Flood Basalts

The break-up of Pangaea was principally facilitated by tensional plate stress acting on pre-existing suture zones. The rifting of Pangaea began during the Early Permian along the southern Tethys margin and produced the lenticular-shaped continent known as Cimmeria. A mantle-plume model is ascribed to explain the rift-related volcanism but the NW-SE oriented Cimmerian rifts do not correlate well with pre-existing suture zones or 'structural heterogeneities' but appear to have a pertinent spatial and temporal association with Late Palaeozoic glacial-interglacial cycles. Mantle potential temperature estimates of Cimmerian rift-related basalts (1410 °C ± 50 °C) are similar to ambient mantle conditions rather than an active mantle-plume rift as previously suggested. Moreover, we find that the distribution of glacial deposits shows significant temporal and spatial concurrence between the glacial retreat margins and rifting sites. We conclude that the location and timing of Cimmerian rifting resulted from the exploitation of structural heterogeneities within the crust that formed due to repeated glacial-interglacial cycles during the Late Palaeozoic. Such effects of continental deglaciation helped to create the lenticular shape of Cimmeria and Neotethys Ocean suggesting that, in some instances, climate change may directly influence the location of rifting. Crustal thinning, narrow rifting characterized by localization of strain through large boundary fault activation, and basin subsidence are the signature features of the early stages of continental break-up 1,2. Transient excess magmatic upwelling and emplacement covering the inboard portions of continental rift margins follow the early stages of rifting 2–4. Continent break-up/rifting requires tremendous amount of energy. In order to conserve energy, initial rifting sites within continental crust are often spatially associated with pre-existing 'structural het-erogeneities' such as suture zones related to ancient orogenic boundaries or regions of the crust with differential thickness 2,5–7. A given 'structural heterogeneity' may be exploited by either a mantle-plume (active extension) or a regional stress field (passive extension) associated with plate boundary forces (slab-pull) that becomes a focal point for flood basalt volcanism before developing into an ocean basin 8. Moreover, icosahedral structures with triple-junction configuration form quasi-hexagonal fractures that minimize the total boundary length and produce polyhedral plate configurations, thus minimizing the energy, area, work and stress required to brittely break plates 9,10. The amalgamation and break-up of Pangaea exemplifies the secular nature of global tectonic processes as plate stresses transitioned from compressional to extensional during the Late Carboniferous to Early Jurassic 6,10–12. There are a number tectonomagmatic models proposed to explain the break-up of Pangaea such as: post-orogenic collapse 13 , dispersal over a mantle-super plume 14 , passive rifting and self-subduction of a super-plate 11,15. However, in many cases, rifting and magmatism were initiated at old suture zones that formed during the amalgamation stage. Some of the hypotheses provide a partial explanation for the break-up of the Gondwana portion of Pangaea specifically during the Jurassic to Cretaceous but do not adequately address an earlier period of rifting 5,11. The break-up of Gondwana began during the Early Permian along a ~13,000 km long NW-SE oriented continental rift just as the Late Palaeozoic continental ice sheet retreated 16–18. The rifting of terranes from the Tethyan margin of Gondwana produced the ribbon-like continent Cimmeria and the Neotethys Ocean. The rifting of Cimmeria and accompanying magmatism are thought to be related to a regional-scale mantle-plume 19–21. However, geochemical and structural studies of the Panjal Traps (Kashmir), the single largest contiguous out-cropping of Early Permian flood basalts associated with Cimmerian rifting, suggest they are related to passive

Abstract[1] Crustal structure in Kenya and Ethiopia has been investigated using receiver function analysis of broadband seismic data to determine the extent to which the Cenozoic rifting and magmatism has modified the thickness and composition of the Proterozoic crust in which the East African rift system developed. Data for this study come from broadband seismic experiments conducted in Ethiopia between 2000 and 2002 and in Kenya between 2001 and 2002. Two methods have been used to analyze the receiver functions, the H-κ method, and direct stacks of the waveforms, yielding consistent results. Crustal thickness to the east of the Kenya rift varies between 39 and 42 km, and Poisson's ratios for the crust vary between 0.24 and 0.27. To the west of the Kenya rift, Moho depths vary between 37 and 38 km, and Poisson's ratios vary between 0.24 and 0.27. These findings support previous studies showing that crust away from the Kenya rift has not been modified extensively by Cenozoic rifting and magmatism. Beneath the Ethiopian Plateau on either side of the Main Ethiopian Rift, crustal thickness ranges from 33 to 44 km, and Poisson's ratios vary from 0.23 to 0.28. Within the Main Ethiopian Rift, Moho depths vary from 27 to 38 km, and Poisson's ratios range from 0.27 to 0.35. A crustal thickness of 25 km and a Poisson's ratio of 0.36 were obtained for a single station in the Afar Depression. These results indicate that the crust beneath the Ethiopian Plateau has not been modified significantly by the Cenozoic rifting and magmatism, even though up to a few kilometers of flood basalts have been added, and that the crust beneath the rifted regions in Ethiopia has been thinned in many places and extensively modified by the addition of mafic rock. The latter finding is consistent with models for rift evolution, suggesting that magmatic segments with the Main Ethiopian Rift, characterized by dike intrusion and Quaternary volcanism, act now as the locus of extension rather than the rift border faults.

