Daniel Bernoulli

From rifting to drifting: tectonic evolution of the South-Alpine upper crust from the Triassic to the Early Cretaceous

Abstract The tectonic evolution of the South-Alpine rifted margin is discussed on the base of four palinspastic upper crustal profiles. Extension, related to the movements between Adria and Europe, began in the Norian after Variscan orogeny, Late Carboniferous to Middle Permian orogenic collapse and continental-scale wrenching. From the Late Triassic to the Early Liassic stretching was mostly limited to the Lombardian basin. During this time, extension was mainly controlled by four major listric faults, symmetrically centred around the Late Carboniferous—Early Permian Collio grabens. Smaller faults which also started in the Norian, were progressively de-activated during the Late Triassic. After the Middle Liassic, faulting in the Lombardian basin gradually ceased and the site of extension shifted westwards, i.e. towards the future site of crustal separation. Extension was then controlled by a set of west-dipping normal faults. Oceanic crust was formed not later than 157 Ma. The overall extension along the profile (which had a final length of 290 km) was 52 km which corresponds to a stretching factor of 1.22 for the whole length of the preserved margin. The strain rate is ca. 1 × 10 −16 / s . The sedimentation history suggests that the extension of the South-Alpine margin was a continuous process from the Norian to the Middle Jurassic. The changes in tectonic pattern are related to progressive hardening of the lithospheric segment undergoing slow extension.

Architecture and tectonic evolution of nonvolcanic margins: Present-day Galicia and ancient Adria

A comparison of the reconstructed southeastern margin of the Tethys ocean with the present-day Galicia margin shows that although both margins are of different age and had a different fate, their architectures and tectonic evolutions are very similar. Along both non-volcanic margins the site of rifting shifted from a broad area in the future proximal margins to a localized area in its distal parts, accompanied by a change in the mode of extension. During the initial phase of rifting, extension was accommodated by symmetrically arranged listric faults which soled at midcrustal levels, indicating that deformation in the upper crust was decoupled from deformation in the upper mantle along a hot and weak lower crust. During advanced rifting, extension was dominated by simple shear along low-angle detachment faults with a top-to-the-ocean sense of movement. These shallow crustal structures formed a series of breakaways in the continental crust and cut into mantle rocks, indicating that now deformation in the upper crust and in the upper mantle was no longer decoupled. Cooling and strengthening of the lower crust during an initial stage of rifting apparently led to localization of deformation and a different style of deformation, documenting that the tectonic evolution of nonvolcanic margins is largely controlled by the thermal state of the lithosphere. Seafloor spreading initiated only after exhumation and exposure of the subcontinental mantle on the ocean floor and may have been accompanied by a loss of the yield strength of the upper mantle, due to a combination of simple shear extension, asthenospheric uplift, and increased melt production.

Mediterranean and Tethys

The boundary between the Eurasian and the African plates, formerly the suture between Eurasia and Gondwana, has been the locus of violent tectonic diastrophism and rapidly changing geography since the Triassic. The Mesozoic seas, and sometimes the Paleozoic seas, of this zone and its extension into the Himalayan region are known as the Tethys (Neumayr, 1883; Bittner, 1896; Suess, 1893, 1901; cf. e.g., Kamen-Kaye, 1972), while Tertiary seas are usually called the Mediterranean. From the viewpoint of plate tectonics, it would appear appropriate to talk in general of the African—Eurasian boundary seas. We can try to trace the history of the Tethyan or Mediterranean seas from the breakup of Pangea at the end of the Triassic through the Mesozoic and Cenozoic. The most ambitious attempt to do this has been by Dewey et al. (1973) as a sequel to a model for the opening of the Atlantic Ocean proposed by Pitman and Talwani (1972). However, although the general postulates are valid and, within the framework of plate tectonics, even obvious, the actual implementation of this kinematic jigsaw puzzle is very difficult. It is also ambiguous because of large gaps in information and in the differences in language and interpretation by the various investigators. Indeed, at present, there is no model that would not be seriously questioned by one part or another of the earth science community.

