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Unformatted text preview: Experimental deformation of partially melted granite revisited: implications for the continental crust C. L. ROSENBERG AND M. R. HANDY Department of Geological Sciences, Freie Universita ¨t Berlin, Malteserstr. 74-100, 12249 Berlin, Germany ([email protected]) 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 Ô melt connectivity transition Õ (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 Ô solid-to-liquid transition Õ (SLT), corresponding to the well known Ô rheologically critical melt percentage Õ . 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. Key words: crustal weakening; experimental deformation; melt; partially melted granite; rheologically critical melt percentage. INTRODUCTION AND BRIEF REVIEW OF THE RHEOLOGICALLY CRITICAL MELT PERCENTAGE Weakening of crustal layers may control the style of deformation, as well as the site and the amount of exhumation within orogens (e.g. Beaumont et al. , 2001). Anatexis deep in the crust is certainly the pri- mary process enabling a drastic weakening at depth on the scale of an orogen. However, the geological con- straints on the amount and consequences of such weakening are based on controversial laboratory experiments performed on centimetre-scale samples. In this paper, we discuss and reinterpret the significance and geological implications of such experiments....
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- Spring '12
- Granite, Paterson, RCMP, Van der Molen, melt fraction