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Yu, YJ, Gee JS.  2005.  Spinel in Martian meteorite SaU 008: implications for Martian magnetism. Earth and Planetary Science Letters. 232:287-294.   10.1016/j.epsl.2004.12.015   AbstractWebsite

Shergotty-Nakhla-Chassigny (SNC) meteorites provide the only available samples of Martian material. The stable permanent magnetization of SNC meteorites has been traditionally attributed to magnetite (Fe3O4) or pyrrhotite (Fe7S8). On the basis of rock magnetic, microscopic, and electron microprobe analyses on rock chips and mineral separates, we suggest that a new material (Fe-Cr-Ti spinel) is responsible for the stable paleomagnetic record of Martian meteorite SaU 008. It is possible that SaU 008 acquired a primary remanence of thermal origin from the Martian crustal field. However, this proposition requires further testing because the effect of shock events on Fe-Cr-Ti spinel is unknown. (c) 2004 Elsevier B.V. All rights reserved.

Yu, YJ, Tauxe L, Gee JS.  2007.  A linear field dependence of thermoremanence in low magnetic fields. Physics of the Earth and Planetary Interiors. 162:244-248.   10.1016/j.pepi.2007.04.008   AbstractWebsite

We tested a linear field-dependence of thermoremanent magnetization (TRM) to saturation isothermal remanent magnetization (SIRM) ratio for magnetite-containing natural samples. The TRM/SIRM shows a linear field-dependence to very low field ranges (<1 mu T). This observation is at odds with a claim of limited sensitivity at low fields in TRM acquisition documented in previous studies. We attribute the difference to poor field control in the ovens used in previous studies. The TRM/SIRM ratio shows a grain-size dependence. For magnetite-containing samples with insignificant anisotropy, the TRM/SIRM is most efficient in pseudo-single-domain magnetites. These results suggest that while the TRM/SIRM ratio is linear at low field strengths, the ratio provides only a crude estimation on the actual paleo-field within two orders of magnitude, suggesting that a careful sample characterization is necessary in applying the TRM/SIRM as a paleointensity proxy. (c) 2007 Elsevier B.V. All rights reserved.

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Winterer, EL, Gee JS, Van Waasbergen RJ.  1988.  The source area for Lower Cretaceous clastic sediments of the Galicia Margin : geology and tectonic and erosional history. Proceedings of the Ocean Drilling Program, Scientific Results. 103:697-732.   10.2973/odp.proc.sr.103.181.1988   Abstract

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

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Varga, RJ, Gee JS, Bettison-Varga L, Anderson RS, Johnson CL.  1999.  Early establishment of seafloor hydrothermal systems during structural extension: paleomagnetic evidence from the Troodos ophiolite, Cyprus. Earth and Planetary Science Letters. 171:221-235.   10.1016/s0012-821x(99)00147-8   AbstractWebsite

Paleomagnetic data from the Troodos ophiolite are used to help constrain models for the relationship between extensional normal faulting and hydrothermal alteration related to production of large-tonnage sulfide deposits at oceanic ridges. We have sampled dikes from the Troodos sheeted complex that have been subjected to variable hydrothermal alteration, from greenschist alteration typical of the low water/rock mass ratio interactions outside of hydrothermal upflow zones as well as from severely recrystallized rocks (epidosites) altered within high water/rock mass ratio hydrothermal upflow zones in the root zones beneath large sulfide ore deposits. These dikes are moderately to highly tilted from their initial near-vertical orientations due to rotations in the hangingwalls of approximately dike-parallel, oceanic normal faults. Comparison of characteristic remanence directions from these dikes with the Late Cretaceous Troodos reference direction, therefore, allows a tilt test to determine whether remanent magnetizations were acquired prior to or subsequent to tilting. Remanence directions for both greenschist and epidosite dikes show similar magnitudes of tilting due to rotational normal faulting and restore to the Late Cretaceous Troodos reference direction upon restoration of dikes to near-vertical positions about a NNW-trending, horizontal axis. These data, along with field observations of focused alteration along normal faults, suggest that epidosite alteration occurred during the early stages of extensional tilting and prior to significant rotation. This sequence of events is similar to that observed for creation of large-tonnage sulfide bodies at intermediate to slow spreading centers which form soon after cessation of magmatism and during the early stages of structural extension. We suggest that the dike-parallel normal faults were initiated as extensional fractures during this early stage of crustal extension, thus providing the necessary permeability for focused fluid flow, and that later slip along these structures during rotational-planar normal faulting caused reduction in permeability due to gouge formation. (C) 1999 Elsevier Science B.V. All rights reserved.

