[Geodynamics Series] Alpine‐Mediterranean Geodynamics Volume 7 || Tectonic syntheses of the...

TECTONIC SYNTHESES OF THE

ALPINE-MEDITERRANEAN REGION: A REVIEW

A.G. Smith and N.H. Woodcock

Department of Earth Sciences, Univ. of Cambridge, Downing Street, Cambridge CB2 3EQ, U.K.

Abstract. Criteria used to identify extensional, compressional and transcurrent zones in regional plate tectonic interpretations are briefly reviewed. The principal features of major tectonic syntheses of the Alpine-Mediter- ranean region are outlined. Puzzling aspects of the region that are not adequately accounted for by current models include: large extensional areas contemporaneous with regional compression (e.g. Pannonian basin); the disappearence (by subduction?) of significant amounts of continental crust (e.g. in the Alps); the metamorphism of shallow water continental sequences to blueschist and higher pressure facies (e.g. in the eastern Hellenides); the scarcity of calc- alkaline igneous activity at former compressional plate margins (a general feature of the central and western circum-Mediterranean chains). Previously unpublished computer-drawn maps based on available ocean-floor and paleomagnetic data show the relative positions of stable Africa and stable Europe at their inferred paleolatitudes throughout Mesozoic and Cenozoic time. These or similar maps must constrain future interpretations of the positions through time of continental fragments between the two stable areas. Poor location of these fragments due to present lack of data is the main source of disagreement among current tectonic syntheses.

Introduction

The greatest recent change in tectonic syntheses of the Mediterranean region is the general acceptance that large-scale relative movements have taken place between Africa, Europe and adjacent areas since the beginning of Mesozoic time. Though such movements were postulated over half a century ago by Argand [1924], and later by Carey [1958], they did not then meet with general acceptance.

Evidence for such movements is given by the least-squares fit of the circum-Atlantic continents [Bullard et al., 1965] which requires Africa to have occupied a different position relative to Europe in earlier Mesozoic time than it does now. The history of the movement pat-

tern of Africa relative to Europe from its location on the least-squares fit to its present-day position is recorded in the magnetic anomaly pattern of the Atlantic ocean floor, described by Pitman and Talwani [1972].

Independent evidence for relative movement is provided by land-based paleomagnetic data [Channell et al., 1979; Zijderfeld, this volume]. The available data support the history inferred from the magnetic anomalies and allow Africa and Europe to be repositioned in their original paleolatitudes throughout the period. The paleomagnetic data show that relative movements have taken place among several small fragments lying between Africa and Europe such as parts of Italy, Corsica/Sardinia and Cyprus.

In this paper we shall refer to all Triassic and younger deformation as 'Alpine' in the broad sense, since Mesozoic as well as Cenozoic deformation is widespread in the Alpine chains. The Alpine-Mediterranean region discussed here is the area bounded to the west by the Atlantic Ocean, to the north and south by the stable, rigid areas of Europe and Africa respectively, and to the east by Arabia (Figure 1). We take the eastern limit of the deformed zone discussed in this

review in central Turkey. Prior to the later 1960's, several syntheses

tried to explain the Alpine orogeny in terms of the 'geosynclinal theory'. In contrast to most Paleozoic geosynclinal sequences, the Alpine 'geosyncline', here taken as the sediment body involved in subsequent deformation, is notably thin. Trumpy [1960] coined the term 'lep- togeosyncline' (starved geosyncline) to emphasize this property. Based on his work in western Greece [Aubouin, 1959], Aubouin [1965] suggested that many geosynclines preserved in the circum- Mediterranean chains formed a distinctive couple. One half of the couple was a miogeosyncline free of igneous rocks, the other was a eugeosyncline with abundant igneous rocks.

Subsequent work has demonstrated that some Al- pine eugeosynclines are deformed ocean-floor with associated deep-water sediments (see below, 'Ophiolites'), and that many miogeosynclines are shallower-water passive continental margin se-

Geodynamics Series Alpine-Mediterranean Geodynamics Vol. 7

Copyright American Geophysical Union

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Fig. 1. Schematic tectonic outline of the Alpine system of Europe, modified from the International Geological Map of Europe and the Mediterranean region [1971].

quences (see below, 'Passive Continental Mar- rigid body deformation have created some of the gins'). Together they may record a complete, tectonic features in the region, but it is not though usually strongly deformed original transi- clear what these processes are. tion from continent to ocean. These margin se- This review examines how the basic tectonic quences are supplying valuable data which are elements in any synthesis can be recognized, difficult or impossible to obtain from comparable outlines the tectonic syntheses themselves, and present-day passive margins because of the great identifies some of the outstanding problems. New expense of deep drilling and seismic profiling. paleocontinental maps are presented showing the Their evolution is shown schematically in relative positions of Africa and Europe based on (Figures 2a-d). published ocean-floor and paleomagnetic data

Because Europe and Africa, outside the Alpine (Figures 7-16). New syntheses could start with orogenic belts, have behaved as parts of rigid such maps, show the tectonic elements that were bodies or plates, more recent tectonic syntheses important at the time concerned and adjust them have generally interpreted the evolution of the in the light of this information. We have not region in terms of plate tectonics. The steps attempted to make new syntheses but merely needed for a plate tectonic synthesis are: provide a review and comment on existing syn-

1) location of the present-day positions of the theses. Readers seeking new solutions to the ancient plate boundaries of a particular period.

2) repositioning of those boundaries on the past geography.

3) estimation of the tectonic rotation poles and angular velocity vectors at the plate boundaries.

Despite considerable international effort, none of these steps has been completed for any past

tectonic problems of the region will therefore be disappointed and are referred to other papers in this volume and recent compilations by Biju- Duval and Montadert [1977], Nairn et al. [1977, 1978] and Closs et al. [1978].

Because several recent publications review the evolution of the Mediterranean basins, their history is only briefly sketched here. The emphasis in this review is mostly on the mountain

period of Alpine history. Partly this is due to belts around the Mediterranean, particularly the lack of basic field data. More fundamen- those in the eastern half. Those around the

tally, it is clear that processes other than western half are not treated in as much detail.

16 SMITH AND WOODCOCK

A B C I I I

1 continental crust

Extended continental crust sedimentary basin

Passive continental margin • Ocean floor --- -- •1 - A B g e

•'m•ogeosynchne ' • --'eugeosynchne '-- 'Alpine geosynchne '

_ • Deformed 'AIp•ne• geosynchne' "1

4 •••,.•,••argin 2

l•"4os/o4e/. e Fig. 2. (a)-(d) show the evolution of an idealized Alpine geosyncline and orogenic belt. (a=l) is the initial stage of normal continental crust. In (b=2) it is extended. After cooling its central region sinks to oceanic depth. Further stretching (c=3) creates two passive continental margins, one of which is shown. The sequences deposited on the margin form a 'miogeosyncline', passing laterally into a 'eugeosyncline' on ocean floor rocks. A subduction zone is created and eventually collision takes place (d=4). The thinned continental crust of one margin overrides that of the other, creating an orogenic belt. Some or all of the tectonic elements present prior to deformation may be recognizable in the deformed zone.

Critical Tectonic Elements

Areas of Thin Crust

Extensional Areas. We assume that most areas

that have crustal thicknesses significantly less than continental crust at sea level, about 35 km, have been formed by extension. The possibility of thinning the crust by some other mechanism is discussed below ('Thinning Without Extension').

That the continental crust is capable of thinning by extension has been realised for some time [e.g. Beck and Lehner,1974; Helwig,1976], but only recently has a simple, quantitative model been proposed to explain the reflection and refraction data obtained from some passive continental margins [Le Pichon and Sibuet, 1980].

