Giant pseudotachylyte breccias of Valsesia (Ivrea Zone, Western Italian Alps)

Pseudotachylytes are solidified frictional melts produced in silicatic rocks during an earthquake (Sibson, 1975). They usually form fault and injection veins with thicknesses ranging from few millimeters to several centimeters. Still, exposures of meter-thick pseudotachylyte breccias with evidence of a single melt pulse and associated with seismic faulting have been documented (i.e., Musgrave Ranges, Australia; Lofoten Islands, Norway; Outer Hebrides, Scotland; Greenland; Ivrea-Verbano Zone, Italy).

In the present study, we perform field, microstructural and mineralogical investigations of both “normal” (i.e., mm- to cm-scale) pseudotachylytes and “giant” pseudotachylyte breccias outcropping in the Ivrea-Verbano Zone moving away from the Canavese Line, a segment of the Insubric Line, the main tectonic lineament of the Alps. The giant pseudotachylyte breccias reach up to ~2 m in thickness, and up to 8 m in length, limited by the outcrop extension, and possibly fill pull-aparts. Despite pseudotachylytes in this area have already been studied in detail, giant pseudotachylyte breccias were somehow overlooked (Techmer et al., 1992; Ueda et al., 2008; Souquière and Fabbri, 2010; Ferrand et al., 2018). We aim to determine (i) the ambient P-T conditions of formation (discussed here), (ii) their geodynamic and seismogenic environment, and (iii) their formation mechanism.

We selected four main outcrops along the Sesia River for detailed field mapping and sampling, moving eastward from the Canavese Line for ~9 km. In fact, no giant pseudotachylyte breccias have been found to the west of the lineament. In detail:

Outcrop I, <500 m from the Canavese Line (altered greenschist facies gabbros) shows:

• multiple generations of pseudotachylyte-bearing faults, including giant pseudotachylyte breccias subparallel to the NNE-SSW striking Canavese Line, containing clasts of the altered host rock;
• matrix of the pseudotachylytes overprinted by greenschist facies minerals (epidote, chlorite, albite);
• late quartz-epidote- and chlorite-bearing faults cutting the pseudotachylyte-bearing faults and breccias.

Outcrop II, ~1 km from the Canavese Line (unaltered gabbros) shows:

• multiple generations of pseudotachylyte veins and giant breccias, the latter subparallel to the Canavese Line;
• cataclasite- and graphite-bearing faults cut by giant pseudotachylyte breccia;
• late quartz-epidote- and chlorite-bearing faults cutting the pseudotachylytes.

Outcrop III, ~2 km from the Canavese Line (Balmuccia peridotite) shows:

• multiple giant pseudotachylyte breccias cutting cataclasite-bearing faults;
• serpentine-bearing veins and pseudotachylytes mutually cross-cutting each other;
• giant pseudotachylyte breccias subparallel to the Canavese line; their matrix includes microlites of olivine, enstatite, and vesicles.

Outcrop IV, ~9 km from the Canavese Line (unaltered tonalite) shows:

• only thin pseudotachylytes overprinting foliated cataclasite-bearing faults;
• well-preserved matrix of the pseudotachylytes (microlites, chilled margins, flow structures).

In conclusion, giant pseudotachylyte breccias are (i) mostly subparallel and only outcropping close to the Canavese Line (<2 km), (ii) made of a relatively homogenous matrix, resulting from the solidification of a continuous melt layer, (iii) not reactivated by ductile deformation, (iv) cut and are cut by brittle faults and, (v) cut by quartz-epidote, chlorite-, serpentine- bearing faults and veins. Thus, they were possibly generated in a shallow (~15 km depth) and cold (<350°C) environment by individual earthquakes of large magnitude, associated with the activity of the Canavese Line.