November 13. The South Atlantic opens

Base from NOAA (annotated by Gibson)

It was the obvious good fit of the coastlines of Africa and South America that helped lead Alfred Wegener to his theory of continental drift back in 1915. The South Atlantic has a different history from the North Atlantic, not least in being rather younger than the northern portion.

The geometry of the coasts reflects differences in the way parts of the South Atlantic opened. The near east-west margin of West Africa turns to north-south at the Gulf of Guinea, the corner where the northeastern tip of South America used to be attached. In fact, that corner is probably really a triple junction, a tectonic location where three relatively distinct rifts began, radiating away from the location of the Niger Delta today.

The combined South America-Africa continent, which we’ve referred to previously as West Gondwana, began to separate in several places during the Jurassic. In the north, the irregular boundary between West Gondwana and North America left a piece of West Gondwana attached to North America – Florida. And the zone between West Africa and northern South America began to crack, too. In the south, also during the Jurassic, we talked last month about the separation of East Gondwana, and that also put the beginnings of a rift between southern Africa and southern South America. But the middles of what are now Africa and South America were still attached to each other until the Cretaceous.

At the corner, today’s Gulf of Guinea, two pull-apart rifts started. One ultimately became the rift that makes up most of the South Atlantic, with the north-south margin of Africa from Gabon down to South Africa on one side and Brazil on the other. The other rift was within the African continent, extending northeast from today’s Niger Delta, practically all the way across Africa, through Chad and Sudan to the Red Sea. In places this was a pull-apart zone, especially in Nigeria, where it is called the Benue Rift, but further into Africa it is a tear or shear zone, with the northern and southern parts of Africa moving alongside each other. If this rift had not failed, we would have two continents today instead of one single African continent.

Another shear zone developed in the third branch of the triple junction. This branch ran west from the Niger Delta today, along the east-west trending coast of West Africa, from Nigeria to Ghana to the Cote D’Ivoire to Liberia. On the southern side of this zone, the northern coast of Brazil, from the tip at Cabo San Roque up to the mouth of the Amazon, that section was sliding to the west. So the tectonic boundary between South America and Africa was mostly strike-slip faults, also called transform faults, where the two big blocks slid alongside each other and ultimately parted, leaving the Middle Atlantic Ocean in their wake. The southern Atlantic Ocean was formed in a more straightforward way, with the two sides pulling apart more or less perpendicularly from the spreading center at the Mid-Atlantic Ridge.

But as usual, there was plenty of variety within those overall general parameters. In the southern, relatively simple rifted system, things were complicated by two hotspots that poured basalt and other volcanics into the widening ocean and on the adjacent continents as well. We talked about one of them, the Tristan Hotspot that produced the Parana Basalts and the Rio Grande-Walvis Seamount Chain, the other day. (November 11) The second one was further north, and today it is reflected in active volcanoes like Mt. Cameroon in Africa and St. Helena, the volcanic island near the Mid-Atlantic Ridge. Between them under the ocean, the St. Helena seamount chain represents the movement of the African Plate over the hotspot as the South Atlantic opened. And there is a conjugate submarine ridge on the South American side too.

The big deal about these two volcanic centers, the Tristan and St. Helena hotspots, is that as the South Atlantic was opening, the volcanics erupted from these centers blocked off, segmented, the widening ocean basin. Instead of a long, narrow oceanic seaway, we ended up with long, narrow segments with differing history. In particular, the central section between the hotspots was periodically cut off from the open sea. You won’t be surprised that in that low-lying part of the rift thick evaporites developed as salty marine waters came in and evaporated. Those evaporite beds are hugely important to the economics of Brazil, and across the ocean, to Angola, Congo, and Gabon. Not for the salt, but for the oil that the salt helps trap beneath it.

Early in the formation of the South Atlantic rift, when it was still part of the combined continent of West Gondwana, the region that would ultimately break the continent apart was a low-lying, down-faulted zone. Think of it like the Triassic grabens that we talked about a lot in Eastern North America as the North Atlantic was beginning to form. In West Gondwana, the low-lying, tropical basins held extensive lakes, and the organic rich material that was deposited in those lakes became an excellent oil source rock. As the region pulled apart more and more, the volcanic centers to north and south restricted the basin allowing for the salt deposits to form and serve as the seal to keep the oil from escaping to the surface.

