The global tectonic pattern

The global tectonic pattern

JOURNAL OF GEODYNAMICS 12, 21-38 (1990) 21 THE GLOBAL TECTONIC PATTERN CARLO DOGLION1 Dipartimento di Scienze Geologiche e Paleontologiche, Univers...

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JOURNAL OF GEODYNAMICS 12, 21-38 (1990)

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THE GLOBAL TECTONIC PATTERN CARLO DOGLION1

Dipartimento di Scienze Geologiche e Paleontologiche, Universita di Ferrara, Ferrara, Italy (Received February 27, 1990; accepted March 23, 1990)

ABSTRACT Doglioni, C., 1990. The global tectonic pattern. Journal o[+Geodynamics, 12:21 38. The relative motion vectors between the lithosphere and the underlying mantle appear to follow global flow lines which can be constructed by linking axes of extension and compression over the Earth's surface. The flow lines for the last 40 Ma are generally W N W - E S E (E-W), with an undulation of an about 15,000 km wavelength, showing a gradual and progressive variation in orientation. The undulation, which is sharper to the east, may reflect the mantle flow around an unstable rotation axis. The westward motion of the lithospheric plates could be interpreted as a result of differential angular velocity induced by the deceleration of the earth's rotation or, in a toroidal field, by the effects induced by lateral heterogeneities both in the lithosphere and in the mantle. In this light, plate tectonics is a consequence of variable decoupling at the base of the lithosphere as a function of mantle anisotropies. Simply stated, when there is compression or transpression between two plates, it is the eastern plate which moves more rapidly westwards relative to the underlying mantle. If there is extension or transtension, it is the western plate that moves faster westwards. Lithospheric subduction, especially if it dips westward, produces an obstacle to the eastward flow of the mantle. This is referred to as the Nail Effect. The eastward roll-back of the subduction hinge due to the mantle push will generate back-arc extension. Subductions following the mantle flow (E or NE-dipping) are associated to thicker thrust belts with huge exposures of basement rocks in the hinterland and shallow foreland basins. The subductions contrasting the mantle flow (W or SW-dipping) are characterized by shallow thrust belts with deep foreland basin and coeval extension in the back. E-dipping subductions are passive responses to actively thrusting plates: the base plate and intra-lithospheric decollements are connected to the surface and can uplift deep rocks. The W-dipping subductions are enhanced by the eastward mantle flow: the base plate detachments are folded, subducted themselves and never connected to the surface, and only superficial detachments occur at the plate boundaries. First-order tectonic features appear to be perpendicular or slightly oblique to the global flow lines. Second-order tectonic features are related to localized rotations of plates. The model is applied to the Mediterranean area, where there are several observations that are not easily explicable in terms of standard plate-tectonic processes: the eastward or northeastward relative migration of the underlying mantle with respect to an inhomogeneous disrupted lithosphere could explain the tectonic evolution of this area.

1. I N T R O D U C T I O N

Plate tectonics has revolutionized the earth sciences, providing a logical tectonic framework for Wegener's hypothesis of continental drift, and it is 0264-3707/90/$3.00

i~:; 1990 Pergamon Press plc.

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now a widely accepted theory (e.g., Hess, 1962; Vine and Matthews, 1963; Heirtzler, 1968; Condie, 1989). Objections to continental drift and plate tectonics have, for many years, focused on the driving mechanism (e.g., Jeffreys, 1975; Gordon and Jurdy, 1986; Jurdy and Stefanick, 1988; Ricard and Vigny, 1989). Mantle convection was originally considered as the conveyor-belt driving the plate-tectonic process (Elsasser, 1969). The role of upper-mantle convection in plate tectonics remains poorly understood, although it is now generally believed that convective drag on lithospheric plates is not an important driving force in plate tectonics (e.g., Forsyth and Uyeda, 1975; Turcotte and Schubert, 1982). Gravitational body forces produced at subduction zones (slab pull) and oceanic ridges (ridge push) are still considered as the primary driving forces of the plate-tectonic process. However, is not clear why slab pull does not operate to the north of the Tonga Trench, where the Pacific oceanic crust has the same age and thickness, or why it should operate in a different way in the Mediterranean, where old Cretaceous (?) trapped crust is subducting both below Southern Italy and in the Hellenic trench with different dips. On the other hand, in the Red Sea and in the Gulf of Suez, it has been demonstrated that uprising of the mantle post-dates stretching in the lithosphere (Bohannon et al., 1989; Moretti and Ch6net, 1987) and consequently the mantle rise seems to be more a passive isostatic mechanism than the primary driving mechanism. The configuration of upper-mantle convection cells remains unclear due to uncertainties in the values of physical parameters applicable to the upper mantle, and to ambiguities in data derived from SEASAT altimetry and from seismic tomography, which provide potential methods for viewing upper-mantle density heterogeneities. Moreover, insufficient attention has been paid to the general westward drift of the plates (Le Pichon, 1968; Bostrom, 1971). This paper is an alternative working hypothesis discussing plate tectonics, using general geologic and geophysical observations. The physical aspects of this model will be discussed in Sabadini et al. (1990).

