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
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© 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 region for some that an east-west facies variation indicated indication of this event. Associated features transport of sediment axially along the Pindos expected might include evidence of compres- Basin because he saw no evidence to support sional deformation and calc-alkaline volcanism either an internal or external origin, but he was and deposition of large volumes of volcaniclastic unable to determine whether transport was from debris eroded from the uplifted arc terrain. the south or the north. Although the term premier flysch is often not used, the same sili- ciclastic interval can be recognized throughout 44 R. Hall
the southern Aegean, much further from the most notably is not restricted to the deepest presumed source region, on Crete, Syrna, Tilos, parts of the Pindos Basin; sections recorded by Symi, and in south-west Turkey. Thiebault (1982) show that it is present at the Throughout the Aegean, this siliciclastic basin margins but not in the axial parts of the interval displays a number of puzzling features. basin, where uninterrupted chert deposition In continental Greece and the Peloponnese, the continued throughout the early Cretaceous. The premier flysch is typically Cenomanian- facies variations observed by Fleury (1970) have Turonian in age, but may be locally as old as late been confirmed elsewhere in the Peloponnese Albian and as young as Senonian, with no (Maillot 1973); a change from coarse calcareous obvious pattern to the beginning and end of sandstones in the west to shales and marls in the siliciclastic sedimentation (Fleury 1980; east is not easily explained by an axial transport Thiebault, 1982). In the southern Aegean, the model. Piper and Pe-Piper (1980) interpreted age of this interval is similarly variable (Fig. 3); the west to east passage in the Peloponnese from in west Crete it is dated as Cenomanian (Seidel coarse channelled to fine-grained lithologies as a 1971), in central Crete as Turonian (Bonneau proximal to distal transition, and palaeocurrent and Fleury 1971, Hussin 1983), and further east evidence indicating eastward transport led them as probably Turonian (Bernouilli et al. 1974) but to advocate an external (western) source region locally possibly as old as Cenomanian (Roussos for the siliciclastic debris. They suggested that 1978). Considering its supposed axial origin, it lack of siliciclastic sediments in the Tripolitza shows surprisingly rapid thickness variations in carbonate platform sequences of the same age to the Peloponnese, is absent in some sections, and the west, the principal reason for Fleury's (1970)
~ basaltic volcanics ,~, pelagic limestones, ~ calcarenites and calcirudites cherts and shales ~ breccia ~ sandstones ~ cherty limestones ~ siliciclastic rocks, ;';',',; calcarenites and calcisiltites principally sandstones ~ shallow marine limestones ~ redeposited oolitic limestones ~ radiolarian cherts ~ marly limestone ~ dolomites
A B D E F G H I Ma Eocene 50- I flysch - Paleocene m flysch 70"Late. --cM2 li!i~I | Cret. -- Sa ~ 90- --d~T CoI~~.'1
" AI 110 - fysc• . Early -- I\ "-, I .... i , ...... \ -, i 130-_~Cretaceous I 1 1 150- Late ~---Z]
. Jurassic ~1 i | i | 170- Middle i | | - Jurassic i | 'i 190-_ Early [~
210- Jurassic
230- Triassic i I
Middle Adra Series Ataviros Subgroup Triassic Karpath0s Rhodes (5) Pindos Series Ethia Series Xind0thi0Series (4, 5) Tilos Syrna KokkimidiGroup ArchangelosSubgroup West Crete CentralCrete Karpath0s (6) (7) Symi Rhodes (1) (2,3) (4,5) (5) (5) Fio. 3. Schematic stratigraphic sections through platform, slope, and basinal sequences in the southern Aegean. The early late Cretaceous extensional event is marked in different ways in platform sequences (D,/), slope sequences (B, F, G, H) and basinal sequences (A, C, E), principally by a brief siliciclastic interval and/or an unconformity. Based on information from: 1: Seidel (1971), 2: Bonneau and Fleury (1971), 3: Hussin (1983), 4: Davidson (1974), 5: Harbury (1986), 6: Roussos (1978), 7: Marnelis (1978). Abbreviations: Al Albian, Ce Cenomanian, Tu Turonian, Co Coniacian, Sa Santonian, Ca Campanian, Ma Maastrichtian. Time scale is that of Harland et al. 1982. High-temperature-low pressure metamorphism 45 rejection of a western source region, could be metamorphic rocks. The lavas are MORB-like explained by transport of siliciclastic sediments tholeiites (Robert and Bonneau 1982), indicat- during times of lowered sea-level. They pointed ing significant partial melting of the underlying to important disconformities in the Tripolitza upper mantle and local breaking of the continen- limestone sequences in the early late Cretaceous tal margin crust. They are overlain by Senonian in support of their hypothesis. pelagic marls. On Crete, Karpathos, and Many of the anomalous features of the Rhodes, tholeiitic dolerites and gabbros of premier flysch are also evident in the basinal early-late Cretaceous age (Mutti et al. 1970; sequences further east in the southern Aegean Baranyi et al. 1975; Koepke et al. 1985) are