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.

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.

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.

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.

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Tauxe, L, Gee JS, Staudigel H.  1998.  Flow directions in dikes from anisotropy of magnetic susceptibility data: The bootstrap way. Journal of Geophysical Research-Solid Earth. 103:17775-17790.   10.1029/98jb01077   AbstractWebsite

One of the first applications of anisotropy of magnetic susceptibility (AMS) was an attempt to determine flow directions from mafic dikes [Khan, 1962]. Since the seminal work of Knight and Walker [1988] defining the expected behavior of AMS in response to magma flow, there has been increasing interest in using AMS for this purpose. Here we present a quantitative method for interpretation of AMS data from dikes, using a parametric bootstrap. First, dikes must be sampled with at least five land preferrably more) samples from within 10 cm of the dike margin. The distributions of the eigenvalues and eigenvectors of the AMS tensor are delineated by calculating eigenparameters of many bootstrapped paradata sets. We generate paradata sets by first selecting a sample at random, then calculating a replacement set of data by drawing tensor elements from normal distributions with the mean and standard deviation of the entire site. The bounds containing 95% of the eigenparameters of the bootstrapped data serve as confidence limits for the parameter of interest. Classification of dikes proceeds as follows: Sites whose maximum and intermediate eigenvalues could not be distinguished are deemed uninterpretable. In addition, sites with principal eigenvectors with angles > 45 degrees away from the dike margin (inverse) or with markedly different directions on either side of the dike (scissored) are excluded. The remaining dikes are classified as having unique flow direction information if the principal eigenvectors from at least one side are distinct from the dike plane based on the distribution of the bootstrapped principal eigenvectors. If neither side has principal eigenvectors distinct from the dike plane, the dikes are classified as having lineation information only. A study comprising 251 dikes from the Troodos ophiolite has 151 sites with directional data, 38 sites with lineations only, 7 inverse sites, 5 scissored sites, and 55 sites not fitting into any other category. The flow directions interpreted from the data were generally southerly, toward a fossil transform zone.

Tauxe, L, Gee JS, Steiner MB, Staudigel H.  2013.  Paleointensity results from the Jurassic: New constraints from submarine basaltic glasses of ODP Site 801C. Geochemistry, Geophysics, Geosystems.   10.1002/2013GC004704   AbstractWebsite

Tholeiite of the oldest oceanic crust was drilled during ODP Legs 129 and 185 at Hole 801C in the western Pacific. Fresh appearing submarine basaltic glass (SBG) was recovered from the tholetiites (~167 Ma; Koppers et al. [2003]) which has been shown to be nearly ideal for determining absolute paleointensity. Paleointensities of the younger, off-axis, alkalic basalts (~160 Ma; Koppers et al. [2003]), overlying the tholeiites, had been studied earlier [Tauxe, 2006]. Here we report results from the older tholeiitic (on-axis) sequence. We subjected a total of 73 specimens from 17 cooling units to absolute paleointensity experiments. Of these, 30 specimens and 6 cooling unit averages met our strictest reliability criteria, yielding an average of 11.9± 3.9 μT. The bulk of evidence suggests a paleolatitude of the site of 14°S (with an uncertainty of 10°). This translates the intensity to a value for the virtual axial dipole moment of 28 ZAm2, slightly lower than values determined from the plagio clase crystals in the three cooling units of the younger alkalic basalts over lying the tholeiites. This value is low when compared to the long-term median value of the field of 42 ZAm2. Our results and those of the published literature therefore support the contention of a low magnetic field strength in the Jurassic (average of 28 ± 14 ZAm2; N = 138 individual estimates), as initially suggested by Prévot et al. [1990]. Our interpretation of the body of available data argue for low field strengths for the entire Jurassic extending into the early Cretaceous.

