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Metamorphism Associated with Extensional Rifting of Gondwana

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Metamorphism Associated with Extensional Rifting of Gondwana Geological Society, London, Special Publications Basement and cover rock history in western Tethys: HT-LP metamorphism associated with extensional rifting of Gondwana Robert Hall Geological Society, London, Special Publications 1988; v. 37; p. 41-50 doi:10.1144/GSL.SP.1988.037.01.04 Email alerting click here to receive free email alerts when new articles cite this article service Permission click here to seek permission to re-use all or part of this article request Subscribe click here to subscribe to Geological Society, London, Special Publications or the Lyell Collection Notes Downloaded by Robert Hall on 26 November 2007 © 1988 Geological Society of London Basement and cover rock history in western Tethys: HT-LP metamorphism associated with extensional rifting of Gondwana Robert Hall ABSTRACT: High-temperature-low-pressure metamorphism of the deep crust is probable during continental lithospheric extension. The 75-65 Ma cooling ages of metamorphic and magmatic rocks preserved in overthrust crystalline slices in the southern Aegean are most plausibly explained by late Cretaceous extension of the Apulian continental margin, rather than by subduction-related magmatism. Metamorphism and magmatism at depth can be correlated with those stratigraphic features of the cover sequences that indicate extension. In particular, the puzzling 'premier flysch' of the region is interpreted as one consequence of peripheral uplift associated with stretching. Introduction observed in overthrust terrains, but evidence of the extensional history of the margin may also be discernible from interpretation of the sedimen- Many recent models for the evolution of passive tary sequences deposited within the former con- continental margins imply the possibility of a tinental margin. If the basement and cover of a metamorphic event recorded by the middle and former continental margin can be reconstructed, lower crust, associated with subsidence and then an extensional event may be recognizable probably with lithospheric extension (Sleep which is recorded by metamorphic rocks of low- 1971; Falvey 1974; McKenzie 1978; Royden et pressure facies, with cooling ages close to, but al. 1980; Sclater et al. 1980). The precise charac- post-dating, a 'significant' event recorded ter of the metamorphic rocks formed during stratigraphically by cover sediments. Strati- such an extensional event will depend on the graphic indicators will vary with position in the mechanism(s) of crustal extension and the margin, and will depend on global sea-level, but thermal history of the stretched region (Mid- could include erosional unconformities, marked dleton 1980), for example whether there is a changes in sedimentation rates, and evidence of heating event associated with extension, and faulting where there is no evidence for compres- also on the rate of sedimentation after extension sional deformation. Basic igneous activity may and subsidence (Fig. 1). However, all stretching occur if stretching proceeds to the point where models imply certain metamorphic features. the crust breaks. If more than one phase of The rocks produced will be of a low-pressure extension occurs during the evolution of the facies series (Thompson and England 1984; continental margin the last extensional event Wickham and Oxburgh 1985) and the metamor- will probably be the one recorded by the con- phic ages recorded by the rocks will be cooling tinental basement; if the effects of stretching are ages which post-date the stretching event uniformly distributed, then early events are (Oxburgh 1982). The resulting metamorphic likely to be overprinted by later events; although fabric could be a regional extensional foliation, a if the effects are heterogeneous then several localized foliation associated with deep crustal stretching events could presumably be recorded shear zones, or a combination of the two. Asso- and preserved by the basement rocks. ciated folding may occur in zones of relative displacement, such as shear zones. The metamorphic rocks formed during exten- Late Cretaceous metamorphism in the sion of continental margins are beyond the reach southern Aegean of direct sampling; changes in the physical properties or fabric of the rocks may enable them to be recognized by geophysical methods The southern Aegean represents part of the (Falvey and Middleton 1981). However, they former Apulian continental margin (Bonneau may be identified in former continental margins 1982), which was extended in several stages which have been deformed in mountain belts. during the Mesozoic. The compressional events, Not only can such basement rocks be directly which formed the present orogenic belt linking From AUDLEY-CHARLES, M. G. & HALLAM, A. (eds) Gondwana and Tethys Geological Society Special Publication No. 37, pp. 41-50. 