Field and microstructural characterization of Valsesia pseudotachylytes (Ivrea Zone, Italy)

Pseudotachylytes are solidified frictional melts produced in silicatic rocks during an earthquake (Sibson, 1975). They form both as fault and injection veins, with thickness ranging from some millimeters to some centimeters. Still, exposures of meter-thick pseudotachylytes associated to seismic faulting have been documented (Musgrave Ranges, Australia; Lofoten Island, Norway; Ivrea-Verbano Zone, Italy).

In this study, we perform field (UAV, photogrammetry, structural geology, etc.) and microstructural/mineralogical (FESEM-BSE, EDS, micro-Raman, etc.) investigations of thin (mm-cm) and giant (up to 1 m thick) pseudotachylytes approaching the Canavese Line (strike ~NNE-SSW), the major tectonic lineament of the Western Alps (Ivrea-Verbano Zone, Italy; Techmer, 1992; Ueda et al., 2008; Ferrand et al., 2018). Though thin pseudotachylytes have been extensively investigated, in-depth studies of the giant pseudotachylytes are lacking. The aim is thus to determine (i) the ambient P-T conditions, the geodynamic setting, and the seismogenic environment (megathrust?) of the giant-pseudotachylytes, and, in the future, (ii) their mechanisms of formation.

We investigated for ~11 km the polished outcrops exposed along the ~E-W trending Valsesia river and other creeks in the area and selected three outcrops (I, II, and III) within ~2 km to the W and ~9 km to the E from the Canavese Line. We found pseudotachylytes only to the E of the Canavese Line. In detail:

Outcrop (I), < 500 m to the E from the Canavese Line (altered gabbro host rock) shows:

• multiple generations of pseudotachylyte-bearing faults, including giant-pseudotachylytes with breccia (suggesting a single melt pulse) overprinting microgabbro schlierens. The giant-pseudotachylytes are sub-parallel to the Canavese Line and include breccia clasts of the altered (greenschist facies) host rock;
• late quartz- and epidote-, and chlorite-bearing faults/fractures cutting the pseudotachylytes;
• matrix of the pseudotachylyte overprinted by greenschist facies minerals (epidote, chlorite and albite).

Outcrop (II), ~2 km to the E from the Canavese Line (Balmuccia peridotite) shows:

• multiple giant pseudotachylytes-bearing faults, sub-parallel to the Canavese Line, and associated with thin pseudotachylyte faults and veins;
• serpentine-bearing faults/fractures cutting and cut by the pseudotachylytes;
• giant pseudotachylytes with homogeneous matrix suggesting a single friction melt pulse. The matrix (altered into serpentinite) includes microlites of olivine and pyroxene plus vesicles;

Outcrop (III), ~9 km to the E from the Canavese Line (unaltered diorite) shows:

• only thin pseudotachylyte overprinting/associated with foliated cataclasite-bearing faults cutting and overprinting aplitic dykes;
• pristine matrix of the pseudotachylyte, with well-preserved microlites, chilled margins and flow structures.

In conclusion, the giant-pseudotachylyte-bearing faults are (i) made of a homogenous layer of pseudotachylyte, (ii) sub-parallel and found only near (< 2 km to the E) of the Canavese Line, (iii) overprint and cut dykes and ductile shear zones, (iv) cut and are cut by (sub-) greenschist facies cataclasite-bearing faults, and, (v) are cut by epidote- and chlorite-bearing fractures and veins. The giant pseudotachylytes could be generated by large in magnitude earthquakes, associated with the activity of the Canavese Line and thus of Alpine age.