Paleomagnetism of the southwestern U.S.A. recorded by 0–5 Ma igneous rocks

Abstract

[1] The issue of permanent nondipole contributions to the time-averaged field lies at the very heart of paleomagnetism and the study of the ancient geomagnetic field. In this paper we focus on paleomagnetic directional results from igneous rocks of the southwestern U.S.A. in the age range 0-5 Ma and investigate both the time-averaged field and its variability about the mean value. Several decades of work in the southwestern United States have resulted in the publication of paleomagnetic data from over 800 individual paleomagnetic sites. As part of a new investigation of the San Francisco Volcanics, we collected paleomagnetic samples from 47 lava flows, many of which have been previously dated. The new data combined with published data are highly scattered. Contributions to the scatter were considered, and we find that removal of data sets from tectonically active areas and judicious selection according to Fisher's [1953] precision parameter results in an axially symmetric data distribution with normal and reverse modes that are indistinguishable from antipodal. Monte Carlo simulations suggest that a minimum of 5 samples per site are needed to estimate the precision parameter sufficiently accurately to allow its use as a determinant of data quality. Numerical simulations from statistical paleosecular variation models indicate the need for several hundred paleomagnetic sites to get an accurate determination of the average field direction and are also used to investigate the directional bias that results from averaging unit vectors rather than using the full field vector. Average directions for the southwestern U.S.A. show small deviations from a geocentric axial dipole field, but these cannot be considered statistically significant. Virtual geomagnetic pole (VGP) dispersions are consistent with those from globally distributed observations analyzed by McElhinny and McFadden [1997]. However, a systematic investigation of the effect of imposing a cutoff on VGPs with large deviations from the geographic axis indicates that while it may reduce bias in calculating the average direction, such a procedure can result in severe underestimates of the variance in the geomagnetic field. A more satisfactory solution would be to use an unbiased technique for joint estimation of the mean direction and variance of the field distribution.

40Ar/39Ar chronology of Late Pliocene and Early Pleistocene geomagnetic and glacial events in southern Argentina

“…None of the transitional flows show randomly scattered NRM directions or high NRM intensities usually attributed to lightning strikes, such as observed in flows from the southwestern United States recently studied by Tauxe et al [2003]. All 3 flows have a 95 values of 8.0° or less, and appear to accurately record the magnetic field at the time of their emplacement.…”

Magnetic properties of fault pseudotachylytes in granites

“…minerals present in the specimens (Table 1a, up to 11% in volume of magnetite). Similar large intensities have been attributed elsewhere to lightning strikes [e.g., Sakai et al, 1998;Tauxe et al, 2003;Maki, 2005]. This hypothesis can be rejected because samples were collected in canyons, gorges and quarries where the probability of ground lightnings is low and also because NRM intensities of adjacent surface material at the sampling sites is not anomalous.…”

Lightning remagnetization of the Vredefort impact crater: No evidence for impact‐generated magnetic fields

“…Lightning produces peak currents up to hundreds of kA lasting for a few tens of milliseconds, thereby generating a strong, circular magnetic field whose axis is oriented perpendicular to the current direction. The current flows nearly orthogonally to the ground surface near the strike location [ Sakai et al , 1998; Tauxe et al , 2003; Verrier and Rochette , 2002], but typically assumes a shallower angle as it runs outward near the ground surface [ Appel et al , 2006; Beard et al , 2009; Graham , 1961; Sakai et al , 1998; Shimizu et al , 2007]. The intensity of the magnetic field decreases approximately as an inverse function of distance from the current line.…”

“…This field will produce a secondary isothermal remanent magnetization (IRM) overprint in nearby ground materials. This “lightning induced remanent magnetization” (LIRM) is characterized by high efficiency as indicated by two measures: it has high Q and has remanent magnetization (REM) values (defined as the ratio of NRM to saturation IRM) [ Kletetschka et al , 2004] approaching 1 [ Appel et al , 2006; Graham , 1961; Sakai et al , 1998; Tauxe et al , 2003; Verrier and Rochette , 2002].…”