Introduction

The question of how distinct, successive, and diverse modes of extensional phases are recorded in an evolving rift system ultimately leading to seafloor spreading is fundamental to unraveling and understanding the tectono-stratigraphic evolution of rifted margins. Indeed, rifting leading to mantle exhumation has been shown to progress through different stages of deformation, initiating with distributed extension (i.e., stretching phase), then strain focusing (i.e., necking phase), in-sequence faulting in < 10 km-thick brittle crust (i.e., hyper-extension phase) to finally to mantle exhumation and possibly seafloor spreading (e.g., Lavier and Manatschal 2006; Péron-Pinvidic and Manatschal 2009). By consequence and despite possible allogenic effects, the spatial distribution and temporal evolution of syn-rift depositional systems are controlled by the succession of tectonic events (e.g., Masini et al. 2013; Tugend et al. 2014; Haupert et al. 2016). Dominant tectonic signatures, expressed in the seismic stratigraphic architecture, can be used to describe the evolution of rift systems. This paper addresses the following key questions for the syn-rift succession of a magma-poor rifted margin:

  • Can we define the migration through time and space across a margin the extension process and the subsequent abandonment of any tectonic activity? What are the consequences for the the syn-rift stratigraphic record and how can this be described (i.e., syn-tectonic vs post-tectonic)?

  • Can we identify in the sedimentary record of rifted margins genetically related packages, referred here to as tectonic system tracts linked to the four successive modes of extension (i.e., stretching, necking, hyper-extension, and mantle exhumation)?

To answer these questions, we present a synthesis of field observations from the Austroalpine and Upper Penninic nappes exposed in Grisons (southeastern Switzerland and northern Italy; Fig. 1). These nappes preserve remnants of the Jurassic NW Adriatic rifted margin of the Alpine Tethys (e.g., Froitzheim and Manatschal 1996; Manatschal 2004; Mohn et al. 2010).

Fig. 1
figure 1

a Tectonic overview map of the Alps showing the distribution of the main paleogeographic domains from Mohn et al. (2011), modified after Schmid et al. (2004). b Paleogeographic map of the Alpine domain at the end of Jurassic (Tithonian: 145 Ma, Mohn et al. (2010)

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This paper reviews sedimentological and tectonic data with the aim of proposing a general overview of the chrono-tectono-stratigraphic framework for the syn-rift succession across this fossil-rifted margin. We document facies distribution and stratigraphic architecture within different rift domains. The correlation of sedimentary packages between the rift domains is based on the identification of correlative stratigraphic intervals linked to global or basin-wide Jurassic events, which correspond to: (1) an early Toarcian Oceanic Anoxic Event; (2) an early Bajocian bio-siliceous event; and (3) the onset of Tithonian carbonate-dominated sedimentation.

The use of these timelines enables differentiating sedimentary packages deposited during the successive stretching, necking, hyper-extension, and mantle exhumation stages and to define a set of four tectonic system tracts bounded by unconformities and their correlative surfaces.

Terminology used in this paper

In this study, we use the term “syn-rift” for all sediments that are deposited between onset and end of rifting. Thus, the term “syn-rift” refers to the timing of deposition and not to the architectural characteristic of the sedimentary sequence. The syn-rift succession can be subdivided into two main sediment packages that are either syn- or post-tectonic, i.e., deposited either in or outside the domain undergoing active extension during the timing of rifting. The stratigraphic architecture of syn-tectonic sedimentary packages is controlled by local fault activity and is characterized by sequences that either thicken into high-angle normal faults or down/on-lapping onto exhumation faults and tectono-sedimentary sequences (e.g., breccias). In the mantle exhumation domain, magmatism related to active faulting may also be part of the syn-tectonic package. In contrast, the post-tectonic sedimentary sequence seals major faults, marking the end of local fault activity. The basal limit of the post-tectonic packages corresponds to the “base of passive infill” as recognized in seismic data. The top of the post-tectonic package is the base of the post-rift sequence, i.e., the first sediments that are deposited over oceanic crust and, therefore, post-date lithospheric breakup or rift cessation in a failed rift system.

A fourfold division of the stratigraphic succession in asymmetric graben bounded by active faults has been previously defined by Prosser (1993). This subdivision, however, which includes rift initiation, rift climax, immediate post-rift, and late post-rift, has been specifically defined for the proximal domain and did not, as this study will show, integrate the different modes of extension of distal margins and their oceanward migration during extension.

For these reasons, we will modify the concept of “tectonic system tract” developed for half-graben by Prosser (1993). The aim of this paper is to define a set of tectonic system tracts linked to each of the main extensional deformation modes and to analyze their specific stratigraphic evolution as a function of distinct and successive deformation phases across the NW Adriatic rifted margin. We will demonstrate that stretching, necking, hyper-extension, and mantle exhumation originate in specific tectono-stratigraphic relationships that can be used to differentiate the “tectonic system tracts” in rifted margins. Time scale used in this study is from Ogg et al. (2016).

Geological setting

The fossil NW Adriatic margin in the Central Alps

The building blocks defining a magma-poor rifted margin include four genetic rift domains, which are from the continent to the ocean the proximal, the necking, the hyper-extended, and the exhumed mantle domains (Fig. 2; e.g., Tugend et al. 2015). Remnants of these four domains, mapped in the magma-poor, NW Adriatic margin of the Jurassic Alpine Tethys, are preserved in the Austroalpine and Upper Penninic nappe stack in SE Switzerland/N Italy (Figs. 1, 2; e.g., Mohn et al. 2011; Epin et al. 2017). A restoration of the nappe stack showed that the margin had to be > 60 km wide (e.g., Epin et al. 2017). Based on the kinematics of detachment systems, a lower plate position of the distal NW Adriatic margin has been proposed (Fig. 2; e.g., Froitzheim and Manatschal 1996; Manatschal 2004; Mohn et al. 2010; Epin and Manatschal 2018). Since Alpine shortening was almost perpendicular to the strike of the ancient margin and both extensional (rifting) and compressional (Alpine orogen) transport directions were top to the west, there is a simple link between the place of a unit in the nappe stack and its original position along the former margin.

Fig. 2
figure 2

a Tectonic map showing the paleogeographic domains of the Austroalpine and Upper Penninic nappe systems in SE Switzerland and N-Italy from Mohn et al. (2012). b Reconstructed Alpine section across the Upper Penninic and Austroalpine nappes Mohn et al. (2011) for cross-sectional A-A’ shown in (a). c Palinspastic reconstruction of the magma-poor Adriatic margin and lower plate setting across the Austroalpine and South-Penninic nappes Mohn et al. (2011)

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The Upper Austroalpine nappe system has preserved half-graben-type sedimentary basins in the Ortler and Ela units interpreted as part of a proximal margin (Fig. 2c; e.g., Eberli 1988; Manatschal and Bernoulli 1999; Mohn et al. 2010). The middle Austroalpine nappe system, including the Grosina-Campo and Languard units, is interpreted as having sampled remnants of the necking domain as mapped by (Fig. 2; Mohn et al. 2011, 2012). The Lower Austroalpine nappe system, including the Bernina and Err units, preserves extensional detachment faults with associated, deep-water supra-detachment basins that are inherited from the hyper-extended rift domain (Fig. 2; e.g., Froitzheim and Eberli 1990; Masini et al. 2011). The Upper Penninic nappe system, including the Platta unit, has been interpreted as reworking an exhumed mantle domain showing an oceanward increase in magmatic activity (Fig. 2; Manatschal and Nievergelt 1997; Desmurs et al. 2002; Manatschal and Müntener 2009). The lithospheric mantle exposed in the Upper Platta unit does not show evidence for melting or interaction with magma during rifting, while the Lower Platta unit shows evidence for magmatic depletion during the earlier Late Variscan orogenic collapse and later infiltrated by MORB-type melts during subsequent Jurassic rifting (e.g., Müntener et al. 2009; Picazo et al. 2016). The basement in the Austroalpine nappes mainly consists of Variscan upper, middle, and lower crystalline crust (e.g., Staub 1946) showing a Late Carboniferous–Early Permian evolution characterized by widespread magmatic additions at different crustal levels (e.g., Schuster and Stüwe 2008; Petri et al. 2017, 2018).

