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

Abstract

The Alpine Tethys rifted margins were generated by a Mesozoic polyphase magma‐poor rifting leading to the opening of the Piedmont‐Ligurian “Ocean.” This latter developed through different phases of rifting that terminated with the exhumation of subcontinental mantle along an extensional detachment system. At the onset of simple shear detachment faulting, two margin types were generated: an upper and a lower plate corresponding to the hanging wall and footwall of the final detachment system, respectively. The two margin architectures were markedly different and characterized by a specific asymmetry. In this study the detailed analysis of the Adriatic margin, exposed in the Serie dei Laghi, Ivrea‐Verbano, and Canavese Zone, enabled to recognize the diagnostic elements of an upper plate rifted margin. This thesis contrasts with the classic interpretation of the Southalpine units, previously compared with the adjacent fossil margin preserved in the Austroalpine nappes and considered as part of a lower plate. The proposed scenario suggests the segmentation and flip of the Alpine rifting system along strike and the passage from a lower to an upper plate. Following this interpretation, the European and Southern Adria margins are coevally developed upper plate margins, respectively resting NE and SW of a major transform zone that accommodates a flip in the polarity of the rift system. This new explanation has important implications for the study of the pre‐Alpine rift‐related structures, for the comprehension of their role during the reactivation of the margin and for the paleogeographic evolution of the Alpine orogen.

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

“…Studies of the Iberian margin suggest that basinward migration of the sedimentation in more and more distal half‐grabens is a possible mechanism for the tectonic evolution of hyperextended margins (Ranéro & Pérez‐Gussinyé, ). Similarly, interpretations from analog Tethyan margins also indicate this diachronous evolution as proposed initially by Masini et al () and Decarlis et al ().…”

“…While the exhumed domain is relatively symmetric, although ill-defined, the coupled area is wider in the Espirito Santo basin. (5) Regarding kinematic constraints, the interpreted exhumation domains across both margins are contemporaneous with the salt deposition, therefore, indicating that in the scenario presented here, exhumation had to occur between 116 and 111 Ma, which is the age of the salt (Davison, 2007;Dias et al, 1994). However, recent studies put some doubt on the age and onset of salt deposition within the South Atlantic (e.g., Tedeschi et al, 2017), suggesting the evaporites were deposited earlier during the early Aptian Oceanic Anoxic Event 1a interval.…”

Oligocene-Miocene extension led to mantle exhumation in the central Ligurian Basin, Western Alpine Domain

“…However, continental crustal thinning is well imaged. Considering these aspects, we rather favour rifting scenario (1) which is also supported by the conceptual model described by Decarlis et al (2017) for the evolution of magma-poor rifted margins. The model includes three phases of extension: (1) An initial stretching phase forming widely distributed half-grabens in the upper crust.…”

Tectono‐thermal Evolution of a Distal Rifted Margin: Constraints From the Calizzano Massif (Prepiedmont‐Briançonnais Domain, Ligurian Alps)

“…(a) Paleogeographic reconstruction of the Alpine Tethys during Late Jurassic (see Decarlis et al, ). (b) Simplified cross section across the Alpine Tethys illustrating the main Alpine paleogeographic structural domains and rifting domains (from Decarlis et al, ) (detachment faults in green).…”

“…During the final phase of hyperextension and exhumation, distal margins can be subdivided in upper plate (hanging wall of the exhumation system) and lower plate (Haupert et al, ; Decarlis et al, ; Figure ). Stratigraphic and structural data reported by Lemoine et al (), Decarlis et al (), and Haupert et al () showed that in the present‐day Alps, the Provençal, Dauphinois, and Upper Austroalpine units (Figure ) represent remnants of the former proximal margins, whereas the internal European units and Lower Austroalpine units are derived from the former distal margin.…”