In essence, the continental crust can extend by

necking until a central region attains 2-3 times its original length, and half to one third of its original thickness (Figures 2a-b). The upper third of this zone is brittle, breaking up into fault blocks bounded by curved (=listtic) normal faults, whereas the lower two thirds seems to behave in a ductile manner. The total width of

the extended zone may range up to 300 km. The crust cannot extend further without a well-

defined fracture forming, along which ocean-floor spreading starts (Figure 2c). Two continental margins, each about 150 km wide, then border a new ocean. Of course, the formation of a spreading ridge may start if the extended zone is less than 300 km wide, and in some cases the extended zone may be wider than 300 km before a well-defined fracture forms. For illustrative purposes a value of 300 km is taken here as

representative of the width of the extended zone, that is, 150 km on each margin. The thinned continental lithosphere is now hot; as it cools the thinnest part of the continental crust can subside to 4 km or more, reaching depths that are normally regarded as diagnostic of ocean floor.

The evidence for this model is best seen where later post-extensional sediments are thin [Montadert et al., 1979]. Where seismic results are inadequate, the nature of a passive margin, particularly the nature of the crust in water depths of 2 km or more, is open to dispute. Those areas of the region in which the continental crust has most probably been thinned by extension are shown in (Figure 20).

If the continental crust is extended without

ever reaching the ocean ridge stage, the subsequent cooling of the extended lithosphere will cause subsidence and the area will become a

potential sedimentary basin (Figures 2b,3a). The subsidence history of such basins has been cal- culated [McKenzie, 1978a].

Thinning Without Extension. Thinning without extension can theoretically take place in at least two ways. In the first, the bulk of the continental crust is chemically transformed into rock whose seismic properties resemble a typical oceanic refraction seismic section, with the Moho some 10 km or so below the surface. The chemical transformations required for such 'oceanization' have never been satisfactorily described in terms of the mineral and chemical changes required, nor observed where ophiolites have been emplaced onto adjacent continents. We do not consider this process to be significant, if it indeed exists. In some areas, volcanics believed to have formed at the beginning of the opening of an ocean have incorporated blocks of continental material in them, ranging up to a few km in size [Robertson and Woodcock, 1980]. This is not a type of oceanization, but a type of extension in which new material has been added over a wide area, rather than along a narrow, well-defined zone.

In the second process, the lower continental crust is envisaged as being replaced physically by upper mantle [Laubscher, 1971b]. For this

TECTONIC SYNTHESES 17

•-Interpretation difficult

L•Extension: ,• ,•, continent ocean

__•.•..•.•.--'""•--•k into mantle• No extension

old, dense lithosphere

__• •• fraction of the volume required to excavate the •• • basins Furthermore, recent data suggest that, No extension ø

asthenospheric diapir

Fig. 3. (a=5), (b=6) and (c:7) create identical sedimentary basins. (a) forms by extension; (b) by the dragging off of large pieces of the lower crust; (c) by a mantle diapir that sweeps crust laterally away from under the basin site. Only by a detailed examination of the structure of the crust under the basin or in adjacent regions can the mechanism responsible be determined.

sion had taken place at the continental margin [e.g. Smith and Eaton, 1978]. This province is also a zone in which the crustal thinning has taken place mostly by extension, rather than subcrustal transfer alone. No such zones of

thinned/thickened crust are known to be as-

sociated with present-day rift valleys or actively spreading zones. The available evidence suggests that areas of continental crust formed by a thinning/thickening process will be contemporaneous with or follow directly on a period of subduction.

Schuiling [1969] envisages thinning taking place by erosion of 20 km or so of continental crust following uplift and anomalous heating of the mantle. After erosion the crust subsides to

become a deep water basin. Though clastic sources appear to have lain in what is now the deep water part of the western Mediterranean [De Booy, 1969], the volume observed is only a small

to occur, the lower continental crust must be firmly attached to the underlying upper mantle and thin enough so that the whole can sink into the asthenosphere (Figure 3b). In effect it is stoping on a gigantic scale. A thickness of 10 km might be subducted by such a process [Le Pichon et al., 1976]. The model has been applied to the creation of the western Mediterranean

basins. However, the subsequent partial melting of such a large volume of continental crust should give rise to very large volumes of silicic magma. Though silicic volcanism of the appropriate age is widespread in the western Mediterranean [Wezel, 1977], the volumes observed are much smaller than would be expected on the mega-stoping mechanism.

Some thinning models envisage a sideways transport of the continental crust from the thinned zone, perhaps by an uprising mantle diapir (Figure 3c), [Van Bemmelen, 1972, 1973; Stegena et al., 1975]. In such models, there need be no lateral separation of the continental crust. The subsidence effects in the thinned zone will

be similar to those of a passive margin, but on its edges contemporaneous mountain belts will form. To cause lateral transport the lithosphere/asthenosphere boundary must rise up toward the Moho. The source of the heat re-

quired is a major problem. The Basin and Range province in the western United States seems to have started to form after a ridge-trench colli-

rather than foundering, these sources still exist at the surface, having been rotated to other areas without having been eroded to the extent required. We consider this process to be of minor importance in the formation of any thinned crust in the region.

Comparisons Between Thinning Mechanisms. The effects of the two basin-forming processes, passive extension and subcrustal transfer, will differ in shape and timing. Passively extended basins will tend to be elongate perpendicular to the stretching direction and not preceded by subduction, whereas those formed by subcrustal transfer due to diapirism could be subcircular, bordered on most of their edges by contemporaneous mountains and contemporaneous with or just following a period of subduction.

Basins in the Alpine-Mediterranean region that may have formed by the thinning of continental crust include the Pannonian Basin of Hungary [Stegena et al., 1975] and the Aegean region [McKenzie, 1978b; Le Pichon and Angelier, 1979; Mercier et al., 1979]. Their shapes, association with subduction and adjacent mountains support a subcrustal transfer origin, similar to that envisaged by Van Bemmelen [1972, 1973], though abundant normal faults in the Aegean show that some extension must have taken place as well. Obviously, if subduction is taking place at the same time as the basin is being extended, the zone will be a type of back-arc basin in which ocean-floor spreading need not begin until the basin itself has been stretched by 300 km or more.

In all the above cases: passive continental margin, passively stretched basin and basin above a zone of subcrustal transfer, at no time during the thinning phase need there be a well-defined plate boundary. For example, shallow seismicity is widely distributed in the Aegean region [McKenzie, 1978b] and plate boundaries are poorly defined. Rigid body deformation is therefore

not taking place. (See below, 'Recent Tec- tonics' ).

However, outside these deforming zones the lithosphere is rigid. Relative motion can be described in terms of Euler poles and angles. Once deformation has ceased, rigid body deformation applies to the thinned zone as well. Only during active extension is the description messy. Of course, if subcrustal transfer takes place without deforming the surface, the area will remain a plate throughout the thinning phase. The current intense interest in these actively thinning areas may overemphasize the problems they pose in terms of plate tectonics. In our view, it ought to be possible to work back to the state prior to the commencement of extension which, on the scale at which it is appropriate to make plate tectonic reconstructions, will have relatively small errors [e.g. Le Pichon & Angelier, 1979].

Ophiolites. That ophiolites are tectonically emplaced slices of extensionally generated mafic/ultramafic lithosphere, probably some form of ocean floor, is now generally accepted [e.g. Coleman 1977]. They must therefore represent former extensional regions whose origins can probably be described in terms of rigid body motions. Though their general distribution in the Mediterranean is well known (Figure 4), their crystallization ages are poorly known and their precise environment of origin is uncertain. Some may be slices of 'normal' ocean-floor, but many examples have a chemistry similar to back- arc basin crust or even to parts of island arcs [Pearce, in press]. The main problem with repositioning these ancient extensional plate boundaries lies in locating the root zones of the ophiolites. Upper estimates of 1200 km of shortening have been given for the pelagic and continental margin rocks associated with the Oman ophiolite [Glennie et al., 1974], implying a similar transport for the ophiolite itself, but their lower estimate of 400 km seems more

probable. Nevertheless, there are several major ophiolite occurrences in the region whose root zones are uncertain by a few hundred km. (See 'East Mediterranean allochthons').

