Chuck DeMets on High-Resolution Plate Motions
Chuck DeMets is Emeritus Professor of Geoscience at the University of Wisconsin, Madison. He studies the magnetic anomalies in sea-floor rocks to reconstruct plate motions at a temporal resolution five times better than has been done hitherto. This has revealed unexpected speed-ups and slow-downs in plate motions that provide juicy puzzles for geodynamicists. In the podcast, he focuses on the detailed motions of the Indian plate that show, among other things, that its northward movement actually sped up for a period after the collision with Eurasia before it settled down to a steady slower speed.
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Podcast Illustrations
Images courtesy of Chuck DeMets unless otherwise indicated.
Magnetic stripes are created symmetrically on each side of a mid-ocean ridge spreading center as illustrated in (b). When the melt emerges from the ridge, the Earth’s field aligns the magnetite crystals it contains. When the rock solidifies, the aligned magnetite crystals are frozen in and thus preserve a record of the field, indicated by the black and white shading. Since the age of the magnetic field reversals of the Earth are known accurately from radiometric dating and the effect of periodicities in the Earth’s orbital motions (Milankovitch cycles), the age of each stripe edge can be determined. This is the principle used by Chuck DeMets to obtain his high-resolution reconstructions of plate motions. An example of measured sea-floor anomalies on either side of the Reykjanes ridge, which is part of the mid-Atlantic ridge just south of Iceland, is shown in (c). In order to determine exactly where the reversals are located in the noisy magnetometer measurements, the measured field is compared with an idealized profile of the known reversals to find the optimal alignment as shown in (a).
Magnetic anomaly measurements measured by Russian ships across the Carlsberg ridge. The observed profile extends northeast from the ridge, and the synthetic magnetic anomaly profile below the map shows the magnetic anomaly pattern resulting from an assumed 30 mm/yr full spreading rate.
High-Resolution Motion Study of the Indian Plate
Maps and Diagrams from DeMets et al. (2021), Geophys. J. Int. 227, 1730
Chuck DeMets with his Russian collaborator, Sergey Merkouriev. The high-resolution reconstructions of the Indian plate motions relied on the detailed magnetic surveys performed by Sergey Merkouriev aboard a Russian navy ship during the 1990s.
The map shows the Indian plate with the three ridges discussed in the podcast: the Carlsberg Ridge, the Central Indian Ridge (Central IR), and the Southwest Indian Ridge, as well as the Sumatra trench where the Indian plate is subducting to the northeast. The map shows how movement of the Africa (Somalia) plate would cause an anticorrelated change in spreading rates at the Carlsberg Ridge, which lies on the northeast side of the plate, and at the Southwest Indian Ridge, which lies on the southeast side of the plate. The black dots indicate the locations of earthquakes with foci shallower than 60 km depth from 1963 to 2003. The shaded areas show diffuse plate boundaries where the deformation that accommodates the plate motion is distributed across many faults in a broad area, as opposed to by a single feature such as a mid-ocean ridge.
The area covered by high-resolution plate motion studies performed using the magnetic survey data from the Russian navy. Plate boundaries are shown in red. AN, Antarctic Plate; AR, Arabia Plate; AU, Australian Plate; CIR, Central Indian Ridge; CP, Capricorn Plate; FZ, fracture zone; SM, Somalia Plate.
Ship and airplane magnetic profile tracks used to perform the high-resolution plate motion analysis. The red, blue, and black lines, respectively, indicate tracks from Russian vessels, from a 1990 aeromagnetic survey, and all other sources. The bold lines indicate active plate boundaries.
The magnetic anomaly grid derived from dense Russian and other magnetic data for the region shown in the figures above. Reds and blues indicate areas of positive and negative magnetic anomalies, respectively.
Results of the high-resolution study of the Carlsberg Ridge showing the motions of India relative to Somalia over the past 60 million years. (a) The spreading rate showing the 100% speed-up at about 50 million years ago followed by a steep slow-down to about 45 million years ago, assumed to reflect the effect of the collision with Eurasia. The plot also shows the short-lived small speed-up at about 18 million years ago, after which the plate motion is a fairly constant 22 mm/yr. (b) Plate slip directions. CIR, Central Indian Ridge; IN, India; SM, Somalia.
Inferring the Motion of the Somalia Plate From Spreading Rates of the Three Ridges Bordering the Plate
Sea-floor spreading rates (a) and slip directions (b) between the Somalia-Antarctic (Sm-An), Somalia-India (Sm-In), and Somalia-Capricorn (Sm-Cp) plates since 52 million years ago. The plots reveal the anticorrelation between the Sm-An spreading rates (red) and the Sm-In spreading rates (black) that would be expected if the variations were caused by motion of the Somalia plate. The results are based on the high-resolution determinations of the spreading rates at the Carlsberg Ridge, the Southcentral Indian Ridge, and the Southwest Indian Ridge.