Sedimentary succession: from Permian to Jurassic deposits

The pre-rift stratigraphic record starts with Permian basins filled with volcano-sedimentary sequences (e.g., Dössegger 1976; Handy et al. 1993). Triassic sediments overlie Permian sediments and/or crystalline basement, are comprised of continental clastics and evaporites at the base (e.g., Fuorn and Raibler Fms), and grade up-section into shallow-marine kilometer-thick platform carbonates (i.e., Ladinian S-charl Fm and Norian Hauptdolomit Fm) with occasional lagoonal paleo-environments responsible for the deposition of shales and interbedded limestones and dolomites (i.e., Kössen Fm.; e.g., Finger 1978; Naef 1987; Eberli 1988; Furrer 1993).

From the Early Jurassic to the Late Jurassic, the Alpine Tethys experienced a rifting period that structured the domain into several sub-basins, usually referred to as the Liguria, Piemonte and Valais basins (Fig. 1b; e.g., Jacquin and de Graciansky 1998; De Graciansky et al. 2011, and references therein). The associated syn-rift sediments show large variations in thickness and sedimentary architecture that resulted from temporal and spatial variations in accommodation rates, sediment supply and relative sea-level changes. The study area was more specifically located along the eastern margin of the evolving Piemonte basin, considered as sediment-starved (Fig. 1; Masini et al. 2013). Jurassic syn-rift deposition was characterized by marine sediments comprising limestones, marls, shales, calcarenites, sandstones and siliceous deposits. A detailed sedimentological and stratigraphic description of the syn-rift succession is provided below. It is generally acknowledged that the cessation of extensional deformation and thus the start of the post-rift succession occurred in the Late Jurassic (e.g., Jacquin and de Graciansky 1998; De Graciansky et al. 2011, and references therein).

Timelines associated to region-wide and global events

The Jurassic period includes a number of global and regional or basin-wide events that are recorded in the Tethyan syn-rift succession. Special attention has been paid in the field to their stratigraphic signature. The signature might vary across the margin, from the deep, distal margin to the shallower-water proximal margin, where those events have generally been mapped and characterized.

Early Toarcian oceanic anoxic event (T-OAE event ~ 183 Ma)

The Toarcian OAE is a global-scale anoxic event characterized by a carbonate crisis, affecting both pelagic and neritic environments, associated with the worldwide distribution of black shales (e.g., Jenkyns 1988; Erba 2004; McArthur et al. 2008; Jenkyns 2010). The T-OAE is associated with a major negative carbon isotope excursion, a faunal turnover (in particular, the collapse of calcareous nannofossils) and a major marine transgression (e.g., Bailey et al. 2003; Huang and Hesselbo 2014; Ullmann et al. 2014).

Bajocian-Bathonian bio-siliceous event (Si event ~ 170 Ma)

In the Tethyan realm, the Bajocian is marked by a drop in the biodiversity of calcareous organisms followed by the onset of bio-siliceous sedimentation (e.g., Bartolini et al. 1996; Muttoni et al. 2005; Chiari et al. 2008). This crisis in carbonate productivity is correlated with a decrease of the Sr87/Sr86 isotope ratio, a eustatic sea-level rise and positive δ13C excursions (e.g., Morettini et al. 2002). The isotopic shifts are interpreted as related to climate-induced primary biological productivity changes in the oceans and suggest eutrophication as a cause of carbonate platform demise (e.g., Bartolini and Cecca 1999; Morettini et al. 2002). The lithological change from siliceous pelagic limestones to lime-free radiolarites, with a diversified radiolarian fauna, occurs in Tethyan margins during the Bajocian (e.g., Bill et al. 2001; Baumgartner 2013). Radiolarite deposition is recorded up to the Kimmeridgian (e.g., Bill et al. 2001; Baumgartner 2013).

Tithonian carbonate event (Ca event ~ 152 Ma)

During the Late Oxfordian-early Tithonian, the Alpine Tethyan margins record a general recovery of carbonate factories both on platforms and in the adjacent deep-water basins (e.g., Bartolini et al. 1996; Weissert and Erba 2004). Sedimentation shifted abruptly from bio-siliceous to carbonate facies in many Tethyan basins (e.g., Bartolini et al. 1999; Baumgartner 2013). This change is thought to be triggered by a rapid decrease in nutrient availability during the Late Jurassic that reduced radiolarian productivity, allowing nannofossil communities to develop (e.g., Cecca et al. 2005). These Tithonian nannofossils form the first planktonic-derived pelagic limestone across the Alpine Tethys (e.g., Aptychus Limestone Fm, Calpionella Limestone Fm) and Central Atlantic Ocean (e.g., Bornemann et al. 2003).

Characterization of the syn-rift succession

Proximal domain: Ortler and Ela units

Structural architecture

The proximal domain (Ortler and Ela units) consists of typical fault bounded tilted blocks with east or west facing normal faults (Figs. 3, 4; Eberli 1987; Froitzheim 1988). The cut-off angle is approximately 60° ± 10° with a vertical offset ranging from a hundred to more than 1000 m (Froitzheim 1988). These faults accommodated stretching of the brittle, upper continental crust. As already proposed by Froitzheim (1988), the occurrence of evaporitic facies associated with the Middle Triassic Raibler Fm may represent a pre-rift unit (e.g., Jammes et al. 2010; Rowan 2014). The influence of halokinesis on the extensional deformation of the Adriatic margin is, however, beyond the scope of this paper.

Fig. 3
figure 3

Palinspastic reconstruction of the Adriatic margin at late Jurassic time, modified from Mohn et al. (2011) and Masini et al. (2011). Restoration of various nappes show inferred relative positions with respect to each other. Platta unit: F Falotta, M Mulegns, PdP Piz Digl Plaz, T Tigias, PP Piz Platta; Err unit: C Carungas, PN Piz Nair, FC Fuorcla Cotschna, PB Piz Bardella, G Grevasalvas, PE Piz Emmat Dadora, R Roccabella; Bernina unit: CG Corn Gess, PA Piz Alv, PdF Piz dal Fain, PM Piz Mezzaun, PS Piz Sassalb, PT Piz Tschüffer, VM Valle del Monte, VL Val Lavirum; Ela unit: PB Piz Blaisun, PM Piz Mitgel, PT Piz Toissa, VT Val Tisch; Ortler unit: MB Munt Blais, IM Il Motto, PC Piz Chaschauna, LP La Paré, MT Monte Torraccia, MP Monte Pettini

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Fig. 4
figure 4

Palinspastic reconstructions across the Ortler (a) and the Ela (b) units. The cross sections summarize the sediment architecture, fault geometry, and position of global timelines observed in these units. Data compiled from Furrer (1985); Eberli (1988); Furrer (1993); Conti et al. (1994) and field observations. Note, the scale for the syn-rift is not the same as for pre-rift and basement

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Facies distribution and stratigraphic architecture

The Triassic pre-rift Kössen and Hauptdolomit Fms are overlain by the Lower Jurassic Lower Allgäu Fm, which coincides with the onset of rifting in the Alpine Tethys domain, consistent with the local appearance of breccias in half-graben with syn-tectonic, wedge-shaped sedimentary packages (Figs. 4, 5). Indeed, the Lower Allgäu Fm is composed of locally sourced, tectonic-induced facies related to normal faults interfingering with a background sedimentation (Fig. 5; Eberli 1988). The tectonic-induced Chaschauna Breccias are directly related to the proximity to a fault scarp (Eberli 1987). It comprises megabreccias close to the fault scarp, grading downdip into finer breccias and calciturbidites (Fig. 5). These intervals are observed from the early Hettangian (Monte Torraccia and Il Motto areas, Conti et al. 1994) to the early Pliensbachian (Piz Toissa area, Furrer 1993). They are interpreted as syn-tectonic packages.