... 6A and B). Of the samples with isotopic analyses, the low-Ti sample 121600 from Sandoy (87Sr/86Sr ... Niels Hald, Marie Drejer-Nielsen and Malan Ellefsen are thanked for their efforts during the field-work. ... Dosso, L., Hanan, BB, Bougault, H., Schilling, J.-G., Joron, J.-L., 1991. ...

The Indian Plate has been the focus of intensive research concerning the flood basalts of the Deccan Traps. Here we document a volcanostratigraphic analysis of the offshore segment of the western Indian volcanic large igneous province, between the shoreline and the first magnetic anomaly (An 28 ∼63 Ma). We have mapped the different crustal domains of the NW Indian Ocean from stretched continental crust through to oceanic crust, using seismic reflection and potential field data. Two volcanic structures, the Somnath Ridge and the Saurashtra High, are identified, extending ∼305 km NE–SW in length and 155 km NW–SE in width. These show the internal structures of buried shield volcanoes and hyaloclastic mounds, surrounded by mass-wasting deposits and volcanic sediments. The structures observed resemble seismic images from the North Atlantic and northwest Australia, as well as volcanic geometries described for Réunion and Hawaii. The geometry and internal seismic facies within the volcanic...

New constraints on the timing of the Cretaceous-Paleogene mass extinction and the Chicxulub impact, together with a particularly voluminous and apparently brief eruptive pulse toward the end of the “main-stage” eruptions of the Deccan continental flood basalt province suggest that these three events may have occurred within less than about a hundred thousand years of each other. Partial melting induced by the Chicxulub event does not provide an energetically plausible explanation for this coincidence, and both geochronologic and magnetic-polarity data show that Deccan volcanism was under way well before Chicxulub/Cretaceous-Paleogene time. However, historical data document that eruptions from existing volcanic systems can be triggered by earthquakes. Seismic modeling of the ground motion due to the Chicxulub impact suggests that the impact could have generated seismic energy densities of order 0.1–1.0 J/m3 throughout the upper ~200 km of Earth’s mantle, sufficient to trigger volcani...

We propose that the Rajahmundry Trap lavas, found near the east coast of peninsular India , are remnants of the longest lava flows yet recognized on Earth (˜ 1000 km long). These outlying Deccan-like lavas are shown to belong to the main Deccan Traps. Several previous studies have already suggested this correlation, but have not demonstrated it categorically. The exposed Rajahmundry lavas are interpreted to be the distal parts of two very-large-volume pahoehoe flow fields, one each from the Ambenali and Mahabaleshwar Formations of the Wai Sub-group in the Deccan Basalt Group. Eruptive conditions required to emplace such long flows are met by plausible values for cooling and eruption rates, and this is shown by applying a model for the formation of inflated pahoehoe sheet flow lobes. The model predicts flow lobe thicknesses similar to those observed in the Rajahmundry lavas. For the last 400 km of flow, the lava flows were confined to the pre-existing Krishna valley drainage system that existed in the basement beyond the edge of the gradually expanding Deccan lava field, allowing the flows to extend across the subcontinent to the eastern margin where they were emplaced into a littoral and/or shallow marine environment. These lavas and other individual flow fields in the Wai Sub-group may exceed eruptive volumes of 5000 km 3, which would place them amongst the largest magnitude effusive eruptive units yet known. We suggest that the length of flood basalt lava flows on Earth is restricted mainly by the size of land masses and topography. In the case of the Rajahmundry lavas, the flows reached estuaries and the sea, where their advance was perhaps effectively terminated by cooling and/or disruption. However, it is only during large igneous province basaltic volcanism that such huge volumes of lava are erupted in single events, and when the magma supply rate is sufficiently high and maintained to allow the formation of very long lava flows. The Rajahmundry lava fields were emplaced around 65 Ma during the later times of Deccan volcanism, probably just after the K/T environmental crisis. However, many lava-forming eruptions of similar magnitude and style straddled the K/T boundary.

