Abstract

We report new measurements of the seismic properties of an upper amphibolite facies metapelite from the Serie dei Laghi basement (southern Alps, northern Italy), presenting evidence for the α-β transition using elastic wave velocity methods and under in-situ conditions. Measurements were made using an internally heated gas apparatus under hydrostatic conditions to 500 MPa and temperatures to 1075 K. P-wave and S-wave velocities were measured parallel and normal to both lineation and foliation at room temperature. An average Vp (at 400 MPa) of 6.4 km/s and Vp anisotropy of 15% were found, and an average Vs1 of 3.6 km/s (8.2% anisotropy) and Vs2 of 3.5 km/s (4.1% anisotropy) were also measured. To observe the effects of the α-β quartz transition under hydrostatic conditions, elastic wave velocity was monitored continuously as the sample was heated to 1075 K. At 400 MPa, Vp was seen to decrease monotonically with increasing temperature to 950 K; as temperature continued to increase, Vp rapidly increased, until 1075 K. The effect was found to be reversible and is interpreted in terms of the α-β transition in quartz, in broad agreement with previous studies. A simple interpretation of the data, taking into account a field case from southern Tuscany as well as natural and elevated geothermal gradient, suggests that thermal and hydraulically driven fracture is a likely cause of “bright spots” seen in these areas. With knowledge of the depth of such features, the transition additionally allows the temperature at depth to be estimated.

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