Fig. 5
figure 5

Representative sedimentary successions of the proximal domain, including the Ortler unit (left) and Ela unit (right), based on literature compilations (Furrer 1985; Eberli 1988; Furrer 1993; Conti et al. 1994) and field observations. Time scale from Ogg et al. (2016)

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Background sedimentation, in which breccias are interbedded, are represented by the Naira, Stidier, Trupchun, and Spadlatscha beds comprising marls and limestones (Figs. 4, 5, 6a). On gentle slopes facing, the fault escarpment, marl, and limestone alternations prevail in the upper part of the Lower Allgäu Fm (Fig. 5), which is contemporaneous with basinal facies of the Trupchun Beds (Fig. 5). The Lower Allgäu Fm in the Ortler and Ela units exhibits the classical features of facies associations in extensional half-graben basins as described by Leeder and Gawthorpe (1987). This formation, a marker for the onset of rifting, is dated as Hettangian to Early Toarcian by biostratigraphy (Fig. 5; e.g., Eberli 1985, 1988; Furrer 1993; Conti et al. 1994). The Lower Allgäu Fm further displays an overall thinning- and fining-upward megacycle (Fig. 5; Eberli 1988), ending with manganese-bearing black shales attesting to anoxic conditions (Figs. 5, 6a, c; Eberli 1985).

Fig. 6
figure 6

Outcrop photographs of representative facies of the proximal domain in the NW Adriatic margin. a Panoramic view of the Lower Allgäu Fm, Mn-rich black shales and Upper Allgäu Fm in the Ela unit at Piz Blaisun. The dashed pink line represents the onset of the Mn-rich black shales. b Panoramic view of the Blais-Radiolarit Fm and Aptychus Limestone Fm in the Ortler unit at Munt Blais. The dashed blue line represents the onset of carbonate sedimentation. c Manganese-rich black shales Fm at Munt Blais, Ortler unit. d Calcarenite beds (a) with secondary silicification occurring as nodules and irregular elongated bands (b) of the Mezzaun beds at Piz Blaisun, Ela unit. e Reddish radiolarite beds of the Blais-Radiolarit Fm at Munt Blais, Ortler unit

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The Upper Allgäu Fm (Mezzaun beds; Figs. 5, 6a, d) consists of calcarenite beds interbedded with shales, marls and shaly limestones (Eberli 1988). In addition, the Mezzaun beds also display an overall thinning- and fining-upward megacycle. Secondary silicification occurs as chert nodules or irregular elongated silicified bands in the calcarenite beds. The silicification develops up-section (Figs. 5, 6d) to thinly bedded radiolarites showing an alternation of 1–10 cm-thick radiolarian-rich beds with millimeter-to-centimeter-thick shale partings (Blais-Radiolarit Fm., Figs. 4, 5, 6b, Eberli 1988). The radiolarites start with blue–green facies interbedded with shales, passing upwards to a red facies typically showing bed amalgamation (Figs. 4, 5). Continuing, the reddish radiolarian-rich beds alternate upward with thin limestone beds, developing finally into the well-known pelagic Aptychus limestone Fm (Fig. 6b, e).

Stratigraphic markers: T-OAE, Si, and Ca events

The manganiferous black shale interval was interpreted by Eberli (1988) as the record of the Early Toarcian OAE (~ 181 Ma) and correlates with similar deposits observed in the Northern Calcareous Alps and Southern Alps (e.g., Jacobshagen 1965; Jenkyns et al. 1985; Jenkyns and Clayton 1986; Rantitsch et al. 2003).

The radiolaritic succession, starting with black-to-green/blue radiolarite facies that pass upward to a reddish radiolarite facies, is interpreted as an expression of the onset of bio-siliceous sedimentation dated as Early Bajocian. This sequence is common throughout the Alpine Tethyan margins, where the blue–green radiolarites are interpreted as indicators of at least dysoxic conditions associated with the occurrence of primary organic matter, while the red facies is interpreted as an indicator of well-oxygenated depositional environments (e.g., Baumgartner 2013). Finally, the transition from radiolarite to limestone deposits (i.e., Blais-Radiolarit Fm. to Aptychus Fm.) likely represents the Tithonian carbonate timeline (~ 152 Ma).

Necking domain: Campo-Grosina and Languard units

The Campo-Grosina and Languard units are interpreted as the remnants of a necking domain (Fig. 3; Mohn et al. 2012). These units comprise upper and middle continental crustal blocks, separated by the upper Early Jurassic Eita shear zone (Fig. 3; Meier 2003; Mohn et al. 2011, 2012). The necking domain is characterized by a crustal-scale detachment system (i.e., Grosina detachment system) exhuming mid crustal basement at the seafloor. The exhumation leads to the creation of a totally new top basement surface allowing syn- to post-rift sediments to be deposited directly on mid crustal rocks (Mohn et al. 2011). The Grosina detachment system shows indurated black gouges and green, silica-rich cataclasites that are characteristic of Jurassic extensional detachment faults (Mohn et al. 2012; Pinto et al. 2015). In the field, no Mesozoic sedimentary cover has been identified above the detachment. Whether a sediment cover existed and has since been eroded or was never deposited over this exhumation surface will be discussed below.

Hyper-extended domain: Bernina and Err units

The Bernina and Err units preserve pristine remnants of the hyper-extended domain of the Adriatic rifted margin (Fig. 3; e.g., Manatschal and Nievergelt 1997; Mohn et al. 2010; Masini et al. 2011). The Bernina unit is considered more proximal, while the Err unit is thought to represent the most distal part of the hyper-extended domain (e.g., Manatschal and Nievergelt 1997; Mohn et al. 2010; Masini et al. 2011).

Structural architecture

In map view, the Bernina and Err units display a complex arrangement of discontinuous slivers of Mesozoic sedimentary cover surrounding or overlying basement rocks. In this domain, both high-angle normal faults and extensional detachment faults are observed (Fig. 7). The high-angle faults have been interpreted as related to the initial stretching phase similar to those observed in the proximal domain (Fig. 7). The extensional detachments often cap the basement units. Where Permian basins occur, extensional detachments can be identified at the boundary between Permian rhyolites and their underlying basement. The extensional detachment faults are associated with characteristic black gouges and silicified cataclasites (Mohn et al. 2011) and are responsible for the creation of new exhumed surfaces available for sedimentation. The extensional detachment faults are generally younger than the high-angle normal faults as shown by cross-cutting relationships, although the opposite situation also occurs. Such late high-angle faults become more common in the exhumed mantle domain (see description below). The combination of both types of extensional structures resulted in small isolated extensional allochthonous blocks (Fig. 7). In the Err unit, the basement is tectonically delaminated by the Err detachment system underlining the base of extensional allochthons (Fig. 10; e.g., Froitzheim and Eberli 1990; Froitzheim and Manatschal 1996; Handy 1996; Manatschal 1999; Masini et al. 2012). The Err detachment system has recently been re-interpreted as an in-sequence detachment system by Epin and Manatschal (2018), which involved at least three successive detachments (Err, Jenatsch, and Agnel detachments, see Fig. 10).