Passive Continental Margins. Indirect but strong evidence of ancient extensional areas is provided by old passive continental margin assem- blages (see below 'Tethyan Continental Margins and Faunas'). These areas show an originally contemporaneous transition, now tectonically dis- rupted, from shallow water sediments deposited on an old felsic continental basement passing laterally into clastic/hemipelagic deeper-water facies deposited on a variable mafic/felsic basement to pelagic facies on newer mafic 'oceanic' crust. Many of these margins originated in Per- mo-Triassic time and some continued into the

Cenozoic. Although the margins have not been extended much, they bordered oceanic areas of in- determinate extent. The maximum width at any one time of these oceans is given by the gap

between stable Africa and stable Eurasia on the

maps (Figures 7-16).

Transcurrent Zones

The Mediterranean region contains a number of major, active strike-slip zones recognisable by their surface expression and seismic charac- teristics. The best known of these are the North

Anatolian Fault [Sengot, 1979a, this volume], the East Anatolian Fault [McKenzie, 1976] and the Dead Sea Rift [e.g. Freund et al., 1970]. An- cient, inactive, possibly deeply eroded examples are difficult to diagnose. Late dip-slip motion may dominate local structural style as on the In- subtic Line in the western Alps [Johnson, 1973], thereby camouflaging the earlier strike-slip displacement. This displacement can be deduced only from regional relationships [Gansser, 1968; Laubscher, 1971a].

Few thrust belts in any orogenic system are purely compressive: most will contain a strike-slip component, to give 'transpressive' movement [Hatland,1971]. In the Alpine region, in the broad sense, important strike-slip components may exist in several belts. For example, the Antalya Complex of southwest Turkey is now interpreted as a strike-slip belt with thrust components [Woodcock and Robertson, in press, Robertson & Woodcock, 1980]. In complexly deformed zones, the interpretation of ancient transcurrent zones as 'transforms', implying parallelism with the contemporaneous displacement vector between two large rigid plates, is hazardous. For instance, the Anatolian faults probably originated at a high angle to the contemporaneous convergence vector of Arabia and Anatolia and accommodate sideways motion of the Anatolian sliver along the convergent zone [Sengot, 1979a; this volume]. They bound a relatively small 'plate' and, as with extensional zones, the problem with such a zone of strike-slip faulting is to know when a plate tectonic model ceases to be appropriate.

Compressional Zones

Compressional zones are here equated approximately with orogenic belts. Six possible in- dicators of compressional zones are discussed below.

Calc-alkaline Igneous Terrains. The igneous activity above present-day subduction zones varies. In the western Pacific, the volcanism is predominantly submarine tholeiitic lavas. Calc-alkaline basalts and andesites do occur, but are much more common within areas of continental

crust such as the Andes. Continental volcanic

arcs are presumed to pass downward into granitic batholiths, as in the western United States. Oceanic island arcs may be underlain by mafic in- trusives. In the Mediterranean region there are two active island arcs: •he Aegean and the Calabrian arcs. The Aegean arc is predominantly

# •e

b• 0

calc-alkaline with some alkalic volcanism but the

Calabrian arc is mostly alkaline [Ninkovitch and Hays, 1972]. The distribution of all forms of Cenozoic volcanism is shown on Figure 5.

One of the peculiarities of the Alpine-Mediter- ranean region is that many compressional zones suggested by low temperature/high pressure and high temperature/high pressure metamorphism, by folding and thrusting and by ophiolite emplacement lack significant calc-alkaline igneous activity. For example, there are very few synorogenic andesites and granites in the Alps. In such areas the position and dip direction of any subduction zone may be uncertain, and the variation of potassium with composition used to determine dip values of the associated subduction zone cannot be applied [Dickinson, 1970].

This scarcity of synorogenic calc-alkaline igneous activity can be speculatively attributed to a number of causes among which are: the small size of the subducted oceans; the soaking up of available water from the sinking slab by serpen- tinization of the upper mantle; compression preventing the uprise of magmas; the lack of a wedge of asthenospheric material above the sub- ducting slab [Barazangi and Isacks, 1976]; or the dominance of transcurrent rather than com-

pressive deformation. Regional Metamorphic Belts. Regional metamor-

phism of 'Alpine' (=Triassic and younger) age is widespread [e.g. Metamorphic Map of Europe, 1973]. However, the authors have not been able to extract from this map or from reviews a sum- mary map showing the distribution of Alpine regional metamoprhism. These metamorphic belts are assumed to lie near former subduction zones.

Of particular interest are the high pressure blueschist and eclogite facies, generally regarded as characteristic of exhumed subduction zones [Ernst, 1977]. The Alpine-Mediterranean region is abnormal in that many of the original rocks from which the blueschists have been made

include shallow water sediments deposited on continental crust (e.g. Pelagonian zone, Greece; Voltri Group, Apennines) rather than the more common ocean floor and trench sediments of cir-

cum-Pacific examples [Ernst, 1977]. Under favourable circumstances the lateral changes in the metamorphic facies can be used to infer the direction of subduction zone dip, as in the Alps [Ernst, 1973; Hawkesworth et al., 1975].

Fold and/or Thrust Belts. The distribution and

probable age of folding in the region is shown in Figure 6. Horizontal transport in some circum-Mediterranean orogenic zones exceeds several tens of kilometers and at times, as in the Alps, may be a few hundred kilometers [e.g. Laubscher, 1971a, b]. Palinspastic reconstructions show that the sedimentary cover, now piled up as thrust sheets and folds, cannot be restored to a visible basement. While this absence might be attributed to subduction of oceanic crust, some sediments suggest a continental basement. Thus in the Alps a few hundred kilometers of con-

tinental crust may have been subducted, or alter- natively overridden by a second slice of continent [Helwig, 1976; Laubscher and Bernoulli, 1977]. (By subduction, we mean the overriding of one plate by another, without any restriction on how much overriding occurs.) If, as Helwig speculated, and recent data suggest, the continental crust at passive margins is thinned by extension, then during subsequent collision, one might need about 150 km of subduction to restore the crust to its original form (Figure 2d). Only after such restoration would the crust start to

increase beyond its normal thickness. The relationship of thrust sense to the dip

direction of subduction zones is poorly under- stood. Some authors favour a 'synthetic' relationship, where thrusting parallels and has the same sense as the inferred subduction zone [Dewey and Bird, 1970]. We consider this the most plausible interpretation of thrusting in collision belts. For example, this would give a south-dipping subduction zone in the Alps in agreement with the independent evidence of the metamorphic facies variations (see above). However, entirely different considerations lead other authors to postulate a north-dipping, antithetic, subduction zone [e.g. Hsu & Schlanger, 1971; Oxburgh, 1972; Oxburgh and Turcotte, 1974]. Antithetic thrusting undoubtedly occurs in non-collisional belts such as the Canadian

Rockies, but its origin has been speculatively attributed to batholithic emplacement [Smith, in press].

As a result of these uncertainties most Alpine chains in the region have been interpreted by a variety of plate tectonic models, invoking both synthetic and antithetic thrusting. The problem of interpretation is further complicated by the readiness with which some geologists envisage an active subduction zone to suddenly cease and start up, or 'flip', in the opposite direction. Flipping seems to us a geometrical device that may not always have had a basis in reality. When flipping is regarded as a possibility, when thrusts can be synthetic or antithetic, and when opposing thrust directions are present, as in a section from Corsica to the Apennines, the variety of possible interpretations of the dip direction and number of subduction zones in-

creases accordingly (e.g. compare Boccaletti et al., [1974] with Laubscher [1971b]).

