The Adriatic continental margin

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The Adriatic continental margin

The Alps are an orogenic chain formed in response of two main phases of deformation: one during Late Cretaceous and the other during Cenozoic time (e.g. Lemoine et al., 1986; Froitzheim et al., 1994; Schmid et al., 2004). In this mountain belt, remnants of former Adriatic and European rifted margins are widely preserved since they did not undergo most of the Alpine deformation and metamorphism (Froitzheim and Manatschal, 1996; Masini et al., 2013). The two paleo-rifted margins were separated one from each other by the Piemonte-Liguria domain, which represents an embryonic ocean developed during Middle to Late Jurassic time (Lemoine et al., 1987; Manatschal and Bernoulli, 1998). In the present-day nappe stack, the former Adriatic and European continental margins are preserved in the Southern Alps and Austroalpine units and in the Helvetic units respectively. Remnants of the Piemonte-Liguria ocean constitute the Penninic units (Fig. 3.1).
Since the focus of this Thesis is on the Triassic-Jurassic evolution of the former Adriatic continental margin and its transition to the oceanic domain, a general description of their former architecture will be given in the following sections.

GEOLOGICAL SETTING

The study area is located in the southeasternmost part of Switzerland and N-Italy. This sector belongs to the Austroalpine and Upper Penninic units, which preserve one of the most outstanding sections of a magma-poor rifted margin. Since they were not involved in the subduction during the Alpine orogeny, they reached metamorphic conditions that never exceeded prehnite-pumpellyite facies. This weak overprint led to the exceptional preservation of all the features related to the Adriatic continental margin during its Triassic-Jurassic evolution. The nappe stack is actually subdivided into Upper, Middle and Lower Austroalpine units that overlie the Upper Penninic units that consist of ophiolitic material (Fig. 3.2).
The thrust faults separating the nappes have a top-to-the-west sense of shear (“Trupchun phase or D1-phase” of Froitzheim et al. 1994, Mohn et al. 2010). Since the overall orientation of the Adriatic margin was SW-NE, the late Cretaceous orogenic event resulted in the stacking of more proximal parts of the former margin onto more distal domains. The study of Mohn et al. (2010) on the Austroalpine units enabled to understand the former architecture of the margin by the definition of different paleogeographic domains (Mohn et al. 2010) tectonically stacked in the nappe pile. In this frame, the less thinned proximal margin represents the highest structural units lying above the units derived from the former necking zone (Campo-Grosina, not studied in this Thesis), the distal margin (Bernina/Err) and the exhumed mantle domain (Platta) (Mohn et al. 2010). Because of the altitude and the outstanding quality of the outcrops, this area became fundamental in the understanding of the architecture of hyper-extended rifted margins. The so-called “Grisons transect” has been widely studied allowing the definition of the four main domains in which usually a magma-poor rifted margin is divided: the proximal margin, the necking zone, the distal margin, the ocean-continent transition (Fig. 3.3).
Fig. 3.3 – Cartoon schematically showing the evolution of the Adriatic continental margin from the first rifting stages in Late Triassic-early Lower Jurassic to the exhumation of mantle-related rocks in Middle Jurassic time. The main stratigraphic record here represented varies in facies and thickness from the proximal margin to the distal margin and mantle exhumed areas. Location of key cross-sections from these areas, shown in figures 3.4, 3.5 and 3.6, are here highlighted as well as 10.3 and 10.4 which refer to fluid pathways within the proximal and distal margin. The section (d) shows also the distribution of the main paleogeographic domains of the margin (red terms) and refers to the major Alpine units (black terms). OCT: Ocean-Continent Transition; Err det.: Err detachment system; Bernina det.: Bernina detachment system. Modified after Mohn et al. (2012).

The proximal margin (Ortler and Ela nappes)

The proximal margin is exposed in the Ortler and Ela nappes (Upper Austroalpine) and consists of classical fault-bounded tilted blocks with well-defined pre-rift sequences and thick Jurassic syn-rift sediments (Fig. 3.4). The pre-rift sequence overlies Paleozoic basement and consists mainly of continental deposits that grade upsection into shallow marine carbonates. Relics of Jurassic high-angle normal faults can be found associated with half-graben basins. These fault-bounded basins were active during initial rifting from late Triassic to Pliensbachian-Toarcian time and were mainly filled by mass flow breccias and calciturbidites interleavead with hemipelagic limestones (Allgäu Formation of Eberli 1988). All the reworked sediments derive exclusively from the pre-rift carbonate platform (Froitzheim 1988). The syn-rift sedimentary sequence is characterized by thinning- and fining-upward cycles as well as a thinning and fining away from the fault zone. Furthermore, an inversion of the clast stratigraphy is observed in the basin due to erosion of the footwall block and redeposition derived clasts (Eberli, 1988). In the present-day architecture, the Ortler nappe is under- and overlain, along Alpine tectonic contacts, by other upper Austroalpine units: the Quattervals nappe above and the Campo nappe below.

