Stratigraphic evolution of the Triassic–Jurassic succession in the Western Southern Alps (Italy): the record of the two‐stage rifting on the distal passive margin of Adria

Abstract

The Triassic–Lower Jurassic succession of the Southern Alps is characterized by rapid thickness changes, from an average of about 5000 m east of Lago Maggiore to about 500 m in the Western Southern Alps. The stratigraphy reflects the Triassic evolution of the Tethyan Gulf and the Early Jurassic rifting responsible for the Middle Jurassic break‐up of Adria from Europe. The succession of the Western Southern Alps starts with Lower Permian volcanics directly covered by Anisian sandstones. The top of the overlying Ladinian dolostones (300 m) records subaerial exposure and karstification. Locally (Gozzano), Upper Sinemurian sediments cover the Permian volcanics, documenting pre‐Sinemurian erosion. New biostratigraphic data indicate a latest Pliensbachian–Toarcian age for the Jurassic synrift deposits that unconformably cover Ladinian or Sinemurian sediments. Therefore, in the Western Southern Alps, the major rifting stage that directly evolved into the opening of the Penninic Ocean began in the latest Pliensbachian–Toarcian. New data allowed us to refine the evolution of the two previously recognized Jurassic extensional events in the Southern Alps. The youngest extensional event (Western Southern Alps) occurred as tectonic activity decreased in the Lombardy Basin. During the Sinemurian the Gozzano high represents the western shoulder of a rift basin located to the east (Lombardy). This evolution documents a transition from diffuse early rifting (Late Hettangian–Sinemurian), controlled by older discontinuities, to rifting focused along a rift valley close to the Pliensbachian–Toarcian boundary. This younger rift bridges the gap between the Hettangian–Sinemurian diffuse rifting and the Callovian–Bathonian break‐up. The late Pliensbachian–Toarcian rift, which eventually lead to continental break‐up, is interpreted as the major extensional episode in the evolution of the passive margin of Adria. The transition from diffuse to focused extension in the Southern Alps is comparable to the evolution of the Central Austroalpine during the Early Jurassic and of the Central and Northern Atlantic margins.

Constraints on the thermal evolution of the Adriatic margin during Jurassic continental break-up: U–Pb dating of rutile from the Ivrea–Verbano Zone, Italy

“…It cannot be excluded that a grainsize-age relationship not resolved at the precision of our in situ analyses is also superimposed on these discrete cooling events, adding further complexity to the data. We postulate that the ~175 Ma cooling may be due to rift-related exhumation, which would be consistent with the fact that the ~175 Ma ages coincide with or only slightly postdate the termination of documented rift-related activity of faults and shear zones in the western Southern Alps (Berra et al 2009;Mulch et al 2002a).…”

“…These ages cannot therefore be related to exhumation-related cooling and are instead interpreted to record an early post-rift heating-cooling cycle during oceanisation. The ~175 Ma age population coincides with the latest documented stages of activity of rift-related faults (Berra et al 2009) and shear zones (Mulch et al 2002a) within the western termination of the Adriatic margin. As a result, this age population is interpreted to record cooling associated with rift-related exhumation.…”

“…Band of crosses marks a period of episodic mantle-derived magmatism and metasomatism (Grieco et al 2001;Lu et al 1997;Mayer et al 2000;Mazzucchelli et al 2010;Morishita et al 2008;Schaltegger et al 2015;Stähle et al 1990;Zanetti et al 2013). Pale blue band marks the period of Tethyan rift-related faulting in the Southern Alps (Berra et al 2009;Bertotti et al 1993); medium blue band marks the timing of movement on the Pogallo shear zone (SZ) (Mulch et al 2002a). Green band delineates the timing of emplacement of the majority of Tethyan oceanic gabbros (Bill et al 1997;Kaczmarek et al 2008;Li et al 2013;Liati et al 2005;Rubatto et al 1998;Rubatto and Scambelluri 2003;Schaltegger et al 2002).…”

“…Following the onset of regional extensional faulting in the Lombardian basin, crustal thinning within the Southern Alps was progressively localised in westerly domains (Bertotti et al 1993;Berra et al 2009), eventually culminating in complete crustal excision and regional exhumation Ewing et al (2013). Constraints from U-Pb geochronology of rutile (this study) are shown as red boxes distinguishing the older and younger age populations, for a T C of Pb in rutile (6) determined from experimental diffusion data (Cherniak 2000).…”

“…Although the width of the sampling area is limited by the original occurrence and preservation of rutile, the studied samples span a distance between 1.5 and 5.0 km from the Insubric line, indicating that cooling occurred contemporaneously (within error) across different former crustal depths. Secondly, within the IVZ, the latest activity of ductile shear zones is constrained to the Lower Jurassic, at ~182 Ma (Mulch et al 2002a), while normal faulting in the neighbouring westernmost Serie dei Laghi is constrained to the Pliensbachian-Toarcian (Berra et al 2009). All documented riftrelated fault activity thus pre-dates our U-Pb rutile ages by ~15 Ma, suggesting that cooling through 550-650 °C at ~160 Ma cannot be associated with rift-related exhumation.…”