Can field data constrain rock viscosities?

In the 1960s it looked as though the ratio of wavelength to thickness of folds along shortened competent single layers might allow constraint of the viscosity ratios between the layers and their hosts when they deformed together. In the 1970s, the possibility arose that simple field measurements of boudins and mullions might also constrain rock viscosity ratios and thereby distinguish deformation facies and map rock viscosities in pressure–temperature–time space. Even more potential tools for constraining rock viscosities appeared in the 1980s but since then progress appears to have stagnated in a welter of problems. empt is made to refocus attention on direct retrospective measurements of rock rheologies during natural deformations by reviewing the potential field tools for constraining rock viscosities, discussing some of their problems, and by a crude application of the most developed approach. Further advances are likely to come from iteration between modellers and structural geologists working in a variety of tectonic settings. As well as constraining the pressure–temperature–time paths of our rocks, we should also be attempting to measure their viscosities.