and Turkey (Fig. 3). In south-west Turkey, exposed, which locally intrude recrystallized Syrna and Tilos, and other small islands of the carbonates. southern Aegean, a coarse, angular, poorly- In shallower-water regions, carbonate deposi- sorted breccia up to 10m thick containing Jur- tion continued throughout the late Cretaceous assic and Cretaceous limestones, chert, and with no indication of arc volcanism. Areas that locally clasts of diabase, quartz, and mica remained topographically high throughout the (Bernouilli et al. 1974) marks the base of the Mesozoic are represented by sequences of plat- siliciclastic interval. On Symi, breccias are com- form carbonates; at a time of rising global sea- mon in the lower part of the siliciclastic interval level, parts of these platforms were uplifted (Harbury 1986). These breccias have a very slightly on Rhodes (Harbury 1986), Karpathos proximal aspect, which is odd if they are derived (Davidson 1974), and in the Peloponnese either axially from the northern Hellenides or (Tsaila-Monopolis 1977) eroding up to a few externally from west of the Tripolitza platform; hundred metres of the early Cretaceous car- they are more plausibly interpreted as sub- bonates. Elsewhere the platform sequences marine fault-scarp breccias. The overlying flysch record continuous sedimentation in shallow sequence consists of graded conglomerates, water on Crete (Leppig 1978), or subsidence to sandstones, and shales (Bernouilli et al. 1974; subtidal depths on Karpathos (Harbury 1986) on Harbury 1986), and a common feature is the faults indicated by redeposited fault-scarp brec- presence of shallow-water carbonate debris and cias, channelled calcirudites and calciturbidites. clasts of continental metamorphics, serpen- Carbonate slope regions which separated the tinites and basaltic rocks; exactly the same type platforms from basins, and had subsided earlier of debris as found to the west on Crete and in the in the Mesozoic, record an increased input of Peloponnese. Volcanic clasts are typically redeposited shelf carbonate material into basaltic, and calc-alkaline debris has not been basinal areas, indicating local uplift at the basin found anywhere. Throughout the southern margins. The siliciclastic interval is missing in Aegean the sandstones are relatively coarse the carbonate slope sequences of the region, but grained and not typically distal deposits. As in on Karpathos and Rhodes there is an abrupt the Peloponnese, in most sequences there is a change in sedimentation rate in the early late return to carbonate deposition in the Senonian, Cretaceous, and coarse redeposited carbonates typically pelagic with redeposited beds, and of Senonian age rest directly on very early Creta- although on Symi and in Turkey the siliciclastic ceous limestones on Rhodes (Mutti et al. 1970; interval is overlain directly by wildflysch of prob- Harbury 1986), suggesting erosion and channel- able Palaeocene age, the presence of pelagic ling. In some of the redeposited limestones, limestones of late Cretaceous age in the wild- basaltic igneous and continental metamorphic flysch suggests the same return to carbonate lithic clasts, similar to those in the deeper basinal deposition in the Senonian (Thorbecke 1976; sequences, can be found (Hussin 1983). Harbury 1986). Also as in the Peloponnese the siliciclastic interval is not identifiable in all basinal sequences; it is missing in the Profitas Compression or extension? Ilias Subgroup of Rhodes, although it may be correlatable with Turonian marly limestones of the Xindothio Series on Karpathos (Davidson The stratigraphy of the platform, slope, and 1974). Although the siliciclastic interval may be basinal sequences of the southern Aegean and absent, the deepest basinal sequences record surrounding regions record evidence of a signifi- renewed extension, and commonly include brec- cant event in the early late Cretaceous. The cias composed entirely of basinal debris suggest- basinal sequences are typically interrupted by a ing intrabasinal fault scarp deposits. siliciclastic interval, which is unusual in their On Crete, blocks of pillow lavas (Bonneau otherwise continuous record of carbonate and 1973) are found in the same m~lange unit as the chert deposition. An unconformity can be 46 R. Hall recognized in many of the shallower-water car- ing, over a several million year period early in bonate sequences. This is marked by erosion on the late Cretaceous, could account for the vari- parts of the platforms, subsidence of other parts ation in age of the siliciclastic interval. Active of the platforms, and increased input of shallow- extensional faults shed debris, which accounts water debris on to the slopes flanking the basins. for the locally very proximal character of breccia The siliciclastic interval has previously been deposits and also their rapid changes in thick- interpreted as debris eroded and transported ness. Uplift at the basin margins caused erosion axially, from ophiolite nappes emplaced during of platform and upper slope sequences, in some the Eo-Hellenic orogenic phase in northern cases eroding through the carbonate cover to the Greece. However