Testing the Inertial Interchange True Polar Wander Hypothesis in the Grand Canyon

Ryan Petterson is now trying to determine whether a paleomagnetic record of the Cambrian Inertial Interchange event (which we originally identified in 1997) exists in the Tapeats Sandstone in the Grand Canyon. Our lab took a combined rafting and sampling trip through the Canyon last summer to sample the Tapeats for this purpose. Here is Ryan's description of his current research:

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.