Australia: The Land Where Time Began
and the Tethyan subduction zone, respectively, appear to be associated genetically.
Flood Basalts, Continental Breakup and Dispersal of Gondwana - Periodic Migration of Upwelling Mantle Flows (Plumes)
In this study the author1 searched for indications of the paths taken by plume activity by making use of igneous provinces, mostly continental flood volcanics or oceanic plateaus, some of which are associated with regional updoming, major rifting and the breakup of continents, as well as with magmatic events that have been dated precisely. Plumes of the first generation, such the European-northwest African (EUNWA) from the Permo-Carboniferous, the Karoo and northwest Australia, dated to the Jurassic, and convergent environments, such as the Variscan Orogen, the Pacific subduction zone.
Segev suggests that this may indicate that mantle plumes have a tendency to be initiated by long-lived slabs of lithosphere that are downwelling, that cause instabilities at the boundary layers of the lower mantle. According to Segev first generation plumes are larger, having a diameter of 3,000 km while the late-generation plumes have a diameter of about 2,000 Km, and appear to disperse magmatism and rifting radially. Theses plumes ramify spatially, a second plume generation commonly splitting to form 2 offshoots, and even a 3rd generation developed, acting in a similar temporal rhythm of plume activity for about 60 My then about 20 My of quiescence.
The Indian path had an exceptionally short-lived, relatively speaking, plume swarm - Rajmahal-Kerguelen, about 20 My, Madagascar, about 8 My, and Deccan, about 7 My, the migration of which changed significantly the course from the main trend. Along this path the recurrent consequential breakup and formation of spreading centres indicates migration below the upper mantle circulation This plume swarm may also indicate a source in the lower mantle influenced by a weak flow to the northeast.
The expansion and magmatism within igneous provinces probably results from plume migration beneath the lithosphere, though plume migration is suggested to have occurrence in the lower mantle.
Discussion & Conclusion
Segev used 2 lines of fractal data in this study:
Surface and submarine geology - igneous province distribution , mainly flood basalts and oceanic plateaus, that were involved in the process of lithospheric breakup of Gondwana.
Geochronology - Chronology of magmatic events in each described igneous provinces, that is reliable and up-to-date.
The synthesis of geological and geochronological evidence of the distribution, spatial and temporal, of igneous provinces, that were associated by domal uplift, rifting and breakup of the continental lithosphere of Pangaea, and later Gondwana, the mantle plume theory being adopted as the main cause of these phenomena (e.g. Storey, 1995; Courtillot et al., 1999; Segev, 2000a). In this paper Segev presented schematically, the plate reconstruction that was produced by several researchers based on geophysical evidence, in order to understand more clearly the consequences of each plume and to show the different tectonic setting in which the later plume acted. Segev says the relationship between plate motions and plume provinces are beyond the scope of this paper.
First generation plumes - their initiation
The first generation of powerful plumes from the lower mantle marks 3 plume provinces since Pangaea assembled in the Permo-Carboniferous.
These plumes are associated closely with convergent environments, 1 of which is an orogen, and the other 2 are subduction zones. They therefore indicate the existence of a cogenetic relationship between convergence and initial plumes, that was actually to the lithospheric slabs that was downwelling.
The large quantities of slab melts that resulted, rifting of lithosphere, the breakup and formation of ocean spreading centres, is typical of all of them, though each of these plumes met different lithospheres - orogen-EUNWA, craton-Karoo, plate margins-northwest Australia.
The subducting slabs cross the discontinuity at 660 km, as shown by the seismic structure of slabs, following which they descend towards the core-mantle boundary (CMB) (van der Hilst & Seno, 1993; van der Hilst et al., 1997; Bijwaard & Spakman, 1998). Significant amounts of instability are caused by such a structure at the depth of 660 km seismic discontinuity, as well as close to the CMB. It is indicated by models of convection that instabilities in boundary layers might possibly trigger the formation of new plumes (White & Chen, 1970; Dubuffet et al., 2000).
