Ductilely deformed bands of quartz. Image courtesy the USGS.

Imaging of the western US using EarthScope seismic data reveals the abundance of quartz in the crust exerts a first-order control on temperature, deformation, uplift and fluid flux in the continents.

(See the 17 March, 2011 paper in Nature!)

Figure 1: Variation in averaged seismic velocity ratio, v_P/v_S, of continental crust in the western United States. Red/orange colors require abundant quartz; blue/green indicates mafic rocks with little or no quartz.


Figure 2: Elevation of the western United States. Relatively stable blocks within the broad, deforming plate boundary zone are labeled, and are recognizable by their low relief reflecting predominantly erosional and depositional processes. Actively deforming regions exhibit high relief dominated by faulting.

New imaging (Fig. 1) of the averaged seismic velocity ratio, v_P/v_S, of continental crust in the western United States shows a surprisingly strong relationship both to deformation (Fig. 2) and to deep temperature (Fig. 3). v_P/v_S in continental crust is predominantly sensitive to quartz abundance and only slightly sensitive to temperature. To the extent that v_P/v_S does depend on temperature, the physics would predict higher v_P/v_S for higher temperature, so finding the opposite relationship is surprising. The most plausible explanation for the observed relationship between v_P/v_S and temperature is a robust dynamical feedback in which ductile strain first localizes in relatively weak, quartz-rich crust, and then initiates processes that promote advective warming, hydration, and further weakening. Such a feedback mechanism not only would explain stationarity and spatial distributions of deformation, but also would lend insight into the timing and distribution of thermal uplift in the Cordillera and observations of deep-derived fluid chemistry in springs.

Figure 1: Flow strength of rocks, based on laboratory measurements. Here we show flow strength versus depth for crustal mineral constituents, assuming a 10^-14 s^-1 strain rate, 1 mm grain size, and a geotherm estimate from the Colorado plateau (the temperature change determines the depth-dependence). Brittle-field failure assumes a frictional coefficient μ = 0.2. Temperature, quartz abundance and water fugacity determine whether the lower crust can flow.

Most of what we know about rock strength comes from laboratory studies, in which rocks are placed in a specially-designed apparatus to examine how they flow over long time-scales in response to applied stress. Laboratory methods developed during the last decade have become particularly good at isolating the effects of ambient temperature, pressure, stress, strain rate, grain size and water on the flow response. These measurements show that rock flow within the crust is particularly sensitive to temperature, the presence of free water, and the abundance of quartz.

Why would the brittle-field (shallow) process of earthquake rupture depend on whether or not rock flows in the deep crust? Earthquake failure requires a build-up of potential energy in the form of stress/strain across the fault. Earthquakes at plate boundaries (which are relatively well-understood) derive this strain from relative plate motion, but earthquakes and mountains in plate interiors require some kind of localized strain source at depth to build up a shallow strain moment. The most likely source for localized strain in plate interiors is localized flow in the lower crust and/or upper mantle.