Lepontine Dome Research Articles

The Aosta–Ranzola extensional fault system and Oligocene–Present evolution of the Austroalpine–Penninic wedge in the northwestern Alps

Post-nappe brittle structures across the Austroalpine–Penninic wedge are representative of two principal deformation stages. The former (D1) developed during the Late Oligocene, the latter (D2) from the Miocene to the Present. Major D1 features are the E–W Aosta–Ranzola half-graben and the NE–SW Ospizio Sottile subvertical normal fault system. Oligocene extension was also characterized by hydrothermal activity and intrusion of calc-alkaline to ultra-potassic dikes. From the Miocene onwards (D2), the Ospizio Sottile system was re-activated as a SE sinistral boundary of the Pennine–Graian Alps block of nappes, bodily moving away from the Simplon detachment to the SW. Inside the escaping block, a number of D2 slightly diverging sublatitudinal splays (Aosta–Piccolo San Bernardo system) was generated, as well as a regular set of NW-trending normal faults from the Simplon to the Gran Paradiso massif. Instead, the Aosta–Ranzola system was poorly reworked in that time. More generally, the short-lived Oligocene extension (D1) was associated to slab break-off of lower plate, wedge rebound, rapid uplift and thermal perturbation. From the Early Miocene onwards the Oligocene extension was replaced by renewed compression, allowing the vertical extrusion of the Lepontine dome and lateral escape of the overlying Pennine–Graian Alps block.

The erosion history of the Central Alps: evidence from zircon fission track data of the foreland basin sediments

Fission track dating on detrital zircons of Alpine debris in the Swiss molasse basin provides information about the erosion history of the Central Alps and the thermal evolution of source terrains. During Oligocene times, only sedimentary cover nappes, and Austroalpine basement units were eroded. Incision into Austroalpine basement units is indicated by increasing importance of Cretaceous cooling ages in granite pebbles upsection. Erosion of Penninic basement units started between 25 and 20 Ma. Early Oligocene zircon FT ages show that Penninic basement units were exposed at ∼20 Ma. Deeper Penninic units of the Lepontine Dome became exposed first at ∼14 Ma, contemporaneously with the opening of the Tauern window in the Eastern Alps. A middle Miocene cooling rate of 40 °C Myr−1 is deduced for the Lower Penninic units of the Lepontine Dome.

Crustal thickening and crustal extension as controls on the evolution of the drainage network of the central Swiss Alps between 30 Ma and the present: constraints from the stratigraphy of the North Alpine Foreland Basin and the structural evolution of the Alps

The combined information about sedimentary petrography from the North Alpine Foreland Basin and structural geology from the Alps allows a qualitative reconstruction of the drainage network of the central Swiss Alps between 30 Ma and the present. This study suggests that crustal thickening and crustal thinning significantly controlled the location of the drainage divide. It also reveals the possible controls of crustal thickening/thinning on the change of the orientation of the drainage network from across‐strike between 30 and 14 Ma to along‐strike thereafter.Initial crustal thickening in the rear of the wedge is considered to have formed the drainage divide between north and south at 30 Ma. Because the location of crustal thickening shifted from east to west between ≈30–20 Ma, the catchment areas of the eastern dispersal systems reached further south than those of the western Alpine palaeorivers for the same time slice. Similarly, the same crustal dynamics appear to have controlled two phases of denudation that are reflected in the Molasse Basin by petrographic trends. Uplift in the rear of the wedge caused the Alpine palaeorivers to expand further southward. This is reflected in the foreland basin by increasing admixture of detritus from structurally higher units. However, tectonic quiescence in the rear of the wedge allowed the Alpine palaeorivers to cut down into the Alpine edifice, resulting in an increase of detritus from structurally lower units. Whereas uplift in the rear of the wedge was responsible for initiation of the Alpine drainage systems, underplating of the external massifs some 50 km further north is thought to have caused along‐strike deviation of the major Alpine palaeorivers.Besides crustal thickening, extension in the rear of the wedge appears to have significantly controlled the evolution of the drainage network of the western Swiss Alps. Slip along the Simplon detachment fault exposed the core of the Lepontine dome, and caused a 50‐km‐northward shift of the drainage divide.

Kinematic, thermal and petrological model of the Central Alps: Lepontine metamorphism in the upper crust and eclogitisation of the lower crust

