Apparent polar wandering with respect to North America since the late Precambrian

Citation

Van Alstine, David Ralph (1979) Apparent polar wandering with respect to North America since the late Precambrian. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/3p9w-p930. https://resolver.caltech.edu/CaltechTHESIS:09152010-083013551

Abstract

To achieve a better understanding of the relative significance of plate tectonics and true polar wandering, it is important that paleomagnetic poles be as accurate as possible and that knowledge of apparent polar wandering be extended to increasingly remote times. In Chapter 1, advantages of using the mode in analysis of paleomagnetic vectors are discussed, and a computer technique is described for contouring and precisely locating the modes of vector distributions that may be highly skewed. In contrast to conventional determinations of the mode, unit vectors from a given data set are treated not as discrete points, but as identical Fisherian probability density functions defined (at an angle θ from the unit vector) by: p=exp(sk(cos θ-1)), where k is the estimate of the Fisherian concentration parameter, and s is an arbitrarily assigned "smoothing parameter." A grid, representing the cumulative probability distribution of the total sample of vectors, is contoured to provide a graphical display of the distribution around the most probable value, the mode. By repeatedly contouring the same sample of vectors with successively larger values of s, and by treating the mode as a vector with length given by the total probability value at the mode, "progressive modal diagrams" can be constructed, to aid in determining the stable position of the mode of skewed distributions. In addition, a new statistic, "β_(95)" is suggested as an error estimator for the mode. The statistic β_(95) is derived from the largest subset of the total sample that has a mean identical with the mode of the total sample. This statistic is defined as the Fisherian half-angle of the cone of 95% confidence for the mean of this subset. In Chapters 2 and 3, results are presented of a paleomagnetic investigation of upper Precambrian through Middle Cambrian sedimentary rocks from the Cordilleran geosyncline. Over 800 oriented samples were obtained from a homoclinal, conformable sequence of terrigenous and carbonate miogeoclinal strata in the Desert Range, Nevada. Thermal demagnetization isolated similar characteristic magnetization directions in red-purple mudstones of the Wood Canyon Formation (Lower(?) and Lower Cambrian) and in gray limestones of the Carrara and Bonanza King Formations (Lower and Middle Cambrian). Lithologic and magnetic evidence suggest these magnetizations were acquired penecontemporaneously with deposition. The similarity of the characteristic magnetizations in these strata implies that little apparent polar wander occurred with respect to North America from early Early through middle Middle Cambrian time. The divergence of these directions from those from the partly coeval Tapeats Sandstone of the Colorado Plateau probably resulted from a net 36° clockwise rotation of the Desert Range section about a vertical axis. This rotation is probably due to mid-Tertiary oroflexural bending, but may in part have been caused by Mesozoic thrusting. The characteristic magnetization of the uppermost Johnnie Formation (upper Precambrian; ~650 m.y.) has two polarities, which allows its direction to be established despite incomplete removal of a secondary component of recent origin. The pole from the uppermost Johnnie is about 47° from the pole for the Wood Canyon Formation. It seems probable that at least 45° of apparent polar wandering occurred with respect to North America between about 650 m.y.B.P. and Early Cambrian time. In Chapter 4, a method for constructing apparent polar wander (APW) paths is presented that includes: (1) grouping data into standard time intervals of about 22 m.y. duration; (2) using new criteria for selection of paleomagnetic poles to be given unit weight; and (3) using the mode to represent the estimated pole position for each time interval.. This method is applied to paleomagnetic data from about 250 references to revise the Phanerozoic APW path for North America. The revised path is documented with an interval-by-interval review of all reliable North American paleomagnetic poles for the Phanerozoic. Unlike the smooth and simple APW paths first obtained in the late 1950's, the revised Phanerozoit APW path for North America is characterized by frequent, abrupt changes in the direction and rate of apparent polar wandering. Some anomalous but apparently reliable poles that lie off the revised path may be attributable to (1) complex tectonic histories of parts of the present North American plate, and (2) possibly, complex structure of the geomagnetic field for periods of time that may have been as long as 10^5 to 10^6 years. In Chapter 5, a speculative synthesis is presented of the revised Phanerozoic APW path for North America and an inferred north polar wandering path for Mars. It has been proposed that quasi-circular features in the Martian polar regions might represent margins of nearly circular caps that had formed symmetrically about the poles. These features would then in effect be "fossil latitudinal circles," and the offsets in their centers of curvature from the present geographic poles might provide evidence for polar wandering on Mars. If this interpretation is correct, then a polar wander path for Mars can be obtained by connecting the centers of curvature of successive margins of the polar caps. A polar wander path was derived by fitting the most prominent quasi-circular features in the north polar region to portions of circles of nearly constant radius (~3.4°) and then connecting the centers of curvature in sequence. An interesting characteristic of the Martian north polar wander path so obtained is its tendency to exhibit two major changes in direction during a time interval estimated as between 50 and 500 m.y. These two bends in the Martian north polar wander path are reminiscent of the "hairpin" bends that occur about every 100 to 130 m.y. in the Phanerozoic APW path for North America. In fact, a comparison between apparent polar wandering with respect to North America and the inferred true polar wandering on Mars reveals a remarkable similarity: both processes can be modeled as the superposition of a quasi-periodic component and a secular component. On both Earth and Mars, the quasi-periodic component is elliptical and has a period in the 10^8-year range; and on both planets, the secular component propagates at a nearly constant speed in a direction nearly perpendicular to the major axis of the oscillatory component. Moreover, the length of the secular shift per cycle is roughly the same as the length of the major axis of the oscillatory component. These unexpected similarities suggest that a highly ordered form of true polar wandering occurs on Earth and Mars as a consequence of a physical process common to both planets. An oscillatory form of mantle convection is tentatively proposed to be the excitation function of quasi-periodic polar wandering on Earth and Mars.

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