Fig. 7
figure 7

Palinspastic reconstruction across Bernina unit modified from Mohn et al. (2011). The cross section summarizes the sediment architecture, fault geometry and the position of global timelines observed in the Bernina unit. Data compiled from Schüpbach (1973, 1976); Zehnder (1975); Mohn et al. (2011, 2012) and field observations. Scale for syn-rift is not the same as for pre-rift and basement

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The syn-rift succession shows complex relationships with the detachment fault system, indicating syn- and post-tectonic relationships (for discussion, see chapters 5 and 6). In the field, the sediments are either cut by the faults and cap the allochthons, down/onlap onto active exhumation surfaces (syn-tectonic) or seal inactive tectonic surfaces (post-tectonic; Figs. 7, 10). Piz Sassalb, Piz dal Fain and Valle Del Monte in the Bernina unit and Piz Bardella and Corn Suvretta (see Fig. 7) are examples of individual blocks comprising basement, pre-rift, and Lower Jurassic syn-rift sediments truncated by extensional detachment faults, which are in turn covered by Middle Jurassic syn-rift and post-rift sediments. Evaporites, attributed to the Late Triassic Raibler Fm, are present across the Bernina and Err units. There are many examples of evaporites found at the base of the extensional allochthons (e.g., Piz Bardella, Corn Alv, and Piz Alv), or occur along north–south alignments of diapir-like structures (e.g., Monte Garone to Gess, Mohn et al. 2011), both suggesting halokinetic influence during extensional deformation (Figs. 3, 7, 10).

Facies distribution and stratigraphic architecture

The allochthonous blocks overlying the extensional detachment faults, in both the Bernina and Err units, are characterized by Permo–Triassic pre-rift successions conformably overlain by thin and well-bedded dark limestone beds of the Agnelli Fm (Piz Alv and Piz Mezzaun in Fig. 7 and Piz Bardella, Crap Alv, and Carungas in Fig. 10). The Agnelli Fm is also named Steinberger Fm by Schüpbach (1973) or Hierlatz-Kalk and Adnet-Kalk by Furrer (1993) and Eberli (1985). Sedimentary structures are generally not observed and less deformed beds only display parallel laminations. Beds generally contain abundant siliceous spiculae, crinoid debris, and sparse detrital quartz grains (Fig. 9a). Schüpbach (1973) and Finger (1978) reported the occurrence of Eparietites sp., of Sinemurian age.

Bernina unit

In the Bernina unit, a thin interval of the Agnelli Fm is observed at Piz Alv, where it is overlain by the Alv Fm, a thick succession of coarse breccia and megabreccia (Figs. 7, 8; Schüpbach 1973). This formation includes angular clasts from the Triassic Kössen and Hauptdolomit Fms embedded in a reddish-to-yellowish matrix, including belemnites and brachiopods (Fig. 9d). The clasts are very heterogeneous in size, ranging from a few centimeters to several tens of meters. Maximum thickness estimates of the Alv Fm are in hundreds of meters. This formation reflects the activity of nearby faulting during Early Jurassic rifting. Lateral facies variations in the Alv Fm occur over short distances, hindering stratigraphic correlations within this formation. The Alv Fm is also observed at Piz Tschüffer and Piz Mezzaun (Fig. 7). More basinal sedimentation is likely represented by the Fain Fm (Schüpbach 1973), which consists of poorly bedded breccia units with up to meter-sized clasts originating from the Hauptdolomit Fm embedded in a finer yellowish-brown micritic matrix. In comparison with the Alv Fm, the Fain Fm shows smaller blocks and lacks the reddish matrix (Schüpbach 1973). A generalized fining-upward trend leads to the disappearance of boulders and pebbles up-section and to the development of fine-grained calcarenites (Fig. 8; Decarlis et al. 2015). These calcarenites contain bioclastic material such as fragments of crinoids, belemnites, brachiopods, and bivalves (Schüpbach 1973). The transition between the Fain Fm and the overlying Mezzaun beds is marked in places by the occurrence of a decametric interval of manganiferous black shales, similar to those observed in the proximal domain (Figs. 7, 8). In the Piz Mezzaun area, the manganiferous black shales are overlain by a thick breccia interval containing large pebbles from the Triassic Hauptdolomit and Kössen Fms (Fig. 7). This unit grades upward into a thick-bedded calcarenite suite, terminated by thinly bedded calcarenites. Bioturbated and abundant echinoderm skeletal debris and marly interbeds also exist within this unit. Locally, well-preserved Zoophycos feeding traces are found in the marls (Eberli 1988). This thinning- and fining-upward megacycle has been correlated with the Mezzaun beds of the proximal domain (Upper Allgäu Fm.; Figs. 7, 8; Eberli 1988).

Fig. 8
figure 8

Representative sedimentary succession of the proximal hyper-extended domain observed in the Bernina unit based on compilation of Schüpbach (1973); Decarlis et al. (2015) and field observations. Time scale from Ogg et al. (2016)

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Fig. 9
figure 9

Outcrop photographs of representative facies of the hyper-extended domain in the Bernina and Err units. a Limestones of the Agnelli Fm containing abundant spiculae and crinoid debris at Piz Bardella, Err unit (a). b Ferromanganese crusts with abundant belemnites (a) at the top of the Agnelli Fm at Piz Bardella, Err unit. c Thin section of the ferromanganese crusts (b) containing abundant crinoidal fragments (a. ossicles and plates), benthic foraminifera (b. Lagenids and c. Involutina liassica). d Coarse breccia of the Alv Fm; angular clasts are from the Triassic pre-rift Kössen and Hauptdolomit Fms embedded in a reddish-to-yellow matrix at Piz Alv, Bernina unit. Pen for scale. e Basement-derived breccias of the Saluver A sub-Fm at Piz Nair, Err unit. f Reddish radiolarite beds of the Blais-Radiolarit Fm at Piz Nair, Err unit

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Where the Agnelli and Alv Breccia Fms are missing, the Mezzaun beds directly overlie basement or slivers of Triassic dolomites (e.g., Piz dal Fain; Fig. 7). Mohn et al. (2011) interpreted such basement exposures as exhumed detachment surfaces. This configuration is also observed at Valle del Monte and Piz Sassalb (Mohn et al. 2011), where massive breccia beds overlie slivers of Triassic dolomites, Permian sediments (i.e., Verucano Fm) and basement rocks (Fig. 7; Zehnder 1975). Interestingly, an inverted stratigraphy is noted in the Piz Sassalb area, where breccias start with clasts of the Triassic Kössen and Hauptdolomit Fms progressively replaced up-section by clasts of Permian detritus and its underlying basement (Zehnder 1975).

Finally, the Mezzaun beds pass vertically from lime-free, green–blue to red radiolarite beds and then to limestones (Blais-Radiolarit Fm and to Aptychus Fm, respectively; Fig. 8), forming a succession similar to that in the proximal domain.

