How Applicable are Existing Flow Laws to the Granitic Continental Crust?

Abstract:

Flow laws relating stress and strain rate are widely used to model lithospheric behavior. However, they suffer two major limitations: 1) Most flow laws are either based on experimental data or derived from theoretical principles. It is unclear how well they can be extrapolated to geological strain rates, which may be several orders of magnitude slower. 2) Many of these flow laws are developed for a single phase, such as pure quartz. However, the lithosphere is a complex entity, and its rheology is likely controlled by mixtures of different minerals in varying proportions.

We therefore investigate the rheology of a natural shear zone developed in metagranite, which is representative of significant portions of the continental crust. The Simplon Shear Zone is a crustal-scale, normal-sense structure in the central Alps, the exhumed footwall of which preserves deformation that occurred over a range of depths. We independently constrain the stress and strain rate for samples deformed at ~4 to ~17 km depth, using quartz paleopiezometry, the width of the shear zone at the time, and the published displacement rate as estimated from thermochronological modeling. The pressure-temperature conditions of deformation are calculated from Ti-in-quartz and Si-in-phengite geobarometry. All samples are granitic, but the proportions of feldspar, quartz, and mica vary.

We first compare our observed strain rates to those predicted by various recently-published quartzite flow laws, at our estimated conditions of deformation. The strain rates of feldspar-dominated samples are similar to those predicted, whereas quartz- and mica-dominated samples match only the fastest predicted rates, and differ from the slower predicted rates by up to ~3 orders of magnitude (implying our samples are “weaker” than expected). We then examine various mixing models and polyphase flow laws, to determine whether flow laws adapted to account for variable proportions of feldspar and quartz are better able to predict the observed strain rates. This will help determine whether the observed discrepancies are due to: 1) poor extrapolation of laboratory data, from experimental- to mid-crustal conditions; 2) the polyphase nature of granite, in which feldspars may strengthen the bulk rheology, and micas weaken it; or 3) some combination of these two factors.