this hypothesis fails to explain underlying metamorphic basement. Channel- the variable thickness and timing of this interval, ling of this debris from adjacent platform regions local facies variations such as those in the explains local facies variations which indicate Peloponnese, and the very proximal character of external source regions. Piper and Pe-Piper the deposits in areas increasingly far from the (1980) note the similarity of the heavy-mineral supposed original source. It is odd that the Eo- assemblages in the premier flysch of the Hellenic mountains, formed at the beginning of Peloponnese to material from the Phyllite the early Cretaceous, should have revived suffi- Series, and such a similarity is probable if, as ciently to provide debris to the southern Aegean suggested by several authors, the Phyllite Series and Turkey, when intervening regions such as of the Peloponnese and Crete is the likely base- the Argolis Peninsula (Bachmann and Risch ment of the Tripolitza and other platform car- 1978) had long since received the products of bonates of the region. Local uplift of platform their erosion, and carbonate deposition had regions at basin margins could be a consequence resumed. A very complex system of sediment of extension of the platform carbonates on listric erosion, transport, and distribution is required faults, allowing subsidence and local uplift of for this explanation to be satisfactory. Piper and rotated parts of these blocks. An alternative Pe-Piper (1980), who drew attention to some of explanation could be local peripheral uplift of the anomalous features of the premier flysch thermal origin at basin margins, as predicted by interval in the Peloponnese, suggested a western several extensional models (Sclater et al. 1980; source for siliciclastic sediment, which explains Royden and Keen 1980; Beaumont et al. 1982). local facies variations but is less attractive when Faults associated with extension provided access such sediment has to be transported as far east as for basic extrusives, and account for the intru- Turkey. Their proposal that intervals of lowered sion of basic rocks into carbonates seen on sea-level allowed channelling of siliciclastic Crete, Karpathos, and Rhodes. Erosion of the debris across a carbonate platform, is also incon- basic igneous rocks which reached the surface, sistent with evidence of a global rise in sea-level and the basement underlying the platform car- in the late Cretaceous after a possible mid- bonates in channels at the basin margins, pro- Cretaceous lowering of sea-level (Vail et al. vided siliciclastic debris for the flysch sediments. 1977; Hallam 1984; Watts and Thorne 1984). In In the deeper parts of some basins, faulting none of the sequences of the region is there formed intraformational breccias in the basinal evidence for a late Cretaceous episode of sequences, and locally, stretching proceeded to compression. the point where the continental crust broke and A simpler and more satisfactory explanation is true oceanic crust may have formed (e.g. the the proposal that the stratigraphy of the region Arvi ocean of Bonneau (1984) on Crete). indicates an important episode of extension in Thermal subsidence and rising sea-level restored the early late Cretaceous. Eastwards of the carbonate deposition to all areas by the southern Aegean, the abundant ophiolites are of Campanian (?80 Ma). early-late Cretaceous age or younger. Geochemical arguments have been advanced to suggest that these ophiolites are subduction- Metamorphism associated with extension related, although there is little stratigraphic or sedimentological support for such an interpreta- tion (Hall 1984; Woodcock and Robertson There is little evidence for subduction and calc- 1984). I suggest that it was late Cretaceous alkaline volcanism in adjacent regions in the extension which led to important and wide- early late Cretaceous. No calc-alkaline volcan- spread ophiolite formation east of Greece, and ism is recorded, no calc-alkaline volcanic debris stratigraphic evidence of this extensional epi- appears in the sedimentary sequences, and there sode is to be expected in adjacent parts of the is no evidence of compressional deformation in southern Tethyan margins. Intermittent fault- the region. Radiometric dating of high-P-low-T High-temperature-low pressure metamorphism 47 metamorphic rocks in the region indicates Jur- Conclusions assic, early Cretaceous and Tertiary ages, but no late Cretaceous ages (Diirr etal. 1978; Maluski et al. 1981). The absence of evidence for subduc- There appears to be little stratigraphic support tion and volcanism could be explained by sup- for a subduction-related and compressional posing that all of the ocean basins were so small origin of the metamorphic event recorded by that they were subducted with little or no trace of continental basement metamorphic and mag- the normal subduction processes. However, this matic rocks. In contrast, the interpretation of requires special pleading for practically every these rocks as a product of lithospheric exten- postulated ocean in the region, and also an sion is consistent with the regional stratigraphy, unusually