Tauxe, L, Gee J, Gallet Y, Pick T, Bown T.  1994.  Magnetostratigraphy of the Willwood Formation, Bighorn Basin, Wyoming - New Constraints on the Location of Paleocene Eocene Boundary. Earth and Planetary Science Letters. 125:159-172.   10.1016/0012-821x(94)90213-5   AbstractWebsite

The lower Eocene Willwood Formation in the Bighorn Basin of Wyoming preserves a rich and diverse mammalian and floral record. The paleomagnetic behavior of the sequence of floodplain paleosols of varying degrees of maturation ranges from excellent to poor. We present a magnetostratigraphic section for a composite section near Worland, Wyoming, by using a set of strict criteria for interpreting the step-wise alternating field and thermal demagnetization data of 266 samples from 90 sites throughout the composite section. Correlation to the geomagnetic reversal time scale was achieved by combining magnetostratigraphic and biostratigraphic data from this section, from a section in the Clark's Fork Basin in northern Wyoming, and from DSDP Site 550, with the isotopic date determined on a tuff near the top of our section. Our correlation suggests that the Bighorn Basin composite section in the Worland area spans from within Chron C24r to near the top of Chron C24n, or from approximately 55 to 52 Ma. This correlation places the Paleocene/Eocene boundary within the vicinity of the base of the section. Cryptochron C24r.6 of Cande and Kent is tentatively identified some 100 m above the base of the section. The temporal framework provided here enables correlation of the mammalian biostratigraphy of the Bighorn Basin to other continental sequences as well as to marine records. It also provides independent chronological information for the calculation of sediment accumulation rates to constrain soil maturation rates. We exclude an age as young as 53 Ma for the Paleocene/Eocene boundary and support older ages, as recommended in recent time scales. The location of a tuff dated at 52.8 +/- 0.3 Ma at the older boundary C24n.1 is consistent with the, age of 52.5 Ma estimated by Cande and Kent and inconsistent with that of 53.7 Ma, from Harland et al.

Tarduno, JA, Gee J.  1995.  Large-Scale Motion Between Pacific and Atlantic Hotspots. Nature. 378:477-480.   10.1038/378477a0   AbstractWebsite

STUDIES of true polar wander (TPW), the rotation of the solid Earth with respect to the spin axis(1), have suggested that there has been 10-15 degrees of relative motion over the past 130 Myr (refs 2-4). In such studies, the orientation of the spin axis is recovered from continental palaeomagnetic poles (corrected for relative plate motions), and compared with a deep-mantle reference frame defined by hotspot locations. But deducing relative plate motions becomes increasingly difficult for older (Mesozoic) time periods, hindering tests of TPW on timescales comparable to those of large-scale mantle convection; moreover, the assumption of hotspot fixity is controversial(5,6). We examine here a more direct approach(7,8), using palaeolatitudes derived from Pacific guyots. Contrary to predictions from TPW models, these data suggest only minor latitudinal shifts of Pacific hotspots during the Cretaceous period. Instead of TPW, relative motion between the Atlantic and Pacific hotspot groups(9) is required at a velocity of approximately 30 mm yr(-1), more than 50% larger than previously proposed(5).

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Staudigel, H, Gee J, Tauxe L, Varga RJ.  1992.  Shallow Intrusive Directions of Sheeted Dikes in the Troodos Ophiolite - Anisotropy of Magnetic-Susceptibility and Structural Data. Geology. 20:841-844.   10.1130/0091-7613(1992)020<0841:sidosd>2.3.co;2   AbstractWebsite

Sheeted dikes play a central role in the formation of oceanic crust. It is commonly assumed that sheeted dikes intrude vertically upward, from elongated mid-ocean ridge (MOR) magma chambers, but there are no direct observational data bearing on this hypothesis. This assumption contrasts with the intrusive behavior of subaerial volcanoes where magmas rise into shallow central magma chambers that laterally feed vertically oriented dikes. We have studied intrusive directions of sheeted dikes in a structural analogue to oceanic crust, the Troodos ophiolite. Structural and magnetic fabric data of 65 dikes provide consistent results and suggest a broad distribution of shallow (<20-degrees) to nearly vertical, upward magma-transport directions. These data suggest that horizontal emplacement has to be considered for sheeted dikes at MORs, implying more centralized MOR plumbing systems than previously thought. Such plumbing systems provide ample opportunity for complex mixing, fractionation, and contamination of MOR lavas in magma chambers and tabular magma-storage volumes. Whether the MOR magma supply is linear or centralized also has a fundamental effect on crustal accretion processes and the geometry of hydrothermal convection systems.

