Extraordinary Polar Wander During the Late Jurassic?

Small amounts of polar wander have occurred during geological history, but whether larger amounts occurred is still controversial. Did a truly large polar wander event really happen?

SOURCE: Journal of Geophysical Research: Solid Earth

Paleogeographic reconstructions at different points in geologic history with regions of interest shown. The “monster shift” occurred between 160 and 148 million years ago (panels a to e). Paleolatitude as a function of time for the regions of interest with arid and humid climate belts indicated (panel f). Credit: Muttoni and Kent [2019], Figure 7

By Mark J. Dekkers 22 April 2019

Knowing how much Earth’s rotation axis has moved in the geological past – the so-called “polar wander” – has important implications for understanding geological processes. For example, it exposes significant areas on Earth to rapidly changing climatic conditions. The polar shift is a consequence of the Earth responding to a changed center of gravity, caused by processes such as slab-break-off.

While small shifts of 5 to 10 degrees have occurred on several occasions during geological history, bigger changes in polar motion have always been considered controversial. A notably large polar shift has been argued to have occurred during the Late Jurassic and is sometimes referred to as the “Jurassic monster polar shift”. However, the paleomagnetic data underlying this monster shift are not always the most robust, and its magnitude has been debated since it was proposed by Kent and Irving [2010].

Muttoni and Kent [2019] provide new paleomagnetic data for the critical late Jurassic period. They show that widely-used late Jurassic data from North America (Morrison Formation, Colorado Plateau) are in fact overprinted and their usage for paleogeographic reconstruction should be discontinued. To constrain the polar motion during this critical late Jurassic time interval, the authors turn their attention to Adria (Italy) of which paleomagnetic poles are essentially ignored in mainstream paleogeographic reconstructions. They embark on a re-analysis of published Adrian paleomagnetic data, keeping in mind that Adria is of African affinity, and its poles closely map to global apparent polar wander path compilations. They can be rotated into North American coordinates using the Central Atlantic reconstruction parameters.

Importantly, Muttoni and Kent subject the extensive pool of Adria paleomagnetic data to modern procedures to correct for so-called inclination shallowing that restores the true paleolatitude of the site (which was previously done only on a limited subset of the data). Following from their re-analysis, Muttoni and Kent provide a series of paleomagnetic poles that neatly track the Jurassic monster polar shift which they consider a firm proof of its reality, 30 degrees in approximately 12 million years (from approximately 160 million years ago to 148 million years ago), followed by a slower rebound.

The authors also discuss geodynamic implications of the large-scale rotation around a Euler pole centered on Bight of Benin on the equator, which rapidly moved regions across climate zones. The Ghawar region (Saudi Arabia) which contains the vast hydrocarbon fields of the Middle East was moved during the monster shift from the humid tropical belt during which source rock was deposited to the subtropical arid belt which induced deposition of anhydrite-bearing sealing rocks. The shift is thus argued to have created rare geological boundary conditions that enabled the formation of the largest hydrocarbon fields in the world.

Citation: Muttoni, G., & Kent, D. V. [2019]. Jurassic monster polar shift confirmed by sequential paleopoles from Adria, promontory of Africa. Journal of Geophysical Research: Solid Earth, 124. https://doi.org/10.1029/2018JB017199

—Mark J. Dekkers, Associate Editor, JGR: Solid Earth, and Department of Earth Sciences, Utrecht University, The Netherlands

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