August GEOLOGY and GSA TODAY Media Highlights

First field evidence of southward ductile flow of Asian crust beneath southern Tibet
Jess King et al., Open University, Department of Earth Sciences, Walton Hall, Milton Keynes, Buckinghamshire MK7 6AA, UK. Pages 717-720.

Major mountain ranges like the Himalaya have thick crustal roots that heat up rapidly due to radioactive decay. The India-Asia collision, which started about 55 million years ago, would have caused rocks in the deep crust beneath the Himalaya to begin to melt about 20 million years later. Theoretical models suggest that these small amounts of magma dramatically weakened a deep layer of rock so that it could flow as a narrow channel, oozing out toward the surface where the summer monsoon vigorously eroded the mountain front. King et al. present the first geological evidence for a southward flow of partially molten deep crust from Asia, north of the line of collision, to beneath Indian crust during the Miocene. Vertical sheets of magma, called dikes, emplaced into Indian crust south of the collisional suture 12–9 million years ago have a geochemical and isotopic fingerprint indicating a source in the Asian plate to the north. The partially molten Asian source material for the dikes must therefore have been involved in a southward flow at depth, so that the dikes could be intruded vertically into Indian crust south of the line of collision. The peculiar geochemistry of the dike magmas also implies that the Indian crustal slab sliding under Asia around 10 million years ago was diving down into the mantle at a very steep angle, rather than almost horizontally.

Coastal dunes in Westland, New Zealand, provide a record of paleoseismic activity on the Alpine fault
Andrew Wells and James Goff, Geological and Nuclear Sciences, Geohazard Solutions, 2 Mclennan Road, Hawea Flat, Otago 7979, New Zealand. Pages 731-734.

Wells and Goff studied coastal dune sequences in southwestern New Zealand that have built up over the past several thousand years. For at least the past 800 years, new dune ridges have formed in the few decades immediately following large earthquakes on the Alpine fault. This is a response to huge volumes of sediment being brought down to the coast, released from the mountainous catchments through landsliding during the earthquakes. The dunes may provide a valuable tool for identifying and dating prehistoric ruptures on the Alpine fault over the past few thousand years. This direct response of coastal plains to earthquakes may be unique to this part of New Zealand, but it is likely that tectonic activity has left some sort of record in other coastal regions too.

Extensive thin sequences spanning Cretaceous foredeep suggest high-frequency eustatic control: Late Cenomanian, Western Canada foreland basin
A. Guy Plint and Michael A. Kreitner, University of Western Ontario, Earth Sciences, London, Ontario N6A 5B7, Canada. Pages 735-738.