Of more fundamental significance is the possibility that processes other than subduction have created some of the Alpine chains. For example, the Betic-Rif arc (Figure 1) would appear to require two adjacent, contemporaneous subduction zones dipping in the opposite directions during its formation, which appears unlikely. The remarkable symmetry of the gravity field [Bonini et al., 1973] implies a symmetric deformation mechanism that is not readily explained by rigid body motions.

Similar geometrical problems arise in trying to account for the shape of the Carpathian arc

A a, es of

++.• Pre- Alpine / Cretaceous • • • • Paleogene ""'" '"'• late Cenozoic • offshore Recent

Fig. 6. Distribution and age of folding in the Alpine system, from Wunderlich [1969].

(Figure 1) by rigid body motions. Because this arc is contemporaneous with the development of the Pannonian Basin, and because the Basin itself may have originated by extension and/or thinning, it could be argued that the Betic-Rif arc has been created by a similar process, in which the crust has been thinned so much as to simulate or

actually produce an oceanic seismic section. If some form of mantle upwelling or doming cannot be precluded as possible causes of such chains, it becomes important to find criteria to distinguish between such chains and those attributable to subduction.

Emplaced Ophiolites. Though of extensional origin, ophiolites require some form of compressional or transcurrent activity for their emplacement. Some authors believe that ophiolites cannot be emplaced without some form of collision between island arcs and/or continents. However, strong evidence exists for pre-collision emplacement of some ophiolites, for example those emplaced in the Maastrichtian in the east Mediter- ranean and Middle East [Stoneley, 1975]. Sug- gested emplacement mechanisms include uplift and thrusting of the arc-trench gap, perhaps during initiation of subduction [Smith and Woodcock, 1976], gravity sliding from an active ridge close to a continental margin [Osmaston, 1977], though

this has not occurred anywhere along the Pacific margin of the United States during the approach and subsequent annihilation of the East Pacific Rise; and isolation and uplift of oceanic crust by transcurrent faults bordering continental margins [Robertson and Woodcock, 1980].

In the region under review, ophiolites commonly lie tectonically on top of Mesozoic carbonate platforms. Between the platform and the ophiolites is a telescoped continental margin sequence. When restored palinspastically the thrust sheets show an ordered lateral sedimentary transition from a carbonate platform, via a slope and basin sequence to ocean-floor schematically shown in (Figure 2b). We believe that where such an ordered transition can be demonstrated, that the ophiolites themselves are slices of the ocean floor that originally lay closest to the continental margin. As such, they will be the oldest ocean-floor; cannot have been pushed across the subduction zone as a tectonic 'flake'

[e.g. Coleman, 1977]; are likely to have been first emplaced during the earliest compressional phase to affect the orogen; and could be expected to be geochemically different from the ocean-floor further out in the basin, as is commonly observed (see above, 'Ophiolites'). Where the ophiolites have been deformed again by a

later orogenic phase, these simple relationships may have been obscured.

Melanges. Melanges have been widely interpreted as the products of tectonic fragmentation and mixing in subduction zones [e.g. Hsu, 1974; Maxwell, 1974]. There is now an increasing realisation that many melanges are primarily sedimentary deposits [e.g. Page, 1978] and, more important, that many have no direct relationship with subduction zones [e.g. Swarbrick and Naylot, 1980]. Moreover remapping of some ophiolitic melanges has revealed coherent stratigraphic relations between supposedly exotic lithologies [e.g. SW Cyprus; Lapierre, 1975; Robertson and Woodcock, 1979]. Indiscriminate interpretation of all melanges as subduction zone sequences will lead to some erroneous tectonic models.

Flysch. Flysch was originally defined in the Swiss Alps [Tr•npy, 1960; Hsu, 1970]. Essen- tially it is a synorogenic detrital sequence, typically deposited in deep water troughs in front of advancing nappes. The term has been applied to similar sequences in many other Al- pine-Mediterranean chains. In these sequences the early flysch is deposited in the internal zones of the orogenic belt. The flysch troughs then migrate systematically outward from the core of the chain and earlier flysch may be redeposited as it is deformed and incorporated into younger flysch. In many cases, trough migration appears to reflect the effects of continental collision, in turn caused by subduction.

The term flysch has been applied not only to sequences similar to the original rocks in the Swiss Alps, but also to detrital sequences on present-day passive continental slopes and rises, with considerable resulting confusion. Sedimen- tologists tend now not to use the term, preferring to recognize specific sedimentary en- vironments such as prodelta slopes, deep-sea fans, basin plains, trenches and the like [Stanley and Kelling, 1978, p.381]. But in its restricted sense of a synorogenic clastic sequence, flysch is a hallmark of Alpine orogenesis, whose fundamental tectonic significance may vary from chain to chain.

Factual Framework

Present-day Distributions of Tectonic Elements

The approximate distributions on the present- day geography of Mesozoic ophiolites, Cenozoic volcanism and Mesozoic/Cenozoic folding are shown on Figures 4, 5 and 6. Such compilations provide a starting point for tectonic syntheses of the region. However they should only be used only with full awareness of the many problems of iden- tifying ancient tectonic elements; the fact that more recent data may be available and that some important tectonic elements such as regional metamorphism and plutonism have been omitted al- together.

Relative Positions of Europe and Africa

Some tectonic syntheses show the relative positions of Europe and Africa with increasing vagueness back through time. Yet these relative positions are well known from the Atlantic ocean- floor data [Pitman and Talwani, 1972]. Therefore it seems preferable to show these relative positions precisely because they provide the next step for all tectonic syntheses.

A series of new paleocontinental maps has been made showing the relative positions of Africa and Europe and their inferred paleolatitudes (Figures 7-16). These maps are drawn by computer from published ocean-floor spreading and continental paleomagnetic data. Subsequent modifications to Pitman and Talwani's Atlantic ocean-floor

spreading data have been made by Barret and Keen [1976], Hayes and Rabinowitz [1975], Kristoffer- son and Talwani [1977], Le Pichon et al. [1977], Talwani and Eldholm [1977] and Williams [1975]. The positions of the smaller areas between the rigid parts of Europe and Africa are not well constrained by paleomagnetic data, and their ar-

20øW 60øE 50øh

Fig. 7. Present. The maps in Figs 7-16 are 'win- dows' spanning a paleolatitude range of 0#o$ to 50#o$, and a palcolongitude strip of 80#o$ of cylindrical equidistant maps in Smith et al. [in press]. They are made automatically in two stages. In the first, the finite rotations required to produce a continental reassembly for the time concerned are found from a file of

available ocean-floor spreading data; the second stage orients the reassembly using a file of paleomagnetic poles from the stable continental areas so that the mean pole is the geographic pole of the map. The maps show the present-day coastlines and 1000 meter (= 500 fathom) submarine contours. The positions of stable Africa and stable Europe are relatively well known. The positions of all the small intervening continental fragments are uncertain and in many cases arbitrary. The reader is free to move them to whatever positions he feels are consistent with the available data. The motion of Africa rela-

tive to Europe is determined entirely by the Atlantic-floor spreading data.

Fig. 8. Early Miocene. The space along the Dead Sea north to Turkey may not have had the shape shown. Corsica/Sardinia nearly joined to France/Spain; Balearics nearly joined to Spain, according to position of Smith [1971].

rangement on the maps is speculative. (See also map captions and [Smith et al., in press]).

Tectonic Syntheses

Introduction

We now attempt to review available tectonic syntheses of the Mediterranean region from three viewpoints. We list firstly the most accessible syntheses; secondly, we examine the changing methods of synthesis; thirdly, we assess the degree of concensus about different parts of the Mediterranean puzzle.

The list of published syntheses (Table 1) is not comprehensive. We have concentrated on accessible publications of the last decade, and have arbitrarily included only papers giving the areal development through time of a substantial

50øN 20øW 60 ø

Fig. 9. Late Eocene. Red Sea closed; Arabia joined to Africa according to rotation in Smith and Hallam [1970]; Corsica/Sardinia joined to France/Spain in position of Smith [1971], which may be erroneous.

