Publications

Export 12 results:
Sort by: Author [ Title  (Asc)] Type Year
A B C D E F G H I J K L M N O [P] Q R S T U V W X Y Z   [Show ALL]
P
Bowles, J, Gee JS, Kent DV, Perfit MR, Soule SA, Fornari DJ.  2006.  Paleointensity applications to timing and extent of eruptive activity, 9 degrees-10 degrees N East Pacific Rise. Geochemistry Geophysics Geosystems. 7   10.1029/2005gc001141   AbstractWebsite

[ 1] Placing accurate age constraints on near-axis lava flows has become increasingly important given the structural and volcanic complexity of the neovolcanic zone at fast spreading ridges. Geomagnetic paleointensity of submarine basaltic glass (SBG) holds promise for placing quantitative age constraints on near-axis flows. In one of the first extensive tests of paleointensity as a dating tool or temporal marker we present the results of over 550 successful SBG paleointensity estimates from 189 near-axis (< 4 km) sites at the East Pacific Rise, 9 degrees - 10 degrees N. Paleointensities range from 6 to 53 mu T and spatially correspond to the pattern expected from known temporal variations in the geomagnetic field. Samples within and adjacent to the axial summit trough (AST) have values approximately equal to or slightly higher than the present-day. Samples out to 1 - 3 km from the AST have values higher than the present-day, and samples farther off axis have values lower than the present-day. The on-axis samples (< 500 m from the AST) provide a test case for using models of paleofield variation for the past few hundred years as an absolute dating technique. Results from samples collected near a well-documented eruption in 1991 - 1992 suggest there may be a small negative bias in the paleointensity estimates, limiting resolution of the dating technique. Possible explanations for such a bias include local field anomalies produced by preexisting magnetic terrain; anomalously high magnetic unblocking temperatures, leading to a small cooling rate bias; and/or the possibility of a chemical remanence produced by in situ alteration of samples likely to have complicated thermal histories. Paleointensity remains useful in approximating age differences in young flows, and a clear along-axis paleointensity contrast near 9 degrees 50'N is suggestive of a similar to 150 - 200 year age difference. Paleointensity values of off-axis samples are generally consistent with rough age interpretations based on side scan data. Furthermore, spatial patterns in the paleointensity suggest extensive off-axis flow emplacement may occur infrequently, with recurrence intervals of 10 - 20 kyr. Results of a stochastic model of lava emplacement show that this can be achieved with a single distribution of flows, with flow size linked to time between eruptions.

Gee, JS, Yu YJ, Bowles J.  2010.  Paleointensity estimates from ignimbrites: An evaluation of the Bishop Tuff. Geochemistry Geophysics Geosystems. 11   10.1029/2009gc002834   AbstractWebsite

Ash flow tuffs, or ignimbrites, typically contain fine-grained magnetite, spanning the superparamagnetic to single-domain size range that should be suitable for estimating geomagnetic field intensity. However, ignimbrites may have a remanence of thermal and chemical origin as a result of the complex magnetic mineralogy and variations in the thermal and alteration history. We examined three stratigraphic sections through the similar to 0.76 Ma Bishop Tuff, where independent information on postemplacement cooling and alteration is available, as a test of the suitability of ignimbrites for paleointensity studies. Thermomagnetic curves suggest that low-Ti titanomagnetite (T(c) = 560 degrees C-580 degrees C) is the dominant phase, with a minor contribution from a higher Tc phase(s). Significant remanence unblocking above 580 degrees C suggests that maghemite and/or (titano)maghemite is an important contributor to the remanence in most samples. We obtained successful paleofield estimates from remanence unblocked between 440 degrees C and 580 degrees C for 46 of 89 specimens (15 sites at two of three total localities). These specimens represent a range of degrees of welding and have variable alteration histories and yet provide a consistent paleofield estimate of 43.0 mu T (+/- 3.2), equivalent to a VADM of 7.8 x 10(22) Am(2). The most densely welded sections of the tuff have emplacement temperatures inferred to be as high as similar to 660 degrees C, suggesting that the remanence may be primarily thermal in origin, though a contribution from thermochemical remanence cannot be excluded. These results suggest that ignimbrites may constitute a viable material for reliable paleointensity determinations.

