ridge (Ocean Drilling Program Leg 121, Sites 756-758) detail the northward movement of the Indian plate for the past 80 m.y. Analysis of the combined paleolatitude-age profile indicates a distinct reduction in India's northward movement rate at 55+ Ma, interpreted as completion of suturing of Greater India and Asia. India's northward motion slowed from 18-19.5 cm/yr to 4.5 cm/yr for the location of Site 758. Comparison of this profile with paleomagnetic data from the wider Himalayan region indicates that initial contact between northwestern Greater India and southern Asia was already established by Cretaceous-Tertiary time. This supports a possible causal link between the India-Asia convergence and the Deccan Traps extrusion.

Klootwijk, CT, Gee JS, Peirce JW, Smith GM.  1992.  Neogene Evolution of the Himalayan Tibetan Region - Constraints from ODP Site-758, Northern Ninetyeast Ridge - Bearing on Climatic-Change. Palaeogeography Palaeoclimatology Palaeoecology. 95:95-110.   10.1016/0031-0182(92)90167-4   AbstractWebsite

Magnetic susceptibility, remanence and lithostratigraphic profiles for the Neogene-Quaternary sequence at Site 758 (ODP Leg 121) on the northern Ninetyeast Ridge show distinct changes, dated from biostratigraphy and detailed magnetostratigraphy, at 17.5, 10.4-10.0, 8.8, 6.5, 5.4-5.1, 2.7-2.5, 1.9, and 1.2-1.1 Ma. These magnetic and lithologic changes appear to reflect changes in the supply and character of terrigenous material from the Himalayan-Tibetan region resulting from changes in gradient of the Ganges, Brahmaputra and probably the ancient Indus drainage systems. The sedimentary changes can be correlated with changes in uplift-sensitive markers such as the oceanic Sr-87/Sr-86 ratio and monsoonal induced upwelling, but not clearly so with sealevel variations. We interpret these sedimentary changes, therefore, to primarily reflect changes in the tectonic evolution of the Himalayan-Tibetan region. The changes in the distal marine sedimentary record of the northern Ninetyeast Ridge are compared with isotopic control on the timing of Himalayan-Tibetan tectonic phases and magnetostratigraphic control on their reflection in the proximal Siwalik molasse record. This comparison indicates that the distal Ninetyeast Ridge record can be used to detail and to place minimal age constraints on tectonic phases in the wider Himalayan region and on evolution of the proximal molasse sequence, with a time lag determined for the four earliest changes at less than 1 m.y. The changes at 17.5 Ma and 5.4-5.1 Ma can be interpreted in terms of the causative chain: enhanced plate motion double-line arrow pointing right uplift and sedimentation change double-line arrow pointing right climatic change, supporting arguments that the Late Cainozoic global climatic deterioration is driven by uplift of large plateaus such as the Himalayan-Tibetan region and the Western Cordillera.

Klootwijk, CT, Gee JS, Peirce JW, Smith GM.  1992.  The origin of Ninetyeast Ridge: palaeomagnetic constraints from ODP Leg 121. Journal of Southeast Asian Earth Sciences. 7:247-252.   10.1016/0743-9547(92)90004-u   AbstractWebsite

Palaeomagnetic results from a lower Turonian to Middle Eocene ash-rich sedimentary sequence on Broken Ridge (ODP Sites 752–755) and ash and basalt sequences on Ninetyeast Ridge (90ER) (Sites 756–758) (1) support a Kerguelen hotspot origin for the 90ER volcanic pile; (2) are consistent with models for a large-scale southward jump of the Southeast Indian Ridge (SEIR) west of the 90ER at about 58 Ma; and (3) suggest a possible Amsterdam-St Paul hotspot origin for magnetic overprints on the 90ER.

Klootwijk, CT, Gee J, Smith GM, Pierce JW.  1991.  Constraints on the India-Asia convergence; paleomagnetic results from Ninetyeast Ridge. Proceedings of the Ocean Drilling Program, Scientific Results. 121:777-884.   10.2973/odp.proc.sr.121.121.1991   Abstract

