Advances in Quantifying Crustal Stress Magnitudes

Steven Kidder, City College New York

Recrystallized grain size has been used to quantify the magnitude of the stresses involved in the ductile deformation of rocks for nearly 40 years. While the practice has experienced a resurgence recently, it remains a subject of considerable skepticism. Oft-cited concerns relate to the accuracy of extrapolations from laboratory conditions to nature, and the applicability of the technique in rocks that may not generally have been deformed at steady state. I provide evidence challenging both these concerns: First, stress estimates based on grain size data from Taiwan are remarkably consistent with independent estimates of stress in the Taiwan orogenic wedge. Second, experiments on quartzite under non-steady state conditions demonstrate that steady state is not required. To demonstrate the potential of the technique and a new, non-genetic approach to collecting and displaying grain size data, I present a data set of quartz grain sizes using ~250 thin sections spanning the length of New Zealand's Alpine Fault. Among other things the data demonstrate that stress magnitudes at the brittle-ductile transition double towards the ends of the fault. Recrystallized grain size piezometry is viable and inexpensive. It should be more widely utilized. Geodynamic models, for example, can be developed in tandem with actual measurements of stress magnitudes in rock samples to provide an extra dimension of testability and to better reveal gaps in our understanding of rock rheology.