Err unit

In the Err unit, the first interval of limestone of the Agnelli Fm is overlain by a 5–15 m-thick layer of calcarenites, largely formed by crinoidal debris (Fig. 11). Intense silicification is expressed as beds with nodules with irregular shapes that in some cases coalesce to form thin layers. The topmost horizons of this formation, when present, show a number of belemnite- and ammonite-rich levels dated as Early-to-Late Pliensbachian at Piz Bardella by Dommergues et al. (2012) and is in turn capped by Early Toarcian ferromanganese crusts (Fig. 9b) containing ammonites (Hildoceras sp., (Finger 1978). The fossil content in the ferromanganese crusts consists of belemnites, crinoidal fragments (ossicles and plates), and benthic foraminifera (Involutina liassica and lagenids; Fig. 9c). The Agnelli Fm is here considered as a time equivalent of the limestone intervals of the Lower Allgäu Fm observed in the proximal domain (i.e., Naira, Stidier, and Spadlatscha Beds, Figs. 5, 8).

The hardground at the top of the Agnelli Fm, observed at Piz Bardella, is unconformably overlain by the Bardella Fm (Finger 1978). This formation consists of fine-to-very coarse breccias interbedded with finer deposits, mainly shales, marls, and fine-grained sandstones (Finger 1978). Carbonate-dominated clasts originate from the Agnelli Fm and the underlying pre-rift strata; a few basement-derived clasts are also present. Based on the stratigraphic position, the clasts are mainly derived from Triassic and Early Jurassic carbonates. The Bardella Fm is interpreted as formed during onset of extension of the late Triassic–Early Jurassic platform in the future distal margin, which initiates after deposition of the Agnelli Fm (Masini et al. 2011; Incerpi et al. 2017). Thus, the Agnelli Fm is the first syn-tectonic deposit in the most distal part of the hyper-extended domain. Away from the extensional allochthon, where the crystalline basement is exhumed to the seafloor, sedimentation occurs directly on exhumed basement. Basement-derived breccias are dominant and constitute the basal part of the Saluver Fm (Fig. 9e; Member A of Saluver Fm of Finger (1978) and Masini et al. (2011)). The Member A of the Saluver Fm, in turn, records the initial exhumation of basement along detachment faults as indicated by the first occurrence of breccias and footwall-derived lithologies in the sediments (Masini et al. 2011). Since the extensional detachments primarily generate horizontal accommodation, the oldest breccias are deposited above the tilted hanging wall block (Bardella Fm) and the younger breccias are deposited on the tectonically exhumed footwall (Member A of Saluver Fm). Given that the facies distribution is tectonically driven by in-sequence detachments, the Bardella Fm is a marker of the onset of the detachment faulting in this domain and the age of this syn-tectonic facies youngs towards the future ocean.

Above both the hanging wall blocks and the exhumed extensional detachments, the amount of breccia decreases progressively up-section and calcarenitic and marly/shaly sedimentation becomes more prevalent (Masini et al. 2011). The latter is attributed to turbiditic sedimentation constituting Members B and C of the Saluver Fm (Fig. 11, Finger 1978).

Finally, the Blais-Radiolarit Fm and the Tithonian to early Berriasian Aptychus Limestone Fm (Finger 1978) overlie the Member C of the Saluver Fm in the Piz Bardella, Fuorcla Cotschna, and Piz Nair areas (Figs. 9f, 10, 11). These two formations overlie also the Agnelli Fm in the Carungas area (i.e., Piz Salteras) or directly on exhumed continental crust in the Rocabella and Piz Dadora areas (Agnel detachment in Fig. 10).

Fig. 10
figure 10

Palinspastic reconstruction across Err unit modified from (Manatschal and Nievergelt 1997; Masini et al. 2011, 2012; Epin and Manatschal 2018). The cross section summarizes the sediment architecture, fault geometry, and the position of global timelines observed in this unit. Scale for the syn-rift is not the same as for pre-rift and basement

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Fig. 11
figure 11

Representative sedimentary succession of the distal hyper-extended domain observed in the Err unit based on compilation of Finger (1978); Handy (1996); Masini et al. (2011) and field observations. Time scale from Ogg et al. (2016)

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Stratigraphic markers: T-OAE, Si, and Ca events

The ferromanganese crusts observed in the Bernina (Piz Mezzaun) and Err units (Fig. 9b, Piz Bardella), dated with ammonites as early Toarcian (Finger 1978), typically occur on topographic highs and correspond to condensed stratigraphic sections (i.e., accretion rates < 1–15 mm/Ma; e.g., Banakar and Hein 2000). It is well known that the development of the ferromanganese crusts occurred during T-OAE, yet their genetic link to the OAE has yet to be verified. Early Toarcian ferromanganese hardgrounds are observed in the central Mediterranean region (e.g., Sicily; Jenkyns 1970; Di Stefano and Mindszenty 2000; Pallini et al. 2005; Sulli and Interbartolo 2016), the Julian Alps (e.g., Sabatino et al. 2009), and the Northern Calcareous Alps (e.g., Ebli et al. 1998), which were all connected during the Jurassic. The transition between the turbiditic Member C of the Saluver Fm and the Blais-Radiolarit Fm is interpreted as the expression of the biosilicesous event. The transition to the Aptychus limestone Fm is interpreted to coincide with the Tithonian carbonate timeline.

Exhumed mantle domain: upper and lower Platta units

Structural architecture

Extensional structures in the Platta unit are interpreted as the oceanward continuation of the Err detachment system (Fig. 3; e.g., Manatschal and Nievergelt 1997). In this unit, which sampled the most distal part of the continental margin, exhumed mantle rocks are outcropping (Fig. 3). The Platta unit can be subdivided into two units: (i) the Upper Platta unit, formed by a sub-continental serpentinised mantle that contains rare magmatic additions and extensional allochthons made of continental crust and pre-rift sediments and (ii) the Lower Platta unit, consisting of infiltrated serpentinised mantle that shows the development of a magmatic system, including gabbroic intrusive and basaltic extrusive rocks (MORB-type), interpreted as embryonic oceanic crust (Fig. 12; e.g., Desmurs et al. 2001; Manatschal and Müntener 2009 and reference therein). Zircons extracted from gabbros and an albitite yielded concordant U–Pb ages of 161 ± 1 Ma (Schaltegger et al. 2002). The mantle is exhumed at the seafloor along two main detachment faults, referred to as the Upper and Lower Platta detachment faults (Epin 2017). The Lower Platta detachment fault shows a dome-type structure that is cross-cut by post-exhumation normal faults (Piz digl Platz in Fig. 12; Epin 2017).

Fig. 12
figure 12

Palinspastic reconstruction across Platta unit modified from Manatschal and Nievergelt (1997) and Epin (2017). The cross section summarizes the sediment architecture, fault geometry, and the position of global timelines observed in Platta Unit. Scale for syn-rift is not the same as for pre-rift and basement

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Basement rocks in the Platta unit comprise serpentinized mantle (peridotite), serpentinite cataclasites and gouges, basalts, gabbros, and ophicalcites. The basement of the Platta unit comprises massive serpentinized peridotites (Fig. 13). Up-section, fractures, and veins filled by syn-kinematic chlorite and serpentine minerals mark the transition into serpentinite cataclasites. The intensity of brittle deformation increases up-section and develops into a core zone, which is formed by serpentinite gouges (e.g., Picazo et al. 2013; Epin et al. 2017). Penetrative replacement by calcite is observed near the paleo-seafloor (Früh-Green et al. 1990; Manatschal and Nievergelt 1997), which forms characteristic type 1 ophicalcites (OC1; Fig. 13; e.g., Bernoulli and Weissert 1985). The proportion of calcite increases up-section, transitioning into a matrix-supported tectono-sedimentary breccia with a calcitic matrix, referred to as a type 2 ophicalcite (OC2; Fig. 13; e.g., Bernoulli and Weissert 1985; Lemoine et al. 1987). Breccia clasts are derived from the serpentinized mantle rocks, some of them showing a cataclastic fabric. This OC1–OC2 interval is interpreted as the remnant of the detachment fault responsible for the exhumation of the mantle to the seafloor (Manatschal and Müntener 2009).

