Ductile and Brittle Fault Rocks in the Upper and Lower Crust, Respectively.

Robert Wintsch, Wesleyan University
Phillip Resor, Wesleyan University
Bryan Wathen, University of Wisconsin
Ryan McAleer, U. S. Geological Survey, Reston

Abstract

Deformation in crustal-scale fault zones spans the complete range of brittle and ductile rheologies. Brittle deformation is classically thought to dominate in faults where depth is less than 15-20 where coseismic slip occurs. Ductile deformation is thought to dominate at depths >~15 km where temperatures are high enough to activate dislocation creep in load-bearing framework silicates. In this contribution we highlight exceptions to this classical idea. First we consider brittle deformation. Brittle deformation can occur at depths ≥ 15 km if fluid pressures are high enough. Anatexis can provide this fluid pressure leading to hydro- or "magma-fracking" and the generation of pegmatites. We show examples from southern New England where oscillating brittle and ductile deformation can be explained by the oscillating strain rates of the earthquake cycle. This process can go far in explaining the weakness at the felsic crust-lithospheric mantle boundary.

Second, we suggest that aseismic creep can operate in the upper 15 m of felsic crust. In this example of the retrograde East Derby shear zone, south central Connecticut, pressure solution creep and grain boundary sliding were able to accommodate the strain at what must have been only moderate strain rates. Evidence for this comes from the behavior of high-grade, Na-rich muscovite in the lower greenschist facies conditions of the deformation. Here the metastable, Na-rich muscovite is never found to be deforming by dislocation creep. On the contrary, it is replaced by low-grade phengitic muscovite as evident from grain-scale truncations. The truncating muscovite ± chlorite defines new foliations with chemically distinct compositions. Evidence for grain boundary sliding comes from flexural slip folding of muscovite-rich folia, with the precipitation of chlorite in hinge zones that accommodated the increase in volume there. The lower solubility of muscovite relative to chlorite calculated using SUPCRTBL (Zimmer et al, 2016) explains both the concentration of muscovite at sites of high normal stress, and the precipitation of chlorite in extensional sites of low normal stress. The mechanism of dissolution-precipitation creep is confirmed by 40Ar/39Ar analysis that shows muscovite in new cleavages crystallized as much as > 90 m.y. after the high-grade Na-rich muscovite cooled below its closure temperature. What is apparent from these studies is that petrologic processes and reactions work in concert with mechanical processes to define the rheology of the crust.

Session

Session 1: Fault Zones from Top to Bottom