Tag: Sea-floor spreading

Hot-spot track beneath the Greenland ice cap

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Around 63 Ma ago, during the Palaeocene Epoch, major igneous activity broke out in what are now both sides of the North Atlantic Ocean. After initial sputtering it culminated massively between 57 and 53 Ma. Relics are to be seen in Baffin Island, West and East Greenland, the Faeroes and north-western parts of the British Islands, in the form of flood basalts, dyke swarms and scattered remnants of central volcanoes. Offshore drilling on the North Atlantic’s continental shelves suggests that the volcanism extended over 1.3 million km2 and blurted out around 6.6 million km3 of magma. Not for nothing have the products of this event been categorised as a Large Igneous Province. Its formation took place before the North Atlantic existed. It began to form as this precursor magmatic paroxysm waned.  Continued basaltic magma production created the ocean floor each side of the mid-Atlantic Ridge system to divide North America and Greenland from northern Europe. Sea floor spreading continues, rising above sea level in Iceland, which is underlain by a large mantle plume.

The plume beneath Iceland may have been present at a fixed position in the mantle for tens of million years. A hot spot over which plate movements have shifted lithosphere to be heated in a similar way to a sheet of paper dragged slowly over a candle flame. The Iceland plume may have left a hot-spot track similar to that involved in the Hawaiian island chain. The ocean floor to the east and west of Iceland is shallower and forms broad rides at right angles to the trend of the Mid-Atlantic Ridge system, judged to be such tracks that are still warm and buoyant after formation over the plume. But are there traces of earlier passage of drifting lithosphere over the plume. A way to detect older hot-spot tracks is through variations in geothermal heat flow through the continental surface, a linear pattern raising suspicions of such trace of passage. There is no sign to the east beneath Europe, so what about to the west. Greenland, being mainly blanketed in ice, is not a good place to conduct such a search as it would involve deep drilling through the ice at huge cost for each hole. But there is a roundabout way of obtaining geothermal information without even setting foot on Greenland’s icy wastes.

The geomagnetic field measured at the surface records anomalies in rock magnetisation in the solid Earth beneath. Near-surface variations due to large variations in rock types that comprise the continental crust appear as sharp, high frequency signals. Aeromagnetic surveys over Greenland are characterised by such noisy patterns because the subsurface geology is extremely complicated. However, the underlying upper mantle beneath all continents is geologically quite bland, but being uniformly rich in iron it contains a high proportion of magnetic minerals such as magnetite (Fe3O4). The upper mantle should therefore leave a signal in the surface geomagnetic field, albeit a commensurately bland one. Like radio signals that span a large range of wavelengths, Earth properties that vary spatially, such as the geomagnetic field, may be analysed using filters. Once the high-frequency geomagnetic features of the crust are filtered out what should remain is a signal that reflects the magnetic structure of the upper mantle. It should be more or less featureless, yet beneath Greenland it isn’t.

greenland hot spot
Estimated Curie depth variation below Greenland (left) converted to geothermal heat flow variation (right). (Credit: Martos et al. 2018; Figures 1b and 1c)

Magnetic anomalies are created by magnetisation induced in magnetic minerals in rocks by the Earth’s magnetic field. Yet minerals lose their ability to be magnetised at temperatures above a threshold known as the Curie point, which is 580 °C for magnetite, the most abundant magnetic mineral. Depending on the geothermal heat flow the Curie point is exceeded at some depth in the lithosphere. So magnetic anomalies can safely be assumed to be produced only by rocks above the so-called Curie depth. Yasmina Martos of the British Antarctic Survey (now at the University of Maryland) and scientists from Britain, the US and Spain used a complex procedure, including gravity data and a few direct measurements of heat flow below Greenland as well as filtered aeromagnetic data, to estimate the variation in Curie depth beneath the ice cap. (Martos, Y.M. et al. 2018. Geothermal heat flux reveals the Iceland hotspot track underneath Greenland. Geophysical Research Letters, v. 45, online publication; doi: 10.1029/2018GL078289). Using that as an inverse proxy for heat flow they were able to map the likely geothermal variation beneath the island. Rather than a random and narrow variation in depth, as would be expected for roughly uniform heat flow, the Curie depth varied in a non-random way by over 20 km, equivalent to roughly 20 mW m-2.

The shallowest Curie depth and highest estimated heat flow occurs in East Greenland around Scoresby Sund where the largest sequence of Palaeocene flood basalts occur. It is also on a line perpendicular to the mid-Atlantic Rift system that meets the active Iceland plume. Running north-west from Scoresby Sund is a zone of locally high estimated heat flow. Martos et al. suggest that this is the track of Greenland’s motion over the Iceland hot spot from about 80 Ma to the period of maximum on-shore volcanism and the start of sea-floor spreading at around 50 Ma.

