EARTH ROTATION AND THE ACCRETION AND DISPERSAL OF PANGEA

 

Miguel DOBLAS
 Científico Titular del CSIC, Instituto de Geociencias (CSIC-UCM), Facultad de Medicina (Edificio Entrepabellones 7 y 8),
c/ del Doctor Severo Ochoa 7, Universidad Complutense de Madrid, Ciudad Universitaria, 28040 Madrid, España,
Email: doblas@mncn.csic.es

Abstract

            Different physical mechanisms related to the rotation of the Earth had a decisive influence in the evolution of the Pangea supercontinent since Paleozoic times: during the Paleozoic welding of the Pangea, kinetic energy requirements forced the drifting of continents and interior orogens and seas toward the equator; and, since the late Paleozoic, both membrane stresses related to an equatorial bulge and inertia-driven toroidal rotations triggered the breakup of the Pangea. In this sense, four stages can be distinguished in the Paleozoic to present assembly and dispersal history of this supercontinent: 1) A Variscan Carboniferous stage representing the initial welding of the clockwise rotating Pangea, bringing the Gondwana and Laurussia continents and interior orogens and seas toward the equator. 2) A Late Variscan (Stephanian-Autunian) stage during which a series of events began to disrupt the central/equatorial Pangean European-Northwestern-African domain (EUNWA): tectonic deformations including the extensional collapse of the previously thickened orogenic welt, and an equator-parallel dextral transcurrent megashear zone; and, thermal insulation processes accumulating the heat beneath the supercontinent, triggering a large-scale mantle upwelling, and finally releasing part of this heat through a major thermal "exhaust valve". 3) A Late Triassic to Early Jurassic stage of Pangean breakup with the following elements: a Central Atlantic Plume (CAP) located on top of the Equator; a thinned corridor following the trends of the collapsed southern branch of the Variscan belt which will be the future site of the central Atlantic; a subcircular and thinned European large thin-spot domain (ELTS); NE-directed large-scale sublithospheric mantle channeling from the CAP exhaust valve towards the ELTS giving rise to a huge tholeiitic province; and the closure of the PaleoTethys by the NE-directed drifting of the Cimmerian continent. 4) The Jurassic to present final dispersal of the former Pangea. The previous NE-directed large-scale sublithospheric plume channeling along the Central Atlantic continued during this stage in the eastern central Atlantic, giving rise to an African-European magmatic zone (AEMZ). This passive-margin-parallel sublithospheric mantle channeling, together with the trends of the closing Tethys and the opening Indian ocean, might have been effective plate-moving mechanisms triggering the generalized NE-directed drifting and anticlockwise rotation of the southeastern fragments of the former Pangea, ultimately closing the Tethys and generating the E-W-oriented Alpine-Himalayan belt (AHB).

Keywords: Earth rotation, Pangea, accretion, breakup, drifting, thermal insulation, membrane stresses, plumes, exhaust valves, orogens. 

1.   Introduction

            The importance of the rotation of the Earth on crustal deformations and continental displacements has been recognized for a long time, involving some mechanisms such as: 1) kinetic energy requirements of the rotating planet that first tend to group the continents and some favorably-oriented interior orogens and seas toward the equator, and then trigger the fragmentation and dispersal of a supercontinent; and, 2) membrane stresses related to the passage of the continents on top of the equatorial bulge.

            The kinetic energy requirements related to the rotation of the Earth will have two major effects on the assembly and dispersal of the Pangea: 1) The Gondwana and Laurussia continents and some favorably-oriented Paleozoic interior orogens with subduction zones (the Variscan belt), and seas (the PaleoTethys) moved progressively toward the equatorial belt and subsequently the continental fragments became accreted (Goldreich and Toomre, 1969; Vroman, 1981; Anderson, 1982;  Le Pichon and Huchon, 1984; Doblas and Vergas, 1992; Bonatti et al., 1993). In this sense, different authors have suggested that the existence of erratically distributed continental masses on the Earth's surface must topple the planet through its axis of rotation, thus increasing the moment of inertia, and these phenomena will ultimately tend to group the continents toward the equator (Sutton, 1963; Vroman, 1981; Le Pichon and Huchon, 1984; Hoffman, 1989; Murphy and Nance, 1992). 2) Once the Pangea was welded together, inertia-driven toroidal rotations tended to fragment and disperse this supercontinent again (Hynes, 1990; Doblas and Vergas, 1992; Murphy and Nance, 1992). The dynamics of such rotations can be controlled by the global balance of the torques driving the plate motions (Hynes, 1990).

            On the other hand, it is known that the centrifugal effect of the Earth's rotation causes an equatorial bulge, which is the principal departure of the Earth from spherical shape (e.g., Starey, 1977). Calculations indicate a difference of 130 km between the equatorial and the polar great circle circumferences (Turcotte, 1974; Starey, 1977). The theory of membrane tectonics suggests that due to the ellipticity of the Earth (which would be an oblate spheroid), the lithosphere must deform when its latitude changes and the radii of curvature is progressively modified (Turcotte and Oxburgh, 1973; Turcotte, 1974; Oxburgh and Turcotte, 1974; Vroman, 1981). According to these authors, strains of the order of 0.1% and stresses of the same order as the strength of plates (kilobars) are to be expected when large plates undergo modest changes in latitude, and they may be sufficient to fracture the lithosphere. The mechanisms of membrane tectonics propose that plates moving from the equator to the pole have their central parts in compression and their outer parts in extension, while motion of a plate toward the equator gives a central zone in extension and an outer zone in compression (Turcotte and Oxburgh, 1973; Oxburgh and Turcotte, 1974). Membrane stresses would explain the East African Rift or the Central Atlantic as propagating fractures when the continents passed over the equator (Turcotte and Oxburgh, 1973; Oxburgh and Turcotte, 1974; Hynes, 1990), and the fact that east-west seafloor spreading is favoured in the Earth with respect to north-south seafloor spreading (Turcotte and Oxburgh, 1973; Vroman 1981). This might also be what happened during the late Proterozoic when the Rodinian supercontinent (Meert and Van der Voo, 1997; Li, 1998) began to breakup in its central realm when it crossed the equatorial bulge. However, membrane tectonics is not universally accepted as an effective disrupting mechanism and some authors argue that such stresses might be dissipated by creep (Keary and Vine, 1990).

