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

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

Granot, R, Cande SC, Gee JS.  2009.  The implications of long-lived asymmetry of remanent magnetization across the North Pacific fracture zones. Earth and Planetary Science Letters. 288:551-563.   10.1016/j.epsl.2009.10.017   AbstractWebsite

Large marine magnetic anomalies accompany the Pacific fracture zones (FZs) for thousands of kilometers. Although the origin of these anomalies is poorly understood, their underlying magnetization contrasts should reflect the temporal record of crustal accretion as well as geomagnetic field variations. Here we present an analysis of archival and newly collected magnetic anomaly profiles measured across three FZs from the North Pacific Cretaceous Quiet Zone (120.6 to 83 Ma) that are characterized by a remarkably uniform shape. Forward and inverse modeling indicate that these anomalies arise from remanent magnetization, with enhanced remanence located on one side of each FZ along the entire studied area. A comparison of geochemical and magnetic data from active ridge discontinuities and transform faults suggests that elevated iron content near segment ends is likely responsible for the observed anomalies in the Cretaceous Quiet Zone as well. A more complex magnetization setting is observed where the FZs contain multiple faults. There, the simple model of one-sided enhancements is only partly valid. Comparison between 3D forward modeling of the Quiet Zone magnetization and the calculated magnetization contrasts found across the Pioneer and Pau FZs suggests that the intensity of the geomagnetic field during the Cretaceous superchron had less than 50 percent variability about its average value. No major trends in the strength of the geomagnetic field during the superchron are observed. The presence of long-duration (> 30 m.y.) zones of enhanced magnetization along the young/old sides of the Pioneer/Pau FZs (both left-stepping) requires some long-lived asymmetry in crustal construction processes near ridge-transform intersections. Although the underlying mechanism that controls this long-lived asymmetry remains unclear, absolute plate motions might explain this asymmetry. Shorter period (few m.y.) variations in the amplitudes of the enhancements probably result from oscillations in crustal construction. (C) 2009 Elsevier B.V. All rights reserved.

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

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