Fig. 10. Paleouene.

portion of the region. Specifically excluded are many plate tectonic interpretations in vertical sectional view only, and single-frame palinspastic reconstructions. Many important con- tributions are thereby omitted, though they may be referred to in other sections of this paper. We hope nevertheless that the list will provide a few stepping stones through the morass of Tethyan tectonics.

Methods of Synthesis

Schematic syntheses. Wegener [1924,p.53] wrote of the 'book-like opening of the Bay of Biscay'. It was discussed in greater detail by Argand [1924]. The rotation of Spain away from France had opened the Bay of Biscay. Prior to rotation he envisaged that Corsica and Sardinia had been joined to northeast Spain and southwest France, with Italy in turn joined onto them. Their dif- ferential rotation since late Oligocene time was believed to have opened the Atlantic Ocean and created the western Mediterranean. He also en-

visaged that the collision of the Adriatic and Arabian promontories on the edge of Africa with Europe had created the Alpine fold belt (Figure 17). Argand regarded areas of intermediate water

Fig. 11. Late Cretaceous (Santonian)

Fig. 12. Late Cretaceous (Cenomanian). Bay of Biscay closed. Position of Spain against France and Newfoundland from Bullard, Everett and Smith [1965].

depth as thinned 'sal' (=sial), with the deepest areas as still thinner sial or 'trous de sima'

(=holes of sima), [1924, P.358]. It is not clear what the guiding principles were for these insights. Though they would be criticized today mainly on the grounds of timing, they are fundamental to any tectonic interpretation.

Carey [1958, p.191-192] believed that many sinuous orogenic belts had been formed by the bending of an originally straight chain. He named such chains 'oroclines', and the wedge- shaped spaces between two subsequently separated stable blocks as 'sphenochasms' [p. 193]. Carey found that by straightening the Alpine oroclines and closing the sphenochasms 'the Tethys has ap- peared unsought as if by a rub on Aladdin's lamp' [p.251]. His configuration prior to the Alpine deformation is similar to Argand's, but the continental fragments of Spain, Corsica, Sardinia and Italy are rigid and do not deform internally (Figure 18). In subsequent deformation the Ligurian and Tyrrhenian 'sphenochasms' form, the Tethys disappears and oroclinal bending creates the sinuous Alpine chains. Carey sought an ex-

Fig. 13. Early Cretaceous (Hauterivian).

20•W 60ø1

W , / , / , Fig. 14. Late Jurassic (Tithonian).

planation for these and other global tectonic movements in an expanding Earth, a hypothesis supported in Owen's [1976] global analysis, but contrary to paleomagnetic evidence [McElhinny, Taylor and Stevenson, 1978].

Nevertheless, Carey highlights one of the major problems of the region: were the sinuous orogenic belts originally straight or do they reflect the original shapes of the fragments that have collided? We discuss the problem no further in this review.

Rigid body syntheses. One principle implicit in these early syntheses is that continental area is approximately conserved. With the advent of plate tectonics it was natural to consider the relative movement of blocks of constant shape as well as constant area [Dercourt, 1970; Hsu, 1971; Andrieux et al., 1971]. There are considerable advantages in this method. The region is small enough to be regarded as flat to a first approx- imation. A synthesis consists of moving the continental fragments in time so that they produce a reasonably consistent agreement between observation and implication. It resembles a jigsaw puzzle in which the continental pieces change positions but not shapes (unless by at- tachment or fragmentation). Preservation of

20øW 60øE

Fig. 15. Mid Jurassic (Callovian).

20øW 60øE

•øø•1• •L..•"' f•/ I • •'• •' 18o

tion of Turkey, Greece and Yugoslavia is visual. The fit of Africa against North America is that of Le Pichon, Sibuet and Francheteau [1977], which places Africa further from North America than does the fit of Bullard, Everett and Smith [1965].

shape means that it is easy to recognize an area as it changes its position in time. This allows relevant data to be rapidly plotted on the reconstructions, giving a vivid picture of the movements in the region [Bosellini and Hsu, 1973]. Finally, such reconstructions avoid the problems of finding consistent plate boundaries, though they imply what these boundaries must have been. It is really continental drift with rigidity and conservation of continental areas.

A similar approach was applied rigorously by Smith [1971]. He attempted to answer the following questions' could the initial fit of the continental fragments around the Mediterranean be discovered by least-squares fitting of their continental edges? Was the resulting reconstruction consistent with the geology, particularly during breakup? Could the region evolve from its initial geometry to its present-day shape •.,.• the action of a single but variably positioned platr boundary between Europe and Africa?

The computed least-squares fit quantified Ar- gand's and Carey's reconstructions in the western Mediterranean and extended them to include areas

to the east. It is not geometrically unique and pushed the fitting technique devised by Everett [in Bullard et al., 1965] beyond its proper limits. Nevertheless, it was possible to derive a •elf-consistent plate boundary for the initial break-up, whose rotation pole lay close to the initial opening pole of the central Atlantic [Pitman and Talwani, 1972; Smith, 1972]. Whether this reassembly could evolve into its present geometry required a knowledge of the Atlantic spreading anomalies which was not then available. However, a fixed pole opening model simulated a pattern of movement that was not very different from the subsequent survey. The model also suggested that the main phases in Alpine

development were directly attributable to changes in Atlantic spreading history. No possible plate boundary compatible with the opening model and with the initial reassembly could transport continental fragments such as Corsica and Sardinia to their present-day positions unless they had behaved as independent plates, a result that might have been expected from the complexity of the region. It would be worth reexamining the approach in the light of new data.

In Hsu's and Smith's approach, geological ob- servations are used to check a geometrically derived reconstruction. The opposite approach is exemplified by Laubscher's [1971a, b] 'kinematic inversion' method, which is the clas- sic palinspastic approach set in a plate tectonic context. It recognises that some parts of plates, particularly their boundaries, are not rigid. Laubscher showed how 'undeforming' the structures of the western Alps gives rise to wide gaps representing the sites of now consumed lithosphere. Because this type of reconstruction uses a different data set from that of continen-

tal fits, the two methods are complementary. Un- deforming of orogenic belts remains the only practical way of numerically estimating minimum palaeo-separation distances of continental fragments. The method continues to prove helpful [e.g. Bernoulli and Laubscher, 1972; Laubscher, 1975; Laubscher and Bernoulli, 1977]. It will, however, seriously underestimate shortening where oceanic basement has been subducted without

leaving a full structural record of its displacement, such as a fully scraped off accretionary wedge of oceanic sediment.

Both Hsu and Smith realised the important con- straint on Mediterranean reconstructions provided by Atlantic ocean-floor spreading data. The data of Pitman and Talwani [1972] allowed much more accurate relative positioning of Africa, Spain and Europe through time than before. Dewey et al. [1973] incorporated these constraints into an •nbitious evolutionary tectonic model. Their method differed from previous Mediterranean modelling in emphasising the identification of old plate boundaries from geological evidence, mainly field data. These boundaries define numerous small 'plates' whose displacement history is constrained only by the nature of their boundaries. Whilst this approach incor- porates a formidable quantity of evidence it presents difficulties not encountered in other

methods. It is critically dependent on correct diagnosis of plate boundary nature and position, an exercise which we have shown to be difficult.

The positions of the minor plates are simply 'educated guesses' [Dewey et al. p.3138]. The emphasis on plate boundaries rather than on continental margins leaves the continents as ill- defined shapes on which it is difficult to locate new data. The consequent low 'testability' of the models is a serious drawback.

More recent efforts using this complex 'plate boundary' approach suffer similar drawbacks [e.g.

