Instability of a lithospheric step beneath western North Island, New Zealand Tim Stern et al., Institute of Geophysics, Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand. Posted online 7 Feb. 2013; http://dx.doi.org/10.1130/G34028.1.
This study shows that classical geological structures develop at Earth's surface that are due to relatively rapid processes in Earth's mantle. If good stratigraphic control exists, values for the viscosity of Earth's upper mantle can be estimated from this type of study. Tim Stern and colleagues highlight geological observations from western North Island, New Zealand, that show a migrating zone of uplift and subsidence. The amplitude of the uplift-subsidence is ~4 km, the wavelength is ~ 300 km, and it developed on a time scale of about 10 million years. Stern and colleagues show that this surface deformation is most readily explained by a mantle instability, or delamination, that started from a strike-slip zone with an element of tension (or what we call a transtensional zone). They develop a numerical model based on dimensional variables that allows us to test for the most important conditions to trigger an instability in this back environment and find that the key parameters are a low viscosity lower crust, and a ratio of crustal to mantle viscosity of about five and an absolute upper mantle viscosity of about 1020 Pa s.
Colorado River chronostratigraphy at Lee's Ferry, Arizona, and the Colorado Plateau bull's-eye of incision
Joel L. Pederson et al., Dept. of Geology, Utah State University, Logan, Utah 84322, USA. Posted online 7 Feb. 2013;
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