Detachment of Gondwana from Pangaea
The plume activity in central Pangaea (Doblas et al. 1998) of the EUNWA plume from the Permo-Carboniferous, about 320-275 Ma, led to the opening of the Neotethys in the Permo-Triassic, and in northern Europe to the intensive rifting, as well as towards the central Atlantic and Levant areas (Ziegler, 1990; Guiraud, 1998; Segev, 2000b). In the Jurassic, a few 10s of millions of years later, 2 adjacent plumes developed almost synchronously, towards the central Atlantic, the southwestern path, and the Western Tethys, the southeastern path. These Jurassic plumes, that were newly formed, are suggested by the spatial overlapping between the EUNWA plume and these newly formed plumes, as well as by periodic age progression, to be connected to the same genetic process of uprising material from the lower mantle. Gondwana was detached completely from Eurasia as a result of the formation of these second-generation plumes, and they mark the development of 2 migrating upwelling flows from the lower mantle, though the plume activities within the latter supercontinent were not part of the present study.
Third generation plumes, that were newly formed in the Cretaceous, beneath the Equatorial Atlantic on the southwest path, 138-68 Ma, and the Levant Nubia, 138-83 Ma, on the southwest path, occurred with the same trend in each path. When the plate tectonic configuration changed in the Late Cretaceous the Levant-Nubia plumes abruptly ceased, though in the Equatorial Atlantic magmatic activity decreased over time, until it ceased in the Tertiary. In the latest Jurassic the magmatism in the Equatorial Atlantic was initiated and acted periodically synchronously with the Paraná-Etendeka Province, the area remaining closed until 119-120 Ma.
Gondwana breaks into east and west sectors and their southern dispersals
As with northern Gondwana, the Jurassic (about 204-142 Ma) Karoo initial plume that caused the split between East and West Gondwana along the proto-oceanic rift between the Weddle Sea and the Somali Basin, developed into southern Gondwana. From the southernmost tip of the the South Atlantic up to the vicinity of the Salado Basin in South America a minor phase of rifting began close to the end of the Karoo Province (Nürnberg & Müller, 1991). The dispersal of magmatism towards Patagonia, at the southern end of South America, and Victoria, southeast Australia, occurred along an almost normal trend that was sub-parallel to the Pacific subduction zone, though the trend of the main breakup of the supercontinent was SSW-NNE.
In the Cretaceous, following the Karoo plume activity 2 second-generation offshoot plumes developed along 2 opposite edges along the Paraná-Etendeka (WNW)-Marie Byrd Land-East Australia (ESE) direction. The Paraná-Etendeka plume, 138-83Ma, is a well-studied feature (Renne et al., 1999), that is suggested by Segev to have caused the breakup between Africa and South America, and the Marie Byrd Land-East Australian plume (133-82 Ma), caused the breakup between Antarctica and New Zealand and the opening of the Tasman Sea (Storey et al., 1999). A third generation plume (Bellamy) overlapped with the latter, Tertiary-recent, that changed the tectonic setting of the previous plume. The magmatism continued in the Paraná-Etendeka province up to 83 Ma in volcanic fields (Alto Paranaiba) and various igneous intrusions that surround the Paraná Basin (Gibson et al., 1995). It is assumed there was a rigid attachment between Africa and South America (Nürnberg & Müller, 1991) until magnetic Chron M4 (ca. 127 Ma), and rifting propagated to the north into the Benue Trough between this Chron and Chron O (118.7 Ma). n both parts of this province there is a lack of evidence, in the form of updoming, new rifting, extensional direction changes and the breakup of continents, for younger third plume evolution. Until 70 Ma intrusion of dykes in the Paraná area have been reported (Deckart et al.,1998), and in Paraguay alkaline magmatism is known until the Tertiary (Comin-Chiaramonti et al., 1999). Following the main period of plume activity, there was magmatism in the Tertiary in the South Atlantic Ocean of oceanic islands (Trinidad-Victoria, Walvis Ridge and Rio Grande Rise), which represents a late stage of alkaline volcanism. Together with arguments for a plume beneath the Paraná Basin existing recently (vanDecar et al. 1995), this evidence indicates a progressive decay, over a period of about 80 My, of the Paraná-Etendeka Plume.
