Thierry Dumont

The continental margin of the Mesozoic Tethys in the Western Alps

Abstract Most of the structural units of the Western Alps were derived from the European Continental margin of the Ligurian Ocean, a segment of the Mesozoic Tethys ocean. Their Mesozoic palaeotectonic and stratigraphic evolution bears witness of the following main stages: (1) Deposition of the Triassic platform carbonates, essentially prerift but nevertheless bearing the imprint of some extensional movements. (2) The Liassic-Middle Jurassic rifting corresponds to the creation of a horst-and-graben system with especially tilted blocks and hence a shoal-and-basin palaeogeography, but without a preliminary doming event. (3) The late Jurassic-Early Cretaceous opening and spreading of the Ligurian ocean began with a general collapse of the continental margin (‘thermal’ subsidence, latest Middle Jurassic and Earliest Late Jurassic). In this paper, emphasis is given to the refting-derived structures, more especially to the major tilted crustal blocks, a few tens of kilometres wide, which can be either reconstructed or directly observed. The rifting stage lasted roughly 40 m.y., with the alternation of extensional tectonic phases and of relatively ‘quiet’ periods. During the extensional phases, movements along fault-planes and related episodes of tilting were followed by sudden and rapid episodes of subsidence: the latter may be interpreted as resulting from the isostatic and thermal readjustment that follows a phase of stretching of the crust and of the lithosphere. The tectonic evolution of the margin continued, but decreased, during the late Jurassic-Early Cretaceous oceanspreading stage. Tectonic activity resumed in the late Cretaceous: this probably resulting from the beginning of contraction of both the ocean and the continental margin, leading progressively to the continental collision in the Tertiary.

First seismic evidence for continental subduction beneath the Western Alps

The first discovery of ultrahigh-pressure coesite in the European Alps 30 years ago led to the inference that a positively buoyant continental crust can be subducted to mantle depth; this had been considered impossible since the advent of the plate tectonics concepts. Although continental subduction is now widely accepted, there remains debate because there is little direct (geophysical) evidence of a link between exhumed coesite at the surface and subducted continental crust at depth. Here we provide the first seismic evidence for continental crust at 75 km depth that is clearly connected with the European crust exactly along the transect where coesite was found at the surface. Our data also provide evidence for a thick suture zone with downward-decreasing seismic velocities, demonstrating that the European lower crust underthrusts the Adriatic mantle. These findings, from one of the best-preserved and long-studied ultrahigh-pressure orogens worldwide, shed decisive new light on geodynamic processes along convergent continental margins.

Short-lived, fast erosional exhumation of the internal western Alps during the late early Oligocene: Constraints from geothermochronology of pro- and retro-side foreland basin sediments

Apatite and zircon fission-track analysis and single zircon fission-track–U/Pb double dating of Oligocene to Miocene pro- and retro-side foreland basin sedimentary rocks provide evidence for short-lived but fast erosional exhumation of at least 1.5–2 km/m.y. in the internal western Alps between ca. 30 and 28 Ma. This period of fast erosion is seen as a result of rapid surface uplift coupled with increasing orographic precipitation during this phase of orogenesis. Surface uplift may have been caused and sustained by different plate-tectonic processes such as a change in convergence direction, intermediate-depth slab breakoff, and emplacement of the Ivrea body during continental collision. The occurrence of contemporaneous volcanic activity on the pro-side of the western Alps on the subducting European plate between ca. 36 Ma and 30 Ma is seen in connection with slab rollback of the Apennine slab and upwelling of hot mantle material beneath the western Alps. Single zircon double dating shows that the exhumational signal in the detrital thermochronologic data is not compromised by volcanically derived zircons, as volcanic grains can be identified and removed from the zircon fission-track data set to obtain a pure exhumational signal. The signal of fast exhumation is observed in the zircon fission-track data of the pro-side foreland basin and in the apatite fission-track data and published 40 Ar- 39 Ar data in the retro-side foreland basin. During late Oligocene times, erosion rates slowed down to rates similar to present-day erosion rates in the western Alps.

