Birth and evolution of the Paleocarnic Chain in the Southern Alps: a review

Abstract

The present work is aimed at reviewing the state of the art of the studies centered on the pre-collisional and collisional, Variscan evolution of the eastern Southern Alps. The main focus is on the Carnic Alps, a geologically spectacular and extremely complex area, which has been a major subject of study for structural geologists, stratigraphers, and paleontologists for more than a century. After decades of field studies aimed at unveiling the birth and evolution of this belt, the tectonic and geodynamic interpretations proposed for almost a century by two different groups of authors remain substantially different. German-speaking authors, over the decades, have consistently proposed a scenario marked by a mature, Devonian passive margin, which later on evolved into an active Mississippian continental margin; the following collisional phase resulted in an accretionary wedge that, in its evolution, might be compared to those generated by Variscan events outside the Alpine domain. On the contrary, Italian authors have mainly put forward a pre-collisional setting dominated by wrench-fault tectonics, followed by the formation of a large, collisional thrust-and-fold belt, arc-shaped in plan view. We illustrate the results accomplished so far and discuss the interpretations formulated by the two different research schools; our review provides a chance to compare the different interpretations and, at the same time, prompts the need for new and targeted data collection in the area.

The Montello Thrust and the Active Mountain Front of the Eastern Southern Alps (Northeast Italy)

“…Lateral Growth of the Montello Anticline to the rheological changes at the expenses of the pre-Alpine basement, Variscan in origin, which grades to the east toward less-and non-metamorphosed terrains (Mariotto & Venturini, 2019). In the study zone, the thickness of the basement thrust sheet is hypothesized around 8 km, similarly to the proposal of previous authors.…”

“…However, moving to the east toward the Friuli region, the spacing of the thrust sheets decreases dramatically (Figure 1, Largaiolli & Semenza, 1966), and the basement no longer shows up along the main thrust. This lateral variation is likely due to the rheological changes at the expenses of the pre‐Alpine basement, Variscan in origin, which grades to the east toward less‐ and non‐metamorphosed terrains (Mariotto & Venturini, 2019). In the study zone, the thickness of the basement thrust sheet is hypothesized around 8 km, similarly to the proposal of previous authors.…”

Raman Spectroscopy of carbonaceous material thermometry in the Carnic Alps (Austria, Italy)

“…The tectonically highest Fleons Nappe (Fig. 2) covers the entire western Carnic Alps (Hubich et al, 2000;Läufer et al, 2001;Mariotto and Venturini, 2019). It comprises Upper Ordovician metasediments and metavolcanics, Silurian deep-water sediments, Devonian limestones and presumably Mississippian flysch sediments (Läufer et al, 2001), thrusted along the Val Bordaglia fault above the Cellon-Kellerwand Nappe (Hubich et al, 2000;Läufer et al, 2001;Mariotto and Venturini, 2019).…”

“…2) covers the entire western Carnic Alps (Hubich et al, 2000;Läufer et al, 2001;Mariotto and Venturini, 2019). It comprises Upper Ordovician metasediments and metavolcanics, Silurian deep-water sediments, Devonian limestones and presumably Mississippian flysch sediments (Läufer et al, 2001), thrusted along the Val Bordaglia fault above the Cellon-Kellerwand Nappe (Hubich et al, 2000;Läufer et al, 2001;Mariotto and Venturini, 2019). Post-Variscan strata overlie the nappe stack unconformably and give evidence of the Variscan age of nappe stacking (Hubich et al, 2000;Läufer et al, 2001, Venturini et al, 2001Schönlaub and Forke, 2007;Mariotto and Venturini, 2019).…”

“…1) exposes an extraordinary record of the sedimentary and paleontological heritage of the Variscan (Pennsylvanian) orogeny (Schönlaub and Heinisch, 1993;Schönlaub and Histon, 2000;Corradini and Suttner, 2015) at the southern margin of the European Variscides (e.g., Mariotto and Venturini, 2019;Neubauer et al, 2022). Following the post-orogenic molasse phase, the Variscan chain of the Carnic Alps drowned (Schönlaub and Forke, 2007), und became later also a segment of the Mesozoic to Cenozoic Alpine chain (Mariotto and Venturini, 2019), dissected subsequently during Miocene-Pliocene strike-slip tectonics along the Periadriatic Fault System (Fodor et al, 1998;Frisch et al, 1998Frisch et al, , 2000Mancktelow et al, 2001;Bartel et al, 2014;Heberer et al, 2017;Klotz et al, 2019). Due to excellent exposures and an extraordinarily rich fossil content, the Carnic Alps represents a key area to understand Paleozoic life and provides a natural laboratory to study orogenic processes.…”