Aftermath of the Variscan Orogeny
Toward the end of the Pennsylvanian (Late Carboniferous), the Variscan Orogeny was over. Traditionally, tectonic movements driving the earliest sedimentation on the deformed and increasingly metamorphosed westwards orogen were related to the relaxation of the orogen itself (Henk et al., 1997). Recognition of elongated, narrow and highly subsiding basins in the latest Carboniferous (Carnic Alps and Karawanken) and in the Early Permian, introduced the idea of the existence of transtentional movements (Vai, 1994; Cassinis and Perotti, 2007; among others). However, growing evidence from palaeomagnetic studies, developing the initial ideas of Irving (1977) and tectonic models (Arthaud and Matte, 1977; Matte, 1986), suggested the importance of large-scale tangential movements at the boundary between Laurussia and Gondwana. As far as the Southern Alps are concerned, where there are the best-preserved exposures for these phenomena, convincing evidence for palaeomagnetics were given by Muttoni et al. (2003, and ref. therein). The overall geodynamic interpretation with radiometric ages was offered by Schaltegger and Brack (2007). The general engine of this scenario, which should be linked in the transformation from the Pangea B in the Pangea A configurations, occurred up to the middle of the Permian (Fig. 1). Deep-seated phenomena are discussed in Sinigoi et al. (this volume).
Figure 1. From Pangea B to Pangea A configuration.
The Pangea B configuration changed to the Pangea A configuration during the Permian, along a megashear zone situated between Laurussia and Gondwana (From Muttoni et al., 2003, modified).
The peak period for the transtentional or occasional transpressive basins was in the second half of the Early Permian (Fig. 2) (Schaltegger and Brack, 2007; Cassinis et al., 2009). The basins are characterized by fault-driven shoulders, allowing sediment and volcanic accumulation up to 2–3 km. As they are now preserved, the basins are a few tens of km wide and perhaps less than 100 km in length, but alpine tectonic cuts make these figures tentative. To the west, in Lombardy, the basin infill is a mixing of volcanics and braider river/delta fan/lacustrine complex (Figs. 3, 4) (Cassinis et al., 2007, 2009; Sciunnach et al., 2003). Very rarely, marine ingressions are recorded (Sciunnach, 2001a). The volcanites of the Collio Basin show a subalkaline, calc-alkaline affinity, with scarse intermediate volcanics. The REE patterns are relatively homogeneous with significant fractionation of LREE and an almost flat HREE profile (Cassinis et al., 2008).
Figure 2. Stratigraphic setting of Late Carboniferous to earliest Triassic.
Chronostratigraphic setting from Late Carboniferous to earliest Triassic in Southern Alps. (From Cassinis et al., 2009, modified) Lithology – (1) conglomerate and breccia; (2) sandstone and siltstone; (3) pelite, siltstone and marlstone; (4) limestone; (5) fossiliferous limestone; (6) oolitic limestone;(7) dolostone; (8) volcanic rocks. Other symbols – (9) unconformity; (10) erosional surface; (11) stratigraphic gap.
Lithostratigraphic units from west to East. BC, basal conglomerate; V, undifferentiated volcanics; GG, Val Ganna granite; CO, Collio Formation; PC, Ponteranica Conglomerate; DGC, Dosso dei Galli Conglomerate (S, Pietra Simona Member); AV, Auccia Volcanics;VL, Verrucano Lombardo. PGC, Ponte Gardena Conglomerate; BV, undifferentiated Bolzano volcanics; TF, Tregiovo Formation; VDC, Val Daone conglomerate;VL, Verrucano Lombardo; VGS, Val Gardena Sandstone; BE, Bellerophon Formation; W, Werfen Formation.
Figure 3. Lower Permian basins in Lombardy.
The Lower Permian basins in Lombardy (from Sciunnach et al., 2001 modified) Note the narrow and elongated basins, filled with volcanics and siliciclatics. OA – Orobic Anticline; T-CA – Trabuchello-Cabianca Anticline; CE – Cedegolo Anticline; CA – Camuna Anticline.
Figure 4. Volcanic bodies in the Collio Basin.
Details of the Collio basins in eastern Lombardy with positions of the volcanic bodies (from Cassinis et al., 2009 modified).
Moving eastwards, the amount of volcanics increases significantly to reach the maximum in the Adige basin where spectacular huge pyroclastic flows form the present shoulders of the Adige valley (Fig. 5). They are presently about 2,000 km2 in area and consist of calc-alkaline volcanic rocks up to >2 km thick. They include basaltic andesites, andesites, dacites, rhyodacites and rhyolites (Bargossi et al., 1998, and ref. therein). The magmatic rocks comprise domes and lava flows, pyroclastic and surge deposits and ignimbrites (Schaltegger and Brack, 2007).
Figure 5. Lower Permian volcanics in Adige Valley.
The imposing walls formed by the ignimbrites and other volcanic products line the Adige Valley south of Bolzano. (Photo M. Gaetani).
Moving more toward the east, volcanics are reduced or absent in the Carnic Alps where a mostly marine succession is cropping out. It starts in the late Moscovian-early Kasimovian, unconformably sealing the variscan folded basement, mildly metamorphosed (Krainer, 1992; Vai, 1994; Venturini and Spalletta, 1998; Forke, 2002) (Fig. 6).
Figure 6. Stratigraphy of post-variscan successions in Carnic Alps.
Updated stratigraphic subdivisions and fusulinids zonation in the post-variscan successions of the Carnian Alps (from Korte, 2002, modified).
Meagre remnants of these volcanic/continental deposit settings are also preserved in Tuscany where they are presently mildly metamorphosed (Pandeli et al., 2008; Aldinucci et al., 2008) and in Sardinia (Ronchi et al., 2008; Cassinis et al., 2009).
For a general review of Permian continental deposits in Italy, refer to the special issue of the Geological Society of Italy (Cassinis, 2008).