HR: 1330h
AN: T42A-0280    [PDF]
TI: Rheological Behaviour and Microstructures of Natural Gypsum Experimentally Deformed in Simple Shear
AU: * Barberini, V
EM: valentina.barberini@unimi.it
AF: Dipartimento di Scienze della Terra "A. Desio", Universita' degli Studi di Milano, via Botticelli 23, Milano, 20133 Italy
AU: Burlini, L
EM: luigi.burlini@erdw.ethz.ch
AF: ETH Geological Institute, Sonneggstrasse 5, Zuerich, 8092 Switzerland
AU: Rutter, E H
EM: e.rutter@man.ac.uk
AF: Department of Earth Sciences, University of Manchester, Oxford Road, Manchester, M13 9PL United Kingdom
AU: Dapiaggi, M
EM: monica.dapiaggi@unimi.it
AF: Dipartimento di Scienze della Terra "A. Desio", Universita' degli Studi di Milano, via Botticelli 23, Milano, 20133 Italy
AB: Gypsum is an important mineral of evaporitic rocks. Evaporites, interlayered within sedimentary sequences, play an important role in localizing the deformation especially in thrust tectonics (Apennines, Zagros, Gulf of Mexico, etc.) since are generally weaker than the other rocks; in some cases the deformation is accompanied by seismicity as in the Northern Apennines extensional systems. In order to determine the rheological and microstructural evolution of gypsum with strain, a set of experiments was performed on natural gypsum samples from Volterra (Tuscany, Italy). Experimental deformation tests were performed at confining pressures up to 300 MPa, at temperatures up to 130$\deg$C and at strain rates ranging between 6$\times$10$^{-4}$ and 5$\times$10$^{-6}$ s$^{-1}$. In order to reach high shear strain values, we deformed gypsum specimens using both the torsion technique in the Paterson apparatus at ETH Zurich (up to $\gamma$ = 5) and the sawcut-type assembly at 35$\deg$ in a Heard-type apparatus at Manchester University (up to $\gamma$ = 1.2). All samples have been studied by optical microscopy, to investigate the evolution of the microstructure with strain, and by XRD analysis, to determine whether and to what extent gypsum dehydrated during deformation. In torsion, the shear stress increased with the strain rate and decreased with the temperature. In general a peak stress was reached at $\gamma$ between 0.5 and 1 (at higher temperatures is reached sooner). After the peak, a various amount of weakening occurred, and mechanical 'steady state' conditions were never reached. Weakening was up to 30-40%. Most of the times the jacket failure ended prematurely the experiment. The microstructure evolved from a deformation microstructure, where grains changed shape according to the bulk strain imposed, into a recrystallization microstructure, where grains were more aequant. Grain boundary migration recrystallization was very effective in resetting the microstructure after $\gamma$ of 1 or 2. In the samples deformed using saw-cut assembly, the shear stress increased together with confining pressure and strain rate; the temperature increase (up to 90$\deg$C) induced a slight decrease in the shear stress, whereas at 130$\deg$C the shear stress is higher because bassanite has formed. Regarding the gypsum-bassanite transition, we could observe that it is controlled by temperature and pressure but also by the strain rate and the applied differential stress.
DE: 3902 Creep and deformation
DE: 8030 Microstructures
SC: Tectonophysics [T]
MN: 2003 Fall Meeting