Intermittent Brittle and Ductile Deformation as Recorded by Dating of Ultramylonites and Pseudotachylytes in Extending Continental Crust (Ivrea-Verbano Zone, Italy)

Abstract

Mylonites and pseudotachylytes play a crucial role in defining the rheology of extending continental crust. In this context, determining the age of brittle-ductile deformation is fundamental to understanding the rifting evolution stages. Here, we investigate the Premosello Shear Zone, an extensional structure of the Ivrea-Verbano lower crust, where ultramylonitic granulites are crosscut by pseudotachylytes. These features record the rift-related evolution of the Adriatic crust. Middle Triassic–Early Jurassic ductile shearing is confirmed by U–Th–Pb dating of syn-kinematic Th-depleted rims of monazite. Ar-Ar dating of a pseudotachylyte along high-angle extensional faults yields an age of 199.2 ± 1.4 Ma, revealing a ‘semi-brittle’ crustal behaviour of this lower crustal level during the onset of Early Jurassic rifting. Our study highlights: (1) the importance of coupling different radiometric dating methods on several rock types to constrain deformation history, and (2) the role of major extensional faults in localising crustal deformation during rift evolution.