2. EART H R O T A T I O N A N D PLATE T E C T O N I C S

The consideration of rotation of the Earth as a driving mechanism for continental drift and plate tectonics has a long history (Wegener, 1915; Bostrom, 1971; Nelson and Temple, 1972; Wilson and Burke, 1972; Moore, 1973; Rutland, 1973; Hargraves and Duncan, 1973; Jordan, 1974; Hamilton, 1979; Uyeda and Kanamori, 1979; Ogniben, 1985). Shells of variable density and rheology wich constitute the Earth (Ranalli and Murphy, 1989) would be expected to show variations in angular velocity about the rotation axis, probably induced by deceleration of the earth's

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rotation or, in a toroidal field, due to lateral heterogeneities both in the lithosphere and in the mantle (Sabadini et al., 1990). An angular velocity contrast between the mantle and the fluid outer core may account for the observed westward drift of centers of secular variation of the geomagnetic field. Similarly, in the presence of a zone of decoupling, a variation in angular velocity would be expected between the lithosphere and the underlying mantle, probably generated by deceleration of the earth's rotation (Knopoff and Leeds, 1972). Variable degrees of decoupling between the lithosphere and the underlying mantle may account for the variable motion of lithospheric plates, and hence provide a primary mechanism for plate motions. Viscosity-related decoupling has been also proposed between upper and lower mantle (Sabadini and Yuen, 1989). The Earth's rotation has largely been ignored in geodynamics, probably because some mountain chains and rifts are oriented at high angles to the rotation axis and their orientation is apparently in conflict with rotationrelated plate tectonics. However, the Earth is a layered imperfect sphere, mainly composed of viscous-fluid material (mantle 1022 23 poises) rotating about a very unstable rotation axis (see Sabadini et al., 1982) and the angular velocity of the lithosphere might be expected to be slightly less than that of the underlying mantle. We note, however, that for Cenozoic rift zones and mountain chains, the extension and shortening axes, respectively, follow well-depicted global flow lines which show a gradual variation in direction (Fig. 1). For example, one such flow line crosses the Aleutian subduction zone with a W N W orientation, crossing North America and the Atlantic with an about E-W orientation, and then the Red Sea rift with SW-NE orientation and returning to an E-W to WNW-ESE orientation in Eastern Asia. The flow line is continuous, showing gradual variation and an undulation with an about 15,000 km wavelength from east Africa to the western Pacific where a sharper bending is shown. The map in Fig. 1 has been compiled on the basis of many papers indicating the directions of plate motions in different areas of the world (i.e. Le Pichon, 1968; Minster and Jordan, 1978; Scotese et al., 1988; Gealey, 1988; Boulin et al., 1988; Platt et al., 1989; Zoback et al., 1989; and references therein): rifts and compressions tend to be perpendicular to the flow lines while transtensive or transpressive areas can be oblique to the overall trend. Axes of tectonic stress which do not follow the flow lines are interpreted as body forces (i.e., in transpressive areas) or induced by local rotations along flow lines, such as the rotation of Iberia to open the Bay of Biscay, or old inactive rifts (e.g., the Benue Basin). The trend of flow lines is also consistent with the main plate motions derived from Satellite Laser Ranging to LAGEOS (Smith et al., 1989; S. Zerbini, pers. comm.). The global pattern of tectonic stress follows a continuity also within plate interiors (Zoback etal., 1989). We