prolonged period of very slow subduc- with the recent suggestion that continental rifts tion of very small oceans. Instead of a subduc- are a probable location of high-temperature- tion-related origin for the late Cretaceous low-pressure metamorphism (Wickham and igneous and metamorphic rocks, I suggest the Oxburgh 1985), and with suggestions that many alternative interpretation that the metamorphic of the deep basins within the southern Tethyan and magmatic rocks record a thermal event margin were narrow (Jenkyns and Winterer associated with lithospheric extension within the 1982) and underlain largely by continental crust Apulian continental margin. The mineral ages (D'Argenio and Alvarez 1980; Hall 1982, 1984). represent, not uplift and cooling, but subsidence Not until the late Cretaceous extensional phase and cooling after heating and extension of the did many of these deep basins within the con- margin early in the late Cretaceous (?95-90 Ma; tinental margin east of the south-east Aegean Cenomanian-Turonian). The mineral ages acquire an oceanic foundation as the Tethyan record the point in the late Cretaceous where the margins were stretched for the last time. Exten- extended and heated basement cooled to sion may have begun as early as the Albian, and appropriate blocking temperatures. may have continued until the early Senonian; in The southern Aegean is unusual, in that con- the south-east Aegean it seems to have been tiental-margin sedimentary cover sequences are most important in the Turonian. Small ocean still well preserved, while overthrust basement basins developed (probably largely within the rocks have not yet been entirely eroded off the continental margin of the southern Tethys) dur- uplifted orogenic belt. The basement rocks were ing this final extensional phase, and closed later thrust over their cover in the late Paleogene, to a in the Cretaceous, finally emplacing the oceanic structurally high position, at an early stage in the crust as ophiolites. In mainland Greece where compression of the continental margin, and thus small ocean basins had opened and closed much escaped the effects of the regional metamor- earlier--by the earliest Cretaceous, the exten- phism associated with collision. It is therefore sion further east was recorded by the premier possible to compare the basement and cover flysch. The premier flysch, which is widespread histories, which is not possible in undeformed in the Hellenides and recognizable throughout passive margins or uplifted continental rift the southern Aegean and in south-west Turkey, zones, where either cover-only or basement- and local unconformities in the platform and only is seen. The deeper basement rocks of the slope sequences, may be the only record of region are generally not exposed, or where regional extension in areas where Alpine meta- exposed are overprinted by Alpine metamor- morphism has overprinted earlier events in the phism, and therefore the regional extent of this basement. metamorphic event is uncertain. However, dis- High-T-low-P metamorphic rocks associated covery of rudists in marbles forming the cover to with crustal extension will be observable in oro- the Menderes Massif of western Turkey indicate genic belts where collision processes have thrust that it formed the basement of a Mesozoic plat- basement rocks over their former cover form sequence, and although overprinted by sequences. However, to escape regional meta- Alpine metamorphism, one 66 + 4 Ma morphism associated with orogenesis, they must muscovite age is reported (Diirr et al. 1978). be emplaced at high structural levels in the Diirr et al. regard this age as somewhat high for mountain belt, and therefore such rocks will be Alpine metamorphism, but the position of the exposed only for a short period after collision, Menderes Massif on the edge of the southern before they are eroded away. In the Aegean, Aegean, and its reported high-T-low-P post-orogenic extension and subsidence during andalusite-bearing rocks (Evirgen and Ataman the Neogene has resulted in the preservation of 1981) give rise to the speculation that the late remnants of the higher thrust slices, but Cretaceous metamorphic event was more wide- elsewhere only the youngest orogenic belts will spread than its preserved relics now suggest. expose such rocks. One such example is in 48 R. Hall
Timor, where collision between the Australian ated with crustal rifting. Such extremely young continental margin and south-east Asia began orogenic belts may offer the best chance to relate only in the last 3.5 Ma, resulting in overthrusting the stratigraphic features of extension to the of basement rocks of the former rifted margin on thermal processes occurring during crustal to the Australian margin sedimentary cover extension. rocks (Audley-Charles 1981). High-T-low-P metamorphic rocks with decompressional P-T ACKNOWLEDGEMENTS: Fieldwork in the paths have been described from Timor by Brown Aegean was funded by NERC and the Central and Earle (1983), and like the Aegean rocks Research Fund of London University. I thank Neil have been attributed to metamorphism associ- Harbury for help and discussion.