            One of the most conspicuous features of the Earth in relation to its rotation is constituted by the present-day geoid which shows a high-gravity potential zone constituting an equatorial-parallel positive belt in the Pacific (Crough and Jurdy, 1980; Anderson, 1982; Chase and Sprowl, 1983; Le Pichon and Huchon, 1984). This feature corresponds to the present location of the equatorial bulge, which, however, seems to be a non-continuous structure along the equator: 1) the geoid shows also a low-gravity northwest-southeast-oriented polar belt (Atlantic-African) which has been related to the former location of the Pangea (Anderson, 1982; Chase and Sprowl, 1983; Le Pichon and Huchon, 1984); and, 2) the equatorial Atlantic displays an anomalous east-west zone of downwelling of cold suboceanic mantle which can be hardly coincidental and suggests that the rotation of the Earth had also a role in this feature (Bonatti et al., 1993). These downwellings might be related to mantle avalanche phenomena (e.g., Condie, 1998; Pysklywec and Mitrovica, 1998).

            The Earth shows a wide east-west zone along its equatorial belt with a series of features which seem somehow related to its rotation, and hence to the existence of an equatorial bulge: 1) The geoid has two long-wavelength highs centered in the equator which contain most of the world's hot spots (Cazenave et al., 1989), as well as some well-known plumes (St. Hellen and Galapagos) and plateau flood basalts (Ontog-Java, and the Caribbean). In fact, if one observes the map of distribution of the Earth's Phanerozoic large igneous provinces (LIPS; Coffin and Eldholm, 1994), most of them are located between latitudes 30ºN and 30ºS. 2) Three of the most important superplumes in the history of the Earth were located precisely on top of the corresponding equators for their times: the Permo-Carboniferous European-northwestern African superplume (Doblas et al., 1998), the late Triassic to early Jurassic central Atlantic superplume (Oyarzun et al., 1997), and the mid-Cretaceous central Pacific superplume (Larson, 1991a,b; Vaughan, 1995). It is interesting to note that the two first plumes are associated with the largest peaks of coal, oil and gas accumulation in the Earth's history (Larson, 1991b). 3) A series of geochemical, thermal and isotopic anomalies found in the oceanic crust, and reflecting deep mantle  upwelling phenomena, show east-west trends near or on top of the equatorial belt: the SOPITA anomaly in the equatorial western Pacific (Staudigel et al., 1991); and, the DUPAL anomaly stretching from the southern Atlantic to the southern Pacific across the Indian ocean (Dupré and Allègre, 1983; Hart, 1984; Weis et al., 1991). 4) Geophysical global tomographic images indicate the existence of mantle-wide circulation patterns locally concentrated in the equator at mid (1300 km) and deep (2750 km) mantle depths (Van der Hilst et al., 1997). 5) It is interesting to note that some of the most important collisional interior orogens in Earth's history had an east-west orientation near or on top of the equatorial belts of their times: the Precambrian Greenville foldbelt, the Paleozoic Appalachian-Variscan orogen, or the Alpine-Himalayan foldbelt. Van Hilten (1964) explained the late Variscan Appalachian-Variscan belt as a result of a dextral "Tethys twist" between Gondwana and Laurussia along the equatorial belt.

            The existence of rotation-related distorting stresses on top of equatorial bulges might also be deduced in other planets of the Solar System: 1) Venus shows a series of conspicuous features along its equator: a pronounced bulge or lumpiness believed to impose the retrograde rotation of this planet; a 9,000-km-long rift-valley (Aphrodite Terra); a whole series of giant radiating dike swarms (Ernst et al., 1995); most of the volcanoes are found in a belt centered in the equatorial belt. 2) Mars is characterized by a major rift-valley (Valles Marineris) and a series of subcircular bulges along its equator: the huge Tharsis uplift with a giant radial pattern of fractures and some of the largest shield volcanoes in the Solar System; and two minor bulges in Syrtis Major (also with radial fractures) and the Elysium region. 3) The Moon shows a pronounced bulge in its equatorial zone oriented toward the Earth, where the only lunar Mare are found in this planet.

            In this paper we will explore the consequences of the rotation of the Earth both on the Paleozoic final assembly of the Pangea and on the late Paleozoic to present dispersal of this supercontinent, involving four contrasted stages: 1) Carboniferous compression; 2) late Carboniferous to early Permian transtension; 3) late Triassic to early Jurassic protorifting of the Central Atlantic; and, 4) Jurassic to present NE-directed drifting of the different fragments of southeastern Pangea (Africa, Arabia, India, SE Asian blocks, Australia).

