SUPERHEATED FRICTION-INDUCED MELTS IN ZONED PSEUDOTACHYLYTES WITHIN THE ADAMELLO TONALITES (ITALIAN SOUTHERN ALPS)

Most pseudotachylytes are the product of wear, comminution, and friction-induced melting along a fault during seismic faulting. Therefore pseudotachylytes may be used potentially to constrain fault plane processes during an earthquake and pseudotachylyte volumes to estimate the dynamic frictional shear stresses during coseismic slip. This latter estimate requires the determination of the melt temperature.

Pseudotachylytes and cataclasites are present along a strike-slip fault zone (Gole Larghe Fault Zone) in the tonalites of the Adamello intrusion (Italian Southern Alps). Ambient conditions during seismic faulting were 0.25-0.3 GPa and 250-300 °C. 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 (x_MD ) of the microlitic domain increases linearly with the half thickness (a) of the entire vein according to the relation:

x_MD=(0.29 ± 0.12) a for a > 0.003 m

The microlitic and spherulitic domains have similar chemical composition. The microlitic domain consists of randomly oriented plagioclase microlites and lithic clasts of quartz and plagioclase. Length, width and morphology of microlites vary gradually across the microlitic domain. The spherulitic domain is matrix-supported and includes different types of spherulites and lithic clasts are mainly of quartz only. 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 of superheated friction-induced melts (T_melt³1450 °C). Melt temperatures estimated by the clast/matrix ratio using the O’Hara’s geothermometer are considerably lower (316-577 °C).

The O’Hara geothermometer (2001) is based on the assumption that clasts and friction-induced melts are produced during the same seismic event (i.e. rupture propagation plus frictional sliding). Therefore, we suggest that the inconsistency between the two temperature estimates is due to the precursor aseismic cataclastic deformation history of the brittle zone. The difference in the temperature estimates using the O’Hara geothermometer compared to other methods could reveal a more complex history than a single seismic slip rupture in an intact rock.