Seismic and seismological studies as well as gravimetric models indicate that a slab of European lithospheric mantle and lower crust is currently underthrust below the Apulian crust. We assume a simple kinematic model in which the lower and upper subducted European crusts are decoupled along a decollement. The lower crust goes into subduction without deformation. The upper crust deforms by pure shear with a horizontal compressional axis. The total erosional flux is adjusted to balance upper crust input so that the belt keeps the same geometry, different distributions of erosion being used. The computed temperature field is steady-state if the kinematic model applies during a minimum time of 40 Myr for a convergence rate of 8 mm/yr. Equilibrium mineral assemblages and densities are determined from the computed P, T conditions for a granodioritic chemical composition of the upper crust and an andesitic composition of the lower crust. Assuming local isostasy, the density model fits the average topographic profile across the Central Alps. The P-T-t paths obtained for the part of the upper crust initially at depths 10 to 16 km are compatible with the medium pressure Oligocene metamorphism in the Lepontine dome. The peak calculated temperature for the deepest non subducted crustal rocks is 600°C for a pressure of 0.8 GPa, near the lower limit of high-pressure amphibolites. We thus propose that the Lepontine metamorphism corresponds to the steady-state thermal regime. However, either faster erosion rates in the internal part of the belt or tectonic denudation are required for exhumation of the deeper portion of the belt. The computed temperature field implies eclogitisation of the lower crust at a depth of 55 to 60 km. We conclude that the Moho limiting the deepest part of the root may correspond to the eclogitisation phase change. Lower crust eclogites have a density comparable to or higher than that of the mantle, depending on their chemical composition (3.37 for andesitic eclogites, 3.56 for gabbroic eclogites). Thus, andesitic eclogites may stay in gravitational equilibrium in the mantle below the root whereas gabbroic eclogites are gravitationally unstable and should sink.

The transition from passive to active margin tectonics: a case study from the zone of Samedan (eastern Switzerland)

The Zone of Samedan is part of a fossil, early Mesozoic rift system originally situated in the distal, Lower Austro-Alpine domain of the Adriatic passive continental margin. An early Mesozoic configuration of asymmetrical rift basins bounded by relative structural highs compartmentalized Late Cretaceous active margin tectonics; Jurassic half-grabens were folded into arcuate synclines, whereas relative structural highs engendered thin, imbricated thrust sheets. West-directed thrusting and folding initiated at the surface and continued to depths favoring mylonitization under lower greenschist-facies conditions. At this time Liguria-Piemontese ophiolites were accreted to Lower Austro-Alpine units directly underlying the Zone of Samedan. Late Cretaceous orogenic collapse of the Adriatic active margin involved the reactivation of west-directed thrusts as low-angle, top-to-the-east, normal faults. These faults accommodated extensional uplift of Liguria-Piemontese ophiolites and Lower Austro-Alpine units beneath and within the Zone of Samedan. During Paleogene collision, some Late Cretaceous faults in the Zone of Samedan were reactivated under lower anchizonal conditions as north-directed thrusts. The latter stages of this early Tertiary thickening were transitional to brittle, high-angle normal faulting associated with top-to-the-east extension and spreading above the warm, uplifting Lepontine dome.

Geochronologic, petrologic and kinematic constraints on the evolution of the Err-Platta boundary, part of a fossil continent-ocean suture in the Alps (eastern Switzerland)

The boundary between the Austroalpine Err nappe and the Penninic Platta unit represents part of a Late Cretaceous continent-ocean suture preserved in the highest stockwerk of the Tertiary Alpine orogen. Subduction and nappe stacking (D1) and subsequent extensional uplift (D2) along part of this suture are dated with the K-Ar method on two generations of syn-kinematic white mica in the Err nappe and with Ar-Ar step-heating of syn- to post-kinematic riebeckitic amphiboles in the Platta nappe. The first generation of white micas associated with isoclinal folds and W-directed mylonitic thrusting along nappe contacts yields a 76 to 89 Ma age range, whereas the second generation of white micas that grew during tight folding and top-to-the-E extensional D2 shearing yields 67 to 80 Ma ages. The K-Ar white mica ages are interpreted as ages of formation at the approximately 300°C temperature of lower green-schist facies regional metamorphism in the southern part of the Err nappe. The two amphibole samples from the Platta nappe yield 69 and 73 Ma Ar-Ar ages, interpreted as syn-D2 formational ages. The Si-content of the white micas in chlorite-K-feldspar-quartz-bearing meta-radiolarites in the southern part of the Err nappe indicate a pressure decrease from D1 to D2 deformation, probably accompanied by an increase in temperature. This can be related to D2 extensional uplift accommodated along reactivated D1 nappe contacts in the footwall of the Julier-Bernina nappe. When combined with stratigraphic evidence, the radiometric ages and deformation-crystallization relations indicate clockwise P-T paths for the Err, Platta, and Margna nappes: Suturing of the Austroalpine continental and Liguro-Piemontese oceanic crust began with near surface thrusting and flysch sedimentation between 88 and 100 Ma and culminated with D1 folding and nappe stacking under high P/T greenschist facies conditions sometime during the period 80 to 88 Ma. The unroofing of this suture was a multi-stage process involving Late Cretaceous D2 extension under greenschist facies conditions, Early Tertiary D3 thrusting under lower anchizonal conditions, and mid-Tertiary D4 normal faulting above the uplifting Lepontine dome.