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Afar: the field lab for continental break-up

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The Afar Depression of Ethiopia and Eritrea is a feature of tectonic serendipity. It is unique in showing on land the extensional processes and related volcanism that presage sea-floor spreading. Indeed it hosts three rift systems and a triple junction between the existing Red Sea and Gulf of Aden spreading centres and the East African Rift System that shows signs of future spalling of Somalia from Africa. Afar has been a focus of geoscientific attention since the earliest days of plate theory but practical interest has grown rapidly over the last decade or so when the area has become significantly more secure and safe to visit. Two recent studies seem to have overturned one of the most enduring assumptions about what drives this epitome of continental break-up.

Perspective view of the Afar depression and en...
Simulated perspective view of the Afar depression from the south (credit: Wikipedia)

From the obvious thermal activity deep below Afar, linked with volcanism and high heat flow, a mantle host spot and rising plume of deep mantle has been central to ideas on the tectonics of the area. A means of testing this hypothesis is the use of seismic data to assess the ductility and temperature structure of deep mantle through a form of tomography. The closer the spacing of seismic recording stations and the more sensitive the seismometers are the better the resolution of mantle structure. Afar now boasts one of the densest seismometer networks, rivalling the Earthscope USArray. http://earth-pages.co.uk/2009/11/01/the-march-of-the-seismometers/ and it is paying dividends (Hammond, J.O.S. and 10 others 2013. Mantle upwelling and initiation of rift segmentation beneath the Afar Depression. Geology, v. 41, p. 635-638). The study  brought together geoscientists from Britain, the US, Ethiopia, Eritrea and Botswana, who used data from 244 seismic stations in the Horn of Africa to probe depths down to 400 km with a resolution of about 50 km.

The tomographic images show no clear sign of the kind of narrow plume generally aasociated with the notion of a ‘hot spot’. Instead they pick out shallow (~75 km depth) P- and S-wave  low-velocity features that follow the axes of the three active rift systems. The features coalesce at depth; in some respects the opposite of a classic plume that has a narrow ‘stem’ that swells upwards to form a broad ‘head’. If there ever was an Afar Plume it no longer functions. Instead, the rifts and associated lithospheric thinning are associated with a mantle upwelling that is being emplaced passively in the space made available by extensional tectonics. This is closely similar to what goes on beneath active and well-established mid-ocean spreading centres where de-pressuring of the rising mantle results in partial melting and basaltic magmatism along the rift system. Perhaps this is a sign that full sea-floor spreading in Afar is imminent, at least on geological timescales.

Simplified geologic map of the Afar Depression.
Simplified geologic map of the Afar Depression. (credit: Wikipedia after Beyene and Abdelsalam (2005))

For once, mantle geochemists and geophysicists have data that support a common hypothesis (Ferguson, D.J. and 8 others 2013. Melting during late-stage rifting in Afar is hot and deep. Nature, v. 499, p. 70-73). This US-British-Ethiopian team compares the trace element geochemistry of Recent basaltic lavas erupted along the axis of the Afar rift that links with the Red Sea spreading centre with equally young lavas from volcanoes some 20 km from the axis. Both sets of lavas are a great deal more enriched in incompatible trace elements that are generally enriched in melt compare with source than are ocean-floor basalts sampled from the mid-Red Sea rift.  Modelling rare-earth element patterns in particular suggests that partial melting is going on at depths where garnet is stable in the mantle instead of spinel. This suggests that a strong layer, about 85 km down in the upper mantle is beginning to melt – magmas formed by small degrees of partial melting generally contain higher amounts of incompatible trace elements than do the products of more extensive melting. Estimates of the temperature of melting from lavas extruded at the rift axis than off-axis are significantly higher than expected at this depth suggesting that deeper mantle is rising faster than it can lose heat.

The depth of melting tallies with the thermal feature picked out by seismic tomography. The two teams converge on passively induced upwelling of hot asthenosphere while the Afar lithosphere is slowly being extended. The degree of melting beneath Afar is low at present, so that to become like mid-ocean ridge basalts a surge in the fraction of melting is needed. That would happen if the strong mantle layer fails plastically so that more asthenosphere can rise higher by passive means. The geochemists persist in an appeal to an Afar Plume for the 30 Ma old flood basalts that plaster much of the continental crust outside Afar. Those plateau-forming lavas, however, are little different in their trace element geochemistry from off-axis Afar basalts. Yet they are not obviously associated with an earlier episode of lithospheric extension and passive mantle upwelling.  Most geologists who have studied the flood basalts would agree that they preceded the onset of rifting but have little idea of the actual processes that went on during that mid-Oligocene volcanic cataclysm.