Avery, MS, Gee JS, Bowles JA, Jackson MJ.  2018.  Paleointensity estimates from ignimbrites: The Bishop Tuff Revisited. Geochemistry Geophysics Geosystems. 19:3811-3831.   10.1029/2018gc007665   AbstractWebsite

Volcanic ash flow tuffs (ignimbrites) may contain single domain-sized (titano) magnetite that should be good for recording geomagnetic field intensity, but due to their complex thermal histories also contain other magnetic grains, which can complicate and obscure paleointensity determination. An initial study of the suitability of the similar to 767ka Bishop Tuff for measuring paleointensity found an internally consistent estimate of 43.03.2T. This initial study also showed a spatial heterogeneity in reliable paleointensity estimates that is possibly associated with vapor-phase alteration and fumarolic activity, which motivated resampling of the Bishop Tuff to examine spatial changes in magnetic properties. Three new stratigraphic sections of the Bishop Tuff within the Owens River gorge were sampled, and the paleointensity results from the initial study in the same locality were reinterpreted. The mean of all sites is 41.911.8T; this agrees with the initial study's finding but with substantially greater scatter. Two sections show evidence of vapor-phase alteration where the presence of titanohematite, likely carrying a thermochemical remanence, produces nonideal behavior. This thermochemical remanence in the upper portion of the section also produces some paleointensity estimates of technically high quality that have significantly higher intensity than the rest of the tuff. Our best estimate for paleointensity, 39.69.9T, comes from the densely welded ignimbrite that was emplaced above the Curie temperature of magnetite. The low permeability of this unit likely shielded it from vapor-phase alteration. Our results suggest that care must be taken in interpreting paleointensity data from large tuffs as nonthermal remanence may be present. Understanding past variations of Earth's magnetic field help us understand processes in Earth's core and help us to better understand current field behavior, which is important to life on Earth. Earth's field is recorded by magnetic-minerals in rocks as they form. Variations in the strength of the magnetic field (paleointensity) are less well known than large variations in direction. This is partially due to the difficulty in identifying rocks that are suitable for paleointensity experiments. Rocks made of volcanic ash (ignimbrites) have been shown to successfully record the field strength during recent volcanic eruptions. However, we show evidence that ignimbrites may not all be suitable for paleointensity studies. The Bishop Tuff, located in eastern California, erupted about 767 thousand years ago, emplacing a large volume (similar to 200km(3), i.e., about 80 million Olympic swimming pools or slightly bigger than Lake Tahoe) of ash and lava over a few days. With samples from the Bishop Tuff we test variations in magnetic-mineralogy that may be related to venting volcanic gas, interaction with water, eruption temperatures, or the degree to which the ash compacted and solidified into rock. These factors affect the magnetic-minerals' ability to record paleointensity and the success rate of our experiments.

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.

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.

Gallet, Y, Gee J, Tauxe L, Tarduno JA.  1993.  Paleomagnetic analyses of short normal polarity magnetic anomalies in the Matuyama Chron. Proceedings of the Ocean Drilling Program, Scientific Results. 130:547-559.   10.2973/odp.proc.sr.130.033.1993   Abstract

