Equatorial paleosecular variation of the geomagnetic field from 0 to 3 Ma lavas from the Galapagos Islands

Kent, Dennis V.; Wang, Huapei; Rochette, P.

doi:10.1016/j.pepi.2010.08.010

Cited by 25 publications

(33 citation statements)

References 42 publications

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“…Previous alternating field (AF) (16) and thermal demagnetization (TD) (13) studies showed shallow average paleomagnetic directions (AF mean I = 1.9°± 3.0°; TD mean I = 2.3°± 3.0°) from 51 lava sites, which agree with the GAD hypothesis. Like many lava samples that have been studied, most of the Galapagos lavas have saturation-remanent to saturation-induced magnetization ratios between 0.1 and 0.3 (13), which is consistent with a mixture of SD and MD properties (17). We also conducted additional comprehensive rock magnetic studies, which suggest most of the Galapagos lavas are good candidates for MD-corrected paleointensity experiments (SI Appendix, Figs.…”

Section: Significancesupporting

confidence: 55%

Weaker axially dipolar time-averaged paleomagnetic field based on multidomain-corrected paleointensities from Galapagos lavas

Wang

Kent

Rochette

2015

Proc. Natl. Acad. Sci. U.S.A.

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The geomagnetic field is predominantly dipolar today, and highfidelity paleomagnetic mean directions from all over the globe strongly support the geocentric axial dipole (GAD) hypothesis for the past few million years. However, the bulk of paleointensity data fails to coincide with the axial dipole prediction of a factor-of-2 equator-to-pole increase in mean field strength, leaving the core dynamo process an enigma. Here, we obtain a multidomain-corrected Pliocene-Pleistocene average paleointensity of 21.6 ± 11.0 μT recorded by 27 lava flows from the Galapagos Archipelago near the Equator. Our new result in conjunction with a published comprehensive study of single-domain-behaved paleointensities from Antarctica (33.4 ± 13.9 μT) that also correspond to GAD directions suggests that the overall average paleomagnetic field over the past few million years has indeed been dominantly dipolar in intensity yet only ∼60% of the present-day field strength, with a long-term average virtual axial dipole magnetic moment of the Earth of only 4.9 ± 2.4 × 10 22 A·m 2 .geomagnetism | paleomagnetism | Thellier paleointensity experiment | multidomain correction | time-averaged dipole field I n 1600, William Gilbert (1) articulated that Earth's magnetic field was well approximated by a bar magnet centered along its rotation axis. This model has become elaborated for the timeaveraged field as the geocentric axial dipole (GAD) hypothesis, which essentially all paleogeographic reconstructions rely on. Paleomagnetic field directions for the past 5 My for both normal and reverse polarity chrons from around the globe show that the mean inclinations closely correspond to the GAD model (2), which predicts a simple relationship between mean inclination, I, and site latitude, Lat: tan(I) = 2 × tan(Lat). The GAD hypothesis also predicts that the mean field intensity should vary with latitude, as [1 + 3 × cos 2 (90°− Lat)] 1/2 , whereby the intensity at the poles is twice that at the Equator. The modern field [i.e., International Geomagnetic Reference Field (IGRF) (3)] is dominated by an axial dipole with steep field directions at the poles about twice the intensity (∼60 μT) compared with that (∼30 μT) of near-horizontal directions in the equatorial belt and corresponds to a virtual axial dipole moment (VADM) of ∼8 × 10 22 A·m 2 . However, the expected latitudinal dependence has yet to be shown in the current global paleointensity database PINT2014.01 (4) even for the most recent past 5 My. Analyses of these data using various selection criteria tend to show a puzzling near uniformity of mean intensities with latitude and largely because of the dominance of data from 15°t o 30°latitude with ages of 0.03-0.5 Ma, which average close to the present field intensity, would suggest the time-averaged field for normal and reversed polarity chrons over the past 5 My was also near the present VADM of ∼8 × 10 22 A·m 2 ( Fig. 1 and SI Appendix, SI Text and Fig. S1).The factor-of-2 difference between the poles and Equator provides the maximum signal to res...

