Search

In the mid-1850s, Thomas Henry Huxley, who coined the term 'coccolith', described their presence in oceanic sediments dredged from the sea floor. Subsequent recognition of their occurrence in the Upper Cretaceous English Chalk, largely unconsolidated pelagic sediment, and similar, more lithified limestones in the Jurassic of the Alps, led to their being considered major rock-forming elements through much of geological time, although they are now known to be limited to the Late Triassic–Recent interval. Unlike Charles Darwin, who did not travel abroad following the voyage of the Beagle, Huxley made a number of trips to Italy and Switzerland and is recorded as a guest in the exclusive Hotel Kursaal Maloja in the Engadin in 1893. While staying there, he made a number of excursions on foot and his presence was thought significant enough that it is recorded in an inscribed granitic stone tablet, erected in 1896, describing him as'the illustrious writer and naturalist'. His rambles would have led him past outcrops of tectonically emplaced true oceanic calcareous sediment of Early Cretaceous age, here shown to have originally contained coccoliths that were largely destroyed under the imprint of Alpine metamorphism of sub-greenschist to greenschist facies. To this extent, Huxley could have come close to recognizing true oceanic sediments exposed on land, but the dissimilarity between these Swiss Alpine deposits and the friable English Chalk would not obviously have led to the investigation of the former for the organisms he had christened as 'coccoliths'. [ABSTRACT FROM AUTHOR]

[1] In the Kraishte region, near the junction of the Balkanides and Dinarides, late Early Cretaceous compression led to the thrusting of the Morava unit over the Struma unit. Structural investigations, combined with zircon fission track and [sup.40]Ar/[sup.39]Ar analyses, have been used to reconstruct the geological history of the area and to clarify the original tectonic position of the main units before Cenozoic extension. The results show that Early Cretaceous lower amphibolite facies metamorphism and deformation in the Osogovo-Lisets Metamorphic Complex were related to top-to-the-NE directed nappe stacking, whereas the deeper parts of the allochthonous Morava unit experienced low-grade metamorphic overprint at temperatures >~260[degrees]C. The structurally intermediate Struma Diorites and their Mesozoic cover experienced temperatures between ~170[degrees]C and 300[degrees]C. Thrusting of the Morava onto the Struma unit started probably in Valanginian times (140-136 Ma), soon after the cessation of preorogenic to synorogenic turbidite sedimentation on Struma. The metamorphic peak was reached before 112 Ma. Subsequent extension-related cooling of both units was probably accompanied by the formation of low-angle normal faults. doi: 10.1029/2009TC002509

[1] Top-basement detachment faults (TBDF) represent exhumed segments of downward-concave detachments accommodating tens of kilometers of offset. TBDFs are key structures of magma-poor rifted margins, but are difficult to identify in seismic profiles, because they coincide with top-basement reflections. We have constructed realistic seismic models of an Alpine example, which were used to simulate full marine seismic surveys and to identify the seismic fingerprint of TBDFs in CMP-stacked and time-migrated sections. Minor extensional allochthons, which locally cap TBDFs, give rise to pronounced diffractions, which are in stark contrast to continuous reflections from uncapped segments of TBDFs. In time-migrated synthetic sections we find a continuous and constant amplitude reflection alternating with a laterally discontinuous reflection of variable amplitude. These diagnostics were also identified in the Lusigal 12 seismic profile imaging the Hobby High detachment, a TBDF in the Iberia Abyssal Plain. Using these diagnostics, small extensional allochthons and high-angle faults cutting the Hobby High detachment are observed in Lusigal 12 and the entire sedimentary cover can be shown to be postrift. INDEX TERMS: 8109 Tectonophysics: Continental tectonics--extensional (0905); 8105 Tectonophysics: Continental margins and sedimentary basins (1212); 3025 Marine Geology and Geophysics: Marine seismics (0935); 0935 Exploration Geophysics: Seismic methods (3025); KEYWORDS: rifted margins, top-basement detachment, faults, extensional allochthons, downward concave detachment faults, seismic response, seismic modeling. Citation: Holker, A. B., G. Manatschal, K. Holliger, and D. Bernoulli, Tectonic nature and seismic response of top-basement detachment faults in magma-poor rifted margins, Tectonics, 22(4), 1035, doi:10.1029/ 2001TC001347, 2003.