We document three short normal intervals in the natural remanent magnetization of sediments within the Matuyama Chron. These three anomalous zones of magnetization between the Jaramillo and Olduvai subchrons were identified from continuous measurements of archive halves from Hole 803 A using the pass-through 2G cryogenic magnetometer at Scripps. The U-channel samples were taken from the three intervals, analyzed using the pass-through system, and then cut into discrete 1 -cm-thick samples. Measurements on discrete samples confirmed the presence of the upper normal polarity zone. Based on sedimentation rate calculations, this zone is confidently correlated with the Cobb Mountain Subchron. For the two other anomalous zones, complete thermal demagnetization revealed a high-stability component (250°-575°C) of reversed polarity. The intensity of the low-stability normal polarity component, normalized by susceptibility, remains roughly constant throughout the entire interval sampled, whereas the intensity of the high-stability reversed component is much lower within the normal zone than outside. We interpret these two normal zones, then, as periods of low (reversed polarity) geomagnetic field intensity resulting in low magnetization of the sediments; the periods of these low magnetization reversed polarity zones are completely masked by the component acquired viscously in a normal polarity field.

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.

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

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

Garces, M, Gee JS.  2007.  Paleomagnetic evidence of large footwall rotations associated with low-angle faults at the Mid-Atlantic Ridge. Geology. 35:279-282.   10.1130/g23165a.1   AbstractWebsite

Exposures of gabbros and mantle-derived peridotites; at slow-spreading oceanic ridges have been attributed to extension on long-lived, low-angle detachment faults, similar to those described in continental metamorphic core complexes. In continental settings, such detachments have been interpreted as having originated and remained active at shallow dips. Alternatively, currently shallow dipping fault surfaces may have originated at moderate to steep dips and been flattened by subsequent flexure and isostatic uplift. While the latter interpretation would be more consistent with Andersonian faulting theory, it predicts large footwall tilts that have not been observed in continental detachment faults. Here we use the magnetization of oceanic gabbro and peridotite samples exposed near the Fifteen-Twenty Fracture Zone on the Mid-Atlantic Ridge to demonstrate that substantial footwall rotations have occurred. Widespread rotations ranging from 50 degrees to 80 degrees indicate that original fault orientations dipped steeply toward the spreading axis.

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°.

Christie, DM, Dieu JJ, Gee J.  1995.  Petrologic studies of basement lavas from Northwest Pacific guyots. Proceedings of the Ocean Drilling Program, Scientific Results. 144:495-512.   10.2973/odp.proc.sr.144.028.1995   Abstract

Leg 144 of the Ocean Drilling Program (ODP) recovered lavas and volcaniclastics from the volcanic basement of five northwest Pacific guyots (Limalok, Lo-En, and Wodejebato guyots in the Marshall Islands group; MIT Guyot, an isolated edifice midway between the Marshall Islands and Japan; and Takuyo-Daisan Guyot in the Japanese seamount group). Most of the lavas have undergone extensive low-temperature alteration, but their petrography, mineral chemistry, and, in some cases, whole-rock chemistry clearly demonstrate that almost all are of alkalic affinity, ranging from highly magnesian basanites to hawaiites. The sole exception is the single lava recovered from Takuyo-Daisan, which is tholeiitic or transitional in character.Tectonic reconstructions suggest that all the seamounts investigated during Leg 144 originated, in Cretaceous time, as intraplate volcanoes in what is now known as the SOPITA (South Pacific isotopic and thermal anomaly) region. One of the principal objectives of basement sampling was to determine whether the present-day manifestations of this region have persisted since the Cretaceous or whether they have evolved through time. Ratios and relative abundance patterns of those incompatible trace elements that are not readily affected by alteration processes are remarkably uniform in the Leg 144 alkalic lavas, falling well within the overall field of modern SOPITA lavas and strongly resembling those of Tahiti in particular. In this uniformity, they differ from the pronounced diversity of modern SOPITA lavas, although the sample is rather small. Thus, the Cretaceous SOPITA mantle source was little different in trace element abundances from today, although the Leg 144 data do not preclude an increase in diversity through time.

Herbert, TD, Gee J, DiDonna S.  1999.  Precessional cycles in Upper Cretaceous pelagic sediments of the South Atlantic; long-term patterns from high-frequency climate variations. Special Paper Geological Society of America. 332:105-120. Abstract
n/a