Mesozoic-Tertiary carbonate platforms, slopes and basins of the external Apennines and Sicily

Mesozoic and Tertiary limestones form the morphological backbone of the Apennines between Umbria and northern Calabria. They are also exposed in a number of tectonic windows below higher nappes along the Tyrrhenian margin of the Italian peninsula; that is, in the Tuscan nappe and in the exhumed metamorphic structural units of the Northern Apennines They further occur along the Adriatic foreland both in the subsurface and on the plateau of the Apulian peninsula. These limestones are the relics of a Mesozoic archipelago of Bahamian-type carbonate platforms, separated by deeper basins and plateaus, which originally were situated on the continental margin to the S and E of the Liguria—Piedmont segment of the (Neo-) Tethys ocean and extended over what is now the external fold-belts of the Apennines, Hellenides, Dinarides and Southern Alps and their Adriatic foreland (Figs 18.1, 18.2). Wherever the original substratum of these limestone successions is exposed or drilled, it is composed of upper Palaeozoic and Triassic elastics, limestones and evaporites, or of continental crust of Hercynian age. There exists a long-lasting discussion on whether the area was a promontory of the African continent throughout most of its Mesozoic history (Argand, 1924; Channell et al., 1979) or an independent microcontinent (Adria or Apulia, Dewey et al., 1973) that became separated from Africa in Permian (Stampfli et al.,1991; Vai, 1994), Triassic (Dewey et al., 1973) or Cretaceous time (Dercourt et al., 1986; Ricou, 1994). Nowadays there is convincing evidence that Adria was indeed separated from the North African margin by a deep oceanic basin that was connected to the Ligurian Tethys (Fig. 18.2; De Voogd et al.,1992). The Mesozoic and Tertiary limestones of Sicily were deposited along this North African margin and are now an important part of the nappes of central and southern Sicily and of their Hyblean (Iblei) foreland.

Sedimentary fabrics in Alpine ophicalcites, South Pennine Arosa zone, Switzerland

Many ultramafic complexes in the South Pennine nappes of eastern Switzerland are covered by serpentinite breccias, known as ophicalcites. The breccias are overlain by radiolarites or younger oceanic sediments. Their fabric and their components document a combined tectonic-sedimentary origin of the breccias. In the Davos area the breccias occur along distinct zones in the ultramafic host rock. Typically, nonfragmented peridotite/serpentinite grades rapidly through a narrow zone of host rock, cut by different generations of dikes, into complex breccias dominated by a carbonate matrix. The breccias are of polyphase origin, as shown by the different phases of fragmentation and generations of cementation and sediment infill. These are distinguished by color differences and by a bimodal grain-size distribution that also exhibits geopetal structures. Comparison with breccias from present-day oceanic fracture zones suggests that the Tethyan serpentinite breccias may have formed in a transform setting. A sinistral transform margin along the northern edge of the Apulian microplate or promontory is in line with current plate-kinematic reconstructions.

Early Marine Lithification and Hardground Development on A Miocene Ramp (Maiella, Italy): Key Surfaces to Track Changes in Trophic Resources in Nontropical Carbonate Settings

ABSTRACT Phosphatic and carbonate hardgrounds occur in a Lower Miocene nontropical carbonate succession in the Maiella carbonate platform margin, located in the Central Apennines. Multiple diagenetic events occurred at or near the sea floor before the deposition of the overlying sediment and included precipitation of inclusion-rich calcite, micrite, iron oxides, and phosphates. Later diagenetic features are limited to chemical compaction and precipitation of clear calcite cement. We relate the development of these features to a two-step model, involving progressive intensification of upwelling on this carbonate margin, which was triggered by regional changes in water circulation and modulated by sea-level changes, leading initially to precipitation of inclusion-rich calcite. With an increase in trophic resources related to the paleoceanographic conditions on the ramp, increased flux of organic matter to the sea floor led to temporary formation of microbial biofilms. These conditions were associated with extensive precipitation of micrite and phosphate microspherules in the uppermost sediment. The lack of sedimentation provides the precondition to accumulate and preserve evidence for organic-matter utilization in the uppermost sediment layer. Our study suggests that hardgrounds in nontropical carbonates might be used as indicators of circulation changes and can provide a useful link to major environmental changes in the ocean-margin environment. The occurrence of microbial micrite and phosphate microspherules precipitated in the absence of sedimentation near the sea floor as a response to higher nutrient supply provides a new and important criterion to differentiate nontropical carbonate facies deposited under the influence of higher nutrient supplies rather than temperature alone. Furthermore, our study shows that local depletion in 13C at a key stratigraphic surface does not necessarily reflect meteoric exposure but may be related to microbial activity at the sea floor.