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hypothesize that the flow lines reflect the general motion of the asthenosphere relative to the lithosphere, and this allows us to reconstruct primary tectonic stress trajectories acting on the lithospheric plates as a result of relative motion of the lithosphere and asthenosphere. The major global undulation of the flow lines could be due to the wobble effect generated by the instability of the earth's axis (Fig. 2). Due to the high values of mantle and lithosphere viscosity the flow lines cannot reequilibrate in regular horizontal parallels, like in other less viscous planets (i.e., Jupiter). The local undulations of the stress pattern (e.g., along the western Cordillera) are localized at plate margins and can be interpreted as due to body forces coherent with the global trend of flow lines. The present configuration of the flow lines could be valid at least for the past 40 Ma, since the Pacific hot-spot tracks follow the present direction. The lithosphere has a net westward rotation (Le Pichon, 1968; Bostrom, 1971 ) and the simplest way to explain this effect is to invoke the earth's rotation, as also suggested by the faster plate velocities in equatorial regions (Solomon et al., 1975). The general trends of magmatic activities (hot spots, magmatic arcs, rifts) follow an eastward oriented direction (e.g., Morgan, 1972; Bostrom, 1971), indicating an eastward relative motion of the underlying asthenosphere with respect to the lithosphere (Nelson and Temple, 1972). The eastsoutheastward decrease in age of Hawaiian and other such volcanic centres is consistent with a migration of volcanic sources in the underlying mantle (Duncan and Clague, 1985). Many problems exist in the interpretation of the hot-spot reference framework which has been constructed on the basis of volcanic centers which are probably not true hot spots (e.g. volcanic centers located along normal rift zones) which would indicate eastward plate motions. Moreover, geochemical parameters cannot easily identify a hot spot and there is no agreement about the depth of the volcanic source: a deep mantle source of the hot-spot magmas located below an eastwardmoving asthenosphere with respect to the lower mantle (Sabadini and Yuen, 1989) would predict both eastward and westward hot-spot tracks at the earth's surface as a function of the decollement operating at the base of the lithosphere with respect to the mantle. For instance, the undoubtabe northward-directed plate motion of India can be interpreted as due to a major coupling with the mantle flow with respect to Africa and Asia. Moving north, India could have experienced the Coriolis effect, enabling its counterclockwise rotation. However, an up-to-date screening and review of the classification of true hot spots is necessary for reconstructions of plate motions. The present author supposes that extension and compression between lithospheric plates are a function of the variations in lithospheric velocity

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.

.

.

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~ - ~ - ~

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Fig. I. Map showing the inferred flow lines along which plates move westwards relative to the underlying mantle through the detachment layer induced by the Earth's rotation at the base of the Lithosphere. The flow lines are deduced by plate-motion vectors, oceanic-ridge directions, regional first-order stress fields, hot-spot tracks, etc. l hese rio,<', lines indicate the westward plate motions Ismall black arrows} relative to the eastward mantle flow {big white arrowsl. The flow lines indicate the direction of first-order plate motions. Plate tectonics occurs because there are differences in velocity of the plates along these paths due to variations in decoupling between lithosphere and underlying mantle (note the variable relative vectors of velocity i. Van der Grinten projection, world simplified Tertiary tectonic map after Bally et al., 1985.

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CARLO DOGLIONI

A

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Fig. 2. The Earth is not a perfect sphere rotating around a stable axis (A). It is rather an imperfect spheroid characterized by wobble of the rotation axis (B). It is hypothesized that this results in undulations in rotational flow of the upper mantle.