There is some argument about the origin of the salt, and it may be more complicated than simple incursions of marine waters and evaporation; some of the mineralization may have come from hydrothermal sources. The salt was deposited mostly during the Aptian Stage of the Early Cretaceous, about 125 to 115 million years ago. Eventually, about 112 million years ago, the South Atlantic became wide enough that we had an open ocean between the two continents, and the salt basins were split into two – one now offshore Brazil, and the other offshore (and in a few places, onshore) Angola to Gabon. These two areas are among the most prolific oil provinces in the world. Almost all of Brazil’s oil production comes from these sources, more than 2 million barrels a day, from fields that probably contain at least 13 billion barrels of oil. Brazil’s oil also comes from some of the deepest water depths ever drilled, with the sea floor around 7,000 feet below the sea surface. On the opposite side, the African coastal offshore from Nigeria down to Angola produces around 4 million barrels a day.

There’s plenty more to explore about the opening of the South Atlantic, and I have a handful of links and references below if you are interested in more.

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The volcano Nevado del Ruiz erupted in Colombia on November 13, 1985. The eruption melted glaciers on the mountain summit, which is more then 5,300 meters, or 17,400 feet above sea level. The eruption was really quite small, only about 3% of the volume erupted from Mt. St. Helens in 1980, but the glacial melting proved catastrophic. The volcanic debris mixed with the glacial meltwater, loose rock, and surface soil on the mountain flank to produce a pyroclastic flow, also called a lahar, which increased in volume as it came down the slopes. The flow wiped out the town of Armero and several villages, with a total death toll of about 23,000, the fourth-deadliest volcanic eruption in recorded history. Tectonically, Nevado del Ruiz is a typical volcano in the Andes, the result of the subduction of the Pacific Oceanic Plate beneath the South American Continental Plate.

—Richard I. Gibson

Links and References:
A new scheme for opening of the South Atlantic

Aptian evaporites (2012)

Tectonic evolution of South Atlantic (2000)

South Atlantic 

Brazil’s offshore oil 

Evaporites Through Space and Time, edited by B. Charlotte Schreiber, S. Lugli, M. Ba̜bel (The Geological Society, 2007)

October 14. The North Atlantic and North Sea

As we discussed on October 7, the North Atlantic Ocean was very definitely forming during the Jurassic. In late Triassic time, you could probably have walked on dry land from what is now New York City to Casablanca, Morocco. You’d have had to deal with lakes, rivers, swamps, and volcanic eruptions, but probably no ocean. By Late Jurassic time, there was an ocean 1000 kilometers wide between the coasts of New York and Morocco.

But today I want to go a little further north, where Europe and North America were breaking apart but were still pretty close together. The initial rifting between Greenland and Scandinavia may date back to the Permian, barely after Pangaea was finally assembled. But it didn’t take off and separate the continents in a big way for millions of years. Even by late Jurassic time, there was probably nothing but a relatively narrow oceanic strait between Greenland and Scandinavia. Further south, Europe was a complex of islands. Map from Ziegler 

Some of those islands were high-standing, continental blocks, like Iberia and Ireland, but most were segmented and separated from each other by some degree of tectonic activity, analogous to the Triassic grabens of eastern North America. By late Jurassic time, active faulting and rifting were producing oceanic troughs between what is now Iberia and Brittany in France – the modern Bay of Biscay – as well as between smaller blocks such as the Grand Banks, off Newfoundland, and the Flemish Cap, Orphan Knoll, and Rockhall-Hatton Bank, all under water today. A narrow island extended approximately from where London is today across the English Channel into central Germany and the Czech Republic today. Together with another block in northern England and Scotland, those areas were separated from Scandinavia by a developing rift system beneath what is now the North Sea.

Some of these rifts would fail more or less quickly – the one between the Grand Banks and Newfoundland, for example, more or less ceased to be active by sometime in the Cretaceous, and the Grand Banks essentially became an extension of Newfoundland, itself part of North America. The Avalon Peninsula of Newfoundland is another example, like Florida, of rocks that were originally part of another continent – Avalonia – that was accreted to North America during the construction of Pangaea but was left back on the North American continent when the rifting began in Jurassic time. The main rift that formed the Atlantic Ocean might have gone through the zone between the Grand Banks and Newfoundland – but it didn’t. It was just a little bit further east, between the Grand Banks-Flemish Cap blocks and Iberia. That’s where the Atlantic Ocean opened eventually.