References
AUBOUIN, J. (1965). Geosynclines. Elsevier, of the eastern Mediterranean (ed. J. E. Dixon and Amsterdam. 335 pp. A. H. F. Robertson), pp. 499-516, Spec. Publ. AUBOUIN, J., BONNEAU, M., DAVIDSON, J., Geol. Soc. London, 17. LEBOULENGER, P., MATESCO, S., and ZAMBETAKIS, BONNEAU, M. and FLEURY, J.-J. (1971). Pr6cisions sur A. (1976). Esquisse structurale de l'arc 6g6en la s6rie d'Ethia (Cr6te, Gr6ce): 6xistence d'un externe: des Dinarides aux Taurides. Bull. Soc. premier flysch m6socr6tac6. C. R. Acad. Sci., Gdol. Fr. (7), 18,327-36. Paris D272, 1840-42. AUDLEY-CHARLES, M. G. (1981). Geometrical prob- BROWN, M. and EARLE, M. M. (1983). Cordierite- lems and implications of large scale overthrusting bearing schists and gneisses from Timor, eastern in the Banda Arc-Australian margin collision Indonesia: P-T conditions of metamorphism and zone. In Thrust and nappe tectonics (ed. K. tectonic implications. J. Metam. Geol. 1, McClay and N. J. Price). Spec. Publ. Geol. Soc. 183-203. London, 9,407-16. CREU~ZBURC, N. and SE1DEL, E. (1975). Zum Stand BACHMANN, G. H. and RlSCH, H. (1978). Late der Geologie des Pr/ineogens auf Kreta. Neues Mesozoic and Paleogene development of the Jb. Geol. Palaeont., Abhl. 149, 363-83. Argolis peninsula (Peloponnesos). In Alps, DAVIDSON, J. (1974). Contribution ~ l'~tude g~ologique Apennines, Hellenides (ed. H. Closs, D. Roeder, de l'arc dg~en: File de Karpathos (Dodecanese and K. Schmidt). E. Schweizerbart'sche Verlag, m~ridional, Grdce). Th6se 3eme cycle, Universit6 Stuttgart, 424-7. de Pierre et Marie Curie, Paris. BARANYI, I., LIPPOLT, H. J., and TODT. (1975). D'AR6ENIO, B. and ALVAREZ, W. (1980). Strati- Kalium-Argon-Datierungen an zwei Magmatiten graphic evidence for crustal thickness changes on von Kalo Chorio, Nord-Ost-Kreta. Neues Jb. the southern Tethyan margin during the Alpine Geol. Palaeont. Mh. 5,257-62. cycle. Geol. Soc. Am. Bull. 91,681-9. BEAUMONT, C., KEEN, C. E., and BOUTILIER, R. DI3RR, S., ALTHERR, R., KELLER, J., OKRUSCH, M., (1982). A comparison of foreland and rift margin and SEIDEL, E. (1978). The Median Aegean sedimentary basins. Phil. Trans. Roy. Soc. crystalline belt: stratigraphy, structure, metamor- London A305, 295-317. phism, magmatism. In: Alps, Apennines, Hel- BERNOULLI, D., DE GRACIANSKY,P. C., and MONOD, lenides (ed. H. Closs, D. Roeder, and K. O. (1974). The extension of the Lycian nappes Schmidt) pp. 455-77, E. Schweizerbart'sche (SW Turkey) into the Southeastern Aegean Verlag, Stuttgart. islands. Eclogae Geol. Helv. 67, 4-90. EVIRGEN, M. and ATAMAN, G. (1981). Study of meta- BONNEAU, M. (1972). Existence d'un lambeau de morphism of the central Menderes Massif: cristallin chevauchant sur la s6rie du Pinde en isograds, pressure and temperature. Yebilimleri Cr6te moyenne (Gr6ce). C. R. Acad. Sci., Paris (Hacatepe Univ.) 7, 15-26. D274, 2133-36. FALVEY, D. G. (1974). The development of continen- BONNEAU, M. (1973). Les diff6rentes 's6ries ophioliti- tal margins in plate tectonic theory. Austral. J. f6res' de la Cr6te; une mise au point. C. R. Acad. Petrol. Expl. 14, 95-106. Sci., Paris D276, 1249-52. FALVEY, D. G. and M1DDLETON, M. F. (1981). Passive BONNEAU, M. (1982). Evolution g6odynamique de continental margins: evidence for prebreakup l'arc 6g6en depuis le Jurassique sup6rieur deep crustal metamorphic subsidence mechan- jusqu'au Mioc6ne. Bull. Soc. Gdol. Fr. (7), 24, ism. Oceanologica Acta 103-14. 