relative to the underlying eastward asthenospheric flow. The main decollement level should coincide with the Low Velocity Zone (LVZ) at the top of the asthenosphere where partial melting operates (Knopoff, 1983) and could be an useful layer of weakness for decollements. For the Pacific plate, which is composed entirely of oceanic lithosphere, and where the LVZ is particularly well developed, plate velocity relative to the hot-spot reference framework is at a maximum, reflecting enhanced decoupling between lithosphere and asthenosphere. For ancient continental shields with poorly developed LVZ, plate velocities relative to the underlying asthenosphere are generally lower, reflecting less effective decoupling. Many references have noted that oceanic plates are currently faster moving (e.g., Forsyth and Uyeda, 1975). Best estimates of plate velocities in the hot-spot reference framework (Minster and Jordan, 1978) indicate motion along the predicted flow lines and the velocities are greatest for the oceanic plates. Variable decoupling between lithospheric plates and the general eastward asthenospheric flow can account for the observed plate boundary configurations. For compression or transpression, the eastward plate is more effectively decoupled from the asthenosphere than the westward plate. For extension or transtension, the converse would be true. Actually, plates on this global decollement plane must have different velocities to produce plate separations or plate collisions; otherwise the system would be immovable. The decollement at the base of one given plate may be active at one

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m o m e n t and inactive a few million years later: the rule is that the process of decollement is effective as long as the overlying plate is moving westward (or southwestward along the bending of the flow lines in the East Africa Indian Ocean region). A possible explanation of the different decoupling at the mantle top could be the mantle anisotropies (e.g., Dziewonski, 1984) as variations in temperatures and viscosity gradients generate long-term mantle convections. If the decollement at the base of the lithosphere were equally active beneath all the earth's plates (all having the same velocity and delay with respect to the lower mantle), then there would be no plate tectonics at all, because all the lithosphere would simply move westwards as a whole shell rotating around the mantle. Probably this situation occurs in other planets where plate tectonics has not yet been demonstrated. Although the upper-mantle LVZ should be the main level of detachment, other important decollement horizons should be located at the base of the crust and within the lower crust. The ubiquitous presence of mylonites in areas where the deeper parts of the crust are now exposed (e.g., Alpine Ivrea Zone), and the retrodeformation of mountain chains and passive continental margins whose tectonic evolution requires decollement horizons at several levels (both pure shear in the lower crust-mantle transition, and simple shear in the middle-upper crust) suggest that the intra-lithospheric decoupling is important.

3. S U B D U C T I O N ZONES A N D THE NAIL E F F E C T

The simple premise of a general eastward motion of the mantle relative to the overlying lithospheric plates can account for a variety of tectonic observations. For example, volcanic arcs with active back-arc basins are characterized by steep west-dipping subducting lithospheric slabs, whereas eastward-dipping subducting slabs have shallower dip and no true back-arc basins (e.g., Uyeda and Kanamori, 1979). These features could be explained by an eastward motion of the asthenosphere relative to the lithosphere (Nelson and Temple, 1972; Moore, 1973). In this model, we could also distinguish between west and east subductions in terms of relative plate motions: west-dipping subductions (both oceanic and continental) actively subduct beneath a western plate, while east-dipping subductions are passively subducted by an actively thrusting eastern plate. E- (or NE)-dipping subductions (following the mantle flow) are associated with thrust belts with huge exposures of basement rocks (also lower crust) and high structural and morphological reliefs in contrast with limited and usually shallow foredeeps (e.g., Western Alps, Dinarides, Zagros