This study details the Late Cretaceous and Tertiary northward movement of the Indian plate. Breaks in India's northward movement rate are identified, dated, and correlated with the evolution of the India-Asia convergence. Paleolatitudinal constraints on the origin of Ninetyeast Ridge are discussed, and limited magnetostratigraphic detail is provided.Nearly 1500 sediment and basement samples from Sites 756, 757, and 758 on Ninetyeast Ridge were studied through detailed alternating field and thermal demagnetization. Primary and various secondary magnetization components were identified. Breakpoint intervalsintheprimarypaleolatitudepatternforcommon-Site758wereidentified at2.7,6.7,18.5,about53,63.5-67,and68-74.5 Ma. Only the breakpoint interval at about 53 Ma reliably reflects a reduction in India's northward movement rate. The onset of this probably gradual slowdown was dated at 55 Ma (minimal age) based on the intersection of weighted linear regression lines. At the locationofcommon-Site758,northwardmovementslowedfrom 18-19.5cm/yr(fromatleast65to55Ma)to4.5cm/yr(from55 to at least 20 Ma). Reanalysis of earlier DSDP/ODP paleolatitude data from the Indian plate gives a comparable date (53 Ma) for this reduction in northward velocity.Comparison of our Ninetyeast Ridge data and Himalayan paleomagnetic data indicates that the initial contact of Greater India and Asia mayhave already been established by Cretaceous/Tertiary boundary time. The geological record of the convergence zone and the Indian plate supports the notion that the Deccan Traps extrusion may have resulted from the ensuing deformation of the Indian plate. W e interpret the breakpoint at 55+ Ma to reflect completion of the eastward progressive India-Asia suturing process.Neogene phases in the evolution of the convergence zone were correlated with significant changes in the susceptibility, NRM intensity, and lithostratigraphic profile of Site 758.These changes are interpreted to reflect and postdate tectonic phases in the evolutionofthewiderHimalayanandsouthernTibetanregion.Thechangesweredatedandinterpretedasfollows: 17.5Ma,initial uplift of the Higher Himalaya following initiation of intercontinental underthrusting; 10-10.4 Ma, increased uplift and onset of Middle Siwaliks sedimentation; 8.8 Ma, probable reduction in influx corresponding with the Nagri Formation to Dhok Pathan Formation changeover; 6.5 Ma, major tectonic phase evident throughout the wider Himalayan region and northern Indian Ocean; 5.1-5.4 Ma, onset of oroclinal bending of the Himalayan Arc,of extensional tectonism in southern Tibet, and of Upper Siwalik sedimentation; 2.5-2.7 and 1.9 Ma, major phases of uplift of the Himalayan and Tibetan region culminating in the present-day high relief.The basal ash sequence and upper flow sequence of Site 758 and the basal ash sequence of Site 757 indicate paleolatitudes at about 50°S. These support a Kerguelen hot spot origin for Ninetyeast Ridge. Consistently aberrant inclinations in the basalt sequence ofSite757mayberelatedtoasouthwardridgejumpataboutthetime(58Ma)thatthesebasaltswereerupted.Thebasalt sequence of Site 756 indicates a lower paleolatitude (about 43°S), as do parts of the basalt sequence of Site 758 which also have reversed polarity overprints. The low paleolatitudes for Site 756 may be explained by late-stage volcanism north of the Kerguelen hot spot or the influence of the Amsterdam-St. Paul hot spot.

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.

Koppers, AAP, Gowen MD, Colwell LE, Gee JS, Lonsdale PF, Mahoney JJ, Duncan RA.  2011.  New Ar-40/Ar-39 age progression for the Louisville hot spot trail and implications for inter-hot spot motion. Geochemistry Geophysics Geosystems. 12   10.1029/2011gc003804   AbstractWebsite

In this study we present 42 new Ar-40/Ar-39 incremental heating age determinations that contribute to an updated age progression for the Louisville seamount trail. Louisville is the South Pacific counterpart to the Hawaiian-Emperor seamount trail, both trails representing intraplate volcanism over the same time interval (similar to 80 Ma to present) and being examples of primary hot spot lineaments. Our data provide evidence for an age-progressive trend from 71 to 21 Ma. Assuming fixed hot spots, this makes possible a direct comparison to the Hawaiian-Emperor age progression and the most recent absolute plate motion (APM) model (WK08G) of Wessel and Kroenke (2008). We observe that for the Louisville seamount trail the measured ages are systematically older relative to both the WK08G model predictions and Hawaiian seamount ages, with offsets ranging up to 6 Myr. Taking into account the uncertainty about the duration of eruption and magmatic succession at individual Louisville volcanoes, these age offsets should be considered minimum estimates, as our sampling probably tended to recover the youngest lava flows. These large deviations point to either a contribution of inter-hot spot motion between the Louisville and Hawaiian hot spots or to a more easterly location of the Louisville hot spot than the one inferred in the WK08G model. Both scenarios are investigated in this paper, whereby the more eastern hot spot location (52.0 degrees S, 134.5 degrees W versus 52.4 degrees S, 137.2 degrees W) reduces the average age offset, but still results in a relatively large maximum offset of 3.7 Myr. When comparing the new ages to the APM models (S04P, S04G) by Steinberger et al. (2004) that attempt to compensate for the motion of hot spots in the Pacific (Hawaii) or globally (Hawaii, Louisville, Reunion and Walvis), the measured and predicted ages are more in agreement, showing only a maximum offset of 2.3 Myr with respect to the S04G model. At face value these more advanced APM models, which consider both plate and hot spot motions, therefore provide a better fit to the new Louisville age data. The fit is particularly good for seamounts younger than 50 Ma, a period for which there is little predicted motion for the Louisville hot spot and little inter-hot spot motion with Hawaii. However, discrepancies in the Louisville age-distance record prior to 50 Ma indicate there is an extra source of inter-hot spot motion between Louisville and the other Pacific hot spots that was not corrected for in the global S04G model. Finally, based on six new Ar-40/Ar-39 age dates, the 169 W bend in the Louisville seamount trail seems to have formed at least 3 Myr before the formation of the Hawaiian-Emperor bend. The timing of the most acute parts of both bends thus appears to be asynchronous, which would require other processes (e. g., plume motions) than a global plate motion change between 50 and 47 Ma to explain these two observations.

