Un database geologico per segnalazioni di pseudotachiliti nelle Alpi

A long-lasting debate in structural geology regards the «statistic» frequency of pseudotachylytes in the geological record. Some Authors (e.g., SIBSON & TOY, 2006) consider that these rocks, interpreted as the product of frictional melting during seismic events, are rather infrequent and produced in particular environments (e.g. fracturing in dry intact rock). It follows that pseudotachylytes record only an irrelevant percentage of the widespread seismic activity produced in the Earth crust. Other Authors believe that pseudotachylytes are commonly produced but rarely preserved in the geological record (SPRAY, 1987), for instance due to intense post-seismic fluid circulation which may obliterate these fault rocks. As a matter of fact, an exponentially growing number of pseudotachylyte occurrences have been reported since the '90s, when an increasing interest and knowledge of these rocks had developed. It is very likely that, particularly in the past, pseudotachylytes were not recognized in the field and/or recorded on geological maps because: 1) as it is evident by looking at large and perfectly polished exposures of pseudotachylyte-bearing faults, most pseudotachylyte fault veins are less than 1 mm thick and easily missed when outcrops are not as good; 2) most basement rocks are dark in color and thin pseudotachylyte veins (black in color) are almost undistinguishable in such host rocks; 3) the observer may not have experience in knowing what to look for; 4) even when recognized, pseudotachylytes are not reported on published geological maps because they are not the main objective of regional geological survey. In our opinion, it is for the above reasons that their frequency is severely underestimated, and this may represent a strong bias in any theoretical study about the relevance of frictional melting processes in earthquakes (DI TORO et alii, 2006). The Alps are the mountain belt which has been mapped, over 150 years, with the highest detail in the world. Moreover, almost all different crustal levels are now exposed in the Alps, and a good knowledge of the tectonic evolution at the regional scale permits to relate outcropping fault rocks to well defined «ambient» conditions (P, T, kinematics, etc.). Turning this favorable situation to our advantage, we are taking up a new project, which involves the implementation of a structured catalogue of the largest number of (known) pseudotachylyte occurrences in the Alps. This database will comprise data about location, geological setting, fault zone architecture and kinematics, association with other fault rocks, ambient conditions (P, T, fluids), presence of precursor structures, vein geometry, texture and mineralogy, bibliographic references, etc. Several analysis will be performed on these data, starting from a simple estimate of the «real» frequency of pseudotachylytes at different crustal levels, and continuing with a deeper analysis of all listed parameters. In this contribution we present the structure of the database and some preliminary examples of data that can be included. Our primary concern, at this stage of the project, is to collect any useful suggestions about the structure of the database and the data covered by our analysis, and to present our project to colleagues who can contribute with their own data.