TABLE 1. Syntheses of Mediterranean tectonics

Date Author Time span Area

1924 Argand 1958 Carey 1970 Dercourt 1971 Andrieux et al. 1971 Hsu 1971b Laubscher 1971 Smith 1972 Bernoulli & Laubscher 1973 Dewey et al. 1974 Alvafez et al. 1974 Boccaletti et al. 1974 Boccaletti & Guazzone 1975 Laubscher 1976 Channell & Horvath 1977 Biju-Duval & Montadert 1977 Biju-Duval et al. 1977 Horvath & Channell 1977 Hsu

1977 Laubscher & Bernoulli 1977 Tapponnier 1979 Dewey & Sengor 1979b Sengot 1979 Vandenberg in press Robertson & Woodcock

Neogene- Recent

late Triassic- Recent

Miocene- Recent

Jurassic- Recent

Cretaceous- Neogene late Triassic- Recent

late Jur. - Neogene late Triassic- Recent

Oligocene- Recent late Jur. - Recent

Cretaceous- Recent

late Triassic - Recent

late Triassic- Neogene Oligocene- Recent

late Triassic- Recent

Jurassic - Recent mid Miocene- Recent

mid Miocene - Recent

Betic/Rif

Alps/N. Apennines Med

Greece

Carpathians/ Balkans W.Med

E. Med

Boccaletti et al., 1974; Biju-Duval et al., 1977]. Indeed future attempts along similar lines may risk being too complicated for any readers to understand, even though their authors coysider that the available evidence can be interpreted in no other way.

Perhaps a more helpful trend in recent modelling is the attempt to work through the con- sequences of a specific kinematic model. This may also provide more useful insights than models that attempt to incorporate all the data. Topics treated in this way have been the possible rotation of Corsica/Sardinia [Alvarez, 1972; Alvarez et al., 1974], the impingement of postulated Adriatic or Arabian promontories of Gondwanaland [Channell and Horvath, 1976; Horvath and Chan- nell, 1977; Tapponnier, 1977; Channell et al., 1979], the westward expulsion of Anatolia [McKenzie, 1978b; Dewey and Sengot 1979; Sengot, 1979a] and the rotation of Cyprus [Robertson and Woodcock, in press].

Comments on Specific Areas

Points of General Agreement. Although many aspects of Mediterranean geology are being actively debated, we begin by listing the major points on which there is now substantial agreement'

(a) A major oceanic area, the Tethys, sensu lato, separated Eurasia and Gondwanaland at the end

of Triassic time (see review by Jenkyns [1980]). This is independently supported by the Jurassic ocean-floor spreading data; by the least-squares fit of the continental edges and by the excellent fit of the late Triassic and early Jurassic paleomagnetic data on the reconstruction.

(b) The present Mediterranean basins are not relics of the Paleotethys (=Paleozoic Tethys), but are later Mesozoic or Cenozoic basins (=Neotethys or Tethys s.s.).

(c) Continental margin sequences and 'Tethyan' (=Paleotethyan and Neotethyan) oceanic basins are found deformed within several Alpine orogens. They show that some 'Tethyan' ocean basins have been partly or wholly destroyed.

(d) Alpine tectonic history in the broad sense spans Mesozoic and Cenozoic time [c.f. Ar- gand, 1924].

(e) The Mesozoic/Cenozoic tectonics of the Mediterranean region have been constrained by, and probably largely controlled by, the relative displacements between Eurasia and Gondwanaland. Changes in the opening poles and opening rates of the central and northern Atlantic are probably reflected in changes in the tectonic pattern of the Alpine-Mediter- ranean region and vice versa.

(f) Some continental fragments between Africa and 'stable' Europe have moved independently of

Fig. 17. The original positions of Spain, Cor- sica, Sardinia and Italy according to Argand [1924, Fig. 26, p.360].

both bounding continents for at least part of post-Palaeozoic time.

(g) There is no simple Tethyan tectonic solution. We now briefly discuss the main controversial

points, starting with those concerning the whole Mediterranean, described approximately forwards through time, then taking geographically more limited problems from west to east.

Late Triassic Reassembly. Although the relative Europe/Iberia/Africa positions through time are now known fairly accurately (e.g. Figures 7-16) and are unlikely to be substantially al- tered by new data, the arrangement of small intervening fragments is still rather arbitrary. Matching of continental edges may have some potential in the western Mediterranean, but major progress awaits new palaeomagnetic data to orien- tate each fragment and refined geological cor- relation between possible severed continental blocks. Comparison of reassemblies in this paper with those by Smith [1971], Dewey et al. [1973], Hsu [1977] and Biju-Duval et al. [1977] reveals the range of schemes possible without these data.

A Mobile Pangea? Some paleocontinental maps show Pangea as a rigid supercontinent undergoing rotation only throughout Permo-Triassic time [e.g. Smith et al., 1973]. This interpretation requires a large wedge-shaped Permian ocean between Gondwanaland and Eurasia, which was equated with the Tethys [Smith, 1971]. Since the 'Tethyan' ophiolites are bordered by Triassic or younger continental margins, they cannot be part of a Permian ocean basin [Smith, 1971, 1973]. Thus the Permian Tethys has disappeared without leaving any trace of its former existence as ophiolites, in the same way as the Pacific is disappearing at the present-day. However, Per- mian and older continental margins should still be detectable, as should the presumed calc- alkaline igneous activity contemporaneous with its disappearence in Triassic or later time.

Smith [1971] pointed out the need to seek the

remnants of the Permian Tethys north of the 'Tethyan' ophiolites. He speculated that closure of this older Tethyan ocean is reflected in the Carpathian-Crimea/Pontide-Greater Caucusus chain, and this line been favoured by, among others, Dewey et al. [1973], Biju-Duval et al. [1977] and Hsu [1977]. Only recently has Sengot [1979b] marshalled convincing evidence of late Triassic to mid-Jurassic subduction along this lineament, but more field data are needed yet before the suture can be accurately defined. It may now be sliced into several segments. The suture need not mark the place where the Paleotethys disappeared, but could mark the suture where Triassic ocean crust has been sub-

ducted shortly after its formation. Owen [1976], Ahmad [1978] and Crawford [1979]

all argue that the geological evidence does not support the large ocean as is implied by the Per- mian reconstruction using a rigid Permo-Triassic Pangea, from which they draw the rather drastic conclusion that the Earth is expanding. Paleomagnetic data acquired in the last decade suggest an alternative view: a mobile Pangea [Irving, 1977]. In essence, in early Permian time the southern and northern continents may have been much closer together so that South America lay south of western Eurasia, rather than of North America (Figure 19). Such a configuration considerably reduces the size of any Permian Tethys in the region, thereby overcoming the main objection noted above. How this Permian Pangea evolved into the late Triassic Pangea is not clear, but a transform zone passing through Pangea close to the northern edge of Gondwanaland is a possible solution [Hurley, unpublished M.Sc. thesis, University of Cambridge].

Southern Continental Margin. Bernoulli [1972] first showed how similar were the facies

penetrated by the DSDP cores on the western Atlantic continental margin to the deformed Mesozoic carbonates in parts of Italy and Greece. Subsequently Bernoulli and Laubscher [1972], Ber- noulli and Jenkyns [1974], Laubscher and Bernoul- li [1977] and Hsu and Bernoulli [1977] have developed this theme, arguing that the thick

• Europe

Africa • I Fig. 18. The original shape of the Tethys and surrounding continents according to Carey [1958, Fig. 31b].

Mesozoic carbonate platforms of the circum- Mediterranean region all lay south of the Mesozoic Tethys and represent deposits on its southern continental margin. By contrast with eastern North America, the passive continental margin deposits of Morocco have been uplifted and exposed. Combined with the DSDP data, the on- shore geology enables a much closer comparison to be made with deformed continental margin deposits nearby [Wiedmann et al., 1978].