229-42. FLEURY, J.-J. (1970). Sur les modalit6s d'installation BONNEAU, M. (1984). Correlation of the Hellenide du flysch du Pinde au passage Cr6tac6-Eoc~ne nappes in the south-east Aegean and their tec- (Grace continental et P61oponn~se septentrio- tonic reconstruction. In The geological evolution hal). Bull. Soc. Gdol. Fr. (7), 12, 1110-17. High-temperature-low pressure metamorphism 49
FLEURY, J .-J. (1980). Les zones de Gavrovo-Tripolitza KATSlKATSOS, G. (1981). 39Ar/4~ dating of glau- et du Pindos-Olonos (Gr6ce continentale et cophanes and phengites in southern Euboa P61oponn6se du Nord). Evolution d'une plat- (Greece) geodynamic implications. Bull. Soc. forme et d'un bassin dans leur cadre structurale. G(ol. Fr. (7), 23,469-76. Soc. G~ol. Nord, Spec. Publ. 4,651 pp. MARNELIS, P. (1978). Contribution d l'dtude gdolo- HALL, R. (1982). Ophiolites and passive continental gique de l'arc dg~en: Les ties d'Astypalea et de margins. Ofioliti 7, 279-98. Syrna (Dodecanese, GrOce). Th6se 3me cycle, HALL, R. (1984). Ophiolites--figments of oceanic Universit6 de Pierre et Marie Curie, Paris. lithosphere? In Ophiolites and oceanic lithosphere MCKENZIE, D. P. (1978). Some remarks on the (ed. I. G. Gass, S. J. Lippard, and A. W. development of sedimentary basins. Earth Planet. Shelton), pp. 393-403, Spec. Publ. Geol. Soc. Sci. Lett. 40, 25-32. London 13. MIDDLETON, M. F. (1980). A model of intra-cratonic HALL, R., AUDLEY-CHARLES,M. G., and CARTER, D. basin formation, entailing deep crustal metamor- J. (1984). The significance of Crete for the evolu- phism. Geophys. J. Roy. Astron. Soc. 62, 1-14. tion of the Eastern Mediterranean. In The geo- Mua-ri, E., OROMBELLI, G., and PozzI, R. (1970). logical evolution of the eastern Mediterranean (ed. Geological studies on the Dodecanese Islands. J. E. Dixon and A. H. F. Robertson), pp. 499- (Aegean Sea). Geological map of the Island of 516, Spec. Publ. Geol. Soc. London 17. Rhodes (Greece), explanatory notes. Ann. G(ol. HALLAM, A. (1984). Pre-Quaternary sea-level Pays Helleniques 22, 77-226. changes. Ann. Rev. Earth Planet. Sci. 12,205-43. OXBURGH, E. R. (1982). Heterogeneous lithospheric HARBURY, N. A. (1986). The stratigraphy and struc- stretching in the early history of orogenic belts. In tural evolution of the SE Aegean. Ph.D. Thesis, Mountain building processes (ed. K. J. Hsii), pp. University of London. 85-93, Academic Press, New York. HARLAND, W. B., Cox, A. V., LLEWELLYN, P. G., PIPER, D. J. W. and PE-PIPER, G. (1980). Was there a PICKTON, C. A. G., SMITh, A. G., and WALTERS, Western (external) source of terrigenous sedi- R. (1982). A geologic time scale. Cambridge ment for the Pindos zone of the Peloponnese University Press, 131 pp. (Greece)? Neues Jb. Geol. Palaeont. Mh. 2, HussIr~, A. H. (1983). Stratigraphy and sedimen- 107-15. tology of the Pindos Nappe in the Asteroussia REINECKE, T., et al. (1982). Remnants of a Late Ori, Crete. Ph.D. Thesis, University of London. Cretaceous high temperature belt on the island of JACOBSHAGEN, V., Df3RR, ST., KOCKEL, F., KoPP, K.