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Himalaya). W- (or SW)-dipping subductions (contrasting with the mantle flow) are instead associated with shallow thrust belts, the upper crust being involved, and very consistent foredeeps (e.g., West Pacific accretionary wedges, Apennines). Note that in the E-dipping subductions, the basal detachment of the eastern actively thrusting plate is transmitted at the surface and provides a mechanism to bring deep crustal levels to the surface (Fig. 3), whereas in the W-dipping subductions the base plate detachment is folded and subducting itself. The W-dipping case produces more superficial detachments in the accretionary wedge forming at the plate margins. Therefore, the two kinds of subductions provide different geometries and probably different metamorphic paths to the relative thrust belts. We note that the convexity of the arcs related to W-dipping subduction zones shows a "flute cast" shape, suggesting them to be local obstacles opposing the general westward lithospheric counterflow. The westward-dipping slabs provide an obstacle to the eastward mantle flow. The slab steepens and the subduction hinge is pushed eastwards, opening up the back-arc basin (e.g., West Pacific back-arc basins, Tyrrhenian Sea). We can also observe that subduction zones like the Caribbean, the Sandwich or the Alboran arcs are all located where there are velocity contrasts between the northern and the southern plates (N- and S-America, S-America and Antartica, Europe and Africa). In the case of eastward-dipping slabs, the obstacle to mantle flow is less severe, the dip of the slab remains more shallow, but it still acts as an impediment to the eastward mantle flow. The thickening of the lithosphere in E-dipping subductions may inhibit or decrease the amount of relatively greater decoupling of the eastern plate, resulting in alternating phases of compression or extension as a function of the relative decoupling between the eastern and western plates. Thus, the continent-continent collision process can induce a radical change in relative plate motion. This may be an explanation for the Wilson cycle of repeated opening and closing of ocean basins. The obstacles to mantle flow produced by plate convergence act as "nails" which are eastward pushed by the mantle itself (Fig. 3). The Nail Effect is a possible explanation for the observed juxtaposition of regions of compression and extension at plate boundaries. In the case of continentcontinent convergence, the Nail Effect will be most important immediately after the major phase of lithospheric thickening when the lithospheric root is well developed. The decoupling of the lithosphere from the underlying mantle will be reduced in this region and the effect may cause extension in the region immediately west of the zone of lithospheric thickening, as the eastward plate is pushed to the east by the lithospheric keel generated by the continent-continent convergence. Seismicity in Benioff planes can be also due not only to dip-slip motions but also to the horizontal shear (Giardini

THE GLOBAL TECTONIC PATTERN

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29

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Fig. 3. Schematic representation of the relative movements between plates due to variable decoupling between lithosphere and underlying mantle. When there is separation of plates, the western plate is moving faster to the west (A > D, like the ongoing opening of the Atlantic) while when there is collision, it is the eastern plate that moves faster westwards (A < D). The passive rise of mantle oceanic ridges is a consequence of continental rifting when A > D. C o m p a r e the relative vectors of plate velocity in Fig. 3. The differences in the decoupling between lithosphere and mantle are probably allowed by the anisotropies in the upper mantle which determine variations in velocity of the overlying plates.

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and Woodhouse, 1986) generated by the collision between the relatively eastward-moving mantle and the cold subducted lithosphere which is cut by almost horizontal shear planes. In other words, plates behave like passive rafts with different relative velocities above an eastward flow of the mantle: the surficial result is a geometry indicating a westward flow of plates due to the differential delay among the lithospheric plates. When subduction is inhibited by the impossibility of retrogradation of the hinge zone for several reasons (i.e. a continent-continent collision, after the consumption of the entire oceanic crust between the two plates and until conditions of strength stability are established), the subducted slab is in the course of time assimilated and annihilated by the warmer mantle. A consequence is that there will be a new horizontal equilibrium between lithosphere and mantle, isotherms will tend to return to the horizontal and the plates will again be allowed to move westwards with respect to the mantle throughout the decollement at their base. The lateral heterogeneity between continental and oceanic crust (Forte and Peltier, 1987) is also a fundamental factor in controlling subductions and relative plate velocities. In this scenario, the oceanic lithosphere is a sort of new skin that the mantle produces as a passive response to continental rifts due to differential plate velocities. The new oceanic lithosphere forms by upwelling and partial melting of mantle material which cools to generate the oceanic lithosphere. In fact, the domal uplift of the mantle in rift zones is post-stretching of the continental lithosphere (e.g., in the Red Sea, Bohannon et al., 1989; and in the Gulf of Suez, Moretti and Ch6net, 1987); the new oceanic basins form when depressure of the crust occurs and the mantle can consequently rise, producing an asthenospheric diapir with small convection cells localized beneath the oceanic ridges (Bonatti, 19841. The abrupt change in orientation of the seamount chain in the Emperor Ridge, which coincides with a change in the plate boundary configuration (Uyeda and Miyashiro, 1974), may represent a sudden adjustment of Pacific-Plate motion in response to 2nd order rotations or a progressive change in the orientation of mantle flow at a time when new zones of subduction are initiated in the western Pacific.