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Section: Significancesupporting

confidence: 55%

Weaker axially dipolar time-averaged paleomagnetic field based on multidomain-corrected paleointensities from Galapagos lavas

Wang

Kent

Rochette

2015

Proc. Natl. Acad. Sci. U.S.A.

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“…This suggests that some of the contribution to shallow bootstrapped mean inclination, particularly in Area A, is from large scatter of the directions; however, even filtering the data does not produce a mean direction with a steeper inclination similar to the expected GAD inclination. Southward motion of the Nazca Plate over the last ∼3 Ma contributes to shallower inclinations; however, the contribution of ∼0.6°change in latitude suggested from paleomagnetic data from the Galapagos Islands [Kent et al, 2010], corresponds to ∼0.8°shallower inclinations and is not sufficient to explain the significant deviation observed. The rotation about a horizontal axis parallel to Pito Deep Rift is one of the only geologically reasonable means of creating more shallow inclinations.…”

Section: Shallow Bootstrapped Mean Remanence Directionsmentioning

confidence: 73%

Paleomagnetic constraints on deformation of superfast-spread oceanic crust exposed at Pito Deep Rift

Horst¹,

Gee²,

Karson³

2011

J. Geophys. Res.

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[1] The uppermost oceanic crust produced at the superfast spreading (∼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°± 8.1°, −16.7°± 15.6°, n = 23 (Area A) and 30.4°± 8.0°, −25.1°± 12.9°, n = 44 (Area B), both are significantly distinct from the Geocentric Axial Dipole expected direction at 23°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°, Area B = 22°), (2) clockwise, vertical-axis rotation of the Easter Microplate (A = 46°, B = 44°), and (3) block tilting at Pito Deep Rift (A = 21°, B = 10°). 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.

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“…Standard deviation of inclination is equal to the α 95 of the mean paleomagnetic direction. Calculation of inclination anomaly from Galapagos Islands was made assuming the location of Fernandina Island (latitude −0.37º) as the position of the hotpot in order to account for the movement of the Nazca plate [ Kent et al ., ].…”

Section: Resultsmentioning

confidence: 99%

Plio‐Pleistocene paleomagnetic secular variation and time‐averaged field: Ruiz‐Tolima volcanic chain, Colombia

Sánchez-Duque

Mejia

Opdyke

et al. 2016

Geochem Geophys Geosyst

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Paleomagnetic results obtained from 47 Plio-Pleistocene volcanic flows from the Ruiz-Tolima Volcanic Chain (Colombia) are presented. The mean direction of magnetization among these flows, which comprise normal (n 5 43) and reversed (n 5 4) polarities, is Dec 5 1.88, Inc 5 3.28, a 95 5 5.08, and j 5 18.4. This direction of magnetization coincides with GAD plus a small persistent axial quadrupolar component (around 5%) at the site-average latitude (4.938). This agreement is robust after applying several selection criteria (a 95 < 10 ; a 95 < 5.5 ; polarities: normal, reversed, and tentatively transitional). The data are in agreement with Model G proposed by McElhinny and McFadden (1997) and the fit is improved when sites tentatively identified as transitional (two that otherwise have normal polarity) are excluded from the calculations. Compliance observed with the above mentioned time-averaged field and paleosecular variation models, is also observed for many recent similar studies from low latitudes, with the exception of results from Galapagos Islands that coincide with GAD and tend to be near sided.

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Equatorial paleosecular variation of the geomagnetic field from 0 to 3 Ma lavas from the Galapagos Islands

Cited by 25 publications

References 42 publications

Weaker axially dipolar time-averaged paleomagnetic field based on multidomain-corrected paleointensities from Galapagos lavas

Weaker axially dipolar time-averaged paleomagnetic field based on multidomain-corrected paleointensities from Galapagos lavas

Paleomagnetic constraints on deformation of superfast-spread oceanic crust exposed at Pito Deep Rift

Plio‐Pleistocene paleomagnetic secular variation and time‐averaged field: Ruiz‐Tolima volcanic chain, Colombia

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