Stuwe, 1994 esfuerzos y trayectorias P-T

Stuwe, 1994 esfuerzos y trayectorias P-T - 1 metamorphic...

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1. metamorphic Geol., 1994, 12, 445-454 Contribution of deviatoric stresses to metamorphic P-T paths: an example appropriate to low-P, high-T metamorphism K. STUWE AND M. SANDIFORD Department Geology and Geophysics, The University of Adelaide, Adelaide, SA 5005, Australia ABSTRACT P-T paths for a simple situation appropriate to many low-P, high-T (LPHT) terranes in which metamorphism and deformation are localized by advection of heat in magmas, has been modelled assuming a medium with a power-law rheology with an inverse exponential dependence of stress and temperature and capable of sustaining deviatoric stress, t, in the order of 100MPa at 400°C and strain rates of up to lo-" s-~'. Numerical simulations and analytical approximations for P-T histories appropriate for simple convergent deformation histories show that the destruction of the deviatoric stress field around large intrusions may result in significant decompression near the metamorphic temperature peak. Moreover, for a specified strain rate and temperature evolution, P-T paths may vary from clockwise to anticlockwise merely as a function of vertical distance from the heat source. Inasmuch as mounting independent evidence suggests that the crust can support deviatoric stresses of up to about 100MPa at temperatures of 400-50O0C, and that the shear strength of the crust is strongly temperature-dependent in the range 400-800" C, these results suggest that caution should be taken in the tectonic interpretation of P-T paths involving decompression of the order of 100 MPa or less in LPHT terranes. The results illustrate a plausible mechanism for the close spatial association of both clockwise and anticlockwise P-T paths documented in some LPHT terranes. Key words: deviatoric stress contribution; low-P, high-T terranes; varying P-T path. INTRODUCTION Pressure-temperature (P-T) paths recording near isothermal decompression after the metamorphic peak (so-called clockwise paths) are typical of a wide variety of metamorphic terranes, including many Barrovian terranes, as well as a number of low-P, high-T (LPHT) terranes (e.g. Stuwe & Powell, 1989a; Clarke & Powell, 1991; Nichols & Berry, 1991). However, such paths are by no means universal, especially in LPHT terranes, in which many anticlockwise P-T paths that show either compres- sion during cooling or essentially isobaric cooling have been recorded (e.g. Phillips & Wall, 1981; Clarke & Powell, 1989; Stuwe & Powell, 1989b; Dirks et al., 1991). In the metamorphic geology literature these different P-T paths have been given considerable tectonic significance. Decompressional P-T paths are generally explained as the consequence of depth decrease through rapid erosion or tectonic thinning of the crust (e.g. Bohlen, 1987; Harley, 1989). In contrast, anticlockwise P-T paths have been explained as the consequence of rapid cooling following the intrusion magmas with or without convergent deformation (e.g. Lux et al., 1986; Sandiford et al., 1991).
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Stuwe, 1994 esfuerzos y trayectorias P-T - 1 metamorphic...

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