INERTIAL INTERCHANGE TRUE POLAR WANDER HYPOTHESIS IN THE GRAND CANYON
One topic which I will address in my research is a possible connection
between the Cambrian Evolutionary Explosion and a rapid burst of true polar
wander, as suggested by Kirschvink et al 1997. The idea is that the whole of the
solid earth (mantle and crust) shifted approximately 90 degrees relative to the
spin axis, in an interval of only about 15 million years. The suggested
geophysical trigger for this was an interchange event, in which the magnitude of
Earth`s principal moment of inertia (about which all planets must spin) became
less than that of the intermediate inertial axis, located on the Equator. The
resulting imbalance causes the solid earth to reorient via True Polar Wander,
termed an inertial interchange (IITPW) event. There is evidence that this shift
may have occurred during early Cambrian time (Tommotian through the Toyonian
stages), which is the classic interval of the Cambrian Explosion. This evidence
takes the form of mostly paleomagnetic data showing a large shift between 534
and 505 Ma on most continents. This is also supported by variations in facies
and sea level that indicate a rapid transition from polar to equatorial
environments, and vice versa. Although there seems to be a sizable amount of
data supporting this hypothesis, it remains highly controversial, and there are
many nuances of the theory that deserve more detailed investigation. The problem
of IITPW is not simply of intellectual interest, as it has rather widespread
implications. Foremost among them is the interaction with biology. The Cambrian
is a time of extreme biological diversification, an explosion of the animal
phyla that we are still trying to understand. It is very possible that the IITPW
event had a large impact, if not a driving force, on the development of early
life. It is, however, unclear exactly how this interaction was produced. The
process of assessing the validity and furthering our understanding of this event
involves the correlation of data from around the world. In order to tease TPW
out of the
paleomagnetic
data one must have very good constraints on continents across the globe. Care
must be taken to sift TPW from apparent polar wander, as well as from ordinary
individual plate motions that were still occurring during the IITPW event
(although at a much slower rate). Specifically, I will re-examine the
paleomagnetism and magnetostratigraphy of the Tapeats Sandstone of the Grand
Canyon. This unit is constrained paleontologically by trilobites to lie very
close to the Early/Middle Cambrian boundary, very near the end of the proposed
IITPW event. Although a paleomagnetic pole for this unit was reported by Elston
& Bressler (1978), it lacks a robust field constraint on the age of the
magnetization. Subsequent advances in sampling and processing techniques since
that work was done now provide an opportunity to achieve a much more reliable,
higher precision constraint for North America, which is necessary for testing
the IITPW hypothesis. I was recently able to collect a preliminary series of
oriented samples from a basal conglomerate of the Tapeats sandstone in the Grand
Canyon, which should provide the basis for a paleomagnetic conglomerate test.
Preliminary Results from the Deer Creek Section
For more information on IITPW, check out this page:
http://www.gps.caltech.edu/~devans/iitpw/science.html
SNOWBALLS IN DEATH VALLEY
A second project that I am currently laying the groundwork for will also focus on Neoproterozoic and Cambrian time. Rocks of this age have been notoriously difficult to correlate due to a lack of biostratigraphic control. Without dateable volcanic beds, age constraints are questionable at best. A possible solution to this is on the horizon, involving the direct, ion-microprobe dating of early diagentic xenotime overgrowths on detrital zircon grains in sandstone. Our understanding about the early evolution of the planet, the timing of `snowball earth` episodes, and calibration the rates of organic evolution could all be constrained better if this technique works even occasionally. This project is currently in the preliminary stages, and I envision being able to make at least two contributions. First I will be working on development of the process itself, finding non-destructive techniques for the extraction, preparation, and analysis of the xenotime overgrowths. Though it sounds simple, there are several important hurdles to overcome, including developing a method to extract xenotimes that are large enough to analyze, and acquiring a standard set of well-dated xenotimes (which currently do not exist) for calibration purposes. Second, I will conduct a field study in the Northern Panamint Mnts., CA, which will consist of detailed mapping of the area and sampling sandstones to extract xenotime. In this area (which has been mapped extensively) there are several field relationships that cannot be resolved without more accurate dating of the units involved. Once completed, this work will provide information critical to understanding the scope and nature of rifting in Laurasia at that time, and contribute to the overall understanding of the Neoproterozoic Snowballs, and associated biologic and tectonic events. Though not explicit in the preparations for either project, there might also be an opportunity to tie the two together by attempting to acquire xenotime dates on samples collected from the Tapeats and thereby provide yet another age constraint for the paleomagnetic poles for North America.
A first look at some of the results of this study in progress.