Mantle Shear Zones







 Our research on mantle shear zones began when Henry Dick and Peter Kelemen stumpled on peridotite mylonites in the Josephine peridotite. Unbeknownst to us, there was already an excellent paper on these (Loney & Himmelburg, GSA Bull 1976). Our own work involved detailed mapping and sampling, and the discovery that channels for focused porous flow of melt followed some of the larger shear zones, and prompted theoretical work on the feedback between localized deformation and melt transport (Kelemen & Dick, JGR 1995).

We've been back to these key outcrops many times over the years, most recently using them to calibrate methods for measuring strain - via the angle between the shear plane and the olivine lattice preferred orientation - in deformed peridotites, and for calibrating the minimum melt fraction in dunites when porous flow ceased (Warren et al., EPSL 2008; Skemer et al., J Petrol 2010; Sundberg et al., J Petrol, 2010).

While hiking in Oman in the 1990's, Greg Hirth and Peter Kelemen developed an idea that pre-existing shear zones in the mantle might focus viscous deformation during grain size sensitive creep, leading to localized shear heating, thermal runaway, and "viscous earthquakes". It took years for a quantitative expression of this idea to appear in print (Kelemen & Hirth, Nature 2007). In retrospect, our delay was rewarded because the relatively recently recognized "dislocation-accommodated grain boundary sliding" (DisGBS) mechanism in olivine provided a crucial piece of the puzzle.

Ongoing work includes visco-elastic 1 and 2D modeling of mantle shear zones, and study of outcrops that provide key information on the rheology of the mantle and lower crust, and constrain extrapolations of laboratory data (e.g., gabbronorite dikes transformed to plagioclase-rich mylonites within less deformed peridotites in the upper mantle section of the Oman ophiolite, Homburg et al., Geology, 2010).