Fig. 13
figure 13

Representative sedimentary succession of the exhumed mantle domain observed in the Platta unit based on litterature compilation (Dietrich 1970; Suana 1984; Manatschal and Nievergelt 1997; Desmurs et al. 2001) and field observations. Time scale from Ogg et al. (2016)

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Facies analysis and stratigraphic architecture

Mantle rocks are overlain by clast-supported breccias containing clasts of serpentinized mantle and type 1 and 2 ophicalcites in a serpentinite sand matrix (Figs. 13, 14a, b). Sedimentary breccias are locally covered by several massive basaltic bodies consisting of pillows, pillow breccias, and hyaloclastites (Fig. 12; Manatschal and Nievergelt 1997). Dark-blue radiolarite deposits, correlated with the Blais-Radiolarit Fm, are interbedded with these basalts, indicating syn-magmatic deposition (Figs. 12, 13). The Blais-Radiolarit Fm also overlies these basalts and can reach tens of meters in thickness. In this case, the formation consists of radiolarite beds (cherts and radiolarian-rich beds) interbedded with reddish shales (Fig. 14c). Few matrix-supported conglomerates with components of serpentinised mantle, ophicalcite, and continental basement are observed in the radiolarite-rich matrix (Fig. 14d). Finally, limestone beds of the Aptychus Limestone Fm are found above the Blais-Radiolarit Fm (Fig. 13). Note that the Aptychus Limestone is dated as Tithonian to Berriasian in the Totalp nappe (Fig. 1), where these sediments also occur over exhumed mantle (Weissert and Bernoulli 1985).

Fig. 14
figure 14

Outcrop photographs of representative facies of the exhumed mantle domain in the NW Adriatic margin. a Panoramic view of the sedimentary breccia above the exhumed serpentinized mantle in the Falotta area, Platta unit. b Sedimentary breccia containing clasts of serpentinized mantle (a), type 1 and 2 ophicalcites (b) in a serpentinite sand matrix, Falotta area. c Reddish radiolarite beds of the Blais-Radiolarit Fm including radiolarian-rich intervals (a), cherts (b) and manganese-rich horizon (c). d Matrix-supported conglomerates with components of serpentinised mantle (a) in a radiolarite-rich matrix, Tigias area, Platta unit

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Stratigraphic markers: T-OAE, Si, and Ca events

The lithospheric mantle is interpreted as being exhumed during the late Middle Jurassic (Schaltegger et al. 2002), i.e., it post-dates the T-OAE event and can, as a consequence, not record this event. The same radiolarite succession as observed in the proximal and hyper-extended domains also occurs in the Platta unit and is interpreted as the initiation of the bio-siliceous event. Finally, where the Aptychus Fm overlays the Blais-Radiolarit Fm, we interpret the contact to coincide with the Tithonian carbonate timeline.

Stratigraphic correlation across the rifted margin

The vertical and lateral stacking patterns of the syn-rift succession in the four studied rift domains reflect specific stratigraphic architectures and evolutions, governed by both local tectonic and external global oceanic anoxic events. Layers related to these events are used to construct the temporal framework across the NW Adriatic margin from its proximal to distal environments (Fig. 15). Indeed, the manganiferous black shales and the ferromanganese crusts observed in the proximal and hyper-extended domains in basinal and paleo-high areas (Figs. 4, 7, 10) are considered signatures of the T-OAE, defining a regional early Toarcian timeline (Fig. 15). Furthermore, the first appearance of radiolarites across the NW Adriatic margin is here considered as a synchronous event, despite the onset of radiolarite deposition along the overall Tethyan margins being slightly diachronous (i.e., from early Bajocian to late Bathonian; e.g., Bill et al. 2001; Baumgartner 2013). Finally, the abrupt shift in sedimentation from the bio-siliceous Blais-Radiolarit Fm to the carbonaceous Aptychus Limestone Fm is interpreted as the expression of the Tithonian carbonate event (Fig. 15). The top of the syn-rift, defined as the cessation of rifting, occurs within the Blais-Radiolarit Fm and the Aptychus Limestone Fm as the first clearly defined post-rift deposit, since it is not affected by any extensional deformation. We conclude that rifting leading to the formation of the NW Adriatic margin in the Piemonte segment of the Alpine Tethys lasted from the Hettangian to the Kimmeridgian over a period of ~ 50 My.

Fig. 15
figure 15

Generalized chrono-tectono-stratigraphic correlation chart of the northwestern Adriatic rifted margin with the distinction between syn- and post-tectonic packages within the syn-rift succession. Syn-rift defined as the time interval between the onset and end of rifting. Biostratigraphic references: 1 (Furrer 1985, 1993); 2 (Eberli 1985, 1988); 3 (Conti et al. 1994); 4 (Schüpbach 1973); 5 (Dommergues et al. 2012); 6 (Finger 1978). Time scale from Ogg et al. (2016). Platta unit: F Falotta, PP Piz Platta; Err unit: C Carungas, PN Piz Nair, PB Piz Bardella; Bernina unit: PA Piz Alv, PdF Piz dal Fain, PM Piz Mezzaun, PS Piz Sassalb; Ela unit: PT Piz Toissa; Ortler unit: MB Munt Blais, IM Il Motto. Time scale from Ogg et al. (2016). Numbers 1–5 correspond to tectonic system tract boundaries: (1) the onset of stretching system tract (rift-onset unconformity), (2) the onset of necking system tract, (3) the onset of hyper-extension system tract, (4) the onset of mantle exhumation system tract, and (5) the Post-Rift surface

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Discussion: tectonic system tracts of hyper-extended margins

At a regional scale and considering individually the four rift domains, the temporal range of the syn- and post-tectonic sediments is variable across the margin (Fig. 15). For instance, the age of the syn-tectonic breccias is Hettangian to Early Pliensbachian throughout the proximal domain, Late Pliensbachian/Early Toarcian to Bajocian in the hyper-extended domain, and grading from Bajocian to Kimmeridgian in the exhumed mantle domain. More generally, a younging of the syn- and post-tectonic packages is identified from the proximal to the exhumed mantle domains. As a consequence, the syn-tectonic package from a given rift domain is deposited coevally with the post-tectonic package of one or several more proximal rift domain(s) (Fig. 15). The distribution of syn-tectonic deposits thus records a continuous trend of migration from the necking zone towards the exhumed mantle domain. Meanwhile, initially widely distributed extensional activity progressively becomes progressively localized, migrating towards the future location of lithosphere breakup (Fig. 15). Finally, the oldest post-tectonic sediments, although still syn-rift at the margin scale, are found in the proximal domain and are of Early Pliensbachian age. In contrast, post-tectonic sediments are post-Bajocian in the hyper-extended units, and post-Tithonian within the exhumed mantle domain (Fig. 15). In the following discussion, we show how the syn- and post-tectonic packages, both representative of the syn-rift sequence, can be spatially organized into four Tectonic System Tracts linked to four successive time intervals typified by distinctive deformation modes at the crustal scale.