Staudigel, H, Tauxe L, Gee JS, Bogaard P, Haspels J, Kale G, Leenders A, Meijer P, Swaak B, Tuin M, Van Soest MC, Verdurmen EAT, Zevenhuizen A.  1999.  Geochemistry and Intrusive Directions In Sheeted Dikes in the Troodos Ophiolite: Implications for Mid-Ocean Ridge Spreading Centers. Geochemistry Geophysics Geosystems. 1 AbstractWebsite

Sheeted dikes at mid-ocean ridge volcanoes represent the link between deep magma production and storage processes and shallow processes such as volcanism and hydrothermal activity. As such, they are crucial for the interpretation of many observations at mid-ocean ridges or other volcanoes with pronounced rift zones, including topography, hydrothermal systems, petrology, and geochemistry. We carried out a structural, magnetic, and chemical investigation of a 4 x 10 km sheeted dike section in the Troodos ophiolite, Cyprus. On the basis of major and trace element geochemistry, we distinguish dikes that may be correlated with the basal high-Ti series (HTS) lavas from those of the overlying low-Ti series (LTS) lavas. All dikes studied are nearly parallel to each other, with vertical or steeply dipping planes whose strike likely indicates the orientation of the spreading center. Anisotropy of magnetic susceptibility measurements suggests that the HTS and LTS dikes intrude in fundamentally different ways. HTS dikes reflect the intrusive behavior of dikes in the vicinity of a magma supply system and define ridge parallel intrusive sheets that radiate out from the magma chamber. LTS dikes show a bimodal, orthogonal set of intrusive directions, one shallow and one near vertical. Near-lateral propagating dikes provide a means for delivery of magma into distant portions of a rift system, and near-vertical dike propagation directions are probably associated with feeder dikes to down-rift surface flows. Our study suggests that the types of dike intrusive behavior in the Troodos ophiolite may also be typical for "normal" mid-ocean ridges or other major shield volcanoes with well-developed rift zones.

Smith, GM, Gee J, Klootwijk CT.  1991.  Magnetic petrology of basalts from Ninetyeast Ridge. Proceedings of the Ocean Drilling Program, Scientific Results. 121:525-545.   10.2973/odp.proc.sr.121.154.1991   Abstract

Given the importance of the inversion of seamount magnetic anomalies, particularly to the motion of the Pacific plate, it is important to gain a better understanding of the nature of the magnetic source of these features. Although different in detail, Ninetyeast Ridge is composed of submarine and subaerial igneous rocks that are similar to those found at many seamounts, making it a suitable proxy. We report here on the magnetic petrology of a collection of samples from Ninetyeast Ridge in the Indian Ocean. Our purpose is to determine the relationship between primary petrology, subsequent alteration, and magnetic properties of the recovered rocks. Such information will eventually lead to a more complete understanding of the magnetization of seamounts and presumably improvements in the accuracy of anomaly inversions. Three basement sites were drilled on Ninetyeast Ridge, with recovery of subaerial basalt flows at the first two (Sites 756 and 757) and submarine massive and pillow flows at the final one (Site 758). The three sites were distinctly different. Site 756 was dominated by ilmenite. What titanomagnetite was present had undergone deuteric alteration and secondary hematite was present in many samples. The magnetization was moderate and stable although it yielded a paleolatitude somewhat lower than expected. Site 757 was highly oxidized, presumably while above sea level. It was dominated by primary titanomagnetite, which was deuterically altered. Secondary hematite was common. Magnetization was relatively weak but quite stable. The paleolatitude for all but the lowermost flows was approximately 40° lower than expected. Site 758 was also dominated by primary titanomagnetite. There was relatively little oxidation with most primary titanomagnetite showing no evidence of high-temperature alteration. No secondary hematite was in evidence. This site had the highest magnetization of the three (although somewhat low relative to other seamounts) but was relatively unstable with significant viscous remanence in many samples. Paleolatitude was close to the expected value. It is not possible, at present, to confidently associate these rocks with specific locations in a seamount structure. A possible and highly speculative model would place rocks similar to Site 757 near the top of the edifice, Site 756 lower down but still erupted above sea level, and Site 758 underlying these units, erupted while the seamount was still below sea level.