The Indian Plate has been the focus of intensive research concerning the flood basalts of the Deccan Traps. Here we document a volcanostratigraphic analysis of the offshore segment of the western Indian volcanic large igneous province, between the shoreline and the first magnetic anomaly (An 28 ∼63 Ma). We have mapped the different crustal domains of the NW Indian Ocean from stretched continental crust through to oceanic crust, using seismic reflection and potential field data. Two volcanic structures, the Somnath Ridge and the Saurashtra High, are identified, extending ∼305 km NE–SW in length and 155 km NW–SE in width. These show the internal structures of buried shield volcanoes and hyaloclastic mounds, surrounded by mass-wasting deposits and volcanic sediments. The structures observed resemble seismic images from the North Atlantic and northwest Australia, as well as volcanic geometries described for Réunion and Hawaii. The geometry and internal seismic facies within the volcanic...

We examine the role that flood basalt eruptions may have played during times of mass extinction through the release of volcanic gases. Continental flood basalt provinces have formed by numerous eruptions over a short period of geologic time, characteristically a few million years. Within this period, a short-lived climactic phase that lasts about 1 Ma typically emplaces a large proportion of the lava volume. This phase consists of a series of huge eruptions, each yielding 103–104 km3 of magma. Each eruption lasted on the order of a decade or more, and built an immense pāhoehoe-dominated lava flow field by eruptive activity along fissures tens to hundreds of km long. High fire-fountains, emanating from vents along the fissures, at times sustained eruption columns that lofted gas and ash into the upper troposphere and lower stratosphere while the lava flows covered huge areas. The combination of large eruption magnitudes, maintained high effusion rates during eruptions, and the repeated nature of the characteristic, large-scale eruptive activity occurs in Earth history only during periods of flood basalt volcanism. Based on recent analogs and determination of volatile contents of ancient flood basalt lavas, we estimate that individual eruptions were capable of releasing 10,000 Tg of SO2, resulting in atmospheric loadings of 1000 Tg a− 1 during a sustained decade-long eruptive event. We apply this model of flood basalt volcanism to estimate the potential mass of CO2 and SO2 released during formation of the ∼ 65 Ma Deccan province. The Deccan lava-pile contains the record of hundreds of enormous pāhoehoe flow-fields erupted within a period of about 1 Ma. Consequently, atmospheric perturbations associated with SO2 emissions from just one of these long-lasting eruptions were likely to have been severe, and constantly augmented over a decade or longer. By contrast, the amounts of CO2 released would have been small compared with the mass already present in the atmosphere, and thus much more limited in effect. Individual eruptions were followed by hiatuses of hundreds to thousands of years during which the gas contributions to the atmosphere would be recycled. It is clear that the nature and potential atmospheric impact of a series of huge-volume, repeated, long-term degassing events requires further investigation in conjunction with appropriate climate models.

The voluminous Proterozoic continental tholeiites of the Coppermine River province (Northwest Territories), which are coeval with the Mackenzie magmatic event (1.27 Ga old) and were emplaced over a short period of time (<5 Ma), have trace element and Sr-Nd-Pb isotopic characteristics comparable to those of Phanerozoic flood basalts. The variations of the compositional parameters are inferred to be due to a mixing of at least two components: mantle and crust. In addition, the mantle component probably includes two end members. The first is a mantle plume, whereas the other represents the base of the continental lithosphere. The crustal component reflects contamination of the magma by Precambrian basement during its ascent to the surface.

We present compositional data on a 1,250-mthick
sequence of sparsely porphyritic lavas that comprise
the Geikie Plateau Formation, part of the ~55-
Ma break-up-related flood basalts in East Greenland.
Major element compositions are relatively restricted
(6.3–7.6 wt% MgO; 2.2–2.4 wt% TiO2), with two
excursions to more evolved compositions (2.4–3.4 wt%
TiO2) that are similar to the inferred parental magma
of the nearby Skaergaard Intrusion. Major and trace
element calculations show that fractional crystallisation
is the principal control on magma compositions, and
the cyclical sequential variations imply regular magma
chamber replenishment events. Isotopic data indicate
minor crustal assimilation, but with different contaminants
for the main group (amphibolitic gneiss) and
evolved cycles (granulitic gneiss). Rifting episodes may
have allowed more primitive magmas to ascend to
shallow crustal levels and subsequently fractionate to
more evolved compositions in a separate chamber,
which was perhaps similar to the source of the
Skaergaard Intrusion.