North Sea Oil and Gas Fields (USGS)

Let’s go back up to the North Sea area. The rift that formed there was pretty aggressive, producing a deep trough between Britain and Norway. That rift also ultimately failed – if it had not, Scotland today might still be attached to Greenland, or it might have become a microcontinent, independent of either Europe or North America, something like Madagascar today. But it did stay attached to Europe.

The heritage of the North Sea rift probably dates back to the Permian, an extension of that early break between Greenland and Scandinavia. The Permian salt beneath the modern North Sea and across northern Germany – the Zechstein, which we talked about on August 22 - was probably related to tectonic sagging that became a real rift system by Jurassic time.

A low-lying sag accumulates a lot of sediment, and in a restricted basin, it can be starved of oxygen in the deep marine waters, so organic matter can build up without being dispersed. That happened in the Jurassic rifts under the modern North Sea. As faulting continued, high and low ridges and basins formed. If you bury all of that under thick piles of later sediment, you have an excellent oil province. Much of the oil in the North Sea today was sourced in Jurassic rocks, and about half of the oil is in Jurassic reservoir rocks.

Because the rift was active, but not extending to produce a wide open ocean as was happening between Africa and North America, abundant sources of sediment were available throughout most of the Jurassic. This provided material for source rocks, reservoirs, and seals – the elements necessary for hydrocarbon accumulations – and later burial provided the thermal maturation that completed the picture. The North Sea oil province was explored beginning in 1964, although the Groningen gas field was discovered just offshore the Netherlands in 1959. Numerous discoveries in the late 1960s and 1970s made the North Sea into a world-class oil province. The North Sea has produced at least 30 billion barrels of oil over time. It’s a mature oil province, meaning that it is in decline in terms of oil production. At its peak of production in the late 1990s the province produced about 6,000,000 barrels per day. Natural gas production appears to have not yet reached its peak.

By the end of the Jurassic, it was apparent that the far northern Atlantic Ocean was forming, but it wouldn’t be for 40 million years or more, well into the Cretaceous, that separation between Europe and North America was significant.

—Richard I. Gibson

References:
J. R. Underhill, Jurassic – Chapter 8 in Petroleum Geology of the North Sea (K.W. Glennie, ed.). Blackwell Science, 1998.

Evolution of the Arctic-North Atlantic and the Western Tethys, by Peter Ziegler (1988, AAPG Memoir 43) 

Map by D.L. Gautier (USGS, public domain) 

September 8. Pangaea begins to break up

The other day, I said that in the Triassic, Pangaea was pretty much still intact as a single huge continent, with the big embayment on the east side, the Tethys Ocean. But I’ve also said there were hints of the great breakup that was about to begin.  

As the Cimmerian blocks began to rift away from the northeastern coast of Pangaea, the southern portion of the supercontinent, the old Gondwana, was probably rotating a bit, so that where Africa and Europe were attached, they began to pull apart at least to some extent, but it’s not really completely clear exactly what was going on there during the Triassic. Further north and west, through the complex mountain ranges that formed during the Caledonian, Alleghenian, and Appalachian Orogenies, over many millions of years, the compression due to collision was giving way to extension.

Triassic Globe by Ron Blakey, NAU Geology, under Creative Commons license (notes by Gibson)

Even as early as the late Permian, a rift, a pull-apart, had begun to form between what is now northeastern Greenland and northwestern Norway. That narrow strait might have allowed sea water to invade the basins of northern Germany and the Netherlands, where the Permian Zechstein salt formed. By the middle of the Triassic, the rifting seems to have extended a long way into the combined North America-European continent.

Tethys reconstruction globe from Stampfli & Borel 2002

How do we know this? There are extensive deposits of terrestrial sediments scattered through the region – mostly offshore today – from southern Greenland to west of Ireland and France and Iberia, and on the North America side, from east of Newfoundland around the margin to south of Nova Scotia – which was still attached to Africa, about where Morocco is today. This was not a complete seaway, but the rifting was making basins, similar perhaps to the basins that received the Old Red Sandstone back in the Devonian, after the first big mountains formed through this zone, the Caledonian Mountains. It was a complex array of zig-zagging rift basins, and I really think it’s fair to think of it like the East African Rift today – long, linear interconnected rift zones, in places with lakes, in places just lowlands receiving eroded debris off the adjacent highlands.