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

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

Selkin, PA, Gee JS, Tauxe L.  2007.  Nonlinear thermoremanence acquisition and implications for paleointensity data. Earth and Planetary Science Letters. 256:81-89.   10.1016/j.epsl.2007.01.017   AbstractWebsite

In paleointensity studies, thermoremanence is generally regarded as a linear function of ambient inagnetic field at low fields comparable to that of the present-day Earth. We find pronounced nonlinearity at low fields for a class of materials with silicate-hosted magnetite that otherwise perforin well in paleointensity experiments. We model this nonlinearity with narrow size ranges of large, acicular single domain grains, which are most likely in a vortex state (i.e. nonuniformly magnetized, sometimes labeled pseudosingle domain). Simple TRM theory predicts that even certain single domain particles will also exhibit a nonlinear response, saturating in fields as low as the Earth's. Such behavior, although likely to be rare, may bias some paleointensity estimates. The bias is especially pronounced when the laboratory field is higher than the ancient field. Fortunately, the fundamental assumption that thermoremanence is proportional to applied field can (and should) be routinely checked at the end of successful paleointensity experiments by adding two extra heating steps. (c) 2007 Elsevier B.V. All rights reserved.

Selkin, PA, Gee JS, Meurer WP.  2014.  Magnetic anisotropy as a tracer of crystal accumulation and transport, Middle Banded Series, Stillwater Complex, Montana. Tectonophysics. 629:123-137.   10.1016/j.tecto.2014.03.028   AbstractWebsite

Fabric studies of layered mafic intrusions have led to improved understanding of the mechanical processes operating in large magma chambers, including crystal accumulation and crystal mush deformation. Such studies, however, are typically limited by a tradeoff between breadth (number of sites studied, characteristic of field-focused work) and sensitivity (ability to discern subtle fabric elements, characteristic of laboratory fabric analyses). Magnetic anisotropy, if analyzed in a systematic way and supported by single-crystal and petrofabric measurements, permits relatively rapid characterization of magmatic fabrics for large numbers of samples. Here we present the results of a study of remanence and susceptibility anisotropy from three transects through the Middle Banded Series of the Stillwater Complex, Montana. All three transects exhibit a magnetic foliation that increases with stratigraphic height up to the top of Olivine Bearing Zone III, consistent with crystal mush compaction. Perhaps more importantly, each tansect is characterized by a subtle lineation in the anisotropy of magnetic susceptibility with a consistent direction within that transect The magnetic lineation directions, which generally coincide with crystallographic preferred orientations of silicate minerals, likely record a pre-compaction fabric. Lineation directions differ from one transect to another, implying that the process generating the lineation - either slumping of a semiconsolidated crystal mush or magma transport - acted on length scales of at most a few km. These results demonstrate the sensitivity of magnetic anisotropy to petrofabric in mafic rocks. (C) 2014 Elsevier B.V. All rights reserved.

Selkin, PA, Gee JS, Meurer WP, Hemming SR.  2008.  Paleointensity record from the 2.7 Ga Stillwater Complex, Montana. Geochemistry Geophysics Geosystems. 9   10.1029/2008gc001950   AbstractWebsite

The record of geomagnetic intensity captured in the 2.7 Ga Stillwater Complex (Montana, USA) provides a statistical description of the Archean geodynamo. We present results of modified Thellier paleointensity experiments on 441 core specimens, 114 of which pass strict reliability criteria. The specimens are from 53 sites spanning most of the Banded Series rocks in the Stillwater Complex. On the basis of thermochronologic and petrologic evidence, we interpret the highest temperature component of remanence to be a late Archean thermoremanence, though the possibility remains that it is a thermochemical remanence. Thermal models indicate that the highest temperature magnetization component at each of the sites averages similar to 20-200 ka of geomagnetic secular variation. The suite of sites as distributed through the Banded Series samples a roughly a 1 Ma time interval. The average of the most reliable paleointensity measurements, uncorrected for the effects of anisotropy or cooling rate, is 38.2 +/- 11.3 mu T (1 sigma). Remanence anisotropy, cooling rate, and the nonlinear relationship between applied field and thermoremanence have a significant effect on paleointensity results; a corrected average of 30.6 +/- 8.8 mu T is likely a more appropriate value. Earth's average dipole moment during the late Archean (5.05 +/- 1.46 x 10(22) Am(2), lambda(pmag) = 44.5 degrees) was well within the range of estimates from Phanerozoic rocks. The distribution of site-mean paleointensities around the mean is consistent with that expected from slow cooling over timescales expected from thermal models and with secular variation comparable to that of the Phanerozoic field.

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

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

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.

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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.

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.

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.

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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°.