According to Channell et al. [1979], these Mesozoic carbonate platforms are elongate, len- ticular areas up to a few hundred kilometers long and about one hundred kilometers wide. They are characterized by syndepositional extensional faulting on their boundaries. The sediments formed in very shallow water, with subsidence rates of up to 100m/Ma in Triassic time, declining to a few m/Ma at the end of Mesozoic. Together with intervening basins, their distribution marks out a continental margin complex on the southern margin of the 'Tethys'

Northern Continental Margin. There was a similar continental margin complex on the northern continental margin, but the complex was less isolated than that to the south and includes

terrigeneous material eroded from the northern continent.

Faunas. In the region reviewed here, maximum total latitudinal extent of any Jurassic oceanic areas is about 1600 km (Figures 14-16). The Jurassic continental margins bordering any such oceans may well have had continuous shallow-water marine connections. Despite their proximity and possible connections, some fossil groups do seem to be restricted to one or other margin [see review by Channell et al., 1979].

Extent and Age of the Southern Continental Margins. The concept of a continuous Mesozoic continental margin complex on the southern Tethyan margin is in dispute only in the eastern Mediterranean, where extensive Mesozoic ophiolites (e.g. Antalya, Troodos) lie south of the 'southern' carbonate massifs [see Laubscher and Bernoulli, 1977, Figure 8 and 'East Mediter- ranean Allochthons' below]. Irrespective of where the ophiolites were rooted, the carbonate platforms provide independent evidence for at least one nearby Mesozoic ocean. Their detailed subsidence history provides a wealth of tectonic data that has yet to be used in any synthesis. Examples of Mesozoic continental margin sequences include: the Apennines [Bernoulli and Jenkyns, 1974; Carmignani et al., 1978]; east central Greece [Smith et al, 1975]; southern Turkey [Delaune-Mayere et al., 1977]; Cyprus [Robertson and Woodcock, 1979].

Analogy with the Atlantic suggests that the subsidence would be most rapid during and shortly after continental breakup. Subsidence was ap- parently most rapid in Triassic time, probably dating the initiation of spreading in many parts of the Mesozoic Tethys. The absence of any proven Triassic Alpine ophiolites, as opposed to

well-dated Triassic lavas of non-ophiolitic af- finities, has led some workers to suggest that significant spreading did not begin until Juras- sic time [e.g. Smith, 1971]. If this is the case, why did the continental margins subside so much in Triassic time? We speculatively suggest that the Triassic continental margins formed along Triassic transform zones created during the transition from the early Permian to late Trias- sic Pangea.

Microplates. A microplate is not necessarily a microcontinent and vice versa. A microplate is a small plate. It must be bounded by active plate margins. When these margins cease to be active the microplate ceases to exist and becomes part of an adjacent plate. A microcontinent is a small continental fragment that need not at any time belong to a microplate. For example, at the present time Baja California is a microcontinent, but it has not been a microplate since it was severed from the American plate: it has merely become part of the Pacific plate.

The exis%ence, rigidity, number, rotations and displacements of possible small fragments between Africa and 'stable' Europe are hotly debated. Setting aside Argand's [1924] and Carey's [1958] semi-ductile fragmentary rearrangement, Dercourt [1970] and Smith [1971] explored the implications of a single Tethyan 'rigid' plate boundary with no microplates. Microplates proliferated rapidly through Hsu's three microplate model to the op- timistic 20 plate scheme of Dewey et al. [1973]. The evidence for a number of independent plates is fairly strong from paleomagnetic data in the West Mediterranean [e.g Alvafez et al., 1974; Boccaletti and Guazzone, 1974; Channell et al., 1979]. Most of the fragments have rotated anticlockwise with respect to Europe (e.g. Spain, Italy, Corsica/Sardinia, western and southern Alps) and often by similar amounts. Significant rotations in the opposite sense have been observed in the Northern Calcareous Alps and parts of the Carpathians [Frisch, 1979, p.131]. The problem of interpreting these data is to decide which,if any, of the fragments have rotated independently with respect to the others and to Africa. Corsica/Sardinia is the fragment most widely believed to have undergone independent rotation [e.g. Alvafez et al., 1974], leaving the Balearic basin in its wake. However, a case against this rotation can still be made [Bernoulli and Laubscher, 1977]. Conversely, the Italian peninsula may have been fixed to Africa, forming Argand's Adriatic promontory for long periods [Hsu, 1971; Channell and Tarling, 1975; Channell and Horvath, 1976; Channell et al., 1979]. Again, this view has been disputed [Dewey et al., 1973; Biju.-Duval et al., 1977].

Further east, evidence f•or microplates is less clear; indeed recent histories of the East

Mediterranean [Dewey and Sengot, 1979; Robertson and Woodcock, in press] allow considerable ductile deformation of 'microplates' in line with interpretations of active tectonics [McKenzie,

1977, 1978b]. Paleomagnetic data from the East Mediterranean are sparse. Only those for Cyprus convincingly demonstrate independent rotation [Vine and Moores, 1969; Moores and Vine, 1971; Lauer and Barry, 1976; Shelton and Gass, in press], by about 90#05 anticlockwise probably during Miocene time. Though established for over a decade, this important rotation has only rarely been incorporated into tectonic reconstructions [Parrot, 1973; Robertson, 1977a, b; Robertson and Woodcock, 1979, in press].

Two important features of a rigid body (=plate tectonic) solution that are commonly overlooked are:

1) at any one time all plate boundaries must form an interconnected self-consistent

network; 2) a single complex boundary that sporadically

jumps from one location to another can produce very complex tectonic effects.

A consequence of (1) is that those plate tectonic 'solutions' in which a plate boundary does not terminate at a triple junction are inad- missible. (The necessity for such boundaries would show that the areas concerned had not

behaved rigidly). Many proposed 'microplates' could be the result of a sudden change in the position of a complex boundary between Africa and Europe. A fragment may have been detached from, say Africa, for a few tens of millions of years and joined to Europe, and then subseqently become part of Africa at a later date. At no time can it be considered as a microplate, though it may well be a microcontinent.

Gondwanaland Promontories. The hypothesis of an Italian/Adriatic promontory to Gondwanaland now implied by the palaeomagnetic data was previously suggested by Argand [1924, p.305]. He believed that its collision with Europe had created the Alps. Despite doubts mentioned in the previous section, this hypothesis has some support [Horvath and Channell, 1977; Tapponnier, 1977] providing a Mediterranean example of 'indentation' tectonics predicted theoretically [Tapponnier and Molnar, 1976] and supposedly observed in the central Asian segment of the Tethyan Belt [e.g. Tapponnier and Molnar, 1977]. A similar mechanical role has been envisaged for an Arabian promontory to Gondwanaland [Argand, 1924; Tapponnier, 1977; Dewey and Sengot, 1979; Robertson and Woodcock, in press; Sengor, 1979a]. Such models also stress the intimate relationship of large strike-slip faults to the indentation process, allowing slivers of continental crust

to move sideways out of the collision zone. Recent Tectonics. The plate model proposed by

McKenzie [1970, 1972] remains the basis for interpretation of active Mediterranean tectonics. It uses the distribution of seismicity and the nature of the associated fault-plane solutions to define the plate boundaries and their nature. Although it has been argued that the recognition of non-rigid deformation may have important implications for the reconstruction of deformed

• Eurasia

••;/ Africa '- • S. America

/•"• The •' / Tethys Gt x

30øS Fig. 19. A reconstruction of Pangea for 240 Ma (late Permian time) that is consistent with the available paleomagnetic data, modified from Smith et al. [in press]. Note the much reduced size of the Tethys. C=Caribbean, I=Italy, G=Greece, T=Turkey. The positions of C,I,G and T are entirely speculative. The transition from this figure to the late Triassic reconstruction of Fig. 16 can be made by a transform zone lying on the northern edge of Gondwanaland.

areas [e.g. McKenzie, 1977, 1978b; Dewey and Sengot, 1979], and that plastic deformation [nay be important [e.g. Tapponnier, 1977; Le Pichon & Angelier, 1979], we believe that rigid plate models may not be so misleading as has been suggested [e.g. McKenzie, 1977]. It is worth noting that all the continental areas in the region that are today deforming 'plastically', or have recently ceased to deform plastically, are all located in areas affected by earlier subduction and within a few hundred kilometers of the in-

ferred surface trace of the subduction zone. Betic-Rif Arc. The Betic-Rif arc includes

deformed Mesozoic rocks. The continuity of the arc has been used to support the view that there has been no significant movement between Africa and Europe since early Mesozoic time. However, the structures in the arc are mostly of later Cenozoic age [Rondeel and Simon, 1974; Choubert and Faure-Muret, 1974]. Their continuity demon- strates the lack of significant movement between Africa and Europe since Oligocene time as shown independently by the Atlantic floor spreading data (Figures 7-9).

