After being exposed to Earth’s magnetic field, some minerals within rocks acquire a magnetic signal that can be used to measure how this field has changed or even reversed through time. More importantly, these signals are frequently used to trace where the rocks formed.
Frequently, however, the migration of fluids and other processes can remagnetize, or overprint, the original signal. Because this overprinting is often difficult to detect, especially in carbonates and other sedimentary rocks, remagnetization can lead to flawed reconstructions of the ancient locations and motions of Earth’s tectonic plates.
Sedimentary rocks play a key role in reconstructing the wanderings of the Tibetan Himalaya prior to its collision with Asia. To evaluate the extent of the remagnetization in these rocks, Huang et al. magnetically, chemically, and visually characterized 72 carbonate and volcaniclastic sandstone samples collected from Jurassic to Paleogene Himalayan strata in the Gamba and Tingri regions of southern Tibet.
Using a series of magnetic and petrographic techniques, including scanning electron microscopy, the team determined that although magnetite is the predominant magnetic material in both types of rocks, its characteristics vary noticeably between the two. The sandstones, for example, contain fragments of chemically unaltered magnetite and show little evidence of pyrite oxidation, whereas the carbonates host predominantly authigenic forms of magnetite that have replaced other minerals, particularly pyrite.
The authors conclude that although the sandstones retained their initial magnetic signal, the carbonates had been extensively remagnetized, most likely because of chemical reactions instigated by fluids circulating before and after the onset of the Himalayan collision. Paleomagnetic estimates of the tectonic evolution of the Tibetan Himalaya and India prior to collision can now be reconciled with geological evidence that sets the onset of collision to have occurred 58 million years ago. This evidence demonstrates the importance of using a combination of techniques to thoroughly evaluate a sedimentary rock’s magnetism prior to applying paleomagnetic data to tectonic interpretations. (Journal of Geophysical Research: Solid Earth, https://doi.org/10.1002/2017JB013987, 2017)
—Terri Cook, Freelance Writer