Pangea

Pangean Reconstruction Problems

(Sunday, 25 November 2007)
Reconstruction of ancient geographic locations of tectonic plates are mainly based on paleomagnetic data. Some ancient rocks got magnetised at the time of their formation. By measuring the magnetic inclination it is possible to determine the latitude (distance from equator) at the time of formation.

A paper in the journal Science of 23 November 2007 highlights some Inconsistencies Between Pangean Reconstructions and Basic Climate Controls. The paper is based on sandstones in the southwestern United States.

The supercontinent Pangea dominated the look of the earth from the Permian into the Jurassic for a period of 100 million years. According to paleomagnetic reconstructions the study area was near the equator in the Early Permian and moved northward to about 20°N by the Early Jurassic. See the Paleomap reconstruction.

Ancient desert dunes provide excellent evidence of the prevailing winds when they formed. Wind directions are governed by the latitudinal distribution of solar heating, which is modified by the rotation of the planet and the distribution of landmasses. (The three major global wind belts are the Tradewinds at around 0-30°, the Westerlies at middle latitudes, and the Polar Easterlies).

Using climate models the wind directions found for 300 million years ago are in accordance with the paleomagnetic results with a position near equator. During the entire interval of 100 million years, the dominant winds in the north came from the northeast, curving to become northwesterly over the southern portion of the outcrops. This suggests that the Colorado Plateau stayed within the same climate zone during the entire time span, and that is inconsistent with a position 200 million years ago of around 20°N (say between 17°N - 28°N according to paleomagnetic data). Northeasterly winds changing southward to northwesterlies would be out of place near 20°N, but would fit well near and just south of the equator, where northwesterly winds represent cross-equatorial flow induced by a strong summer monsoonal circulation in the southern hemisphere.

The authors also point to a fact that further may aggravate the discrepancy:

“More recently, geophysicists have called attention to the importance of sediment compaction to paleomagnetic interpretations, especially for rocks in which the paleomagnetic signal is carried by detrital hematite. Comparisons of paleomagnetic data from such sedimentary rocks with those from igneous rocks (which do not compact) indicate that sedimentary rocks are likely to yield paleolatitudes that are too low. Because the sedimentary rocks from the plateau that provided the evidence for a low-latitude position contain detrital hematite, most paleomagneticists would now favor the higher-latitude interpretation for the plateau, thereby aggravating the discrepancy with our earlier climate-based interpretation.”

Conclusion: At least one of the following scenarios must be true: The latitude based on paleomagnetism is incorrect; the interpretation of how winds shaped the dunes is mistaken; the basic climate controls in the Jurassic were different from those of today; or the paleogeographic reconstructions available are insufficient to adequately reproduce the wind fields responsible for dune formation.