Kinematics and extent of the Piemont–Liguria Basin – implications for subduction processes in the Alps

Abstract

Abstract. Assessing the size of a former ocean of which only remnants are found in mountain belts is challenging but crucial to understanding subduction and exhumation processes. Here we present new constraints on the opening and width of the Piemont–Liguria (PL) Ocean, known as the Alpine Tethys together with the Valais Basin. We use a regional tectonic reconstruction of the Western Mediterranean–Alpine area, implemented into a global plate motion model with lithospheric deformation, and 2D thermo-mechanical modeling of the rifting phase to test our kinematic reconstructions for geodynamic consistency. Our model fits well with independent datasets (i.e., ages of syn-rift sediments, rift-related fault activity, and mafic rocks) and shows that, between Europe and northern Adria, the PL Basin opened in four stages: (1) rifting of the proximal continental margin in the Early Jurassic (200–180 Ma), (2) hyper-extension of the distal margin in the Early to Middle Jurassic (180–165 Ma), (3) ocean–continent transition (OCT) formation with mantle exhumation and MORB-type magmatism in the Middle–Late Jurassic (165–154 Ma), and (4) breakup and mature oceanic spreading mostly in the Late Jurassic (154–145 Ma). Spreading was slow to ultra-slow (max. 22 mm yr−1, full rate) and decreased to ∼51 mm yr−1 after 145 Ma while completely ceasing at about 130 Ma due to the motion of Iberia relative to Europe during the opening of the North Atlantic. The final width of the PL mature (“true”) oceanic crust reached a maximum of 250 km along a NW–SE transect between Europe and northwestern Adria. Plate convergence along that same transect has reached 680 km since 84 Ma (420 km between 84–35 Ma, 260 km between 35–0 Ma), which greatly exceeds the width of the ocean. We suggest that at least 63 % of the subducted and accreted material was highly thinned continental lithosphere and most of the Alpine Tethys units exhumed today derived from OCT zones. Our work highlights the significant proportion of distal rifted continental margins involved in subduction and exhumation processes and provides quantitative estimates for future geodynamic modeling and a better understanding of the Alpine Orogeny.

Neogene kinematics of the Giudicarie Belt and eastern Southern Alpine orogenic front (northern Italy)

“…Taken together, these events have been interpreted to indicate that Adriatic indentation triggered the orogenparallel extrusion of the Eastern Alps towards the Pannonian Basin (Ratschbacher et al, 1989;Frisch et al, 1998Frisch et al, , 2000Linzer et al, 2002;Rosenberg et al, 2007;Favaro et al, 2017). Contrasting estimates have been proposed for the amount of lateral extrusion, ranging from 14-62 km (Rosenberg and Garcia, 2011;disputed by Fügenschuh et al, 2012), 65-77 km (Favaro et al, 2017), 100 km (Scharf et al, 2013), to as much as 120 km (Linzer et al, 2002) and 160 km (Frisch et al, 2000).…”

“…It ends to the north at the western boundary of Neogene post-nappe folding of the Tauern Window and lateral escape of the Eastern Alps (Frisch et al, 2000;Linzer et al, 2002;Scharf et al, 2013). Previous studies have established a direct kinematic link between sinistral motion along the NGF and Neogene exhumation in the Tauern Window (Ratschbacher et al, 1989;Frisch et al, 1998Frisch et al, , 2000Linzer et al, 2002;Rosenberg et al, 2007;Scharf et al, 2013;Schmid et al, 2013;Handy et al, 2015;Favaro et al, 2015Favaro et al, , 2017Hülscher et al, 2021). This complex response of the orogenic crust to indentation calls for detachment of both sedimentary cover and metamorphic units above one or more detachments located at or above the Moho Discontinuity (Oldow et al, 1990).…”

“…The fold-and-thrust belt of the eastern Southern Alps formed due to indentation of the Adriatic plate into the Eastern Alps (Schönborn, 1992(Schönborn, , 1999Picotti et al, 1995;Frisch et al, 1998Frisch et al, , 2000Castellarin and Cantelli, 2000;Linzer et al, 2002;Rosenberg et al, 2007;Pomella et al, 2011Pomella et al, , 2012Favaro et al, 2017). Indentation is defined as the postcollisional penetration of a relatively stiff part of an upper plate into a weaker, already existing orogenic edifice (e.g.…”

“…Based on the calculated amount of shortening, we propose a kinematic sub-division of the Giudicarie Belt (Sect. 6) and discuss the relationship between Neogene shortening along the Giudicarie Belt and Neogene motion along the NGF, a topic which has fascinated many researchers in the past (Castellarin and Vai, 1981;Picotti et al, 1995;Prosser, 1998Prosser, , 2000Müller et al, 2001;Linzer et al, 2002;Stipp et al, 2004;Pomella et al, 2011Pomella et al, , 2012.…”

Regional centroid moment tensor inversion of small to moderate earthquakes in the Alps using the dense AlpArray seismic network: challenges and seismotectonic insights

“…The rotation results in varying convergence rates across the Alps. Le Breton et al (2017Breton et al ( , 2021 infer a rotation of about 5.25 • during the last 20 Myr resulting in convergence rates ranging from 5.5 mm yr −1 in the NW Adria (Western Alps) to 7.5 mm yr −1 in the NE Adria (eastern Southern Alps) (Fig. 1).…”