2. Four-stage model for the assembly and dispersal of Pangea

            The Paleozoic to present evolution of Pangea can be explained in terms of a four-stage model (figs. 1 to 5): 1) a Carboniferous Variscan orogenic stage representing the beginning of the Pangean accretion (figs. 1 & 5a); 2) a late Carboniferous to early Permian  (late Variscan) stage characterized by transtensional disruptions, thermal insulation processes, and the progressive release of the heat accumulated beneath the supercontinent through an exhaust valve located in the center of Pangea (figs 2 & 5b); 3) a late Triassic to early Jurassic stage involving protorifting processes in the former western branch of the Variscan belt (and future Central Atlantic ocean), with a Central Atlantic superplume releasing the heat towards a European large thin-spot to the NE giving rise to a huge tholeiitic province (figs 3 & 5c); and, 4) a Jurassic to present stage with a generalized NE-directed drifting of the southeastern fragments of the former Pangea, closing the Tethys, and generating the Alpine-Himalayan belt (figs. 4 & 5d). 

 Fig. 1: Idealized sketch showing the plate reconstructions during the Carboniferous events that led to the Variscan orogenic collisions between Gonwana and Laurussia. Base maps from Ziegler (1989).

2.1. Carboniferous Variscan stage (figs. 1 & 5a)

             Pangea began its welding during the Variscan orogeny (Windley, 1995). The central sector of this supercontinent (European Variscan Orogen; Fig. 1) was the site of particularly complex deformation involving subduction, collision, indentation, obduction, and block rotation, thus conferring on this area a highly contorted global geometry, far more complex than the rest of this Pangean Paleozoic orogen (Fig. 1; Laubscher and Bernoulli, 1977; Seyfert and Sirkin, 1979; Zonenshain et al., 1985; Matte, 1986, 1991; Windley, 1995). This is partially explained by the fact that this area was the approximate site of the first collision between Laurussia and Gondwana, which subsequently continued their northward convergence and drifting (Zonenshain et al., 1985), accompanied by a conspicuous clockwise rotation of both continents pivoting about a pole located precisely in the center or neck of the future "crescent-shaped" Pangea, near the Paleotethys embayment (Le Pichon and Huchon, 1984; Zonenshain et al., 1985; Matte, 1986; Hynes, 1990). This clockwise rotational component has been ascribed to the oblique collision of Gondwana and Laurussia (Matte, 1986). Coeval with the Variscan compressions described in this chapter, the Norwegian-Greenland zone was subject to indentation-related extension along a north-south rift oriented perpendicular to the Variscan foldbelt (Fig. 1; McClay et al., 1986; Doblas and Oyarzun, 1990a; Rey et al., 1997), much in the way of the Cenozoic Himalayan collisional foldbelt which triggered orthogonal extensional zones in mainland Asia (Tapponnier and Molnar, 1977).

            Kinetic energy requirements related to the rotation of the Earth explain two characteristics of the assembly of the Pangea during this stage: 1) the Gondwana and Laurussia continents were aggregated along the equator (Anderson, 1982); 2) favorably-oriented Paleozoic interior orogens with subduction zones (the Variscan belt), and seas (the PaleoTethys) had a tendency to move progressively toward the equatorial belt (Goldreich and Toomre, 1969; Vroman, 1981; Le Pichon and Huchon, 1984; Bonatti et al., 1993).

            Thus, as a result of this stage, the European Variscan sector of this Paleozoic foldbelt was established as a preferentially overthickened crustal zone which was further destabilized and overheated by massive syn- to post-tectonic plutonic intrusions. We will see later that this central area of Pangea will continue its peculiar evolution during the following stages of disruption, breakup, and final dispersal of the Paleozoic supercontinent.

Fig.2: Idealized sketch depicting the plate reconstructions within the Pangea during the Late Variscan (Stephanian-Autunian) transtensional disruption of this supercontinent (modified from Doblas et al., 1998). EUNWA: Europe northwestern Africa. Base maps modified from Ziegler (1989), Klein et al. (1994), and Scotese and Langford (1995).

2.2. Carboniferous to Permian late Variscan stage (figs. 2 & 5b)

            This stage truly represents the initial disruption of the Pangean supercontinent, and it will have a decisive influence on the future breakup of Pangea. Paleogeographic reconstructions for this period (Briden et al., 1974; Zonenshain et al., 1985; Livermore et al., 1986; Ziegler, 1990; Klein et al., 1994; Scotese and Langford, 1995)  show that the central narrower "neck" of Pangea was located on top of the equator, following the trends of the Variscan foldbelt to the west, and the Tethys embayment to the east (Fig. 2). This central sector of the Pangea (the European-northwestern Africa province; EUNWA; Doblas et al., 1998) constituted a highly peculiar and differentiated region with distinctive tectonic, sedimentary, and magmatic characteristics.

 2.2.1 Tectonic scenario

            The EUNWA province continued its differential tectonic evolution in the center of the Pangea during the late Variscan as a result of a mixed tectonic scenario involving the extensional collapse of the Variscan crustal welt and the dextral disruption the central Pangean realm by an equatorial-parallel megashear zone.

            The whole EUNWA Variscan orogenic edifice collapsed through simple-pure shear low-angle extensional detachments during the late Variscan, giving rise to a Basin-and-Range-type extensional province in Europe (Lorenz and Nicholls, 1984; Doblas et al., 1988; Menard and Molnar, 1988; Doblas, 1991; Malavieille, 1992; Burg et al., 1994; Doblas et al., 1994) and Morocco (Lagarde et al., 1993), involving major low-angle detachment faulting, unroofing of large metamorphic core complexes, and syn-extensional plutonism and volcanism. This extensional frame has been related to the gravitational collapse of the previously overthickened and weakened Variscan orogenic welt (Dewey, 1988; Menard and Molnar, 1988; Doblas, 1991; Doblas et al., 1988, 1994; Malavieille, 1992; Burg et al., 1994).