The Indian path - dispersal of northern East Gondwana
In northwestern Australia the Jurassic igneous province is not well defined, as it includes either continental margins and oceanic plateaus (Rowley Terrace-Scott Plateau and Exmouth Plateau) that are poorly accessible or, towards the north, lithospheric fragments that have been subducted. Intensive magmatism, extension, rifting and breakup of this area is suggested by evidence that has been published (Powell et al., 1988; Mutter et al., 1989; Veevers et al., 1991). For this first generation plume the final magmatism (White & McKenzie, 1989), that dates to about 155 Ma, and the possible expansion of it towards the Pacific (exterior Gondwana) was not included in this study. The second generation plume from the Early Cretaceous (Rajmahal-Kerguelen) migrated south towards the area separating India from Australia, that at this time remained attached to Antarctica. The Wallaby and Naturaliste Plateaus were the first to be produced at this new position (Crawford & von Rad, 1994), as well as the Bunbury Basalt, that was defined better (130 ± 0.5 Pringle et al., 1994), the oldest rock studied. Measurement of the initiation of these oceanic plateaus and Kerguelen Plateau was prohibited by the limited access to their lower parts. An earlier initiation of magmatism is indicated by the contemporaneous initiation off the Western Australia margin of seafloor spreading (Powell et al., 1988) indicating an earlier beginning of magmatism. About 138 Ma is the age predicted by Segev, which is the synchronous magmatic event that initiated the igneous provinces of the South Atlantic, Equatorial Atlantic, Levant-Nubia, and Marie-Byrd Land-Australia (Segev, 2000a). Between 117 Ma and 109 Ma the main eruption of basalt along the India-Antarctica boundary occurred (Rajmahal, Bengal Basin and Kerguelen) took place (reviewed by Segev, 2000a), alongside the developing oceanic spreading centre (Gopala Rao and Krishna, 1997). Representing the late-stage volcanism after the main plume activity, in the eastern Indian Ocean, ocean island magmatism in the eastern Indian Ocean (Ninetyeast Ridge and Broken Ridge) continued until the Tertiary.
Along the same westward trend a third generation plume produced large quantities of melts in Madagascar, Madagascar Plateau and the area of the Seychelles since 92 Ma. According to Segev the evidence indicates that migration was relatively rapid, about 40 My, and the lower mantle current that jumped northward was reactivated, for its 4th time, below western India and the Seychelles about 24 My later, lasting 69-62 My. According to Segev each of these plumes is treated separately because of the complete evolutionary sequence produced by them, from updoming to flood basalts, breakup and opening of new spreading ridges. The last generation, the 5th, of this current that migrates rapidly, is the Afar Plume, beginning at 40 Ma and continuing to the present.
Relative to the African Plate the position of these plumes demonstrates a swarm of plumes that are close, migrating predominantly along the same trend to the west.
The initial plumes, the EUNWA and the Karoo, that are better known, the Northwest Australia, that is lesser known, differ from the others as they are larger, being about 3,000 km wide compared to about 2,000 km in diameter, and by having what appears to be radial magmatism and rifting dispersion, though all of them have a single spreading system along their main rift. The proposal that the plumes of Gondwana led to an extensional regime, that was newly established, that resulted from the divergent circulation of the upper mantle is strengthened by the study of all Gondwana plumes forming such a spreading system (Segev, 2000a). "Passive rifts" that do not connect directly with mantle plumes are not negated by this interpretation. The Southeast Indian Ridge located between southern Australia and Antarctica is an example that may possibly represent such a case, though the plume activity of the Balleny Plume might have had a strong influence on it.
The exceptional behaviour of the Indian Plume swarms were part of the magmatic events, Madagascar and Deccan, which continued for about 8 My, representing the entire evolution of the plume, as is emphasised by comparisons between the studied plumes and their paths. This is the only path in which the course from the main trend was changed significantly by the migrating current. The authors1 suggest a similar deep source, the Lower Mantle, is indicated by the plume swarm, that was relatively close, and short-lived, and an interruption of a weak flow to the northeastwards might have led to the course change. Migration beneath the circulation of the Upper Mantle, that later led to divergent circulation in it and lithospheric breakup, is indicated by the recurring consequential breakup and formation of spreading centres along the Indian Path. A spatial difference between the plume province and the wider, igneous province, is emphasised by the results of the present study, the igneous province generally being about 3 times greater along its extended axis than that of the plume province. The Afar Plume Head is defined as the 5th plume generation along the Indian path, migrating within the lower mantle, as suggested above. The authors1 suggest the sub-lithospheric migration of the Afar Plume is probably represented by the large northwards volcanic fields (Harrats), mostly along the Arabian Plate's western margin, and the southwards volcanism along the Western and Eastern African rifts, these rifts being a continuous dispersion of volcanic activity outwards from the plume province.
|Author: M.H.Monroe Email: firstname.lastname@example.org Sources & Further reading|