4. P A L E O M A G N E T I C C O N S T R A I N T S

Plates moving above the global flow lines of Fig. 1 will show a rotation when they pass through the bending now present from E-Africa to the W-Pacific. The motion of Africa relative to the pole shows a SE trend from Middle Jurassic to Middle Cretaceous, followed by a north-easterly trend. This is consistent with the predicted path of Africa along the flow lines

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(Fig. 1). Changes in motion of the other continents relative to the pole are, in general, consistent with the flow lines pattern. This kind of rotation is here called a first-order rotation to distinguish it from local rotations induced by body forces (see Oldow et al., in press) or fulcrum effects (e.g., the rotation of Spain), referred to as second-order rotations and tectonic features. Second-order tectonic features, such as rifts, which are not perpendicular to the trend of flow lines are destined to abort (i.e. the Benue Basin). The present position of the flow lines is most probably different from what it was during the Mesozoic, especially in the Pacific area. True polar wandering, meteoric impacts, etc., could have modified the global configuration. The Atlantic area seems to have been quite stable since at least the Mesozoic. For Mesozoic and Cenozoic time, our knowledge of relative plate motions is derived largely from magnetic anomalies and paleomagnetic data. The mantle-flow lines are constructed such that Cenozoic relative plate motion is constrained to lie along the flow lines. It is, however, important to note that the flow lines show smooth progressive changes in orientation, compatible with rotation-induced differential motion of the lithosphere and underlying mantle. We can test the resilience of the flow lines by comparing them with pre-Cenozoic relative plate motions. Analysis of global paleomagnetic data indicates that true polar wander must have been rather low since Cretaceous time and we would therefore expect the general flow lines configuration to have been maintained during this period. For Paleozoic time, relative plate motion is less well defined, depending largely on continental paleomagnetic data, paleobiogeography and paleoclimatic indicators (Van der Voo, 1986). Laurentia appears to have remained in low northerly latitudes during much of Paleozoic times, whereas the Gondwana continents (including Armorica) were in high southerly latitudes during the Ordovician. Late-Silurian northward movement of Gondwana culminated in the Middle-Devonian (Acadian) continental collision. Undulation in the present flow lines pattern (Fig. 1) permits northward (latitudinal) motion, such as that exhibited by India during Cenozoic time, but the abrupt northward motion such as that exhibited by Gondwana in Silurian time may be triggered by changes in the flow lines configuration. True polar wander (e.g., Sabadini et al., 1982) would alter the flow lines configuration relative to the lithospheric plates. Although true polar wander appears to be minor for Mesozoic-Cenozoic time, it may have played a part in determining pre-Mesozoic plate motions. It is interesting to note that the most convincing exemples of large-scale true polar wander are those of the Moon and Mars (e.g., Runcorn, 1967; Schultz and LutzGarihan, 1982; Schultz, 1986). A problem exists in the location of the poles of the mantle rotation shown by the flow lines. It is evident that they are displaced with respect to the J O G 12/I 3

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geographic poles. The northern pole appears to be located in northeastern Siberia, while the southern one is more difficult to place, perhaps in Antartica or south-east of the Australian plate.

