Possible seismic signature of the α–β quartz transition in the lithosphere of Southern Tuscany (Italy)

This paper intends to throw some doubt on the consolidated practice of interpreting seismic data without considering mineral phase transitions. Reference is made to Southern Tuscany, where seismic surveys revealed the presence, within the crystalline basement of a prominent high-amplitude reflector exhibiting local "bright spot" features, known as the K-horizon. According to some researchers, the K-horizon is a layer of fractured rocks hosting high-pressure fluids, but its ultimate origin is unclear. On the basis of the extrapolation of measured temperatures in the Larderello and Amiata geothermal fields and gas geotemperatures from the San Pompeo-2 deep well, we propose that the K-horizon might be largely governed by the α–β transition of quartz and by the very high volumetric thermal expansion of quartz, a main constituent of local crystalline rocks, with respect to other minerals. The effects of these processes are probably suppressed almost everywhere by the abundance of minerals other than quartz and by the occurrence of partial melting, but might be evident in relatively quartz-rich rocks and in regions of very high geothermal gradient, such as Southern Tuscany. As a consequence of these processes, a layer of fractured rocks roughly following the α–β quartz transition temperature would develop and locally host high-pressure fluids, thus justifying in part the high reflectivity of the K-horizon. This hypothesis concords with previous interpretations, although the ruling factors in this case would be the α–β transition and the peculiar physical characteristics of quartz compared to other minerals. In our interpretation, the K-horizon is expected to fade away in rocks where quartz is scarce or absent, where microfractures are not produced by the phase transition. The local disappearance of the K-horizon could, therefore, depend on lithology, whereas its reflectivity would depend on the quantity and type of fluids trapped in the porous layer micro-fractured by the volumetric expansion of quartz.