            Coevally with this extensional scenario, the EUNWA was affected by a complex system of conjugate strike-slip faults (NW-SE dextral and NE-SW sinistral) which partially disrupted the Variscan edifice, resulting in a new Permo-Carboniferous stress pattern with the principal horizontal compressional axis oriented N-S (Arthaud and Matte, 1975, 1977). This episode was accompanied by sediment deposition in transtensional and pull-apart basins (Arthaud and Matte, 1975, 1977). This episode resulted from the dextral transcurrence of an equator-parallel intracontinental megashear zone located between Gondwana and Laurussia, an early proposal of Van Hilten (1964) fully developed by Arthaud and Matte (1975,1977). This right-lateral megashear had two compressional terminations in the Urals and the Appalachian-Mauritanides (Arthaud and Matte, 1977), and two extensional terminations that we suggest in our paper: the Neotethys, and the US Great Basin (Pindell and Dewey, 1982; Smith and Miller, 1990). The Tethyan realm comprised two contrasted domains during the late Variscan (Fig. 2; Scotese and Langford, 1995): a southern propagating rift (the NeoTethys), and a northern convergent/subducting border (the northern boundary of the closing Paleotethys).

 2.2.2. Anorogenic magmatism

            Widespread intrusive and extrusive anorogenic magmatism, characterized by a highly variable chemistry, pervaded the central part of Pangea (Europe and norhwestern Africa) during the late Variscan. Westphalian to Saxonian magmatism (with a peak in the Stephanian-Autunian) in this central Pangean realm comprises the following (e.g., Cailleux et al., 1986; Ziegler, 1990; Neumann et al., 1992; Broutin et al., 1994; Veevers and Tewari, 1995; Youbi, 1998; Doblas et al., 1998): 1) tholeiitic dykes, plateau alkaline olivine basalts, tholeiitic basalts, and silicic plutons in Sweden and the northern North Sea; 2) tholeiitic dykes and sills, olivine basalts, dacites, tholeiitic basalts, dacites, rhyolites and post-orogenic plutons in western and central Europe; 3) aluminous basalts, andesites, dacites, rhyolites and silicic plutonic rocks in the Iberian Peninsula; and, 4) granites, andesites, basalts, spilites, rhyolites, and dolerite and microdiorites dykes in Morocco. Apart from the EUNWA site, other Permo-Carboniferous volcanism in this Pangean realm is also found in northeastern North-America, and in the southwestern border of the Neotethys (Veevers and Tewari, 1995). It is notorious that the Neotethys was the site of enhanced volcanism in its western border during later time-spans (Permian-Triassic; Veevers and Tewari, 1995).

            As a whole, the Permo-Carboniferous volcanism of the EUNWA province might be explained within a progressively evolving extensional scenario (Doblas et al., 1998). This volcanism is similar to the Neogene volcanic suites of two other areas of the world: SE Spain (Doblas & Oyarzun, 1989a; Oyarzun and Doblas, 1991) and the SW United States (Thompson et al., 1986; Wernicke et al., 1987; Gans et al., 1989), in that: 1) the calc-alkaline series are highly enriched in LILE with respect to the HFSE; and, 2) they are associated with an overall extensional setting within a collapsing orogenic belt, involving a lithospheric mantle with a relict/inactive subducted slab. As extension proceeds the later magmas (alkaline/subalkaline) exhibit an HIMU-type signature, and thus the participation of lithospheric components is scarce to inexistant.

  2.2.3. The EUNWA exhaust valve

            The distribution of the Permo-Carboniferous extension and magmatism in the EUNWA central sector of Pangea might be related to long-lived and widespread thermal anomalous conditions within the upper mantle associated to large-scale mantle upwelling processes since the Variscan, as shown by the following (Doblas et al., 1998): 1) A grossly elliptical geometry defined by the spatial distribution of the volcanics and dyke swarms, and the multi-directional pattern of fractures (Ziegler, 1989, 1990). 2) The concentric distribution pattern of the ages of volcanism which tends to decrease away from the axis of the large mantle upwelling region. 3) An initial volcanism involving, with a few exceptions, crustal and/or lithospheric mantle signatures, followed by HIMU-type melts (e.g. Cabanis et al., 1990; Innocent et al., 1994). 4) An European-wide early Stephanian sedimentary hiatus indicating a fundamental change in the stress pattern which may reflect the global uplift of the center of Pangea. 5) A major reverse polarity superchron on the Earth's magnetic field coinciding with the onset of the late Variscan extension and associated magmatism. Magnetic polarity changes have been associated with mantle upwelling events such as mantle plumes (Larson and Olsen, 1991). 6) A major anomalous thermal event during the time span 286-270 (Veevers, 1989; Broutin et al., 1994), as indicated by the magmatism and by widespread occurrence of fossil fuels. A Permo-Carboniferous plume facilitating the generation of these fuels has been already suggested by Larson (1991b). 7) The existence of a short-lived warm (greenhouse) interglacial event during Stephanian-Autunian time, related to an increase of the atmospheric concentration of CO₂ (Gastaldo et al., 1996; Oyarzun et al., 1999).