42 R. Hall Greece and Turkey, began in the early Tertiary Mica Amphibole/ (Aubouin et al. 1976; Jacobshagen et al. 1978); I I siliciclastic flysch sediments were deposited dur- ', ing the Palaeogene and orogenic deformation culminated in the Oligocene and early Miocene (Creutzburg and Seidel 1975; Bonneau 1982; Bonneau 1984). Most of the nappes found on the islands of the southern Aegean are composed of carbonate platform, slope, and basin lithologies representing the cover sequences of the extended continental margin, which were I \ detached from their basement and stacked, in two principal deformation events in the late Paleogene (Hall et al. 1984). The structurally highest unit in the southern Aegean (Fig. 2) is sporadically preserved on the islands of Crete, Anafi, Nikouria, Donoussa and possibly Syros (Bonneau 1984) and has been described as a Mica Amphibole/ m~lange. It includes slices of metamorphic rocks and granitoids. The metamorphic rocks are of high-temperature-low-pressure facies, and include garnetiferous schists containing cor- dierite, andalusite and sillimanite; clinopyrox- ene amphibolites, meta-ultrabasics, calc- A silicates and marbles (Bonneau 1972; Seidel et al. 1976, 1981; Diirr et al. 1978; Reinecke et al. 1982; Koepke and Seidel 1984). Metamorphic I I assemblages indicate maximum pressures of 4- I 5 kbar at 650-700~ The metamorphic rocks are closely associated with I- and S-type granitoids and their contact hornfelses. Amphibole ages range from 79 to 66 Ma, whereas mica ages range 75-64 Ma. The intimate association of magmatic and contact metamorphic rocks in the same crystalline complex, and the systematically MicaI Amphlib~/ greater ages of amphiboles compared with micas, indicate that these are cooling ages. ,' ',K/ Using blocking temperatures of 500~ for amphiboles and 300~ for micas, and a linear , s I cooling rate, indicates that the metamorphic maximum occurred before 78 Ma (Crete) and 'A\ / 72 Ma (Anafi). The beginning of this thermal event can only be estimated, but if the assump- tion is made that the heating rate to the , , \ metamorphic maximum was equal to the cooling I I rate, then the thermal event recorded began before at least 85 Ma (Crete) or 80 Ma (Anafi) (early Campanian or older). Previous authors (Bonneau 1982; Reinecke et FIG. 1. Schematic pressure-temperature- time paths for rocks metamorphosed during al. 1982) have argued that these late Cretaceous rifting: (a) extension and cooling according to the model of McKenzie (1978), (b) exten- sion associated with magmatism, according to the model of Royden et al. (1980), (c) heating followed by erosional thinning of kyanite, S: sillimanite) are shown for the crust, according to the model of Sleep reference. The lines labelled mica and (1971). The stability fields of the aluminium amphibole indicate approximate blocking silicate polymorphs (A: andalusite, K: temperatures for these minerals. High-temperature-low pressure metamorphism 43 38N. 31 Sros onou2 36N ~J ,.X,S ,:~ 4~Anafi Tilos~,~ 9 Q~ ~Rhodes 36 01 km 1001 ~ ~:~--@Karpath~ I 124 ~ I If 127 I FI6.2. The southern Aegean region. Principal localities from which high-temperature-low-pressure metamorphic rocks of late Cretaceous age have been reported are shown in black. metamorphic and magmatic rocks are remnants Late Cretaceous stratigraphy of the of a high-temperature belt, generated by sub- southern Aegean duction processes beneath part of the Pela- gonian realm. If they do record such processes, and the subsequent rapid uplift and cooling of an The most notable feature of the deep-water arc terrain as suggested, subduction must have basinal sequences in the region is a brief sili- begun before 85-80 Ma in order to subduct suf- ciclastic interval of early-late Cretaceous age ficient oceanic lithosphere to initiate melting 9 which interrupted otherwise pelagic carbonate- Reconstructions of the region vary considerably, chert sedimentation. This siliciclastic interval is and the continuation of the Pelagonian zone well known in the Hellenides, where it is known south-eastwards is still problematical, but all as 'premier flysch' and is considered to be reconstructions now agree that any ocean basins characteristic of the Pindos zone; the presence of that did exist in the Mesozoic were small (see greenstone detritus led Aubouin (1965) to link it Robertson and Dixon 1984 for review). In this with the ophiolite complexes of the internal case the effects of subduction of one of these Hellenides, which were emplaced at the begin- ocean basins ought to be recorded over a wide ning of the Cretaceous. For continental Greece area, and it is worth examining the late Creta- and the Peloponnese, Fleury (1970) suggested ceous stratigraphic record of the
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