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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.

Lawrence, K, Johnson C, Tauxe L, Gee J.  2008.  Lunar paleointensity measurements: Implications for lunar magnetic evolution. Physics of the Earth and Planetary Interiors. 168:71-87.   10.1016/j.pepi.2008.05.007   AbstractWebsite

We analyze published and new paleointensity data from Apollo samples to reexamine the hypothesis of an early (3.9-3.6 Ga) lunar dynamo. Our new paleointensity experiments on four samples use modern absolute and relative measurement techniques, with ages ranging from 3.3 to 4.3 Ga, bracketing the putative period of an ancient lunar field. Samples 60015 (anorthosite) and 76535 (troctolite) failed during absolute paleointensity experiments. Samples 72215 and 62235 (impact breccias) recorded a complicated, multicomponent magnetic history that includes a low-temperature (< 500 degrees C) component associated with a high intensity (similar to 90 mu T) and a high temperature (> 500 degrees C) component associated with a low intensity (2 [LT). Similar multi-component behavior has been observed in several published absolute intensity experiments on lunar samples. Additional material from 72215 and 62235 was subjected to a relative paleointensity experiment (a saturation isothermal remanent magnetization, or sIRM, experiment); neither sample Provided unambiguous evidence for a thermal origin of the recorded remanent magnetization. We test several magnetization scenarios in an attempt to explain the complex magnetization recorded in lunar samples. Specifically, an overprint from exposure to a small magnetic field (an isothermal remanent magnetization) results in multi-component behavior (similar to absolute paleointensity results) from which we could not recover the correct magnitude of the original thermal remanent magnetization. In light of these new experiments and a thorough re-evaluation of existing paleointensity measurements, we conclude that although some samples with ages of 3.6 to 3.9 Ga are strongly magnetized, and sometimes exhibit stable directional behavior, it has not been demonstrated that these observations indicate a primary thermal remanence. Particularly problematic in the interpretation of lunar sample magnetizations are the effects of shock. As relative paleointensity measurements for lunar samples are calibrated using absolute paleointensities, the lack of acceptable absolute paleointensity measurements renders the interpretation of relative paleointensity measurements unreliable. Consequently, current paleointensity measurements do not support the existence of a 3.9-3.6 Ga lunar dynamo with 100 mu T surface fields, a result that is in better agreement with satellite measurements of crustal magnetism and that presents fewer challenges for thermal evolution and dynamo models. (c) 2008 Elsevier B.V. All rights reserved.

Lawrence, RM, Gee JS, Hurst SD.  1997.  Magnetic anisotropy in serpentinized peridotites from Site 920; its origin and relationship to deformation fabrics. Proceedings of the Ocean Drilling Program, Scientific Results. 153:419-427.   10.2973/odp.proc.sr.153.041.1997   Abstract

Anisotropy of magnetic susceptibility (AMS) measurements on serpentinized peridotites from Ocean Drilling Program Site 920 reveal a strong magnetic "fabric," typically characterized by an oblate susceptibility ellipsoid. Curie temperatures and max- imum unblocking temperatures near 580°C, as well as petrographic observations, suggest that magnetite is the sole magnetic carrier in the serpentinites. Because the magnetic mineralogy is dominated by coarse-grained magnetite, the susceptibility ellip- soid should provide a three-dimensional image of the average elongation of magnetite grains or grain clusters. Petrographic studies of three orthogonal thin sections from a limited number of samples indicate that the preferred shape orientation of mag- netite grain clusters correlates well with the apparent susceptibility maxima and minima in these planes. The magnetite long- axis preferred orientation is typically within -20° of the maximum principal axis of the susceptibility ellipsoid. The close corre- spondence between the magnetic foliation and the orientation of magnetite-bearing serpentine veins, together with the petro- graphic evidence for the distribution of magnetite, suggests that the magnetic "fabric" is primarily a reflection of the orientation of these veins. Hence, the AMS ellipsoid may be a more accurate descriptor of the integrated three-dimensional vein orienta- tions than visual orientation measurements made on the split cores.