            The Late Paleozoic aggregation of the Gondwana and Laurasia fragments into the Pangean supercontinent imposed upon the mantle hotter-than-normal thermal conditions, a mechanism referred to as "insulation"(e.g., Anderson, 1982; Chase and Sprowl, 1983; Le Pichon and Huchon, 1984; Gurnis, 1988; Hoffman, 1989). This led to the accumulation by conduction of radiogenic heat beneath the blanketing continental cap which reacted by a generalized broad upwelling of the mantle (e.g., Veevers, 1989), triggering a topographic Pangean megaswell and the destabilization of this supercontinent (Fig. 2). The anomalous thermal conditions imposed beneath the supercontinent were never completely released, as shown by the persistance until today of part of this megaswell in the form of the African-Atlantic Geoid high (Anderson, 1982; Gurnis, 1988; Veevers, 1989; Duncan, 1991) and the African superswell (Nyblade and Robinson, 1994). Part of the stored heat was liberated along previous weak zones which may be termed "exhaust valves", triggering scattered magmatic outbursts, the maturation of coal and hydrocarbons, and a major greenhouse effect. Thus, the Permo-Carboniferous EUNWA domain can be interpreted in terms of an exhaust valve relieving and focussing the heat accumulated beneath Pangea (Doblas et al., 1998; Oyarzun et al., 1999). These large-scale mantle upwelling processes might have influenced the development of the late Variscan extensional disruption and uplift of the European-northwestern-African province, similar to what is described for the western US extensional cordilleras in relation to the Yellowstone upwelling hotspot zone (Parsons et al., 1994).

            The location of this exhaust valve in the EUNWA may have been facilitated by two additional factors (Doblas et al., 1998): 1) The singular Variscan to late Variscan evolution which ultimately triggered a weakened, heated crustal section in the EUNWA. 2) The generalized late Variscan destabilization was further enhanced by the latitudinal location and shape of the recently welded Pangean supercontinent. Pangea had an overall crescent-shaped geometry with a narrow embayment in its neck connecting from E to W a series of elements aligned along the equator: the Paleotethys, its central point in the EUNWA province, and the Variscan belt (Morel and Irving, 1978; Matte, 1986; Zonenshain et al., 1985; Laubscher and Bernoulli, 1977; Veevers, 1994; Klein et al., 1994; Windley, 1995). As we have already seen, membrane tectonics might give rise to disrupting membrane stresses in the equatorial bulge (Turcotte and Oxburgh, 1973; Oxburgh and Turcotte, 1974; Turcotte, 1974), and this is what seem to have happened in the central Pangean province. We suggest that this tectonomagmatic late Variscan event might be the result of the close interplay between extensional membrane stresses resulting from the passage of the supercontinent over the equatorial bulge, and the clockwise rotation of Pangea. The equatorial bulge was subject to enhanced membrane stresses with dramatic effects upon the weaker and narrower "neck" of Pangea, in the form of transtensional deformations which are to be expected according to the membrane tectonics theory in a continent located on top of the equator (Turcotte and Oxburgh, 1973; Oxburgh and Turcotte, 1974).  Moreover, the clockwise rotation of Pangea was probably refrained when both the Variscan overthickened crustal "anchor", and the Tethys embayment (bounded by subducting slabs; Le Pichon and Huchon, 1984; Hynes, 1990) were guided along the equatorial bulge track. The inertial rotation energy induced a coherent and clockwise disruption of the Variscan foldbelt, by means of the right-lateral EUNWA province paralleling the equator (Van Hilten, 1964; Arthaud and Matte, 1975, 1977; Doblas et al., 1998), while corresponding compressional and extensional shear-zone-ends developped. Thus, this late Variscan event can be understood as "a sprain of the Pangea who made a blunder when crossing the equator, an event leaving a scar on its neck from which it will never heal", and thus this constitutes the true initial disruption of this supercontinent. 

Fig. 3: Idealized sketch showing the plate reconstructions during the breakup of Pangea in the transition from the Triassic to the Jurassic (modified from Oyarzun et al., 1997). CAP: Central Atlantic Plume. ELTS: European large thin spot. Insets: a) NE-directed sublithospheric mantle channeling from the CAP to the ELTS, accompanied by the opening to the west of the Central Atlantic, and the closure to the east of the Tethys; b) location of the Variscan belt which constituted a collapsed preferential corridor for the NE-directed mantle flow. Base maps modified from Ziegler (1989), Klein et al. (1994), and Scotese and Langford (1995). Legend: 1) CAP; 2) tectonic sedimentary troughs; 3) seas; 4) peripheral active foldbelts.

2.3. Late Triassic to early Jurassic protorifting  stage (figs. 3 & 5c)

            The late Triassic to early Jurassic transition was characterized by the activity of an equatorial Central Atlantic superplume (CAP) releasing the heat along the collapsed southern corridor of the Variscan belt, towards an European large thin-spot domain (ELTS), thus inducing north-northeast-directed large-scale sublithospheric plume channeling and widespread tholeiitic magmatism within a giant irregular zone of ~3000 x 4000 km (Oyarzun et al., 1997). The central corridor represents the protorift phase of the future Central Atlantic. During these times, the Paleotethys was closed by the NE-directed drifting of the Cimmerian continent.

            The CAP was located on top of the equatorial bulge in the triple junction between Africa, North America, and South America. This area was characterized by a nearly radial pattern of dikes within a giant tholeiitic province, which broadly converge toward this megaplume (May, 1971; Greenough and Hodych, 1990; Hill, 1991; Ernst et al., 1995; Oyarzun et al., 1997), although the overall spatial distribution of these dikes is highly irregular as they predominate in western North Africa (Fig. 3). In this sense, the CAP superplume originated probably in the core-mantle boundary. As we will see later, the shifting of the CAP and its effects in terms of tholeiitic magmatism might be explained by a mechanism of NE-directed sublithospheric plume channeling (Oyarzun et al., 1997). Additionally, and similar to what happened during the previous stage (late Variscan) with the EUNWA province, it is conceivable that the CAP might also be understood in terms of an exhaust valve further contributing to release the heat accumulated beneath the Pangea by insulation processes (it is located in the western boundary of the present Atlantic-African geoid high). In this sense, it is usually assumed that plumes constitute mantle upwellings with spatial distributions unrelated to specific crustal tectonic conditions, as they seem to depend essentially on instabilities either in the core-mantle boundary, or in the 670 km-deep thermal boundary layer (Kearey and Vine, 1990; Ziegler, 1993). However, it has been suggested that thermal blanketing by large continents might also result in the preferential generation of plumes beneath them (Le Pichon and Huchon, 1984; Worsley et al., 1984; Duncan and Turcotte, 1994). We suggest that this might have been the case for the CAP. We also propose that the CAP might result from extension-related membrane stresses on top of the equatorial bulge, which were particularly effective in this "weak" triple junction located in the intersection of three resistant cratons.

