March/April Geological Society of America Bulletin media highlights

A common approach used to determine which rocks were eroded and deposited as sediment in ancient sedimentary basins is to study grains of zircon, a mineral that survives erosion, sediment transport, deposition, and transformation into sedimentary rock. The ability to determine the age of zircons means a spectrum of ages for tens of individual grains within a sedimentary rock can be matched with rock ages within potential source regions, and thus the most likely source region can be determined. For Paleozoic sedimentary rocks that were deposited at the margin of the supercontinent of Gondwana (comprising parts of South America, Antarctica, Australia and New Zealand), the detrital age spectrums are remarkably similar along the length of the margin, over a distance of thousands of kilometers, so confident characterization of the source is not possible. But, with the ability to measure the hafnium isotope composition of a zircon grain in addition to its age, a more detailed geological fingerprint of the source region is revealed. Paleozoic-aged sedimentary rocks of the Ellsworth Mountains, Antarctica, have similar detrital zircon age populations throughout the sequence; however, the additional hafnium data reveal that the sediment source has changed through the evolution of the basin: older sedimentary rock has a source region that was proximal to the basin, being derived from rock similar to that which underlies the basin, whereas the younger parts of the sequence have a source that is more distal, predominantly from within southern Africa and parts of South America, rather than East Antarctica. The inference is that source regions from within west Gondwana and east Gondwana can be better distinguished with the additional hafnium data. Moreover, this technique can be used to correlate sedimentary rock units. Analyses from two-isolated rock exposures south and west of the Ellsworth Mountains suggest they correlate with the older parts of the Ellsworth Mountains sequence, whereas another locality apparently has a different source, located within East Antarctica. These results suggest that such an approach to provenance analysis can better identify the source region, substantiate changes of sediment provenance up-stratigraphy, and can also be used as a broad correlation tool.

The Bend: Origin and significance
Rex H. Pilger, Landmark Graphics, R&D, Highlands Ranch, CO 80129, U.S.A. Pages 314-328.

Keywords: plate tectonics, hotspots, Hawaii, plumes, kinematics, plate reconstructions.

Is there another way to look at hotspots, especially the Hawaiian-Emperor island-seamount chain? Rex Pilger shows how sets of hotspots may be “embedded” in the upper mantle and move together. These sets thereby define a few large stable reference frames he calls “mesoplates.” A key to this inference is the distinctive “bend” in the chain and its correspondence with motion paths of the Pacific and adjacent continental plates over the past 75 million years.

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