5. R E G I O N A L I M P L I C A T I O N S O F T H E M O D E L

The Mediterranean area has long been an enigma in the plate-tectonic global framework. The recent evolution of the Mediterranean area has generally been considered as a result of the N-S collision of Africa and Europe. Although Africa has moved more or less northward relative to Europe since the late Cretaceous, we consider that the dominant plate motion of Adria is E-W (ENE-WSW) relative to both Europe and Africa, and coincides with the mantle flow lines (Fig. 4) which are consistent with present-day plate-motion satellite data (Smith et al., 1989; S. Zerbini, pers. comm.). Kinematic indicators in the central Alps give a dominant E-W or NW-SE sense of relative motion from Late-Cretaceous time ( P l a t t e t al., 1989), suggesting that the motion of Adria and Africa were more or less independent from that time. The predominant first-order eastward motion of Adria relative to Europe gives rise to second-order Jurassic to MiddleCretaceous sinistral transtension at the E-W oriented Adria/Europe plate boundary. Subsequently, the first-order westward motion relative to Europe, from Late-Cretaceous time, gives rise to second-order transpression at this plate boundary. The observed juxtaposition of extension and compression in the Mediterranean episutural basins, such as the Tyrrhenian Sea, does not have a simple explanation in terms of classic plate-tectonic theory. The Nail Effect, accompanied by the hypothesized eastward flow of mantle provide possible explanations for various enigmatic Mediterranean tectonic problems (Doglioni et al., 1990). We note that the tectonic patterns (both tensional or compressive) have the same orientation in the DinaridesApennines, in the Aegean and Cyprus subductions, in the Red Sea rifting and Zagros Mountains, in the Indian Ocean and Himalaya Chain, suggesting once again a global meaning of such an orientation. The Nail Effect could explain why oceans open and close several times: extension can be a product of collision because the more active decollement plane at the bottom of two colliding plates is first that of the eastern plate, then this becomes smaller or inactive with respect to that located below the western plate (Fig. 3). This evolution would be in agreement with the Wilson cycle which predicts opening and closing several times. Why, for instance, did the Tethys open widely where the Variscan orogen formed (Bernoulli and Lemoine, 1980)? The collapse of orogenic chains is typical

--W---

/

2nd or:d,

/

N-AFRICA

to abort.

Fig. 4. With respect to the global flow lines, the Atlantic is a first-order ocean developed perpendicularly to the mainstream mantle flow. The western Mediterranean basins are also first-order features developed progressively eastwards in time as back-arc basins. The Apennines and the Dinarides are firstorder tectonic features also. The Aegean subduction follows the mainstream and is located where the flow lines tend to bend northeastward, as in the east Africa-Indian ocean region. The rotation of Iberia is considered a second-order local rotation producing extension in the Bay of Biscay and shortening in the Pyrenees. These features are considered second-order features because they are not perpendicular to the trend of flow lines and are therefore destined

1 st order . . . . . .

, N-AMERIC,

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,-4

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CARLO DOGLIONI

all over the world (Dewey, 1988). The post-collisional extension could be the result of the Nail Effect (the subducted lithosphere) into the mantle which pushes eastwards all the chain and activates the decollement plane in the western plate: therefore once an ocean is closed and a chain is formed, it is quite natural to expect a new basin formation because the major decollement motion shifts from the eastern to the western plate (Fig. 3). The westward drift of N-America should be a consequence of a relatively greater detachment at its base with respect to Africa and Europe, with the rapid Cretaceous opening of the Atlantic. N-America was relatively faster than the eastern Pacific oceanic plate producing the Laramide orogeny. Another consequence was that the activity of the base-plate decollement was responsible for the production of new oceanic crust which, in turn, generated the 2nd-order (long-term) Cretaceous sea-level rise. When the eastern Pacific plate subducted, the N-America plate became relatively slower with respect to the western Pacific oceanic plate, enabling extension (Basin and Range) and dextral oblique movements at the western N-American margin.

6. C O N C L U S I O N S

Global flow lines drawn along the axes of Cenozoic extension and shortening show a smooth and gradual variation (Fig. 1). The flow lines may approximate the path of eastward mainstream mantle flow relative to the overlying lithosphere, the long-wavelength undulation being due to instability of the rotation axis (Fig. 2). The westward delay of the lithosphere with respect to the mantle could be due to a minor angular velocity of the lithosphere relative to the underlying mantle, as a result of deceleration of Earth's rotation or, in a toroidal field, due to lateral heterogeneities within the lithosphere and within the mantle. Variations in the upper-mantle Low Velocity Layer allow variable decouplings between lithosphere and asthenosphere. It is supposed that the plate-tectonic process may be driven by differential plate velocities due to differential coupling between individual lithospheric plates and the underlying mantle (Fig. 3). A simple rule is that when there is compression or transpression, the eastern plate is moving faster westwards, while if there is extension or transtension it is the western plate that moves faster westwards (Fig. 3). Convection in the mantle (e.g., Spohn and Schubert, 1982; Cserepes and Rabinowicz, 1985; Sabadini and Yuen, 1989) would help the model in producing lateral heterogeneities in the upper mantle which consequently generate different degrees of development of the LVZ and the relative lithosphere-mantle decoupling. Relative plate motions are allowed by