            Another important element of this tectonomagmatic scenario is constituted by the NE-oriented thinned/weakened corridor following the collapsed southern branch of the Variscan belt between eastern North America and western North Africa (Appalachians-Mauritanides belt). This NE-trending corridor constituted a preferred sublithospheric channel along which the head of the CAP was laterally dragged toward the NE (Oyarzun et al., 1997). This corridor was characterized by a progressive asymmetric continental breakup through detachment systems, which will give rise to the protocentral Atlantic rift zone (Manspeizer, 1994; Piqué and Laville, 1995; Oyarzun et al., 1997). Wilson (1966) suggested that the Atlantic closed and opened several times in the Earth's history following precisely the relic scars or geosutures generated by interior collisional orogens (Miyashiro et al., 1982; Van Andel, 1985; Duncan and Turcotte, 1994), a process commonly referred as the Wilson cycle. The opening and closing of oceans roughly along the same lines has been attributed to the presence of eclogite-facies roots of partially collapsed orogens (Ryan and Dewey, 1997). According to these authors, such roots will weaken the orogenic lithosphere relative to that of the adjacent foreland for hundreds of millions of years, and make it a preferred site for later rifting. Costa and Rey (1995) suggested lower crustal rejuvenation processes with intrusion of mantle-derived magmas and granulite-grade metamorphism as a result of the post-thickening extensional collapse of the European Variscan belt, part of which constituted the future trend of the central Atlantic. This protorifting of the Central Atlantic can result from an additional mechanism in terms of a submeridional propagating fracture relevant to membrane stresses imposed on the Pangea drifting through the equator (Turcotte and Oxburgh, 1973; Vroman, 1981), much in the way of what has been suggested for the East African Rift System (Oxburgh and Turcotte, 1974).

            A third element in the late Triassic to early Jurassic tectonomagmatic evolution of the Pangea is constituted by a large European realm with the following characteristics (Fig. 3): 1) As shown by Ziegler's (1989) reconstructions, it constituted a peculiar zone pervaded by rift-type shallow-marine ensialic basins with heterogeneously distributed blocks broken by irregular fracture patterns, as compared to the more stable blocks surrounding it (Laurentia/Greenland, North-Africa, Fenno/Sarmatia, and Tethys). Thus, part of this area was affected by the central European block-faulting-type tectonics that was active during most of the Jurassic (Sëngor, 1979). 2) The pole of the clockwise rotation of the Pangea was located there (Laubscher and Bernoulli, 1977; Matte, 1986; Hynes, 1990). 3) This spot corresponds also to the so-called triple point between Eurasia, Africa, and the Paleotethys (Burek, 1972; Laubscher and Bernoulli, 1977). 4) Its eastern part contains the pole of rotation of the Cimmerian continent which closed the Paleotethys and opened the Neotethys (Sëngor, 1979). 5) Scattered basaltic volcanism is found within and around this realm: Morocco, Algeria, Balearic islands, Iberian Ranges, Cantabrian mountains, Carnic region, and northern Rumania (Dewey et al., 1973).   

            This peculiar European realm has been interpreted in terms of a large thin-spot-type domain (ELTS; Oyarzun et al., 1997), triggering the NE-directed sublithospheric plume channeling from the CAP. Thompson and Gibson (1991) defined the term "thin-spot" as a domain characterized by tectonic lithospheric thinning. Under these conditions, the space vacated by the thinned lithosphere results in a horizontal pressure gradient during the early stages of extension, thus creating a plume suction effect toward the low-pressure zone (Thompson and Gibson, 1991). The ultimate consequence of this phenomenon is the triggering of tholeiitic or alkaline magmatism by decompression melting. It is interesting to note that the ELTS domain (Fig. 3) corresponds aproximately to the same site of the complex Variscan central Pangean zone (Fig. 1), and the late Variscan EUNWA exhaust valve (Fig.2).

            As already mentioned, another important element of this late Triassic to early Jurassic scenario is constituted by the NE-directed closure of the Paleotethys at the expense of the final opening of the Neotethys which began already during the late Variscan stage (Sëngor, 1979; Scotese and Langford, 1995). Note also that the NE-directed Tethys closing/opening events is subparallel to the sublithospheric plume channeling described here from the CAP to the ELTS. 

            Finally, an additional effect of the rotation of the Earth might have been active during these initial times of the breakup of Pangea: according to Hynes (1990), kinetic energy requirements related to inertia-driven toroidal rotations tended to fragment and disperse this supercontinent, which had already become an unstable mass for the rotation of the Earth.

Fig. 4: Highly simplified diagram depicting the generalized NE-directed drifting of the SE fragments of the former Pangea that triggered the closing of the Tethys and the collisional Alpine-Hymalayan foldbelt. Legend: 1) hypothetical pole of rotation of the SE sector of the former Pangea; 2) African European magmatic zone (AEMZ); 3) NE-directed features (shear zones, fractures, transform faults, hot spot tracks, aseismic ridges, etc.); 4) NE-directed drifting of the SE fragments of the former Pangea following the same direction of flow in the mantle (Alvarez, 1982); 5) Alpine-Hymalayan foldbelt. AS: Angola fault system. BR: Bouvet ridge. CF: Cocos fault. CLR: Chagos-Laccadive ridge. GBS: Guinea-Bissau fault system. LMS: Lüderitz-Mombasa fault system. MF: Mozambique fault. NR: Ninetyeast ridge. OF: Owen fault. PS: Pelusium fault system. SH: St. Hellen hot spot track. SLR: Sierra Leone rise. WR: Walvis ridge.