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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.

Meurer, WP, Gee J.  2002.  Evidence for the protracted construction of slow-spread oceanic crust by small magmatic injections. Earth and Planetary Science Letters. 201:45-55.   10.1016/s0012-821x(02)00660-x   AbstractWebsite

Gabbroic cumulates drilled south of the Kane Transform Fault on the slow-spread Mid-Atlantic Ridge preserve up to three discrete magnetization components. Here we use absolute age constraints derived from the paleomagnetic data to develop a model for the magmatic construction of this section of the lower oceanic crust. By comparing the paleomagnetic data with mineral compositions, and based on thermal models of local reheating, we infer that magmas that began crystallizing in the upper mantle intruded into the lower oceanic crust and formed meter-scale sills. Some of these magmas were crystal-laden and the subsequent expulsion of interstitial liquid from them produced 'cumulus' sills. These small-scale magmatic injections took place over at least 210 000 years and at distances of similar to3 km from the ridge axis and may have formed much of the lower crust. This model explains many of the complexities described in this area and can be used to help understand the general formation of oceanic crust at slow-spread ridges. (C) 2002 Published by Elsevier Science B.V.

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.

Morris, A, Gee JS, Pressling N, John BE, MacLeod CJ, Grimes CB, Searle RC.  2009.  Footwall rotation in an oceanic core complex quantified using reoriented Integrated Ocean Drilling Program core samples. Earth and Planetary Science Letters. 287:217-228.   10.1016/j.epsl.2009.08.007   AbstractWebsite

Oceanic core complexes expose lower crustal and upper mantle rocks on the seafloor by tectonic unroofing in the footwalls of large-slip detachment faults. The common occurrence of these structures in slow and ultra-slow spread oceanic crust suggests that they accommodate a significant component of plate divergence. However, the subsurface geometry of detachment faults in oceanic core complexes remains unclear. Competing models involve either: (a) displacement on planar, low-angle faults with little tectonic rotation; or (b) progressive shallowing by rotation of initially steeply dipping faults as a result of flexural unloading (the "rolling-hinge" model). We address this debate using palaeomagnetic remanences as markers for tectonic rotation within a unique 1.4 km long footwall section of gabbroic rocks recovered by Integrated Ocean Drilling Program (IODP) sampling at Atlantis Massif oceanic core complex on the Mid-Atlantic Ridge (MAR). These rocks contain a complex record of multipolarity magnetizations that are unrelated to alteration and igneous stratigraphy in the sampled section and are inferred to result from progressive cooling of the footwall section over geomagnetic polarity chrons C1r.2r, C1r.1n (Jaramillo) and C1r.1r. For the first time we have independently reoriented drill-core samples of lower crustal gabbros, that were initially azimuthally unconstrained, to a true geographic reference frame by correlating structures in individual core pieces with those identified from oriented imagery of the borehole wall. This allows reorientation of the palaeomagnetic data, placing far more rigorous constraints on the tectonic history than those possible using only palaeomagnetic inclination data. Analysis of the reoriented high temperature reversed component of magnetization indicates a 46 degrees +/- 6 degrees anticlockwise rotation of the footwall around a MAR-parallel horizontal axis trending 011 degrees +/- 6 degrees. Reoriented lower temperature components of normal and reversed polarity suggest that much of this rotation occurred after the end of the Jaramillo chron (0.99 Ma). The data provide unequivocal confirmation of the key prediction of flexural, rolling-hinge models for oceanic core complexes, whereby oceanic detachment faults initiate at higher dips and rotate to their present day low-angle geometries as displacement increases. (C) 2009 Elsevier B.V. All rights reserved.

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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°.

