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A Type Sequence Across an Ancient Magma-Poor Ocean–Continent Transition: the Example of the Western Alpine Tethys Ophiolites

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A Type Sequence Across an Ancient Magma-Poor Ocean–Continent Transition: the Example of the Western Alpine Tethys Ophiolites Tectonophysics 473 (2009) 4–19 Contents lists available at ScienceDirect Tectonophysics journal homepage: www.elsevier.com/locate/tecto A type sequence across an ancient magma-poor ocean–continent transition: the example of the western Alpine Tethys ophiolites Gianreto Manatschal a,⁎, Othmar Müntener b a CGS-EOST, CNRS-ULP, 1 rue Blessig, F-67084 Strasbourg, France b Institute of Mineralogy and Geochemistry, University of Lausanne, CH 1015 Lausanne, Switzerland article info abstract Article history: The ophiolites from the Alpine Tethys are incompatible with the definition of the classical 3-layered Penrose Received 16 February 2008 ophiolite sequence, but they also show features that are inconsistent with ultraslow-spreading ridge Received in revised form 11 July 2008 sequences or transform settings. The existence of pre-rift contacts between subcontinental mantle and Accepted 19 July 2008 continental crust, the association of top-basement detachment faults with continent-derived blocks Available online 5 August 2008 (extensional allochthons) and tectono-sedimentary breccias overlying subcontinental mantle, and a post- rift sedimentary evolution identical to that of the adjacent distal margin enable to characterize some of the Keywords: Alps Alpine Tethys ophiolites as remnants of a former Ocean Continent Transition (OCT). Therefore, we propose Ophiolites that at least some of the Alpine Tethys ophiolites are formed by remnants of an ancient Magma-Poor-Ocean Ocean–continent transitions Continent Transition, referred to as a MP-OCT sequence. The type sequence consists of the Platta, Tasna and Detachment faults Chenaillet ophiolite units, the former two representing the OCT of the ancient Adriatic and European/ Tethys Briançonnais conjugate rifted margins, the latter representing a more developed “oceanic” domain. All three units escaped Alpine subduction and preserve pre-Alpine contacts between exhumed basement and a volcano-sedimentary cover sequence. These units preserve the structural, magmatic, hydrothermal and sedimentary record of continental breakup and early seafloor spreading. The observations compare well with those made along the magma-poor Iberia–Newfoundland rifted margins, which are the only example in an OCT where drill holes penetrated into basement. At present, magma-poor rifted margins form up to 50% of all rifted margins worldwide. We argue that MP-OCT sequences are more common in the geological record but were, in part mistaken as either Mid Ocean Ridge or tectonically dismembered Penrose-type ophiolite sections. © 2008 Elsevier B.V. All rights reserved. 1. Introduction ophiolites. These ophiolite units are quite distinct from the “classical” ophiolite sequence observed in the Semail ophiolite in Oman (Glennie Since the establishment of the new paradigm of plate tectonics in et al., 1974; Nicolas and Boudier, 1995) or Troodos in Cyprus (Moores the late sixties of the last century, ophiolites in Alpine-type collisional and Vine, 1971). In contrast to these “magma-rich” ophiolites, the mountain belts were often equated with former oceanic crust. At Magma-Poor-Ocean Continent Transition (MP-OCT) ophiolites are present it is generally accepted that the classical ophiolite stratigraphy distinguished by the scarcity of mafic plutonic rocks, the absence of a as conceived by the Penrose conference definition (Anonymous, 1972) sheeted dike complex, the occurrence of exhumed, ancient subconti- is just one among various types. Ophiolites may derive from back-arc, nental mantle capped by top-basement detachment faults and fore-arc or mid-ocean ridge settings, the latter showing a large overlain by extensional allochthons, tectono-sedimentary breccias variability ranging from fast to slow to ultraslow-spreading environ- and a post-rift sedimentary sequence similar to that of the adjacent ments. In this paper, we describe ophiolites that record the final stage distal margin. Along present-day rifted margins, the OCT is covered by of rifting and incipient seafloor spreading in magma-poor rift systems a thick pile of sediments at abyssal depth. A ‘complete’ OCT sequence based on observations made in the Platta, Tasna and Chenaillet across a magma-poor rift system has only been drilled across the Iberia–Newfoundland rifted margins. However, drilling along these margins is limited to basement highs. This, and the fact that geophysical imaging has very low resolution at deep margins, makes ⁎ Corresponding author. Fax: +33 3 90 24 04 02. that at present very little is known about the nature of rock types and E-mail address: [email protected] (G. Manatschal). structures