I. Detailed Records of Geomagnetic Field Behavior from Death Valley and Hawaii. II. An Age Constraint on Gulf of California Rifting from Santa Rosalía, Baja California

Citation

Holt, John William (1997) I. Detailed Records of Geomagnetic Field Behavior from Death Valley and Hawaii. II. An Age Constraint on Gulf of California Rifting from Santa Rosalía, Baja California. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/y9sj-ca84. https://resolver.caltech.edu/CaltechTHESIS:09072023-170005385

Abstract

Detailed records of geomagnetic field behavior were produced and analyzed from Plio/Pleistocene sedimentary rocks of Death Valley, California and from Pleistocene/Holocene volcanic rocks of Hawaii. These records provide new information about geomagnetic field polarity reversals, excursions, secular varia­tion, and the paleomagnetic recording process in sediments. In addition, a magnetostratigraphic and geochronlogic study of Mio/Pliocene marine sedimen­tary rocks in Baja California, Mexico was performed in order to provide a new age constraint on Gulf of California rifting.

The paleomagnetic studies in Death Valley were undertaken on siltstones, sand­ stones, and evaporites exposed along the southern Death Valley fault zone in the Confidence Hills. These rocks contain multiple records of the Réunion and Olduvai normal-polarity subchrons (2.15 - 2.13 Ma and 1.95 - 1.79 Ma, respec­tively) within rocks formed as a result of deposition in different subenvironments of a saline ephemeral lake, with an average deposition rate of ~ 30 cm/kyr. Variations in bedding attitudes between the exposed sections allow a fold test of paleomagnetic directions. Two records of the upper Olduvai polarity reversal were obtained which agree despite differences of lithology, depositional environ­ment, and structural tilting. These records indicate several phases of polarity shifts during the transition of the geomagnetic field from normal to reversed polarity. Transitional virtual geomagnetic poles (VGP's) lie in longitudinal bands ~ 90° away from the sampling site longitude, vastly different from VGP's pro­duced by studies in other locations around the world but consistent with site­ dependent trends of VGP paths observed in global data compilations. Studies of the anisotropy of anhysteritic remanence conclude that inclination shallowing in sediments during periods of low ambient magnetic field intensity is a possible cause for this site dependence of VGP paths. However, some aspect of the transitional geomagnetic field is recorded by the sedimentary rocks of Death Valley whether or not inclination shallowing took place.

A detailed record of the Réunion normal-polarity subchron was obtained from one of the sections in the Confidence Hills. This record shows that the Réunion subchron was a single normal-polarity event of ~ 20 kyr duration, which is a significant finding due to the lack of previous data in this time interval. The pres­ence of a lithofacies which contains disruptive anhydrite crystals dispersed throughout the matrix creates two small gaps in the record just prior to the nor­mal-polarity Réunion interval. Remagnetizations within this lithofacies facilitate the interpretation of the depositional environment responsible for growth of the disruptive evaporites.

Samples of the 1 km core produced by the Hawaii Scientific Drilling Project (HSDP) provided the basis for a study of geomagnetic field excursions and secular variation during the past 400 kyr at Hawaii. This core consists of over 200 lava flows erupted from Mauna Loa and Mauna Kea volcanoes. In contrast to previous hypotheses that the non-dipole field has been anomalously low in the central Pacific region for the past few hundred kyrs, the results of this study show that secular variation, and hence, the non-dipole field component, at Hawaii is consistent with secular variation elsewhere on the globe for the past 400 kyr. In addition, the data show evidence for a persistent axial quadrupole in the time­ averaged field. This research also resulted in the first records of geomagnetic field excursions in the central Pacific which may be correlated with those found elsewhere on the globe, lending support to the hypothesis that these are global events rather than local perturbations of the geomagnetic field.

Paleomagnetic and geochronologic studies of marine sedimentary rocks in the Santa Rosalía basin, Baja California Sur, show that despite the presence of local copper ore deposition, primary magnetic remanence directions may be obtained from most of the marine sandstones overlying the basement in that area. Using preliminary magnetostratigraphy and an ⁴⁰Ar/³⁹Ar isotopic age of 6.76 ± 0.45 Ma (1σ) obtained for a volcanic unit interbedded with the sandstones, a correla­tion with the geomagnetic polarity time scale was made possible, yielding an age of 7.1 ± 0.05 Ma for the base of the marine section. This provides a new age constraint on Gulf of California rifting and may help to refine models of North America - Pacific plate boundary interactions during the period 12 - 3.5 Ma.

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