Continuity of the Alpine slab unraveled by high-resolution P wave tomography

The question of lateral and/or vertical continuity of subducted slabs in active orogens is a hot topic partly due to poorly resolved tomographic data. The complex slab structure beneath the Alpine region is only partly resolved by available geophysical data, leaving many geological and geodynamical issues widely open. Based upon a finite-frequency kernel method, we present a new high-resolution tomography model using P wave data from 527 broadband seismic stations, both from permanent networks and temporary experiments. This model provides an improved image of the slab structure in the Alpine region and fundamental pinpoints for the analysis of Cenozoic magmatism, (U)HP metamorphism, and Alpine topography. Our results document the lateral continuity of the European slab from the Western Alps to the central Alps, and the downdip slab continuity beneath the central Alps, ruling out the hypothesis of slab break off to explain Cenozoic Alpine magmatism. A low-velocity anomaly is observed in the upper mantle beneath the core of the Western Alps, pointing to dynamic topography effects. A NE dipping Adriatic slab, consistent with Dinaric subduction, is possibly observed beneath the Eastern Alps, whereas the laterally continuous Adriatic slab of the Northern Apennines shows major gaps at the boundary with the Southern Apennines and becomes near vertical in the Alps-Apennines transition zone. Tear faults accommodating opposite-dipping subductions during Alpine convergence may represent reactivated lithospheric faults inherited from Tethyan extension. Our results suggest that the interpretations of previous tomography results that include successive slab break offs along the Alpine-Zagros-Himalaya orogenic belt might be proficiently reconsidered.

Upper Triassic Tethyan Carbonates off Northwest Australia (Wombat Plateau, ODP Leg 122)

SummaryLeg 122 of the Ocean Drilling Program (ODP) recovered Upper Triassic (Carnian to Rhaetian) sediments at the sediment-starved passive continental margin off Northwestern Australia.The early-rift series at the Wombat Plateau, a northern sub-plateau of the Exmouth Plateau, consists of Upper Triassic fluviodeltaic siliciclastics and shallow-marine carbonates including reefal facies. Twenty-five microfacies types could be distinguished.These sequences are capped by an erosional ‘post-rift unconformity’ with a 70 m.y. hiatus during the Jurassic. The Wombat Plateau bears only a thin post-rift sedimentary cover of Cretaceous to Cenozoic age.The Carnian and Norian sequences are dominated by fluviodeltaic sediments that contain many carbonate intercalations. Their frequency and the kind and amount of allochems allow the reconstruction of a storm-influenced deltaic to prodeltaic environment with restricted estuarine (intradeltaic) lagoons and high-energy carbonate sand shoals in front of the delta lobes.The presence of the foraminifersTriasina oberhauseri andTriasina hantkeni in Sites 762 and 764 indicate a Norian to Rhaetian age. The reefal platform can be differentiated in a lagoon to patch reef environment with abundant Aulotortidae, and a patch reef to shelf zone with smaller foraminifers.The ‘Rhaetian’ starts with a global sequence boundary. Several shallowing-upward cycles from bioturbated wackestones to dolomitic algal bindstones suggest a shallow-subtidal to intertidal environment at Site 761. Typical reef development was observed at the more “distal’ Site 764. The limestone-marl alternations of the open marine shelf grade into local bioclastic and oolitic grainstones, which are the base for the incipient carbonate buildup. Calcisponge patch reefs developed into coral reefs. Several cycles characterize a ‘catch-up’ system grading into a ‘keep-up’ carbonate system. The reef growth ended abruptly with the second sequence boundary (211 Ma afterHaq et al., 1987), coinciding with the worldwide latest Rhaetian sea level fall, followed by renewed transgression.By comparison with Upper Triassic carbonates of the western Tethys (e.g., the Northern Calcareous Alps), several microfacies types could be combined to characteristic facies units: biolithite facies, different reef talus types, grapestoneoncoid facies, and calcareous algae-foraminifera detritus facies showing the reef-backreef/fore-reef-lagoon transitions.Detailed investigations of microfacies, wireline logs and high-resolution seismics allow the determination of depositional sequences (sequence stratigraphy). We distinguish influences of regional or global tectonics and/or eustatic sea-level changes. The results show that regional tectonic movements are of minor importance in the Rhaetian and that theHaq et al. (1987) cycle chart could also be used at the passive margin of Northwest Australia.