In 1890, former students of Arnold Guyot at the University of Neuchàtel dedicated a 'large glacial boulder from Neuchâtel' to his memory, now in front of Guyot Hall at Princeton University. We established that this block originated with all certainty from Upper Carboniferous conglomerates of the Salvan–Dorénaz Basin, the relics of which crop out in the Alps of western Switzerland and eastern France. During the Last Glacial Maximum, the boulder travelled from the lower Rhône Valley on the back of the Pleistocene Rhône Glacier to Neuchâtel from where, later, it crossed the Atlantic and was transported to Princeton. Guyot's name is familiar to geologists because of the term 'guyot', introduced 1946 by Harry Hess for 'curious flat-topped peaks scattered over millions of square miles in the Pacific basin', many of which are now known to represent drowned carbonate platforms. [ABSTRACT FROM AUTHOR]

The use of white phengitic micas with Si=3.3 p.f.u. to indicate Alpine high-pressure metamorphism is insufficient proof for the erosion of high-P metamorphic source rocks. Such a method is questionable because it does not provide buffer documentation and does not accurately establish the polytype of mica. That only locally preserved Variscan basement of the Alps does not contain blueschist relics does not connote that blueschist metamorphism during Variscan convergence did not occur.

The Upper Cretaceous (Campanian–Maastrichtian) bioclastic wedge of the Orfento Formation in the Montagna della Maiella, Italy, is compared to newly discovered contourite drifts in the Maldives. Like the drift deposits in the Maldives, the Orfento Formation fills a channel and builds a Miocene delta‐shaped and mounded sedimentary body in the basin that is similar in size to the approximately 350 km2 large coarse‐grained bioclastic Miocene delta drifts in the Maldives. The composition of the bioclastic wedge of the Orfento Formation is also exclusively bioclastic debris sourced from the shallow‐water areas and reworked clasts of the Orfento Formation itself. In the near mud‐free succession, age‐diagnostic fossils are sparse. The depositional textures vary from wackestone to float‐rudstone and breccia/conglomerates, but rocks with grainstone and rudstone textures are the most common facies. In the channel, lensoid convex‐upward breccias, cross‐cutting channelized beds and thick grainstone lobes with abundant scours indicate alternating erosion and deposition from a high‐energy current. In the basin, the mounded sedimentary body contains lobes with a divergent progradational geometry. The lobes are built by decametre thick composite megabeds consisting of sigmoidal clinoforms that typically have a channelized topset, a grainy foreset and a fine‐grained bottomset with abundant irregular angular clasts. Up to 30 m thick channels filled with intraformational breccias and coarse grainstones pinch out downslope between the megabeds. In the distal portion of the wedge, stacked grainstone beds with foresets and reworked intraclasts document continuous sediment reworking and migration. The bioclastic wedge of the Orfento Formation has been variously interpreted as a succession of sea‐level controlled slope deposits, a shoaling shoreface complex, or a carbonate tidal delta. Current‐controlled delta drifts in the Maldives, however, offer a new interpretation because of their similarity in architecture and composition. These similarities include: (i) a feeder channel opening into the basin; (ii) an excavation moat at the exit of the channel; (iii) an overall mounded geometry with an apex that is in shallower water depth than the source channel; (iv) progradation of stacked lobes; (v) channels that pinch out in a basinward direction; and (vi) smaller channelized intervals that are arranged in a radial pattern. As a result, the Upper Cretaceous (Campanian–Maastrichtian) bioclastic wedge of the Orfento Formation in the Montagna della Maiella, Italy, is here interpreted as a carbonate delta drift. [ABSTRACT FROM AUTHOR]

The reconstructed southeastern margin of the Tethys ocean was compared with the present-day Galicia margin and their architectures and tectonic evolutions were found to be very similar. It is argued that asymmetric rifting was dominated by simple shear extension in the later stages of rifting, resulting in the exhumation of subcontinental mantle.

An obituary for Hans P. Laubscher, Professor emeritus of Geology at the University of Basel, is presented.

