Pseudotachylyte Diversity in the Ivrea-Verbano Zone: Insights into Crustal Rheology from DIVE Drill Core DT-1A

Pseudotachylytes are commonly interpreted as evidence of seismic rupture, and there are numerous studies documenting their occurrence under a wide range of P-T conditions throughout the entire lithosphere. Pseudotachylytes, which appear to have formed under different ambient conditions, have previously been reported in exhumed fault zones within the mafic and ultramafic rocks of the lower crustal section of the Ivrea-Verbano Zone. However, an unexpectedly high abundance of pseudotachylytes was encountered in core DT-1A, obtained at Megolo as part of the Drilling the Ivrea-Verbano Zone (DIVE) project. Along its total length of 900 m, pseudotachylytes can be observed as distinct generation surfaces, anastomosing networks and breccias, not only in major fault zones but also in virtually undeformed parts of the core. While commonly found pristine, some pseudotachylyte are overprinted by ductile shearing, and in a few cases exhibiting cross-cutting relationships with both other pseudotachylytes and ultramylonites.

Samples (up to 20 cm long and 9 cm wide) collected from different depths and with different characteristics have been polished and prepared for SEM analysis, including BSE, EDS, CL, possibly EBSD and CT. By carrying out microstructural and petrological investigations on these large samples, the context of the observations is preserved and the results can be reliably interpreted on a larger scale.

The study of a wide range of pseudotachylytes within DT-1A aims to: (1) improve our understanding of the (metamorphic) conditions that govern the formation of pseudotachylytes and (2) better understand the interaction between different deformation mechanisms within the continental crust.