The role of the Periadriatic Line in the tectonic evolution of the Alps

Abstract

Summary The Periadriatic Line and related lineaments formed as a result of post-collisional deformations which severely modified the Alpine chain. This post-late Oligocene deformation is the result of dextral transpression between the Adriatic sub-plate and the European foreland. Indentation of the western edge of the southern Alps caused uplift, related to backthrusting and associated deformations of the Lepontine region combined with E-directed escape of the central Alps. In the eastern Alps the response to dextral transpression is mainly by lateral escape along conjugate strike slip zones with minor or no vertical movements. Older deformations along this essentially late Alpine lineament can still be inferred locally and include: extension and transfer faulting in the late Palaeozoic to early Mesozoic, Cretaceous deformations, and Tertiary phases of compression (Eocene) and possibly extension (Oligocene). The geometry of crustal thinning associated with the formation of the passive continental margin of the southern Alps (associated with initial uplift of the Ivrea zone) has a profound influence on strain localization and the kinematics of movements along and north of the present day Periadriatic Line.

Geological-structural map of the Orobic and Porcile thrust junction, central Southern Alps (N Italy)

“…The reverse right-lateral faults measured along the Orobic Thrust crosscut all previous structure; we tentatively relate this tectonic phase, which took place at shallow crustal level, to the well-documented dextral transpressive activity of the Insubric Fault during the Oligocene (Schmid et al, 1989).…”

“…They initially formed during the Late Cretaceous, in the hangingwall of the Tethyan subduction; they were later re-activated in Late Palaeocene to Middle Eocene during the ongoing Africa-Europe collision and, finally, accommodated in the cSA the dextral transpression of the Insubric Fault during the Oligocene (Schmid, Aebli, Heller, & Zingg, 1989).…”

Late Oligocene to early Miocene lateral extrusion at the eastern border of the Lepontine dome of the central Alps (Bergell and Insubric areas, eastern central Alps)

“…The postkinematic intrusion of acidic dykes crosscutting the Insubric mylonites and the SSB of the Lepontine dome at $25 Ma further documents the end of the Insubric ductile backthrusting and dextral deformation phase [Gebauer, 1996;Romer et al, 1996;. The youngest dated event is an early Miocene cataclastic overprint [Müller et al, 2001;Zwingmann and Mancktelow, 2004] giving rise to a master fault plane at the immediate contact with the Southern Alps, the Tonale fault, in relation with purely brittle dextral strike-slip and downfaulting of the central Alps (D4, Figure 1b) [Fumasoli, 1974;Schmid et al, 1989;Werling, 1992;Schmid et al, 1996a]. Near Passo San Jorio (Figure 2a) a 23 Ma old age from K-Ar dating of <2 mm illite fractions in a fault gouge related to a normal fault parallel to and immediately south of the Tonale master fault [Zwingmann and Mancktelow, 2004] suggests that brittle normal faulting in the vicinity of the Tonale fault might actually have started slightly before the strike-slip movement on it.…”

“…This late Oligocene exhumation event was associated with local sets of brittle-ductile south dipping steep pro-thrusts, considered as antithetical conjugate sets to the vertical movements on the Insubric Mylonites (D3, Figure 2b) [Huber and Marquer, 1996]. South and east of the Bergell pluton, vertical offset is progressively replaced by pure dextral strike-slip along the greenschist facies Insubric mylonites, claiming for an eastward change from transpressional to pure strike-slip kinematics [Wiedenbeck, 1986;Schmid et al, 1989;Werling, 1992;Meier, 2003;Stipp et al, 2004].…”

“…Although the kinematics of individual segments of the PFS in the Bergell Alps have been studied in details [Schmid and Froitzheim, 1993;Schmid et al, 1996a;Meyre et al, 1998;Ciancaleoni and Marquer, 2006], the main difficulty arises from linking fault Figure 1. Tectonic map of the Alps with major ductile and brittle faults related to postcollisional deformations, active during the late Oligocene and Neogene (modified from Schmid et al [1989], Ratschbacher et al [1991b], Decker and Peresson [1996], Viola et al [2001], and Handy et al [2005] [1995], Huber and Marquer [1996], and Meyre et al [1998] and Marquer [1996] and Schmid et al [1996b]). Regional deformation phases (D1 -D4) are discussed in the text.…”