A crustal‐scale view at rift localization along the fossil Adriatic margin of the Alpine Tethys preserved in NW Italy

Abstract

Fossil rifted margins, whereby originally extended continental crust is subsequently stacked in orogenic belts, provide the opportunity to track rift-related tectonics across different crustal levels. In this study, the tectonothermal evolution of the fossil Adriatic continental margin, sampled in the Italian Southern Alps, is investigated combining new (U-Th)/He zircon (ZHe) thermochronology from upper crustal rocks with existing data from the originally underlying lower crust, to shed light on the processes responsible for rift localization in the Alpine Tethys system. The Adriatic microplate records a protracted rift evolution, whereby distributed upper crustal stretching at 245-190 Ma was followed by rift localization along its future western edge, culminating in mantle exhumation at 165-160 Ma. A progressive westward younging of ZHe ages, from 280-240 Ma in the Lombardian Basin to 215-200 Ma near the Sostegno and Fenera Basins, indicates that anomalously high thermal gradients were established in the Late Triassic in the area where rifting later localized. The inferred episodic heating was contemporaneous with protracted fluid flow, minor magmatism, and ductile shearing within the originally underlying lower crust. Subsequent normal faulting was initiated post-185 Ma, as constrained by exhumation-related ZHe ages in detrital zircons from a syntectonic sandstone. The spatial distribution of the detected heating-cooling cycle suggests that rift localization along the western edge of the Adriatic Plate was probably favored by a crustal-scale thermal anomaly, established at 215-210 Ma, followed by thermal decay by 200-190 Ma. Subsequent crust-wide extension, starting at 185-180 Ma, led to excision of continental crust and mantle exhumation.

Architecture of the Distal Piedmont‐Ligurian Rifted Margin in NW Italy: Hints for a Flip of the Rift System Polarity

“…The Mesozoic sedimentary record of the western Southalpine is extremely lacunous if compared to the successions of the Lombardian basin. This marked difference has been described in the literature (Beltrando, Stockli, et al, ; Berra et al, ; Fantoni et al, ). The syn‐rift sequence is visible only at selected locations.…”

“…The depositional age for these sandstones has been determined by palinomorphs as Pliensbachian‐Toarcian (Berra et al, ). Interestingly, (U‐Th)/He analysis on zircons in volcanic grains of the sandstone gave comparable absolute cooling ages (171 ± 14 Ma and 177 ± 14 Ma, respectively, for Fenera and Sostegno; Beltrando, Stockli, et al, ). These data demonstrate that the process of exhumation, cooling, erosion, and sedimentation was very rapid.…”

“…Red boxes indicate the two main Jurassic phases of the Alpine rifting respectively acting in the proximal (Lombardian basin) and distal margin (Cusio‐Biellese‐Canavese). References for the stratigraphy of the different sectors: (1) Elter et al (), (2) Beltrando, Stockli, et al (), (3) Ferrando et al (), (4) Sturani (), (5) Berra et al (), (6) Casati (), (7) Montanari (), (8) Bertotti et al (), (9) Berra et al (), and (10) Berra and Carminati ().…”

New insights on the Monte Fenera Palaeolithic, Italy: Geoarchaeology of the Ciota Ciara cave

“…Geologically, the Monte Fenera succession lies almost horizontally over the metamorphic basement (the “Serie dei Laghi”). From bottom to top, the following main geological formations are identified (Beltrando, Stockli, Decarlis, & Manatschal, ; Berra, Galli, Reghellin, Torricelli, & Fantoni, ; Fantoni & Fantoni, ): Permian Volcanic Complex (lava, tufa and other volcanic or pyroclastic rocks, 100–200‐m‐thick); Triassic gray/green sandstone (mostly quartzarenite), with intercalations of pelite and doloarenite (“Fenera Annunziata Sandstone” formation, few meters thick); about 300‐m‐thick succession mostly consisting of dolostone, with thin intercalations of clay, dolorudite and thin volcanoclastic horizons, dating from the Middle Triassic (“San Salvatore Dolomite” formation). A stratigraphic hiatus marks the Upper Triassic, while the Jurassic is represented by a thin basal unit of limestone/dolomite breccia (“Brecce del Monte Fenera”), covered by a Sinemurian formation of sandstone and microconglomerate (“San Quirico Sandstone,” 25–60‐m‐thick) and by the thick (about 250 m) Pliensbachian layers of the “Calcari spongolitici” formation, which consist of limestone with chert nodules and lenses (mostly spongolite) and intercalations of sandstone.…”

Controls on the Thermomechanical Evolution of Hyperextended Lithosphere at Magma‐Poor Rifted Margins: The Example of Espirito Santo and the Kwanza Basins

“…Understanding the factors controlling decoupling, coupling and hyperextension should provide critical information on the mechanics of lithospheric necking and breakup. This exercise is critical at a time when new thermochronological data of fossil magma‐poor margins in the Alps and the Pyrenees have shown that reheating of the crust and the sediments occurs during lithospheric necking (thinning; Smye & Stockli, ; Beltrando et al, ; Seymour et al, ; Hart et al, ; Smye et al, ). Some authors suggest that these thermal events can be explained by rapid, active mantle upwelling during necking (Smye et al, ) and suggest that structures coupling fault in the crust and mantle are linked to both hyperextension and an actively upwelling mantle that could increase heat flow at the base of the extending crust and mantle lithosphere (Chenin et al, ; Gallacher et al, ; Hart et al, ; Huismans & Beaumont, , ; Royden & Keen, ; Svartman Dias et al, , ).…”