A multiple fluid history recorded in Alpine ophiolites

Petrographic and cathode luminescence studies of Alpine ophicalcites are combined with stable isotope data of pelagic sediments and associated ophiolite relicts, to document multiple phases of fluid-rock interaction related to sea-floor processes and Alpine orogeny. Internal sediments and primary carbonate microstructures in ophicalcites provide evidence for early sea-floor fragmentation, cementation and fluid activity in the Jurassic Tethys. Ca-carbonate cementation is comparable to cementation in modern transform setting analogues, where alkaline and Ca-enriched fluids are associated with serpentinites. Local mineralization and isotopic compositions of serpentine in ophicalcites are interpreted as signs of hydrothermal activity at temperatures of 100–150 °C. Differences in the degree of oxygen isotope re-equilibration reflect differences in fluid/rock interaction during Alpine accretionary tectonics and continent-continent collision. Regional-scale homogenization of oxygen isotopes in carbonates and cherts indicate the presence of pervasive metamorphic fluids in the pelagic sediments during early Alpine recrystallization. In contrast, a lack of oxygen isotope re-equilibration in serpentine may reflect lower permeabilities and more limited fluid flow in serpentinites during Alpine metamorphism. Cathode luminescence and preliminary oxygen isotope data of late vein-forming phases suggest that late metamorphic fluids were channelled along discrete brittle fractures in a closed, rock-dominated system.

The upper Hawasina nappes in the central Oman Mountains: stratigraphy, palinspastics and sequence of nappe emplacement

AbstractIn the Oman mountains, a succession of sedimentary decollement nappes, the Hawasina nappes, is sandwiched between the Samail ophiolite nappe and its underlying melange and the “autochthonous” sequences of the Arabian platform. The sediments of the Hawasina nappes document the Mesozoic evolution of the northeastern Arabian continental margin and the adjacent Tethys Ocean. In earlier paleogeographic reconstructions, based on simple telescoping of the tectonic units, the upper Hawasina nappes represent the distal part and the lower nappes the proximal part of the margin. New stratigraphic data suggest a revision of the paleogeography and a more complex model for nappe emplacement in the central Oman mountains. The lower Hawasina nappes with their Jurassic and Cretaceous base of slope and basin sediments (Hamrat Duru, Wahrah) form the original cover of part of the upper Hawasina nappes. In the latter (Al Ayn, Haliw), Triassic pelagic sediments, locally overlain by massive sandstone successions are pre...

Stratigraphy, Facies, and Significance of Late Mesozoic and Early Tertiary Sedimentary Rocks of Fuerteventura (Canary Islands) and Maio (Cape Verde Islands)

Mesozoic deep water sedimentary rocks uplifted and exposed in basement complexes on the islands of Fuerteventura (Canary Islands) and Maio (Cape Verdes) help document the early Atlantic ocean and the volcanic history of these islands.

The Steinmann Trinity revisited: mantle exhumation and magmatism along an ocean-continent transition: the Platta nappe, eastern Switzerland

Abstract The close association of serpentinites, basalts and radiolarites, later known as the Steinmann Trinity, was clearly described by Steinmann from the south Pennine Arosa zone and its southern prolongation, the Platta nappe of the eastern Swiss Alps. This classical ‘ophiolite’ is distinctly different from typical fast-spreading ridge associations and can be compared with the transitional crust occurring along non-volcanic passive continental margins in present-day oceans. It includes serpentinized peridotites that we interpret as subcontinental mantle rocks, which were exhumed along low-angle detachment faults and locally overlain by extensional allochthons of continental crust, minor gabbroic intrusions, tholeiitic pillow lavas and flows and a succession of oceanic sediments. The serpentinized peridotites record deformation at falling temperatures during extension leading to final exposure of the mantle rocks at the sea floor and their inclusion in tectono-sedimentary breccias (ophicalcites). Field relationships, and mineral-chemical and radiometric data show that the gabbros intruded already serpentinized mantle rocks at shallow depth 161 Ma ago. They are Mg gabbros, Fe gabbros and Fe-Ti gabbros, cut by dioritic pegmatoid veins and albitite dykes, which originated by differentiation from the same parental magma. All gabbros show the same metamorphic evolution, i.e. intrusion at relatively low pressure, oceanic hydration at elevated temperature and a subsequent static oceanic alteration. The pillow lavas stratigraphically overlie the exhumed mantle rocks and the tectono-sedimentary breccias related to the exhumation of both mantle rocks and gabbros. However, both gabbros and pillow basalts are characterized by εNd values typical for an asthenospheric mid-ocean ridge-type source of the melts. They may be the products of a steady process that combined extensional deformation with magma generation and emplacement. They appear to document the onset of sea-floor spreading across an exhumed subcontinental mantle during the earliest phases of a slow-spreading ridge.