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Parker, RL, Gee JS.  2002.  Calibration of the pass-through magnetometer - II. Application. Geophysical Journal International. 150:140-152.   10.1046/j.1365-246X.2002.01692.x   AbstractWebsite

We describe the experimental procedure we use to calibrate a cryogenic pass-through magnetometer. The procedure is designed to characterize the magnetometer sensitivity as a function of position within the sensing region. Then we extend a theory developed in an earlier paper to cover inexact observations and apply it to the data set. The theory allows the calculation of a smooth, harmonic, internally consistent interpolating function for each of the nine components of the response tensor of the magnetometer. With these functions we can calculate the response to a dipole source in any orientation and position, and predict the magnetometer signal from any kind of specimen. The magnetometer in the paleomagnetic laboratory onboard the research vessel Joides Resolution is the subject of one such experiment and we present the results. The variation with position of sensitivity is displayed in a series of plane slices through the magnetometer. We discover from the calibration model that the X and Z coils are misaligned so that the magnetic centre of the coils is displaced from the geometric centre by approximately 0.7 cm. We synthesize the signal expected from the magnetometer when a variety of simple cores are measured. We find that, unless appropriate corrections are made, changes in magnetization direction can appear as variations in magnetic intensity, and conversely, fluctuations in the magnetization strength can produce apparent swings in declination and inclination. The magnitude of these effects is not small and is certainly worth taking into account in the interpretation of records from this kind of instrument. In a pilot study on data from a core measured with the shipboard magnetometer, we observe some large distortions, particularly in declination, that are attributable to uncorrected instrumental effects.

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.

Pringle, Staudigel H, Gee J.  1991.  Jasper Seamount: Seven million years of volcanism. Geology. 19:364-368.   10.1130/0091-7613(1991)019<0364:jssmyo>2.3.co;2   AbstractWebsite

Jasper Seamount is a young, mid-sized (690 km super(3)) oceanic intraplate volcano located abut 500 km west-southwest of San Diego, California. Reliable super(40)Ar/ super(39)Ar age data were obtained for several milligram-sized samples of 4 to 10 Ma plagioclase by using a defocused laser beam to clean the samples before fusion. Gee and Staudigel suggested that Jasper Seamount consists of a transitional to tholeiitic shield volcano formed by flank transitional series lavas, overlain by flank alkalic series lavas and summit alkalic series lavas. Twenty-nine individual super(40)Ar/ super(39)Ar laser fusion analyses on nine samples confirm the stratigraphy: 10.3-10.0 Ma for the flank transitional series, 8.7-7.5 Ma for the flank alkalic series, and 4.8-4.1 Ma for the summit alkalic series. The alkalinity of the lavas clearly increases with time, and there appear to be 1 to 3 m.y. hiatuses between each series. The age data are consistent with the complex magnetic anomaly of Jasper; however, the dominant reversed polarity inferred from the anomaly suggests that most of the seamount formed at ca. 11 Ma, prior to the onset of Chron C5N.

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Schoolmeesters, N, Cheadle MJ, John BE, Reiners PW, Gee J, Grimes CB.  2012.  The cooling history and the depth of detachment faulting at the Atlantis Massif oceanic core complex. Geochemistry Geophysics Geosystems. 13   10.1029/2012gc004314   AbstractWebsite

Oceanic core complexes (OCCs) are domal exposures of oceanic crust and mantle interpreted to be denuded to the seafloor by large slip oceanic detachment faults. We combine previously reported U-Pb zircon crystallization ages with (U-Th)/He zircon thermochronometry and multicomponent magnetic remanence data to determine the cooling history of the footwall to the Atlantis Massif OCC (30 degrees N, MAR) and help establish cooling rates, as well as depths of detachment faulting and gabbro emplacement. We present nine new (U-Th)/He zircon ages for samples from IODP Hole U1309D ranging from 40 to 1415 m below seafloor. These data paired with U-Pb zircon ages and magnetic remanence data constrain cooling rates of gabbroic rocks from the upper 800 m of the central dome at Atlantis Massif as 2895 (+1276/-1162) degrees C Myr(-1) (from similar to 780 degrees C to similar to 250 degrees C); the lower 600 m of the borehole cooled more slowly at mean rates of similar to 500 (+125/-102) degrees C Myr(-1) (from similar to 780 degrees C to present-day temperatures). Rocks from the uppermost part of the hole also reveal a brief period of slow cooling at rates of similar to 300 degrees C Myr(-1), possibly due to hydrothermal circulation to similar to 4 km depth through the detachment fault zone. Assuming a fault slip rate of 20 mm/yr (from U-Pb zircon ages of surface samples) and a rolling hinge model for the sub-surface fault geometry, we predict that the 780 degrees C isotherm lies at similar to 7 km below the axial valley floor, likely corresponding both to the depth at which the semi-brittle detachment fault roots and the probable upper limit of significant gabbro emplacement.