New perspectives on 'geological strain rates' calculated from both naturally deformed and actively deforming rocks

A value of similar to 10(-4) s(-1) is commonly cited as an average geological strain rate. This value was first suggested for finite strain across an orogen, but based on more limited information than the combined geophysical, geological, and experimental data now available on active and ancient rock deformation. Thus, it is timely to review the data constraining strain rates in the continents, and to consider the quantifiable range of crustal strain rates. Here, where resolution allows, both spatial and temporal strain rate variations are explored. This review supports that a strain rate of 10(-14 +/- 1) s(-1) arises from geological estimates of bulk finite strains. Microstructural arguments combining laboratory-derived piezometers and viscous flow laws, however, imply local rates that are orders of magnitude faster. Geodetic rates, in contrast, are typically similar to 10(-15) s(-1) in actively deforming areas, about an order of magnitude slower than the bulk rates estimated from geological observations. This difference in estimated strain rates may arise from either low spatial resolution, or the fact that surface velocity fields can not capture strain localisation in the mid to lower crust. Integration of geological and geodetic rates also shows that strain rates can vary in both space and time, over both single and multiple earthquake cycles. Overall, time averaged geological strain rates are likely slower than the strain rates in faults and shear zones that traverse the crust or lithosphere.