Search

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.earscirev.2010.06.002 Byline: Mark R. Handy (a), Stefan M. Schmid (a)(c), Romain Bousquet (b), Eduard Kissling (c), Daniel Bernoulli (d) Keywords: Tethys; Alps; Mediterranean; plate motion; subduction Abstract: A new reconstruction of Alpine Tethys combines plate-kinematic modelling with a wealth of geological data and seismic tomography to shed light on its evolution, from sea-floor spreading through subduction to collision in the Alps. Unlike previous models, which relate the fate of Alpine Tethys solely to relative motions of Africa, Iberia and Europe during opening of the Atlantic, our reconstruction additionally invokes independent microplates whose motions are constrained primarily by the geological record. The motions of these microplates (Adria, Iberia, Alcapia, Alkapecia, and Tiszia) relative to both Africa and Europe during Late Cretaceous to Cenozoic time involved the subduction of remnant Tethyan basins during the following three stages that are characterized by contrasting plate motions and driving forces: (1) 131-84Ma intra-oceanic subduction of the Ligurian part of Alpine Tethys attached to Iberia coincided with Eo-alpine orogenesis in the Alcapia microplate, north of Africa. These events were triggered primarily by foundering of the older (170-131Ma) Neotethyan subduction slab along the NE margin of the composite African-Adriatic plate; subduction was linked by a sinistral transform system to E-W opening of the Valais part of Alpine Tethys; (2) 84-35Ma subduction of primarily the Piemont and Valais parts of Alpine Tethys which were then attached to the European plate beneath the overriding African and later Adriatic plates. NW translation of Adria with respect to Africa was accommodated primarily by slow widening of the Ionian Sea; (3) 35Ma-Recent rollback subduction of the Ligurian part of Alpine Tethys coincided with Western Alpine orogenesis and involved the formation of the Gibraltar and Calabrian arcs. Rapid subduction and arc formation were driven primarily by the pull of the gravitationally unstable, retreating Adriatic and African slabs during slow convergence of Africa and Europe. The upper European-Iberian plate stretched to accommodate this slab retreat in a very mobile fashion, while the continental core of the Adriatic microplate acted as a rigid indenter within the Alpine collisional zone. The subducted lithosphere in this reconstruction can be correlated with slab material imaged by seismic tomography beneath the Alps and Apennines, as well as beneath parts of the Pannonian Basin, the Adriatic Sea, the Ligurian Sea, and the Western Mediterranean. The predicted amount of subducted lithosphere exceeds the estimated volume of slab material residing at depth by some 10-30%, indicating that parts of slabs may be superposed within the mantle transition zone and/or that some of this subducted lithosphere became seismically transparent. Author Affiliation: (a) Institut fur Geologische Wissenschaften, Freie Universitat Berlin, Malteserstrasse 74-100, D-12249 Berlin, Germany (b) Institut fur Geowissenschaften, Universitat Potsdam, Postfach 601553, D-14415 Potsdam, Germany (c) Institut fur Geophysik, ETH-Zentrum, Sonneggstrasse 5, CH-8092 Zurich, Switzerland (d) Geologisch-Palaontologisches Institut, Universitat Basel, Bernoullistrasse 32, CH-4056 Basel, Switzerland Article History: Received 23 November 2008; Accepted 1 June 2010

[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.

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 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.

Abstract: Before the theory of plate tectonics took hold, there was no coherent model for ocean–continent transitions that included both extant continental margins and fragmentary ancient examples preserved in orogenic belts. Indeed, during the early 1900, two strands of thought developed, one relying on the antiquity and permanence of continents and oceans, advocated by the mainstream of the scientific community and one following mobilist concepts derived from Wegener''s hypothesis (1915) of continental drift. As an illustration of the prevailing North-American view, the different composition and thickness of continental and oceanic crust and the resulting isostatic response showed “how improbable it would be to suppose that a continent could founder or go to oceanic depth or that ocean floor at ±3000 fathoms could ever have been a stable land area since the birth of the oceans” [H.H. Hess, Trans. R. Soc. London, A 222 (1954) 341–348]. Because of the perceived permanence of oceans and continents, mountain chains were thought to originate from narrow, elongated, unstable belts, the geosynclines, circling the continents or following “zones of crustal weakness” within them, from which geanticlines and finally mountain belts would develop. This teleological concept, whereby a geosyncline would inevitably evolve into a mountain chain, dominated geological interpretations of orogenic belts for several decades in the mid-twentieth century. However, the concept of permanence of oceans and continents and the concept of the geosyncline had already met with the critiques of Suess and others. As early as 1905, Steinmann considered the association of peridotite, “diabase” (basalt/dolerite) and radiolarite (a typical ocean–continent transition assemblage), present in the Alps and Apennines, as characteristic of the deep-ocean floor and Bailey (1936) placed Steinmann''s interpretation into the context of continental drift and orogeny. Indeed, in both authors’ writings, the concept of ophiolites as ocean crust is apparent. Between 1920 and 1930, the stage was thus potentially set for modern mobilist concepts that were, however, to prove attractive to only a small circle of Alpine and peri-Gondwanian geologists. After the Second World War, the 1950s saw the rapid progress of the geophysical and geological exploration of oceans and continental margins that provided the data for a reevaluation of the geosynclinal concept. Actualistic models now equated the former preorogenic miogeosyncline of Stille (1940) and Kay (1951) with passive continental margins [C.L. Drake, M. Ewing, G.H. Sutton, Continental margin and geosynclines: the east coast of North America, north of Cape Hatteras, in: L. Ahrens, et al. (Eds.), Physics and Chemistry of the Earth 3, Pergamon Press, London, 1959, pp. 110–189], the (American version of the) eugeosyncline and its igneous rocks with “collapsing continental rises” [R.S. Dietz, J. Geol. 71 (1963) 314–333] and the ophiolites, the Steinmann Trinity, of the (European) eugeosyncline with fragments of oceanic lithosphere [H.H. Hess, History of ocean basins, in: Petrologic Studies: a Volume to Honor A.F. Buddington, Geol. Soc. Am., New York. 1962, pp. 599–620]. The concept of sea-floor spreading [H.H. Hess, History of ocean basins, in: Petrologic Studies: a Volume to Honor A.F. Buddington, Geol. Soc. Am., New York. 1962, pp. 599–620; H.H. Hess, Mid-oceanic ridges and tectonics of the sea-floor, in: W.F. Whittard, R. Bradshaw (Eds), Submarine Geology and Geophysics, Colston Papers 17, Butterworths, London, 1965, pp. 317–333] finally eliminated the weaknesses in Wegener''s hypothesis and, with the coming of the “annus mirabilis” of 1968, the concept of the geosyncline could be laid to rest. Ocean–continent transitions of modern oceans, as revealed by seismology and deep-sea drilling, could now be compared with the remnants of their ancient counterparts preserved in the Alps and elsewhere. [Copyright &y& Elsevier]

Albert W. Bally, Professor emeritus of Geology at Rice University in Houston, passed away on the 30th of July in Houston. In Switzerland, Bally obtained his baccalaureate and began to study Geology at Zurich University. As Bally noted in his curriculum vitae, 'August 1981 marked my return as geologist to Italy, almost 30 years after publication of my Ph.D. thesis in 1954'. [Extracted from the article]

The article presents the author's comments on the contribution of the association Geologische Vereinigung to the development of the field of geology and earth sciences. He says that the organization is celebrating its 100th anniversary in 2010. Its first meeting was held in 1910 at the Senckenberg Museum in Frankfurt, Germany. The author also talks about the periodical "Geologische Rundschau," published by the association.