Stretching system tract (~ 201–191 Ma)

The stretching system tract is characterized by widely distributed high-angle normal faults that break the pre-rift succession and the brittle upper crust (Fig. 16; e.g., Lavier and Manatschal 2006; Duretz et al. 2016; Brune et al. 2017). The onset of the stretching phase marks the start of the syn-rift stratigraphic succession that initiated in the Late Triassic/Early Jurassic (~ 201 Ma).

Fig. 16
figure 16

First-order schematic tectono-sedimentary evolution and associated Tectonic System Tract represented in a tectonic “Wheeler diagram” for the NW Adriatic rifted margins during the Jurassic rifting, based on Mohn et al. (2012) and Masini et al. (2013). Numbers 1–5 correspond to tectonic system tract boundaries: (1) the onset of stretching system tract (rift-onset unconformity), (2) the onset of necking system tract, (3) the onset of hyper-extension system tract, (4) the onset of mantle exhumation system tract, and (5) the Post-Rift surface. Time scale from Ogg et al. (2016). Platta unit: F Falotta, PP Piz Platta; Err unit: C Carungas, PN Piz Nair, PB Piz Bardella; Bernina unit: PA Piz Alv, PdF Piz dal Fain, PM Piz Mezzaun, PS Piz Sassalb; Ela unit: PT Piz Toissa; Ortler unit: MB Munt Blais, IM Il Motto

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Related facies are characterized either by: (i) breccia deposits composed of reworked clasts from the pre-rift section intercalated with calciturbidites and hemi-pelagites or (ii) hemi-pelagic carbonate deposits occurring over basement highs, the footwall of various fault bounded basins (e.g., Ortler and Ela Units; Figs. 6, 16).

Such half-graben basins and platform accumulations dominate the proximal domain, where they have been extensively preserved, while they are less well preserved in the (subsequent) hyper-extended distal domain. In the latter position, the stretching system tract essentially corresponds to discontinuous intervals capping extensional allochthons. In the proximal domain, the youngest normal faults are sealed by Lower Pliensbachian sediments, marking the end of local tectonic activity and the transition from the syn- to the post-tectonic stage (Figs. 15, 16). The time interval of the stretching system tract is from Late Triassic/Early Jurassic to the Early/Late Pliensbachian, corresponding to about 10 My.

Necking system tract (~ 191–185 Ma)

The transition between the stretching and the necking system tracts relates to the localization of deformation along a number of major crustal-scale faults with contemporaneous abandonment and sedimentary sealing of fault structures in the proximal system tract (Fig. 16; Chenin et al. 2015). Faults associated with the necking system tract are, like those of the stretching tract, decoupled at a crustal scale (Sutra and Manatschal 2012). In the study area, the necking zone does not show a sedimentary cover above the tectonically exhumed Grosina detachment (Mohn et al. 2012). Neither pre-rift nor syn-tectonic sediments exist. These sediments likely were never deposited. Depositional systems registering the formation of the necking system tract are pending, at least for the NW Adriatic margin. Using the Early/Late Pliensbachian end of tectonic activity in the proximal domain as the initiation of the necking system tract, and assuming a pre-Early Toarcian termination, the necking system tract corresponds to an approx. 6 My time interval. Coeval sedimentation in the proximal domain is characterized by distal calciturbidites constituting a locally conformable, syn-rift post-tectonic succession.

Hyper-extension system tract (~ 185–170 Ma)

The onset of hyper-extension reflects the change from focused and decoupled crustal deformation to a deformation mode characterized by a complete coupling of crustal deformation and a general oceanward in-sequence stepping of detachment faults. New real estate was formed, which resulted from the unroofing and exhumation of upper to lower crustal rocks to the seafloor (Fig. 16; e.g., Ranero and Pérez-Gussinyé 2010; Reston and McDermott 2011; Mohn et al. 2015; Epin and Manatschal 2018). The hyper-extension system tract sediments overlie older pre-rift to earlier syn-rift sediments over extensional allochthons or directly on tectonically exhumed basement (Fig. 16). Within the proximal and distal segments of the hyper-extended domain, syn-tectonic deposits of the hyper-extension system tract are interbedded with the Early Toarcian manganese-rich black shales, or overlie the ferromanganese crust marking the Early Toarcian OAE, respectively. Both are characterized by thick breccia packages containing clasts derived either from the basement or from the pre-rift succession. The younger breccia intervals are overlain by a well-sorted sandstone that is considered post-tectonic (Figs. 15, 16).

Coeval sedimentation related to the hyper-extension system tract in the proximal domain, and most probably in the necking domain, is characterized by distal calciturbidites constituting a conformable continuation of the post-tectonic succession. The hyper-extension system tract, considered to start during the pre-Early Toarcian (T-OAE) and end before the Bajocian bio-siliceous event, is a ~ 15 My time interval.

Mantle exhumation system tract (~ 170–152 Ma)

The extensional deformation characterizing the mantle exhumation system tract is accommodated by in- and out-of sequence extensional detachment faults exhuming mantle rocks at the seafloor (e.g., Reston and McDermott 2011; Sauter et al. 2013; Gillard et al. 2016). During this phase, the syn-tectonic sedimentary architecture is thought to be controlled by detachment faults and minor high-angle normal faulting, magmatism and hydrothermal activity (e.g., Gillard et al. 2015). This stage is also named the syn-OCT (Ocean-Continent transition) sequence by Nonn et al. (2017) based on seismic interpretation.

In the exhumed mantle domain of the NW Adriatic margin (Platta unit), new real estate was progressively established along extensional detachment faults exhuming mantle rocks (Fig. 16). The first sediments observed above the exhumed mantle are radiolarites and/or sedimentary breccias with ophicalcites and serpentinized mantle clasts occurring in a serpentinite- or radiolarite-rich matrix (e.g., Dietrich 1970; Manatschal and Nievergelt 1997). In turn, these sediments are overlain and/or interbedded with MORB basaltic flows (Figs. 12, 15). As a consequence, the syn-magmatic, radiolarian-bearing sediments of the mantle exhumation system tract are considered syn-tectonic and constitute the latest part of the syn-rift succession (Schaltegger et al. 2002). This sequencing is similar to that observed in the Ligurian Alps (e.g., Principi et al. 2004; Decarlis et al. 2018). Evidence for the timing of rift cessation is contained within the post-tectonic sediments and is not simply defined due to a lack of age constraints of the extruded magmatic rocks and related sediment starvation. In that respect, the cessation of magmatism before the Early Tithonian establishes a minimum age for the onset of the post-rift stage (Figs. 15, 16). In the proximal and hyper-extended domains, the mantle exhumation system tract is characterized by the typical conformable radiolarite sequence constituting a post-tectonic succession. The development of the mantle exhumation system tract, which starts in the Bajocian bio-siliceous event, ends before the Tithonian carbonate event, lasting ~ 18 My.

Bounding surfaces of tectonic system tracts

The four successive tectonic system tracts migrate spatially through time and the consequent bounding surfaces reflect the abandonment of one mode of tectonic activity and the subsequent oceanward continuation of tectonic activity and establishment of the next deformation mode. Following this scheme, five distinct surfaces, corresponding to the tectonic system tract boundaries, can be tracked across the NW Adriatic rifted margin, namely: (1) onset of stretching, (2) onset of necking, (3) onset of hyper-extension, (4) onset of mantle exhumation, and (5) onset of post-rift sedimentation; the latter corresponding to the end of the syn-rift sequence. It is essential to investigate the whole rifted margin, because these surfaces extending into the whole syn-rift sequence include unconformities (mainly in the eponym rift domain) and related conformities (in more proximal rift domains) depending on their position across the margin.

