Superheated friction-induced melts in zoned pseudotachylytes within the Adamello tonalites (Italian Southern Alps) Giulio Di Toro a , Giorgio Pennacchioni a,b, * a Dipartimento di Geologia, Paleontologia e Geofisica, Universita ` di Padova, Padova I-35137, Italy b CNR Istituto di Geoscienze e Georisorse (Sezione di Padova), Padova, Italy Received 28 April 2003; received in revised form 15 October 2003; accepted 20 January 2004 Available online 12 May 2004 Abstract Pseudotachylytes and cataclasites are present along a strike-slip fault zone in the tonalites of the Adamello intrusion (Italian Southern Alps). Ambient conditions during faulting were 0.25 – 0.3 GPa and 250 – 300 8C. Pseudotachylyte veins thicker than 6 mm are zoned and consist of two symmetric microlitic domains towards the vein walls and a central spherulitic domain. The thickness of the microlitic domains (x) increases linearly with the half total thickness (a) of the pseudotachylyte vein (commonly , 2 cm) according to the relation: x ¼ð0:29 ^ 0:12Þa. The spherulitic and microlitic domains have similar chemical composition but the microlitic domain has a lower amount of plagioclase clasts. A numerical model explains zoning as the result of the different cooling rates and clast/melt interactions at the center and periphery of thick veins. Zoning is compatible with the injection of a single pulse of superheated friction-induced melt (T melt ø 1450 8C). Melt temperatures estimated by the clast/matrix ratio (O’Hara, 2001) are considerably lower (316 – 577 8C). It is suggested that the difference in the temperature estimates reflects a more complex slip history than a single seismic slip event along the fault during production of cataclasites/pseudotachylytes. q 2004 Elsevier Ltd. All rights reserved. Keywords: Adamello batholith; Cataclasite; Pseudotachylyte; Earthquakes; Numerical modeling; Clast size distribution 1. Introduction There is general agreement today that most pseudo- tachylytes are the product of wear, comminution and friction-induced melting during seismic slip along a fault (e.g. Spray, 1995; Ray, 1999; Wenk et al., 2000; O’Hara, 2001). Therefore, pseudotachylytes may be potentially used to constrain fault plane processes during an earthquake (Magloughlin and Spray, 1992). In exhumed paleoseismic (i.e. pseudotachylyte-bearing) faults, the dynamic shear stress resistance achieved during slip along the fault plane may be calculated from the volume of friction-induced melt (Sibson, 1975), assuming that most of the mechanical work is converted to heat (e.g. Scholz, 1990). This would allow the comparison of single-jerk paleoseismic data from field studies with indirect seismological data from present-day earthquakes (Wenk et al., 2000). The calculation of shear stress from pseudotachylyte volumes requires an estimate of melt temperature (T melt ). For tectonic pseudotachylytes, estimates of peak T melt are in the range 750–1450 8C. Since friction-induced melts are produced by non-equilibrium melting on the whole rock scale (Spray, 1992), melt temperatures were calculated by SiO 2 glass composition (T melt $ 1450 8C; Lin, 1994a), by microlite mineralogy (T melt ¼ 890–1100 8C: two pyroxene geothermometer (Toyoshima, 1990); T melt ¼ 790–820 8C: omphacite–garnet geothermometer (Austrheim and Boundy, 1994); T melt . 1000 8C: pigeonite crystallization (Camacho et al., 1995)) and by the mineralogy of survivor clasts (T melt $ 1000 8C; Maddock, 1983). In experimentally generated pseudotachylytes, the measured T melt were 1100 – 1550 8C(Lin and Shimamoto, 1998). Recently, O’Hara (2001) has suggested the use of the volume ratio between lithic clasts and matrix in pseudotachylytes for estimating the friction-induced melt (or host rock) temperature. Tectonic pseudotachylyte veins decorate fault planes discontinuously and their thickness typically ranges from millimeters to a few centimeters. As a consequence, melt lifetime is commonly in the range of a few to hundreds of 0191-8141/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsg.2004.03.001 Journal of Structural Geology 26 (2004) 1783–1801 www.elsevier.com/locate/jsg * Corresponding author. Correspondence address: Dipartimento di Geologia, Paleontologia e Geofisica, Universita ` di Padova, Padova I- 35137, Italy. Tel.: þ 390-49-827-2053; fax: þ 390-49-827-2070. E-mail address: giorgio.pennacchioni@unipd.it (G. Pennacchioni).