The necking zone

The necking zone, exposed in the Grosina-Campo nappe (Middle Austroalpine), is the domain where major crustal thinning occurred (Mohn et al., 2010, 2011). This zone juxtaposes crust that is little or not thinned (proximal margin) against strongly thinned, less than 10 km thick crust of the distal margin. Within this domain, the pre-rift sequence starts to be highly extended and can occur as extensional allochthons lying above top-basement detachment faults.

The distal margin (Err and Bernina nappes)

Exposed in the Bernina nappe (inner distal) and the Err nappe (outer distal; Lower Austroalpine), it consists of hyper-extended continental crust. Rift-related low-angle detachment faults are the most characteristic rift structures. Two main detachment systems are described from this domain, the Bernina and the Err detachment systems. These faults separate continental crust and exhumed subcontinental mantle in the footwall from extensional allochthons made of basement and pre-rift sediments in the hangingwall (Fig. 3.5).
The ocean-continent transition is exposed in the Platta nappe (Upper Penninic). This domain is formed by exhumed mantle rocks intruded by gabbros and capped by detachment faults (Fig. 3.6). The Platta domain is interpreted to be associated with the same detachment system affecting also the hyper-extended domain. Such an interpretation is supported by the occurrence of extensional allochthons made of continental crust and pre-rift sediments over exhumed subcontinental mantle (Manatschal and Nievergelt, 1997). Further oceanward, the exhumed mantle rocks are overlain by MORB extrusives, indicating that this domain may have developed into an embryonic oceanic domain (Desmurs et al., 2002; Manatschal & Müntener, 2009).

THE ADRIATIC DISTAL MARGIN: THE ERR NAPPE

The present-day Alpine architecture

Like in the reminder of the Austroalpine units, the Err domain was overprinted by deformation phases, which are classically referred to as D1 to D5 phases (e.g. Froitzheim et al. 1994; Fig. 3.7). The major D1 structure in the study area is the thrust separating the Julier half-klippe, belonging to the Bernina nappe, from the underlying Err nappe. Second order D1 thrusts can also be found in the Err nappe subdividing it into an Upper, Middle and Lower Err unit. The Upper Err unit is only discontinuously preserved as klippen in post-thrust (D3) synclines (e.g. Padella-Schlattain and Piz Bleis Marscha; Manatschal and Nieverglet 1997). The middle Err unit represents the main body of the Err nappe. The Lower Err unit can be observed in the so-called Jenatsch tectonic window and also along the frontal contact of the Err nappe with the Platta nappe to the west. All three units preserve primary relationships between footwall and hangingwall rocks, including syn-tectonic sediments of the rift-related Err detachment system.
D2 structures reactivate, but also cut, D1 thrusts as extensional top-to-the-SE normal faults (Froitzheim et al. 1994, Handy et al. 1993, Handy 1996).
D3 structures consist of north to northwest vergent folds and steep S-dipping thrusts showing displacements in the order of hundreds of meters.
The latest Alpine deformation consists of high-angle faults limiting the northern border of the Zone of Samedan (ZoS, already described in Cornelius 1935). The total amount of normal displacement along this fault system can exceed half a kilometre in the center of ZoS and decreases eastwards. This phase is presumably linked to activity along the Engadine line (e.g. Schmid and Froitzheim 1993, Handy 1996).

The Jurassic architecture: the Err detachment system

The weak Alpine overprint enables to map and describe a Jurassic detachment system that is exceptionally well exposed and preserved in the Err nappe. The detachment system consists of brittle fault zones that are made of cataclastic damage zones and a peculiar core zone that separates the footwall from a hangingwall in which the pre- and syn-rift sedimentary sequences are locally preserved.

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The footwall

The footwall is constituted of Paleozoic rocks inherited from the late to post-Variscan history. They are made of calcalkaline Albula granitic bodies (Cornelius 1935, Staub 1948, Von Quadt et al. 1994) that intruded a Paleozoic polymetamorphic host-rock consisting of schists and gneisses. The Albula granite is by far the most common rock in the footwall. Since it preserves primary stratigraphic contacts to Permo-Triassic sediments and volcanic rocks, these basement rocks had to be in the pre-rift upper crust before onset of rifting in Jurassic time.

