Posted on 17/03/2011
Seismic properties: red/orange colours require abundant quartz
Common mineral quartz may be behind earthquakes, mountain building and other continental tectonics, according to a new study by Geophysicists Marta Pérez-Gussinyé from Royal Holloway, University of London and Tony Lowry from Utah State University.
The research, published in Nature today (17 March), suggests that quartz, familiar from sandy beaches, may hold the key to the persistent zones of weakness along which the Earth’s continents deform. This conclusion comes from large-scale survey of seismic velocities, gravity and heat-flow measurements in the western United States, which has revealed surprising correlations between these properties and the pattern of deformation of this mountainous region.
“The key to our discoveries has been to devise new ways to jointly analyze many geophysical data sets that were previously analyzed separately. This in combination with the US-array seismic experiment, which has enabled us to obtain an image of the Earth's subsurface at a 70-km lateral spacing over the whole surface of the United States, has been decisive to decipher the relative abundance of quartz in the crust.” says Dr Pérez-Gussinyé, from the Earth Science Department at Royal Holloway.
“In particular, it was not clear why deformation repeatedly focuses in some areas, while others remain undeformed. Our analysis shows that it is the places where quartz, the weakest crustal mineral, is most abundant, that experience repeated cycles of deformation during Earth’s history”, she adds.
The researchers say the tectonic plate itself is formed by two different layers: the crust which is the shallowest layer and goes to a depth of around 30 km, and the mantle lithosphere which extends from the base of the crust to around 120 km depth. There has been much discussion on the relative roles of crust vs. lithospheric mantle in controlling where deformation occurs.
The scientists’ analysis shows that deformation first localises where the crust is quartz-rich crust and relatively weak, and then initiates processes that promote warming, hydration, leading to further weakening and focusing of deformation. While this process has led to the complete deformation of the whole tectonic plate in places like the Basin and Range extensional province, in others like the Wyoming fold and thrust belt, the mantle lithosphere remains strong, suggesting a role for the mantle in controlling the pattern of deformation.