2.4. Jurassic to present NE-directed drifting stage (figs. 4 & 5d)

            The CAP activity and its channeling toward the ELTS that began during the previous stage continued in the eastern margin of the central Atlantic realm from the Cretaceous to the present, leading to alkaline magmatism along a general north-northeast propagating magmatic vector from the Senegal basin and Cape Verde islands to Europe (African-European magmatic zone; AEMZ; Fig. 4; Oyarzun et al., 1997).

            Most of the tectonomagmatic and tectonosedimentary activity in the western north African margin concentrated along two well differentiated active segments (Senegal and Tarfaya-Laâyoune basins). One of the most unusual features of these segments are the Cretaceous to Tertiary basal complexes of the Cape Verde and Canary islands (Stillman et al., 1982; Araña and Ortíz, 1991), which were dramatically uplifted 4 to 7 km, and comprise the following elements: sheeted dike swarms, mafic-ultramafic units, mid-ocean ridge pillow basalts within a 400-km-wide dome-shaped ocean-floor swell in the Cape Verde islands, and carbonatites cropping-out along ductile shear zones (Canary islands). The sedimentary basins of Senegal and Tarfaya-Laâyoune (Morocco) constitute thick and long-lived basins; more than 12 km of  Triassic-Jurassic synrift sediments have been accommodated along west-dipping listric normal faults, and there were Late Cretaceous postrift sedimentary infillings along second-order half grabens (Heyman, 1989; Bellion and Crevola, 1991). Major positive gravity anomalies in the Senegal basin have been interpreted in terms of west-dipping shallow asymmetric mantle-rooted massifs of mafic-ultramafic rocks (Guetat, 1981). Detailed seismic and/or stratigraphic analyses from Morocco to the Cape Verde abyssal plain indicate the existence of west-dipping detachment faults (Heyman, 1989; El Khatib et al., 1996; Reston et al., 1996). Alkaline volcanism reached the European Volcanic Province in Paleocene-Oligocene times and continued to the present (Wilson and Downes, 1991), while the southern European realm was undergoing a generalized extensional collapse during the Cenozoic (Doblas and Oyarzun, 1989a,b, 1990b).

            Two possible mechanisms account for the long-lived and asymmetric character of the sublithospheric plume channeling processes occurring during the passive margin alkaline stage (Oyarzun et al., 1997): (1) north-northeast-directed plume-suction effects generated by the footwall uplift of the Senegal and Tarfaya-Laâyoune segments according to a rolling hinge model (sensu Wernicke and Axen, 1988; King and Ellis, 1990); and (2) additional north-northeast-directed plume-suction effects triggered by the initial opening of the southern branch of the northern Atlantic in the Late Cretaceous, and the extensional disruption of Europe during Tertiary time. According to the model of Oyarzun et al. (1997), the Hoernle et al. (1995) S-wave mantle anomaly described beneath Europe and the eastern central Atlantic may represent a present instant picture of these long-lasting Triassic to present phenomena. Additionally, the eastern central Atlantic margin would represent an anomalous less-passive Atlantic-type margin with an unusual tectonic activity involving dramatic uplifts which ultimately led to the unroofing of deep igneous assemblages (Oyarzun et al., 1997).

            Apart from this NE-oriented margin-parallel sublithospheric plume channeling along the AEMZ, this last stage of fragmentation of the Pangea was characterized by a series of episodes happening subparallel to the NE-trending AEMZ (Fig. 4): the generalized counterclockwise rotation and drifting of the SE sector of the supercontinent, the opening of the NE-trending Indian ocean (Binks and Fairhead, 1992), the closing of the Tethys, and the generation of the Alpine-Himalayan orogen (Fig. 4). It is conceivable that this generalized NE-directed conterclockwise rotational phenomena might have had a pole of rotation in the central/northern Atlantic, somewhere near the Mesozoic pole of rotation of the disgregating Pangea (plotted in this region by Hynes, 1990), which might correspond to the present location of the Great Meteor Group hot spot in the central Atlantic (Fig. 4; Müller et al., 1993). The NE-directed drifting of the SE sector of Pangea (Africa, Arabia, India, Australia, SE-Asian block) has been recognized for a long time (Dietz and Holden, 1974). This generalized drifting is evidenced by a series of NE-oriented conspicuous features (Fig. 4): 1) hot spot tracks in the central Atlantic, the southern Atlantic, and the Indian Ocean (O'Connor and Duncan, 1990); 2) a series of intracontinental-scale shear zones  transecting Africa and the Arabian peninsula (Neev et al., 1982); 3) transform faults or fractures of the oceanic crust (Dietz and Holden, 1974); 4) aseismic ridges in the oceanic floor; and, 5) NE-directed large-scale mantle flow of this whole SE sector of the dismembered Pangea (Hager and O'Connell, 1979; Alvarez, 1982). It is conceivable that the opening of the Atlantic and Indian Oceans was done at the expense of the closing of the Tethys (Le Pichon & Huchon, 1984).