The detachment faults

The detachment faults correspond, at least in the areas where the footwall is formed by the Albula granite, to well characterized brittle fault zones (Fig. 3.8). The faults are formed by tens of meter-thick damage zones constituted of characteristic green cataclasites and core zones made of black fault gouges (Manatschal, 1999). The green cataclasites result from the progressive cataclastic overprint of the host rock, which is related to fluid and reaction-assisted breakdown processes of feldspar (Manatschal, 1999). The textures show a progressive change from fractured host rock, to a clast-supported cataclasite to a matrix-rich green cataclasite. In the last uppermost meters, quartz veins are common. Since these veins occur only within the damage zone crosscutting cataclastic structures and are also observed to have a cataclastic overprint, they are interpreted to be coeval with detachment faulting (Manatschal and Nievergelt 1997). This is also confirmed by geochemical studies (Manatschal et al. 2010) as well as by their occurrence as clasts in the black gouges and reworked into the Jurassic syn-rift sediments. The black gouges occur along a centimeter- to some meter-thick core zone at the top of the green cataclasite corresponding to the main zone of displacement. Where the detachment is exhumed at the seafloor it can be either eroded or directly overlain by syn- to post-rift sediments. The occurrence of clasts of black gouge in the syn-rift sediments points to the erosion of the detachment fault. In the black gouge itself, hangingwall clasts made of Triassic dolomite (Fig. 3.9) exist, but are rare compared to more abundant footwall-derived material. The discovery of clasts of the black gouge in the Jurassic sediments enabled to demonstrate that these rocks were pre-Alpine.
detachment fault. The thickness of the lithologies in the profile are average values, since the real thickness changes considerably along strike (Manatschal, 1999).

The hangingwall

The hangingwall of the detachment system (Fig. 3.10) consists mainly of poly-metamorphic basement that shows primary contacts to either volcanic and volcano-sedimentary sequences or siliciclastic deposits. Ladinian to Norian platform carbonates (Hauptdolomit Formation) and Rhaetian limestones and shales (Kössen Formation) constitute the youngest parts of the pre-rift sediments. The Middle-Upper Triassic succession can be up to 500 m thick; however, it is important to note that throughout the margin, the Triassic dolomites are dismembered and never preserve the original thickness. As shown in Mohn et al. (2010) this is due to the onset of the major activity along the detachment faults. The Triassic to Lower Jurassic sedimentary units (Hauptdolomit-Kössen Fms. and Agnelli Fm. respectively) are very discontinuous in the Err nappe despite the fact that they were deposited as continuous carbonate platforms or hemipelagic sediments. In some areas, the syn- to post-rift sediments overlie directly the exhumed footwall of the detachment faults (e.g. Piz Nair, Pass Suvretta or Fuorcla Cotschna areas in Handy et al. 1993, Handy 1996, Manatschal and Nievergelt 1997) and the Triassic to Lower Jurassic cover is reworked in the syn-rift sediments. The pre-rift succession was disrupted as discontinuous blocks, with a very complex 3D shape over the detachment surface. These structures have been referred to as “extensional allochthon blocks” (Manatschal and Nievergelt 1997, Manatschal 2004).
The syn-rift sediments consist of complex gravitational to hemipelagic sedimentary deposits that occur either unconformably over extensional allochthons (e.g. Bardella block) or directly over the tectonically exhumed basement. Finger (1978) distinguished these syn-rift sediments in two formations, the Bardella and Saluver Fms. The subdivision was mainly based on their composition: the Bardella Fm. is made of reworked Triassic to Early Jurassic carbonates (pre-rift rocks > 185 Ma) whereas the Saluver Fm. includes mainly basement-derived material. The observation that the Bardella Fm. is interleaved with the Saluver Fm. shows that the two sedimentary systems co-existed. It implies that during the evolution of the basin, at least two clastic sedimentary sources where available, one resedimenting the pre-rift Mesozoic platform carbonates and the other reworking detachment fault rocks and tectonized basement (Fig. 3.10). Finger (1978) further subdivided the Saluver Fm. into a coarse-grained Saluver A sub-formation at the base, an intermediate Saluver B and a fine-grained Saluver C at the top. The author defined three different facies tracts, namely the basal, the intermediate and the top tracts corresponding approximately to the Bardella and Saluver A Fm. (basal), Saluver B (intermediate) and Saluver C and post-rift deposits (top).

Table of contents :