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horizontal and vertical viscosity and density gradients both in the lithosphere and the mantle (i.e. between continental and oceanic lithosphere, etc., Forte and Peltier, 1987). Tectonics and plate rotations of 1st order are defined as those localized along the global flow lines; tectonics and plate rotations of 2nd order are defined as those induced by localized body forces and/or fulcrum effects (Fig. 4). The flow lines seem to be quite stable since at least Permian times, especially in the Atlantic and european-asiatic regions. However the westPacific plate underwent several rotations (i.e. the 40 ma Haway-Emperor rotation): a different decoupling in some parts of the base plate could account for rotation of this huge oceanic plate. Lithospheric subduction into the mantle, particularly westward-dipping oceanic subduction, strongly enhances the coupling between the lithospheric plate and the underlying eastward mantle flow. Hence, subduction zones act as "nails" into the mantle which strongly modify the relative plate velocities (Fig. 3). The Nail Effect can result in extension on the western side of the obstacle, as the westward dipping plate is eastward pushed by the mantle flow. Note that W or SW-dipping subductions are characterized by arcs with eastward vergent convexity supporting the westward counterflow of the lithosphere with respect to the mantle. The eastward dipping oceanic subduction z6nes have shallower dip and do not provide such a formidable obstacle to mantle flow. However, lithosphere thickening can provide variations in the relative decoupling with the mantle and can generate alternating phases of compressions and extension. Back-arc extension and related magmatism (rather semicircular) is migrating toward the sense of mantle flow (eastward, s.l.), due to the "window" that the eastward mantle flow is producing at its top pushing back the subduction hinge of the eastern plate. Linear normal rift zones are instead accompanied by frequent westward rejuvenation of the rift, due to the relative westward migration of the greater detachment. The eastward rejuvenation of lithospheric stretching (Oligocene LigurianProvencal Seas, Late Miocene-Quaternary Tyrrhenian Basin) is in agreement with an eastward relative migration of the underlying mantle~ confirmed also by the eastward decrease in age of the magmatism. The Nail Effect of the Apenninic subduction is considered to be the cause of the opening of the western Mediterranean basins. The present Calabrian subduction would be the Nail which is pushed eastward by the mantle, and the back-arc Tyrrhenian basin opens as a consequence. The coeval eastward Apenninic migration is allowed by the presence to the east of an anisotropic southward thinning lithosphere which can subduct. In other words, the Mediterranean tectonic setting would be better explained in terms of general E-W (WSW-ENE) motions rather than N-S compressions: localized N-S

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compressions (e.g., central-eastern Alps, Sicily, etc.) are generated by local body forces (transpressions) at plate margins. In this light, we should be able to distinguish, for instance, between east-vergent and west-vergent thrust belts, or between east-vergent thrust belts related to west-dipping subduction (i.e., the Apennines, Bally et al., 1986) and east-dipping subduction (i.e., the eastern Andean Cordillera, Roeder, 1988).

ACKNOWLEDGMENTS

This paper is dedicated to my parents, Fausta and Leonisio. In the last years this idea has been discussed with several patient people. I particularly thank Jim Channell who re-edited the english text and contributed with useful suggestions. Thanks for the stimulating discussions to R. Sabadini, E. Passaglia, L. Brigo, G. Rivalenti, G. V. Dal Piaz, E. Sommavilla, E. Locardi, E. Bonatti, A. W. Bally, L. Beccaluva, G. Biino, D. Bernoulli, A. Bosellini, R. Brandner, R. Catalano, R. Crane, J. Debelmas, P. Elter, F. Ghisetti, H. P. Laubscher, F. Massari, K. McKlay, I. Moretti, R. Nicolich, X. Le Pichon, G. Ranalli, Y. Ricard, F. Roure, F. Sabat, M. Sacerdoti, M. Schumacher, J. Stoner, J. Suppe, R. Trumpy, P. Verrall, W. and C. Wallace, and S. Zerbini. Two anonymous referees are also acknowledged. The Italian "Ministero dell' Universitfi e della Ricerca Scientifica" supported this research.

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