The eastern Tonale fault zone:: a 'natural laboratory' for crystal plastic deformation of quartz over a temperature range from 250 to 700°C

Near the eastern end of the Tonale fault zone, a segment of the Periadriatic fault system in the Italian Alps, the Adamello intrusion produced a syn-kinematic contact aureole. A temperature gradient from similar to250 to similar to700 degreesC was determined across the Tonale fault zone using critical syn-kinematic mineral assemblages from the metasedimentary host rocks surrounding deformed quartz veins. Deformed quartz veins sampled along this temperature gradient display a transition from cataclasites to mylonites (frictional-viscous transition) at 280 +/- 30 degreesC. Within the mylonites, zones characterized by different dynamic recrystallization mechanisms were defined: Bulging recrystallization (BLG) was dominant between similar to280 and similar to400 degreesC, subgrain rotation recrystallization (SGR) in the similar to400-500 degreesC interval, and the transition to dominant grain boundary migration recrystallization (GBM) occurred at similar to500 degreesC. The microstructures associated with the three recrystallization mechanisms and the transitions between them can be correlated with experimentally derived dislocation creep regimes. Bulk texture X-ray goniometry and computer-automated analysis of preferred [c]-axis orientations of porphyroclasts and recrystallized grains are used to quantify textural differences that correspond to the observed microstructural changes. Within the BLG- and SGR zones, porphyroclasts show predominantly single [c]-axis maxima. At the transition from the SGR- to the GBM zone, the texture of recrystallized grains indicates a change from [c]-axis girdles, diagnostic of multiple slip systems, to a single maximum in Y. Within the GBM zone, above 630 +/- 30 degreesC, the textures also include submaxima, which are indicative of combined basal (a)- and prism [c] slip. (C) 2002 Elsevier Science Ltd. All rights reserved.