1. Introduction
1.1 General remarks
1.2 The study areas: Remnants of fossil rifted margins
1.3 Aims of the Thesis
1.4 Thesis structure
2. Materials and methods
3. The Adriatic continental margin
3.1 Introduction
3.2 Geological setting
3.2.1 The proximal margin (Ortler and Ela nappes)
3.2.2 The necking zone
3.2.3 The distal margin (Err and Bernina nappes)
3.2.4 The ocean-continent transition (OCT, Platta nappe)
3.3 The Adriatic distal margin: the Err nappe
3.3.1 The present-day Alpine architecture
3.3.2 The Jurassic architecture: the Err detachment system
3.3.3 The major time lines in the Adriatic distal margin
3.4 The Adriatic ocean-continent transition: the Platta nappe The Results
4. The central distal margin
4.1 Piz Val Lunga area
4.1.1 Stratigraphy and petrography
4.2 Fuorcla Cotschna area
4.2.1 Stratigraphy and petrography
4.3 Isotope geochemistry and fluid inclusion data
4.3.1 O and C isotopes
4.3.2 Sr isotopes
4.3.3 He isotopes
4.3.4 Fluid inclusion microthermometry
4.4 First order interpretation
4.4.1 Dolomite
4.4.2 Dedolomitization
4.4.3 Breccias
4.4.4 Calcite cement
4.4.5 Veins
4.4.6 Silicification
4.4.7 Fe-Mn oxide coating
5.1 Mal Pass area
5.1.1 Stratigraphy and petrography
5.2 Isotope geochemistry and fluid inclusion data
5.2.1 O and C isotope
5.2.2 Sr isotopes
5.2.3 He isotopes
5.2.4 Fluid inclusion microthermometry
5.3 First order interpretation
5.3.1 Dolomite
5.3.2 Neptunian dykes and breccias
5.3.3 Veins
5.3.4 Septarian-like concretions and silicification
6. The proximal margin
6.1 Il Motto area
6.1.1 Stratigraphy and petrography
6.2 Isotope geochemistry and fluid inclusion data
6.2.1 O and C isotopes
6.2.2 Sr isotopes
6.2.3 Fluid inclusion microthermometry
6.3 First order interpretation
6.3.1 Dolomite
6.3.2 Breccias
6.3.3 Dolomite and calcite cements
6.3.4 Veins
7. The inner distal margin
7.1 Piz Alv area
7.1.1 Stratigraphy and petrography
7.2 Isotope geochemistry
7.2.1 O and C isotopes
7.2.2 Sr isotopes
7.3 First order interpretation
8. U-Pb dating
8.1 The dataset
8.2 Discussion and interpretation
9. Trace elements and REE
9.1 Introduction
9.2 Analysed samples
9.2.1 Reference samples
9.3 The Hauptdolomit Fm. along the Adriatic continental margin
9.4 The Hauptdolomit Fm. in the distal margin
9.5 Dolomite veins in the distal margin
9.6 Silicification
9.7 Fe-Mn oxides
9.8 Discussion and interpretation
9.8.1 The distal margin
10. Discussion: Fluid characteristics and flow pathways
10.1 Introduction
10.2 Data constraints on the hydrothermal features of the fluids
10.3 Evolutionary model
10.3.1 Similar conditions but different products: why?
10.3.2 The two diagenetic stages along the Adriatic continental margin
11. The Pyrenean hyper-extended rift system
11.1 Introduction
11.2 The Bay of Biscay-Pyrenean domain
11.2.1 Large-scale rift architecture of the Arzacq-Mauléon system
11.2.2 The stratigraphy of the Arzacq-Mauléon basin
11.2.3. Mantle-derived rock occurrences in the Mauléon Basin
11. 3 The Chaînons Béarnais
11.3.1 The study area
12. Black Dolomites Unit
12.1 Stratigraphy and petrography
12.2 Stable isotope geochemistry
12.2.1 O and C isotopes
12.3 First order interpretation
12.3.1 Dolomite
12.3.2 Breccias
12.3.3 Dolomite and calcite cements
12.3.4 Veins
13. Black Limestones Unit
13.1 Introduction
13.2 The Quarries area
13.2.1 Stratigraphy and petrography
13.3 Isotope geochemistry and fluid inclusion data
13.3.1 O and C isotopes
13.3.2 Fluid inclusion microthermometry
13.4 First order interpretation
13.4.1 Breccias
13.4.2 Calcite and dolomite cements
13.4.3 Veins
13.5 The Riverbed site
13.5.1 Stratigraphy and petrography
13.6 Stable isotope geochemistry
13.6.1 O and C isotopes
13.7 First order interpretation
13.7.1 Marbles and Carbonate mylonites
14. Sedimentary Breccias Unit
14.1 Stratigraphy and petrography
14.2 First order interpretation
14.2.1 Breccias
14.2.2 Fluid-related products
15. Discussion: Stratigraphy and Fluid flow evolution
15.1 Introduction
15.2 Data constraints on the hydrothermal features of the fluids
15.2.1 Black Dolomites and Black Limestones Units
15.2.2 The mylonites and marbles
15.2.2 Sedimentary Breccias Formation
15.3 New interpretation of the stratigraphic setting of the study area
15.4 Evolutionary model
16. Summary of the results and future perspectives
16.1 Aim of the Thesis
16.2 The Results
16.2.1 The Adriatic rifted margin
16.2.2 The Pyrenean hyper-extended rift system
16.3 Open questions and Future perspective
References

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