Experimental deformation of partially melted granite revisited: implications for the continental crust

Abstract

A review and reinterpretation of previous experimental data on the deformation of partially melted crustal rocks reveals that the relationship of aggregate strength to melt fraction is non-linear, even if plotted on a linear ordinate and abscissa. At melt fractions, U < 0.07, the dependence of aggregate strength on U is significantly greater than at U > 0.07. This melt fraction (U ¼ 0.07) marks the transition from a significant increase in the proportion of melt-bearing grain boundaries up to this point to a minor increase thereafter. Therefore, we suggest that it is the increase of melt-interconnectivity that causes the dramatic strength drop between the solidus and a melt fraction of 0.07. We term this drop the Ômelt connectivity transitionÕ (MCT). A second, less-pronounced strength drop occurs at higher melt fractions and corresponds to the breakdown of the solid (crystal) framework. This is the Ôsolid-to-liquid transitionÕ (SLT), corresponding to the well known Ôrheologically critical melt percentageÕ. Although the strength drop at the SLT is about four orders of magnitude, the absolute value of this drop is small compared with the absolute strength of the unmelted aggregate, rendering the SLT invisible in a linear aggregate strength v. melt-fraction diagram. On the other hand, the more important MCT has been overlooked in previous work because experimental data usually are plotted in logarithmic strength v. melt-fraction diagrams, obscuring large strength drops at high absolute strength values. We propose that crustal-scale localization of deformation effectively coincides with the onset of melting, pre-empting attainment of the SLT in most geological settings. The SLT may be restricted to controlling flow localization within magmatic bodies, especially where melt accumulates.

Extreme ductility of feldspar aggregates—Melt‐enhanced grain boundary sliding and creep failure: Rheological implications for felsic lower crust

“…Dell'Angelo et al [1987] have shown transition from dislocation creep to meltenhanced diffusion creep in fine-grained granitic aggregates at small volume fractions of melt F = 0.01-0.03 contemporaneously with strength drop below the limit of detection. Rosenberg and Handy [2005] have shown that the ''melt connectivity transition'' marked by melt fraction F = 0.07 is critical in mineral aggregate strength drop at experimental conditions. These results imply that small amount of melt can be responsible for considerable weakening of crustal rocks, which can explain, e.g., the development of largescale shear zones bounding regions of rapid uplift [Hollister and Crawford, 1986].…”

“…This critical melt volume corresponds to creation of interconnected network of melt between the framework of grains, so that the melt can start increasing its hydrostatic pressure toward the level of maximum compressive stress [Renner et al, 2000]. This melt volume is given by the melt connectivity transition (MCT) of F = 0.07 [Rosenberg and Handy, 2005] or similar ''liquid percolation threshold'' (LPT) of F = 0.08 [Vigneresse et al, 1996], although this critical level is likely to depend on several variables (e.g., dihedral angles). It is possible that the melt migrates from locally overpressured isolated pockets at high angle to the maximum compressive stress (s 1 ) to intergranular boundaries subparallel with this direction (s 1 ).…”

Crustal deformation induced by mantle dynamics: insights from models of gravitational lithosphere removal

“…Hydration and partial melting will result in lower effective viscosities than shown in Figs 1(c) and (d) (e.g. Rosenberg & Handy 2005). In the calculations, strain rates of 10 −14 -10 −16 s −1 are used, as the crustal strain rates in the numerical models in Section 3 generally fall within this range, with the weak channel having a higher strain rate.…”

Building the central Andes through axial lower crustal flow

“…More steeply dipping slab segments allow the development of a mantle wedge, higher temperatures at the base of the crust, and the introduction of fluids and partial melt [Gutscher, 2002]. The higher temperatures and possible introduction of fluids or partial melt will weaken the crust above steeply dipping slab segments and make the lower crust more susceptible to weakening and ductile flow [Kohlstedt et al, 1995;Rosenberg and Handy, 2005]. The effect of slab dip can be seen in the pattern in heat flow in the Andes, as several studies find that heat flow is highest in the Altiplano region and drops off to the north and to the south [Henry and Pollack, 1988;Springer and Förster, 1998].…”

“…This enables a lateral flux of deep crustal material beneath the plateau with minimal deformation of the upper crustal layer and may effectively decouple the motion of the upper crust from that of the mantle. Experimental, observational, and theoretical studies suggest that the strength of continental crust can decrease significantly at high temperatures and/or in the presence of fluids or partial melt [Kohlstedt et al, 1995;Rosenberg and Handy, 2005].…”