  1. 1.

    The ‘onset of stretching’ surface (~ 201 Ma) corresponds to the rift-onset unconformity, separates pre- and syn-rift strata (e.g., Falvey 1974; Driscoll et al. 1995; Franke 2013). This surface is recorded at the base of the syn-tectonic packages in the proximal domain and locally shows erosion of the footwall of tilted blocks. In the hyper-extended domain, the Triassic/Early Jurassic transition occurs as a dissected and, thus, discontinuous surface capping the extensional allochthons [Fig. 16 (1)].

  2. 2.

    The formal characterization of the ‘onset of necking’ surface (~ 191 Ma) should theoretically correspond to the base of a sedimentary unit and is expected to be directly overlie exhumed footwall (basement) blocks in the necking domain (not observed in the NW Adriatic margin). This surface passes continentward into the proximal domain, where the onset of necking is represented as the transition between syn- and post-tectonic depositional units. The ‘onset of necking’ surface is dated Late Pliensbachian in the Adriatic margin [e.g., Ortler and Ela units; Fig. 16(2)]. In the proximal domain, the transition between syn-tectonic wedge-shaped deposits and overlying passive infill has been referred to as the “necking unconformity” by Chenin et al. (2015).

  3. 3.

    The ‘onset of hyper-extension’ surface (~ 185 Ma) corresponds to the unconformable base of the syn-tectonic breccias in the hyper-extended domain. It merges with the top of the exhumed basement along detachment faults, and also extends into the syn-rift succession of the extensional allochthons. The ‘onset of hyper-extension’ surface passes continentward to the base of the post-tectonic package in the necking domain [Fig. 16(3)] and is represented by a conformable surface within the proximal domain (uppermost part of the post-tectonic Lower Allgäu Fm). The ‘onset of hyper-extension’ surface has a pre-Early Toarcian age.

  4. 4.

    The ‘onset of mantle exhumation’ surface (~ 170 Ma) is a timeline corresponding to the crustal separation of the two conjugate margins (Fig. 17). Leading to the emplacement of sub-continental mantle, it is also referred to as lithospheric breakup on magma-poor rifted margins (e.g., Sutra et al. 2013). This surface passes laterally to the hyper-extended domain and is found as the transition between the syn-tectonic Member B and the post-tectonic Member C of the Saluver Fm [Fig. 16(4)]. This interpretation is consistent with the geochemical signature of an enrichment in mantle derived elements (Ni–Cr–V) in the Member C of the Saluver Fm. suggesting deposition during the initial stage of mantle exhumation and pre-Bajocian in age (Pinto et al. 2015). The mantle exhumation system tract surface in the proximal domain is represented as a conformable surface within the uppermost part of the post-tectonic Upper Allgäu Fm.

    Fig. 17
    figure 17

    Idealized tectonic system tract terminology representation for conjugate rifted margins using the two-dimensional kinematic model of the evolution of the southern Iberia and Newfoundland margins proposed by Mohn et al. (2015). (1) The onset of stretching system tract (Rift-onset unconformity), (2) the onset of necking system tract (Necking Unconformity), (3) the onset of hyper-extension system tract, (4) the onset of mantle exhumation system tract (Crustal breakup unconformity), and (5) the Post-Rift surface (lithospheric breakup surface)

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  5. 5.

    The ‘onset of the post-rift’ surface (~ 152 Ma) marks the rift cessation and in theory, the onset of seafloor spreading. In the Liguria–Piemonte domain, no direct evidence for oceanic crust or depleted oceanic mantle has been found, implying that there is no direct evidence for lithospheric breakup (e.g., Picazo et al. 2016).

Implication for other rifted margins

This study details how the Jurassic syn-rift succession of the NW Adriatic margin currently exposed in the Alps can be subdivided in four Tectonic System Tracts separated by unconformities and their correlative surfaces. Expressing the development of rift processes linked to progressive focusing of extensional deformation, our scheme contrasts with earlier sedimentary models of syn-tectonic extensional basins that have often focused on tilted-block/half-graben geometries (e.g., Leeder and Gawthorpe 1987; Lemoine and Trümpy 1987; Prosser 1993; Jackson et al. 2006; Martins-Neto and Catuneanu 2010; Alves and Cupkovic 2018). Tilted-block/half-graben geometries, while typical for the proximal domain, do not capture the fundamental differences of syn-rift depositional environments, facies distributions, and stratal architectures associated with hyper-extended continental crust and/or exhumed continental mantle. These margins have been relatively inaccessible and thus less studied (e.g., Principi et al. 2004; Lagabrielle et al. 2010; Clerc et al. 2012; Masini et al. 2014). Only a few studies have investigated the syn-rift sequence throughout the rifted margin from the proximal to the most distal parts of the extensional system (e.g., Lemoine et al. 1986; Bertotti et al. 1993; Decarlis et al. 2013).

It is thus difficult to corroborate our observations and development model by comparison with other field-based case studies; the pertinent geological data just does not exist. Nevertheless, we argue that our reconstruction can be generalized and integrated into the study of any magma-poor margin by studying changes in deformation mode style, timing and location (Fig. 17). Finally, our study demonstrates that the simple definition of a “breakup unconformity” (Falvey, 1974) needs to be seriously revised and cannot be applied to hyper-extended rifted margins.

Conclusions

The aim of this study was to integrate and reconcile sedimentological, stratigraphic, and tectonic data from the Austroalpine and Upper Penninic nappes to provide a tectono-sedimentary framework for the evolution of the Jurassic syn-rift succession of the NW Adriatic rifted margin. We show how the syn-rift succession of the margin can be subdivided into sediment packages referred to as syn- and post-tectonic. Temporal relationships between the syn-tectonic packages across the margin and three margin-wide isochronous events (i.e., T-OAE, Si event, and Ca event) clearly demonstrate that the syn-tectonic packages relate to diachronous extensional deformation of continental crust with a continuous trend of migration towards the distal part of the rifted margin. Consequently, the base of the post-tectonic deposits records a similar trend showing a younging from the proximal to the distal exhumed mantle domain of the rifted margin. This diachroneity is interpreted as the result of successive phases of deformation during rifting and allow the syn-rift sequence to be subdivided into four Tectonic System Tracts, namely, the stretching, necking, hyper-extension, and mantle exhumation system tracts. We show that the base of passive infill, often termed the breakup unconformity, is not a single surface and is, in fact, diachronous across, and likely along, a passive margin system. It is worth noting that the entire syn-rift development of the NW Adriatic rifted margin lasted as much as 50 My, though syn-tectonic packages in individual rift domains developed over a 5-18 My time range. Numeric models need to take these observations into account, as it will dramatically affect the thermal structure and related rheological properties of the evolving lithosphere during extension. Although further work is needed to completely understand the architecture and evolution of syn-rift systems in rifted margins, we envision that the recognition of the above-described Tectonic System Tracts and related bounding surfaces may help to deconvolve and help understand the stratigraphic record of rifted margins. In particular, as seismic and well data from distal domains become increasingly available for research, we can expect much progress to be made allowing comparison of worldwide rifted margin development with the so far unique field-based data set of the Tethyan Alps.