West Mediterranean Basins. The Cenozoic age of the Alboran, Balearic and Tyrrhenian basins is now established, but their mechanisms of formation are still in debate. Evidence for a rifted

origin for the Alboran and Balearic Basins is particularly strong [Hsu, 1977] though not une- quivocal [Channell et al., 1979]. Bernoulli and Laubscher [1977] still argue strongly for a vertical foundering origin for the West Mediter- ranean basins (see above 'Thinning Without Exten-

sion'). Hsu [1977] points out that 'foundering' and 'rifting' are parts of a spectrum of basin formation processes, and that their distinction may be more semantic than real.

Structures of the Alps. Little controversy now exists about the major structure of the Alpine chain. It is essentially a pile of north facing nappes. Over half a century ago some geologists attributed their origin to the collision and overriding by 'Africa' of 'Europe'. In plate tectonic terms, the structures result from the long-continued southward-subduction of an ocean and its adjacent continental margin, though northward subduction has been postulated by Ox- burgh [1972] and Oxburgh and Turcotte [1974]. Recent plate models of the eastern Alps suggest that more than •e oceanic basin may have been present in Lower Cretaceous time (Frisch, 1979, Figure 3, p.128). Significant strike-slip motion has been suggested by Laubscher [1971a, b]. The relationship of the Alps to the Apennines is ob- scure, but is discussed by Scholle [1970] and Laubscher [1975].

Aegean and Pannonian Basins. As in the western Mediterranean, both extreme rifting and foundering models have been applied to the Aegean and Pannonian basins. Recent models for the Aegean [McKenzie, 1978a, b; Dewey and Sengot, 1979] favour an extended continental crust. True

ocean-floor spreading is not proposed, though igneous intrusion and extrusion may play a significant part in the process. It is unclear whether the Aegean is a precursor of a true ocean basin or whether it reflects specifically intra- continental processes associated with a nearby subduction zone. As noted above, a stretched continental origin is also supported by recent analysis of the Pannonian Basin [Royden & Sclater, in press], though previously both in- tracontinental [Stegena et al., 1975; Bernoulli and Laubscher, 1977] and oceanic nature [Dewey et al., 1973] have been proposed. Papers dealing with the geology of adjacent regions include those on Romania by Burchfiel [1976, 1980] and Herz and Savu [1974], and Bulgaria by Hsu et al. [1977].

Levantine Basin (=Eastern Mediterranean). This is the most problematic of the Mediterranean basins. Once considered to be a remnant of the

Palaeozoic Tethys, it is now thought to be either a remnant of a Mesozoic ocean basin created south

of the Paleotethys or a subsided portion of thin continental crust. A thick sediment cover com-

plicates seismic interpretation, but these data strongly favour an oceanic or transitional crust in the deeper parts of the basin [see Lort, 1977; Channell et al., 1979 for reviews]. Geophysical and geological evidence suggests an early Mesozoic continental edge beneath the present Levant margin [e.g. Bein and Gvirtzman, 1977] that could have been contemporaneous with spreading in the Levantine basin. Land-based evidence (see below) is also equivocal. The hypothesis of a Mesozoic ocean south of Anatolia

was accepted as recently as 1972 (the Pamphylian Basin of Dumont et al. [1972]), but some recent opinion does not favour it [e.g. Ricou, Ar- gyriadis & Marcoux, 1975]. Side-scan sonar sur- veys show that large areas of the eastern Mediterranean floor are being actively deformed [Stride, Belderson and Kenyon, 1977], and seismic refraction lines [review by Biju-Duval et al., 1977] show active southward thrusting within the Mediterranean ridge.

East Mediterranean Allochthons. A problem common to all Tethyan orogens but particularly acute in the Hellenides and Taurides, is the location of root zones for major allochthonous sheets. Those allochthons that are ophiolitic must root in what were once oceanic areas. One hypothesis suggests that they were all derived from one linear root zone now marked by the Vardar Zone in Greece and continuing eastward through central Anatolia into Iran; along a line that is approximately that of the Tethyan suture (Figure 4). This hypothesis requires that ophiolites of a more southerly alignment, stretching from the Othris zone in Greece through the Antalya Complex (SW Turkey), Cyprus and Hatay (Figure 4), have all been transported southwards over an intervening continental autochthon from an ocean that originally lay north of of the authochthon [e.g. Bernoulli & Laubscher, 1972; Ricou et al., 1975; Vergely, 1975]. The opposing view is that these southern ophiolitic allochthons formed parts of a separate southern Mesozoic ocean basin [e.g. Dumont et al., 1972; Brunn, 1974]. The present Levantine basin could also be a remnant of this

same ocean. In this opposing hypothesis this southern ocean has no place and the Levantine Basin is seen as a foundered bit of the northern

edge of Gondwanaland, contiguous with the continental fragments now between the two ophiolite zones. Brunn [1974] summarised the evidence for

both views and Robertson and Woodcock [in press] give a more recent discussion.

Black Sea. The Black Sea is another enigmatic region. Refraction seismic work shows the central area to have an oceanic seismic structure.

The associated magnetic anomalies have a long wavelength and small amplitude [Ross, 1977] and are of uncertain age. The Crimean region was an orogenic belt throughout much of Jurassic and Cretaceous time, but these structures are now abruptly truncated at the Crimean margin. For this reason, Smith [1971] speculated that the Black Sea was a late Cretacous structure formed

during the dispersal elsewhere of a once-continuous orogenic belt. An alternative view is that the Black Sea is a 'marginal' (=back-arc) basin formed in Jurassic and earlier Cretaceous

time, with partial subduction in later periods [Letouzey et al., 1977].

Discussion.

One reason why the Mesozoic and Cenozoic tectonics of the Alpine-Mediterranean region is so

8:O-IA-CF

Fig. 20. Extensional zones in the circum-Mediterranean region. 1: probably Recent/late Cenozoic; 2: probably mid- to late Cenozoic; 3: probably late Cretaceous/early Cenozoic; 4: possibly Mesozoic; the strip shown lies about 150 km from the 200 m submarine contour; 5: Cenozoic/Mesozoic areas lying more than 150 km offshore from the 200 m submarine contour. These areas are most likely to be oceanic; 6: SCM = Mesozoic southern continental margin complex; 7: NCM = Mesozoic northern continental margin complex; 8: O-IA-CF = ophiolites and pelagic sediments/island arcs/continental fragments. The ophiolites, pelagic sediments and island arcs are mostly Mesozoic in age. Based partly on Channell et al. [1979]. GK = Grand Kabyle; PK = Petit Kabyle; P = Pelitoran massif; C = Calabrian massif: all four are continental slivers which may have migrated southwards from the southern edge of Europe to Africa and Sicily. Arrows show direction of tectonic transport of thrust sheets.

complex is that many continental margins have been created in it and subsequently destroyed. We can assume that most geological complexities will lie near such margins. Thus each kilometer of margin brings into existence a 150 km or so wide strip of potential complexity (Figure 20). The area of potential complexity exceeds 20 per cent of the total area. If methods can be dev-

loped to reconstruct the shapes of these areas

Prouse for drafting the figures. This is Cam- bridge Earth Science Contribution Number ES11.

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