            Plate-moving mechanisms that might account for this NE-directed drifting of the lithospheric plates of this sector of the former Pangea can be understood in terms of drag-forces and shear-traction excerted on the base of the lithosphere by directional asthenospheric convective mantle flow (Ziegler, 1993), and by the outward flow from the axis of superplumes (Phipps Morgan et al., 1995). This seems to be the case in the SE sector of the former Pangea, where NE-directed sublithospheric plume flow from the CAP to the ELTS continued till the present in the form of the AEMZ, together with a generalized NE-oriented mantle flow beneath the whole sector from Africa to Australia (Alvarez, 1982).

            Other major events probably associated to the equatorial bulge and to large-scale mantle upwelling processes occurred in the antipodal superoceanic Panthalassan hemisphere since the Jurassic. There, the breakup of the Panthalassa superocean began by the early Jurassic (190 ma) with the birth of the Pacific plate in its center at an rrr triple junction above the equator (Moores and Twiss, 1995). These equatorial-centered large-scale mantle upwelling processes continued later with the mid-Cretaceous impingement of a megaplume in the central Pacific realm (giving rise among others to the Ontong-Java LIP), accompanied by global geological overturns and major circum-Pacific deformation and uplift (Larson 1991a; Vaughan, 1995; Larson and Kincaid, 1996; Tatsumi et al., 1998). In this sense, the global deformations of the Pacific hemisphere during the mid Cretaceous (central extension and peripheral compression; Vaughan, 1995) are well in accordance with the membrane tectonics theory which advocates that these contrasted stresses are typically generated by a plate moving on top of the equatorial bulge (Turcotte and Oxburgh, 1973; Oxburgh and Turcotte, 1974). Large-scale mantle upwelling processes continued till the present beneath the equatorial Pacific as shown by the existence of a series of long-lived instabilities, upwelling processes, and elevated thermal gradients, such as: the Darwin Rise (Menard, 1984), the Polynesian superswell (McNutt and Judge, 1990), and a major positive geoid anomaly (Cazenave and Feigl, 1994). 

 Fig. 5: Schematic diagrams summarizing the four-stage assembly and breakup evolution of Pangea: A) Variscan stage; B) Late Variscan stage; C) Late Triassic to early Jurassic stage; D) Jurassic to present stage. AEMZ: African-European magmatic zone. AHB: Alpine-Hymalayan belt. AM: Appalachian-Mauritanides. ARS: Atlantic ridge system. CAP: central Atlantic Plume. ELTS: European large thin spot. EQ: equator. EUNWA: European northwestern African domain. EVO: European Variscan orogen. GB: Great Basin. NGR: Norwegian-Greenland rift. NT: Neotethys. PAAGH: present Atlantic-African geoid high. PT: Paleotethys. T: Tethys. UR: Urals.

3. Discussion and conclusions

            In this paper we argue that the rotation of the Earth might have had a preponderant role in the aggregation and dispersal of the Pangea through two different mechanisms: 1) during the Paleozoic welding of the Pangea, kinetic energy requirements had a role in the amalgamation of the continents and some favorably-oriented interior orogens and seas toward the equator; 2) since the late Paleozoic, disrupting membrane stresses triggered by the equatorial bulge and inertia-driven toroidal rotations facilitated the breakup of the Pangea. It is important to remind here that the velocity of the Earth's rotation has been slowly decreasing during its history (407 to 365 days a year for the lower Silurian and the present, respectively; e.g., Coulomb and Jobert, 1972). Thus, the possible effects of the equatorial bulge on the disruption of drifting plates should have been greater during the Paleozoic than at present.

            Another additional factor that enhanced the rupture of the Pangea is related to thermal insulation and blanketing effects which prevailed since the end of the Palaeozoic beneath the supercontinental cap, triggering large-scale mantle-wide upward convection which has lasted until the present in the form of the Atlantic-African residual geoid high. This heat was progressively released by means of two exhaust valves located on top of the equatorial bulge: the late Paleozoic EUNWA province, and the Triassic/Jurassic CAP.

            An important question highlighted in this paper is whether large-scale and long-lived sublithospheric lateral plume channeling phenomena might have been important as plate-moving mechanism contributing to breakup the Pangea and triggering the generalized NE-directed drifting of the SE fragments of Pangea.

            The European realm represented a long-lived Variscan to present sublithospheric singularity which has yet to be explained within the scenario of moving plates: the European complex Variscan orogenic sector was later the site of the EUNWA exhaust valve during the late Variscan, the ELTS realm during the Triassic/Jurassic transition, and the European volcanic province sector of the AEMZ from the Cenozoic to the present.

            The Variscan and AHB east-west-oriented interior orogens might be somehow related to each other, the first one associated to the clockwise amalgamation of the Pangea, and the second one triggered by the counterclockwise drifting apart of Pangea and closure of the Tethys. In this sense, it is interesting to note that the genesis of the Alpine-Hymalayan Belt (AHB) might be ultimately understood in terms of a far-reaching consequence of the initial breakup of the Pangea along the equatorial belt, following the weakened trend of the Variscan belt.

            The cyclicity of the processes of breakup and coalescence of continents might be explained within the hypothesis suggested here for the Pangea, as an additional mechanism to other theories already proposed: insulation-induced heating (Anderson, 1982), earth-rotation stresses (Le Pichon and Huchon, 1984), breaking-healing plates (Gurnis, 1988), pulsation tectonics (Sheridan, 1997), lower/upper mantle contrasted exchange rates (Stein and Hofmann, 1994), deep mantle plumes (McNutt and Judge, 1990; Sleep, 1990; Vaughan, 1995), or slab-induced disruption of the 670 km and D" layers (Larson and Kincaid, 1996).

 Acknowledgments.  We thank Professors Nasserdinne Youbi and José López-Ruiz for their useful comments. We acknowledge the drafting of the figures by José Arroyo.

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