in present-day OCT. Therefore, the sequence described in 0040-1951/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2008.07.021 G. Manatschal, O. Müntener / Tectonophysics 473 (2009) 4–19 5 this paper is not only of interest for the study of ophiolites, but also “Steinmann Trinity”, in geodynamic terms. In his view, the “con- represents a unique natural laboratory that documents continental sanguineous” association of ultramaficandmafic material was breakup and the formation of oceanic crust in a magma-poor rift characteristic for the axial part of the “geosyncline” and the deep system. ocean floor (Steinmann, 1905). Steinmann interpreted the diabase, spilite, and variolite (variolitic pillow lavas) as intrusive rocks 2. Historical perspective distinctly younger than the associated sediments, and he stressed their characteristic association with deep-sea sediments, notably The concept and definition of ophiolites as remnants of former radiolarian cherts and deep-water limestones of Maiolica facies oceanic domains is of first order importance to interpret paleogeo- (Steinmann, 1925, 1927). Argand (1924, p. 299) noted that “A graphic domains and their evolution during continental collision. geosyncline will result, in general, from horizontal extension which With the advent of plate tectonics, many observations made stretches the raft of sal [continental crust] … This explains the previously in orogens were ignored and replaced by simple concepts frequent association of greenstones with bathyal or abyssal sediments. that came along with the new ideas of plate tectonics. Examples for … If extension continues, … the geosyncline makes place to an ocean”. such simple concepts are the homogeneous, 3-layered oceanic crust, Until the late sixties, this classic interpretation did not change as defined by the Penrose conference (Anonymous, 1972), or the idea fundamentally. that the transition from rifting to drifting is abrupt, implying that the The major revolution in the interpretation of the Alpine Tethys transition from continental to oceanic crust is a sharp and well defined ophiolites occurred in the late sixties and early seventies, when Italian boundary. Although these simple concepts might explain in a general geologists showed evidence for stratigraphic contacts between all the way continental breakup in magma-rich systems, they are insufficient individual terms of the ophiolite suite (basalts, gabbros and mantle to explain magma-poor systems presently observed at ultraslow to rocks) and pelagic sediments in the Apennines (Passerini, 1965; slow-spreading ridges or in MP-OCT. Thus, despite the undisputed Decandia and Elter, 1969; Elter, 1971, 1972; Decandia and Elter, 1972). success of plate tectonics, in the case of the ophiolite research, this In the benchmark paper by Decandia and Elter (1972) on the “zona theory had the side effect that many detailed observations were ofiolitifera del Bracco” in the area between Levanto and Val Graveglia overlooked and/or ignored in more recent literature. This can be well in the Ligurian Apennines, these authors concluded that mantle rocks demonstrated with the history of the Alpine Tethys ophiolites. were exhumed tectonically and overlain by Jurassic pelagic sediments Gustav Steinmann was not the first to use the term ophiolite (see (Fig. 1). They described a major unconformity between mantle rocks Bernoulli et al., 2003 for a review), but he was the first who noted the and overlying basalts and sediments and recognized that the close association of serpentinites, diabase and radiolarian cherts along basement was capped by tectono-sedimentary breccias (e.g. ophical- the western boundary of the Austroalpine nappes in eastern Switzer- cites). They were also the first who interpreted mantle exhumation as land and to interpret this “rock association”, later referred to as related to a tectonic process and proposed that mantle exhumation Fig. 1. The four classical models proposed to explain mantle exhumation in the Alpine ophiolites. 6 G. Manatschal, O. Müntener / Tectonophysics 473 (2009) 4–19 had to predate the emplacement of basalts. This interpretation explain the asymmetry and in particular the subsidence history of the anticipated the discovery of mantle rocks during ODP Leg 103 along two Alpine margins (Fig. 1). The discovery of subcontinental mantle in the Galicia margin (Boillot et al., 1987) and was at the forefront of direct contact with marginal sequences in the Tasna (Florineth and concepts that are today proposed to understand continental breakup Froitzheim, 1994) and Err-Platta (Manatschal and Nievergelt, 1997) at magma-poor rifted margins. However, the pioneering work of the and Malenco nappes (Hermann and Müntener, 1996), in the Ligurian Apennine geologists remained largely ignored. The major reason was domain (Molli, 1996; Marroni et al., 1998; Marroni and Pandolfi,2007) that
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