Garnet peridotite and eclogite Sm-Nd mineral ages from the Lepontine dome (Swiss Alps): New evidence for Eocene high-pressure metamorphism in the central Alps

In the central Alps the upper Pennine Cima Lunga nappe (Lepontine dome, Swiss Alps) underwent eclogite facies conditions up to 850-900 °C and 3.5-4.2 GPa. Sm-Nd dating of garnet peridotites and an eclogite from the Cima Lunga nappe yields consistent mineral ages of ca. 40 Ma. The mineral ages are probably cooling ages, but petrological arguments indicate that they approximate the time of eclogite facies metamorphism, implying that the latter is related to the collision of the Adriatic plate with Europe. Emplacement of the upper Pennine nappe stack with the hot Cima Lunga nappe at its base onto European basement (lower Pennine units) is suggested to be the primary cause for the middle Tertiary Barrovian-type metamorphism in the central Alps. This process accounts for the different metamorphic evolution of the Cima Lunga nappe compared to underlying lower Pennine units and explains the time difference between Sm-Nd mineral ages from the Cima Lunga nappe and U-Pb monazite ages from the lower Pennine units.

The kinematic history of the Pennine Nappes east of the Lepontine Dome: Implications for the tectonic evolution of the Central Alps

The Pennine zone of the eastern Central Alps (South, Middle, and North Pennine from top to bottom of sequence) underwent a complex structural and strain history. A first phase (D1) is characterized by E‐W oriented stretching and is of Cretaceous/lower Tertiary age. D1 structures affected most, but not all, of the South and Middle Pennine units, were probably obliterated in the North Pennine Adula nappe, and are absent in the basement nappes underneath the Adula nappe. Further deformation led to N/NW directed nappe movement (D2), followed by incipient strike‐slip motion along the Periadriatic line (late D2). Subsequently, large‐scale folding (D3) affected mainly the nappe fronts. In the Adula nappe F3 folds verge towards the foreland, whereas the overlying units were backfolded towards the hinterland. The D2/D3 events were accompanied by a complex strain pattern that is best seen in the Adula nappe and includes plane to prolate strains in the upper frontal part and oblate strains in both the lower frontal part and the interior of the nappe. D3 backfolding was associated with the beginning of backthrusting at the Periadriatic line and was accompanied and/or followed by rapid uplift in the Lepontine dome and subsequent unroofing of the latter which led to E/NE directed extensional movements (D4) in the eastern Central Alps. The D1 event is related to the anticlockwise rotation of the Adriatic plate relative to Europe. This caused dextral transpression between the Adriatic and the European plates and partitioning of the deformation into convergence and strike‐slip. It is suggested that the North Pennine nappes below the Adula nappe were situated along a major strike‐slip fault and escaped this event. D2 resulted from continental collision between Adria and Europe. D2 displacement directions are subperpendicular to the Alpine arc and are best approximated by WNW/NW oriented motion of Adria. The structures and the strain pattern of D3 are interpreted as the result of gravitational force. Late D2 strike‐slip deformation at the Periadriatic line, D3 backthrusting at the southern steep belt, and the unroofing stage (D4) are generally attributed to dextral transpression between Adria and Europe. This approach implies that the Alpine evolution of the Central Alps was largely governed by tangential convergence between the Adriatic and the European plates.

Tertiary kinematics in the Lepontine dome

Summary In the Pennine zone of the central (Lepontine) Alps, the mineral-zone boundaries of Tertiary metamorphism and the regional attitudes of foliation and thrust sheets define a dome. We investigate the Tertiary kinematic history of the dome using structural, metamorphic and geochronological data. Thus we distinguish two main ductile deformations, that occurred under differing metamorphic conditions. The first high-temperature deformation (HTD) developed under amphibolite facies conditions. It is responsible for (1) a radiating pattern of stretching lineation trends at the scale of the Lepontine area, and (2) structures indicating overthrusting towards the European foreland in the lineation direction (i.e. N and NW). The HTD event is attributable to NW-SE Tertiary convergence between Europe and the Adriatic sub-plate. The second retrograde deformation (RD) is responsible for differential uplift of the Pennine zone, with formation of an earlier subdome to the E (Ticino subdome, 25-20 Ma) and a later one to the W (Simplon subdome, 15-10 Ma). In the Ticino subdome, kinematic indicators reveal a horizontal shearing top W. Some major structures such as the Maggia steep zone and the vertical duplication of the Antigorio basement nappe (Bosco-Gurin area) may be related to this westward phase. Dextral transcurrent shearing along the southern steep belt and backthrusting of the central Alps over the southern Alps along the N dipping Insubric mylonite belt are probably coeval with the retrograde deformation recorded in the Ticino subdome. In the Simplon subdome, kinematic indicators reveal a radial motion parallel to the lineation, which plunges towards the SSW in the Verampio window and towards the WSW at the Simplon Line. Serial uplift of Ticino and Simplon subdomes is attributed to continuing dextral transpression at the boundary between Europe and the Adriatic sub-plate. Finally, the back-folding event located at the northern limit of the Pennine zone postdates both HTD and RD deformational events described above. It is part of a later pop-up responsible for uplifting the external crystalline massifs.