In contrast to Northern Europe where, in the 19th Century, a coherent lithostratigraphy and biostratigraphy was established, many of the ‘odd rocks of mountain belts’ (as christened by Alfred G. Fischer) of the Alpine-Mediterranean region defied an easy interpretation in terms of the simple English layer-cake stratigraphy mapped by William Smith. However, as early as 1862, Eduard Suess recognized close affinities between Mesozoic faunas of the Alps and those of the Himalaya. Suess also described many such Alpine sedimentary rocks as pelagic (a term discussed by Antoine Lavoisier in the context of sea-level change), and the results of the Challenger Expedition (1872 to 1876) convinced the Viennese school of geologists of their deep-water nature. Based on the composition of Jurassic–Cretaceous faunas, Melchior Neumayr postulated the existence of an equatorial ocean, the ‘Central Mediterranean’ which extended from Central America through the Alpine belt to the Himalaya and beyond, an ocean which Suess subsequently named ‘Tethys’. The question of whether or not deep-sea sediments could occur on land, as deliberately posed by Gustav Steinmann in 1925, became central to the long-lasting controversy on the permanency of continents and ocean basins. Indeed, the occurrence of deep-sea deposits in mountain belts required the disappearance of oceanic areas and a mobilist concept of orogeny that was subsequently applied to the Tethyan region by Emile Argand in the wake of the continental drift hypothesis of Alfred Wegener. Implicit in the views of Wegener and Argand is the concept of an oceanic crust fundamentally different from that of the continents. Although ophiolites had been recognized as a special group of rocks in the early 19th Century, Steinmann was the first to interpret the association of (serpentinized) peridotite, ‘diabase’ (dolerite–basalt) and radiolarite (Steinmann Trinity) in a geodynamic context and to consider it as characteristic of the deep ocean. The comparison of Alpine radiolarites with Recent radiolarian oozes deposited below the calcite compensation depth did not remain unchallenged, because of the local association of these siliceous rocks with coarse clastic deposits. However, after the advent of the turbidity-current hypothesis, these detrital deposits were reinterpreted as submarine mass-flow deposits. Consequently, such typical Tethyan facies as Rosso Ammonitico, Maiolica/Biancone and black shales, found in close stratigraphical association with the radiolarites, could be interpreted as deposited on deeply submerged continental and/or oceanic crust where they recorded faithfully a range of interdependent palaeoceanographic phenomena: changes in the relative proportion of siliceous, carbonate and organic-walled biota, in carbonate saturation of sea water, in oceanic circulation and in the impact of orbital-climatic cycles. The results of the Deep Sea Drilling Project, with its discovery of typical Jurassic and Cretaceous Tethyan facies in the Atlantic Ocean, dramatically confirmed the early interpretations of European geologists of the 19th and early 20th Centuries. [ABSTRACT FROM AUTHOR]

The concept of turbidite has evolved so much since its original definition by Kuenen and Migliorini in 1950 – i.e. the deposit of turbidity currents exemplified by the sandy flysch successions of the Northern Apennines – that it is now used to define a variety of deposits, some of which have little in common with sandy flysch formations in terms of facies, geometry and geological significance. The extension of the concept to other geodynamic settings and deposits of non-siliciclastic composition is considered only briefly in the concluding sections. With the diffusion of the concept of turbidity current, in the 1950s and early 1960s, an entirely new branch of sedimentology came into being, concerned with the inventory of sedimentary structures, palaeocurrent measurements and bedding patterns. The most representative expression of this branch came from the ‘Dutch school’ of Philip H. Kuenen and his students. Between the late 1960s and the mid-1970s, there was a new development: facies analysis, in terms of modern environments and depositional systems. This development led to the introduction and discussion of ‘fan models’ that became an increasingly thorny issue with the accumulation of data from modern deep-marine settings. In particular, most researchers emphasized the importance of channel and lobe elements and their mutual relationships in space and time... [ABSTRACT FROM AUTHOR]

We challenge the former interpretation of the ‘sedimentary mélange’ of the Makran accretionary complex as a tectonic mélange diapirically emplaced from below and provide evidence for its sedimentary gravitational emplacement from the north during Tortonian–Messinian times (between 11.8 and 5.8 Ma). It is an olistostrome that includes blocks of ophiolites and oceanic sediments derived from the ‘coloured mélange’ to the north, and reworked chunks of the turbidites on which it rests with an erosional truncation. The chaotic scattering of blocks of any size and lithology and the weak, soft-sediment deformation of the matrix argue against a tectonic emplacement of the chaotic formation. Its size and internal structure, together with the size of the individual blocks, make the olistostrome a fossil equivalent of the large, gravitationally emplaced debris flows observed along present-day continental margins and unstable volcanic edifices. [ABSTRACT FROM AUTHOR]

We provide new geological and isotope geochemical constraints on the evolution from continental rifting to sea-floor spreading along a segment of the Jurassic Tethyan margin exposed in the Platta and Err nappes (eastern Central Alps). Field observations show that the ocean–continent transition zone is characterized by oceanward-dipping detachment faults leading to the exhumation of subcontinental mantle rocks subsequently intruded by gabbro bodies and dolerite dikes, and covered by pillow basalts and radiolarites. Zircons extracted from gabbros and albitite yield concordant U–Pb ages of 161 ± 1 Ma; their initial ℇHf (+ 14.4 to + 14.9) as well as bulk rock ℇNd values of from gabbros and basalts (+ 7.3 to + 9.5) point to a MORB-type depleted mantle source. These data suggest that the onset of magmatic activity coincides with the latest phase of mantle exhumation along low-angle detachment faults and may be controlled by upwelling asthenosphere beneath a zone of exhumed continental mantle. [ABSTRACT FROM AUTHOR]

Comment. Provides information on the results of a study on the metamorphic age and geologic significance of the blueschist source terranes. Opposition to the results of this study.