Tracing the Oligocene-Miocene Evolution of the Western Alps Drainage Divide with Pebble Petrology, Geochemistry, and Raman Spectroscopy of Foreland Basin Deposits

The formation of the western Alps topography is the result of continental collision between the Apulian and European plates. In this study, we trace the Early Oligocene to Early Miocene development of topography and the position of the drainage divide in the southern western Alps by analyzing the erosion products preserved in the pro-side (Montmaur and Barrême) and retro-side (Torino hills) foreland basins of this orogen. Using petrologic and geochemical analyses of basalt pebbles and Raman spectroscopy of serpentine sand grains and pebbles, we identify source lithologies, which are not easily detected with more commonly used detrital thermochronology. Lower Oligocene sediments of the pro-side foreland basin contain numerous basalt pebbles that share strong geochemical similarities with the Chenaillet obducted ophiolite (Montgenèvre massif). Other ophiolite-suite derived clasts, e.g., radiolarite or serpentinite appear widely in pro- and retro-side foreland sediments since about 30 Ma. This suggests a wider distribution of Chenaillet-type obducted ophiolite rocks in the western Alps during the Oligocene, but the exact locations, except for the Chenaillet, are unknown. Raman analysis on serpentine grains and pebbles from the retro-side foreland basin deposits documents a systematic trend from antigorite (high-grade metamorphic source rocks) to lizardite (low-grade metamorphic source rock) from the Early Oligocene to the Early Miocene. This trend is attributed to a westward growth of the paleo-Dora Riparia drainage basin in the southern western Alps. Ophiolite erosion and drainage divide shift were caused by the topographic evolution of the western Alps, which we suggest to be linked to the shift in convergence direction between the Apulian and Eurasian plates from N–S to E–W and the presence of the so-called Ivrea body mantle splinter acting as a vertical indenter beneath the western Alps at that time. The drainage patterns of the paleo-Durance and paleo-Doria Riparia Rivers seem to have remained stable since the Early Miocene. In comparison to the central Alps, the drainage divide shift in the southern western Alps occurred earlier than in the central Alps, but in both locations the trend from an internal to a more external position is the same.

Un escarpement sous-marin permanent du Lias à l'Eocène, dans la dorsale calcaire Péloritaine (Sicile, Italie)

The schistosity of the Palaeozoic basement of the Capo San Andrea unit (Peloritan Dorsale) is cut by Liassic to Eocene neptunian dykes which developed along a pelagic fault scarp exposed on the sea bottom for 140 My. The Mesozoic structures determined the location of the Alpine deformation.

Foreland exhumation controlled by crustal thickening in the Western Alps

In alpine-type collision belts, deformation of the foreland may occur as a result of forward propagation of thrusting and is generally associated with thin-skinned deformation mobilizing the sedimentary cover in fold-and-thrust belts. Locally, foreland deformation can involve crustal-scale thrusting and produce large-scale exhumation of crystalline basement resulting in significant relief generation. In this study, we investigate the burial and exhumation history of Tertiary flexural basins located in the Western Alpine foreland, at the front of the Digne thrust sheet (southeast France), using low-temperature apatite fission-track and (U-Th)/He thermochronology. Based on the occurrence of partially to totally reset ages, we document 3.3–4.0 km of burial of these basin remnants between ca. 12 Ma and 6 Ma, related to thin-skinned thrust-sheet emplacement without major relief generation. The onset of exhumation is dated at ca. 6 Ma and is linked to erosion associated with significant relief development. This evolution does not appear to have been controlled by major climate changes (Messinian crisis) or by European slab breakoff. Rather, we propose that the erosional history of the Digne thrust sheet corresponds to basement involvement in foreland deformation, leading to crustal thickening. Our study highlights the control of deep-crustal tectonic processes on foreland relief development and its erosional response at mountain fronts.

Une preuve d'extension contemporaine de l'expansion océanique de la Téthys ligure en Briançonnais : les failles du Vallon Laugier

Extensional faults of Late Jurassic to Cretaceous age in the Brianconnais postdate the initial opening of the Tethyan ocean. Such faults could record the distant effects of the North Atlantic, the Bay of Biscay and the Valais rifting and oceanic opening.