Douwe van Hinsbergen on What Drives the Motions of Tectonic Plates
Douwe van Hinsbergen is Professor of Global Tectonics and Paleogeography at the University of Utrecht. He has reconstructed the plate movements over the past 250 million years of the regions that contain today’s mountain belts. He explains how these reconstructions appear to be consistent with seismic tomography of the mantle and the geochemical signatures of lavas at mid-ocean ridges, above subductions zones, and at hot spots only if the mantle is relatively static over geological time. This in turn suggests that plate motions are not driven by a vigorously convecting mantle.
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Podcast Illustrations
All illustrations courtesy of Douwe van Hinsbergen unless otherwise noted.
Diagram of a subduction zone showing slab pull and ridge push, the main gravitational forces invoked to explain plate motions. Slab pull is caused by the negative buoyancy of the old, cold, dense oceanic lithosphere, which becomes denser still as it descends into the mantle when basalt transforms into eclogite. Ridge push arises from the gravitational pull of the lithosphere down the slope at ridges and down the gentler slope as the lithosphere cools and sinks before it subducts.
Plate reconstruction showing the presence of a former continent called Greater Adria in the Mediterranean region between Northern Africa and Southern Europe. Douwe van Hinsbergen used reconstructions such as this one to predict the location of past subduction zones.
Van Hinsbergen et al. (2020), Gondwana Research 81, 79-220
Enlarged detail of the Mediterranean region shown in the above reconstruction.
The Grasberg gold, copper and silver mine in Papua New Guinea. These magmatic rocks display a geochemical signature associated with subduction zones, but there is no subduction occurring there. Douwe van Hinsbergen’s research suggests that the mantle below the mine was enriched by subduction of a slab (the Arafura slab) 30-25 million years ago, whose relics appear in seismic imagery of the mantle 500-650 km below the mine. As the Australian plate moved north over the Arafura slab, it “ploughed” through the mantle, with the leading edge causing melting of the mantle and forming the magmas that carry the signature of the past subduction. The magmas ascended to the surface and cooled to form the rocks of the Grasberg mine.
Courtesy of Freeport McMoRan
These diagrams present the evidence for the suggestion that the geochemical signature of the rocks in the Grasberg gold mine of Papua New Guinea results from a subduction zone in the region 30-25 million years ago. The chemical imprint of that subduction on the mantle remained stationary to the present day, when the northward movement of Australia triggered mantle melting that brought material sourced from the mantle to the surface, thus providing a window into the composition of the mantle below. A: absolute motion of the Australian plate. B: plan-view seismic tomogram at a depth of 586 km. Blue areas are regions of higher seismic velocities and are interpreted as lithosphere of the Australian continent and of the Arafura and Halmahera slabs. C: location of the seismic tomogram cross section of D. Gr/Er refer to the Grasberg and Erstberg magma bodies. D: Seismic tomogram of a vertical section parallel to the direction of the absolute motion of the Australian plate over the past ~25 million years. White dots represent earthquake locations. E: Interpretation of the seismic tomogram of D, indicating the locations of the Arafura and Halmahera slabs and Australian continental lithosphere.
Van Hinsbergen et al., (2020), Geophysical Research Letters, 47, e2020GL087484
The diagrams present a model of how the rocks bearing the imprint of a subduction zone might have formed in New Guinea. A. An edge of the Australian lithosphere was formed when a slab broke off 50 million years ago. B. Australia moves north, pulled by an intra-oceanic subduction zone to the north that creates a subduction zone signature in the mantle. C. Over the period from 25 to 8 million years ago, the northern continental edge of Australia reaches the mantle region enriched by the subduction zone to the north. D. The continued ploughing of Australia through the enriched mantle causes melting, and magmas rise through the faults associated with a plate boundary.
Van Hinsbergen et al., (2020), Geophysical Research Letters, 47, e2020GL087484