Let’s take a break for a minute and talk about rifts. When I say “rift,” I mean a major break in a continent, where two parts of the continent pull apart from each other. Ultimately, such a rift might become an ocean basin, with the two continental fragments bordering it on each side. This process is often driven by the generation of new oceanic crust at a mid-ocean ridge. Heat rising in convection currents from the deep mantle brings molten material to the surface – or at least near the surface – in a linear zone. As more and more such material rises, the previous material has to move out of the way – and the crust of the ocean spreads apart, away from the mid-ocean ridge. If that ridge started beneath a continent, the inexorable force of rising heat and magma will eventually break even thick continental crust. That’s what’s happening today in East Africa, and it’s what was beginning to happen during the Triassic where North America and Europe were attached. This is the birth of the modern Atlantic Ocean.

Rifting. (I have tried and failed to
determine the owner of this image;
if you are the copyright owner, please let me know.)

But during the early and middle Triassic, we didn’t have much in the way of open oceans yet. Probably just that narrow strait next to Greenland in the north, and possibly some ocean between North Africa and southern Europe. The rest was a diverse lowland, a sag, with linear mountains surrounding lake basins and continental river systems. Think of it like a big mass of cold caramel – soft enough to stretch some, but brittle enough to break eventually. As you pull the caramel apart, the middle will sag, and finally will break with a pretty sharp edge, assuming the consistency is just right. Since most of the rocks are under the Atlantic Ocean today, they are known only from remote sensing studies, including seismic data, and from wells drilled for oil and gas exploration. Not quite the same as having them exposed for geologists to take rock hammers to.

I think two big questions might be occurring to you at this point. First, what makes a rift start? And second, in this case, why did the rift run more or less along the zone where the original collision had created a huge mountain uplift that went from northern Greenland all the way to West Texas and probably beyond?

Oceanic rifts start where the linear edges of mantle convection currents rise toward the surface. The ultimate controls on the geometry, size, and position of convection currents are poorly understood – it’s the distribution of heat down in the mantle, and the complex response of the solid earth to that. And the solid earth is not uniform, so variety will be the name of the game. It’s also possible that some rifts begin because isolated mantle plumes, or hot spots like those at Yellowstone and Iceland, rise and weaken the crust, essentially encouraging the rift to radiate from that location. To break continental crust, much thicker and stronger than oceanic crust, probably depends on some special circumstances, such as a pre-existing state of stress, but it seems possible that a mantle plume might initiate such a continental break-up. This is still a controversial topic. Here's a 2014 paper on this idea, and see also this paper from 2014 for an opposing view.

As for the second question, why did the Atlantic Rift begin to form pretty much right along the zone where the continents had come together, one simple rationale is that such a zone, full of faults and inhomogeneities, would be the weak point in the system. The central cores of the continents – the cratons, which we outlined in January, and the word craton means “strong” – would have been much more resistant to breaking apart than the collision belt. You might argue that the collision zone made the crust even thicker, and with lots of igneous rocks and metamorphism, the suture zone, where the continents were welded together, ought to be the strongest part. Maybe it was. But it’s an observational fact that the break-up of Pangaea – at least between Europe and North America – followed the old collision, more or less. There are some interesting exceptions that we’ll talk about as the break-up proceeds over the next month or so.

* * *

Today’s birthday is Raphael Pumpelly, born September 8, 1837, in Oswego, New York. His geological work was wide-ranging, from Chinese coal fields to the copper country of Michigan, but he focused on economic geology of mineral deposits. He was the first to explore the Gobi Desert scientifically and he was also in charge of the Northern Transcontinental Survey of Dakota, Montana, and Washington Territories in the early 1880s. Pumpellyite, a low-grade metamorphic calcium-iron silicate mineral, was named for him.

—Richard I. Gibson

Tethys reconstruction globe from Stampfli & Borel 2002:    http://www-sst.unil.ch/research/plate_tecto/alp_tet_main.htm#Introduction 

Globe by Ron Blakey, NAU Geology, under Creative Commons license (notes by Gibson)


References: P.A. Ziegler, Evolution of the Arctic-North Atlantic and the Western Tethys, AAPG Memoir 43, 1988.

Mantle plumes cause rifts?