- Anafi (Cyclades, Greece). Neues Jb. Geol. O., KOWALCZYK, G., BERKHEMER, H., and BUT- Palaeont., Abhl. 145,157-82. TNER, D. (1978). Structure and geodynamic ROBERT, U. and BONNEAU, M. (1982). Les basaltes des evolution of the Aegean region. In Alps, Apen- nappes du Pinde et d'Arvi (Crete) et leur signifi- nines, Hellenides (ed. H. Closs, D. Roeder, and cation dans l'6volution g6odynamique de la K. Schmidt), pp. 537-64. E. Schweizerbart'sche Mediterran6e orientale. Ann. Gdol. Pays Hel- Verlag., Stuttgart. leniques 31,373-408. JENKYNS, H. C. and WINTERER, E. L. (1982). Palaeo- ROBERTSON, A. H. F. and DIXON, J. E. (1984). ceanography of Mesozoic ribbon radiolarites. Introduction: aspects of the geological evolution Earth Planet. Sci. Lett. 60, 351-75. of the Eastern Mediterranean. In The geological KOEI't
SEIDEL, E. (1971). Die Pindos-Serie in West-Kreta, Hellenides externes en P61eponn~se m6ridional auf der Insel Gavdos und im Kedros-Gebiet (Mit- (Gr6ce). Soc. Gdol. Nord, Spec. Publ. 6 (574 tel-Kreta). Neues Jb. Geol. Palaeont., Abhl. 137, PP.). 443-60. THOMPSON, A. B. and ENCLAND, P. C. (1984). Press- SEIDEL, E., OKRUSCH, M., KREUZER, H., RASCHKA, ure-temperature-time paths of regional meta- H., and HARRE, W. (1976). Eo-alpine metamor- morphism II. Their inference and interpretation phism in the uppermost unit of the Cretan nappe using mineral assemblages in metamorphic rocks. system--petrology and geochronology. Part 1. J. Petrol. 25,929-55. The Lendas area (Asterousia Mountains). Con- THORBECKE, G. (1976). Zur Geologie der Insel Simi/ trib. Mineral. Petrol. 57, 259-75. Griechenland. Neues Jb. Geol. Palaeont., Mh. 1, SEIDEL, E., OKRUSCH, M., KREUZER, H., RASCHKA, 31-40. H., and HARRE, W. (1981). Eo-alpine metamor- VAIL, R. P., et al. (1977). Seismic stratigraphy and phism in the uppermost unit of the Cretan nappe global changes of sea level. Mere. Am. Assoc. system--petrology and geochronology. Part 2. Petrol. Geol. 26, 49-50. Synopsis of high-temperature metamorphics and WArtS, A. B. and THORNE, J. (1984). Tectonics, global associated ophiolites. Contrib. Mineral. Petrol. changes in sea-level and their relationship to 76,351-61. stratigraphical sequences at the US Atlantic con- SLEEP, N. H. (1971). Thermal effect of the formation tinental margin. Mar. Petrol. Geol. 1,319-39. of Atlantic continental margins by continental WICKHAM, S. M. and OXBURGIa, E. R. (1985). Con- break-up. Geophys. J. Roy. Astron. Soc. 24, tinental rifts as a setting for regional metamor- 325-39. phism. Nature 318,330-3. TSAILA-MONOPOLIS, S. (1977). Micropalaeontological WOODCOCK, N. H. and ROBERTSON, A. H. F. (1984). study of the Tripolitza (Gavrovo) zone in the The structural variety in Tethyan ophiolite ter- Peloponnesus. Geol. Geophys. Res., Inst. Geol. rains. In Ophiolites and oceanic lithosphere (ed. I. Subsurface Res., Athens 20, 1-99. G. Gass, S. J. Lippard, and A. W. Shelton), pp. THIEBAULT, F. (1982). Evolution g6odynamique des 321-30, Spec. Publ. Geol. Soc. London 13.
ROBERT HALL, Department of Geological Sciences, University College London, Gower Street, London WCIE 6BT, UK.