The 366 daily episodes in 2014 were chronological snapshots of earth history, beginning with the Precambrian in January and on to the Cenozoic in December. You can find them all in the index in the right sidebar. In 2015, the daily episodes for each month were assembled into monthly packages, and a few new episodes were posted. Now, the blog/podcast is on an occasional schedule with diverse topics, and the Facebook Page showcases photos on Mineral Monday and Fossil Friday. Thanks for your interest!

Monday, September 8, 2014

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? 

2 comments:


  1. Because of your interest in Permian times, you may find interesting a series of articles that discuss the possible large impact that development of the ability to digest cellulose by roaches may have had on Permian ecology and climate, and on the Triassic coal hiatus shown below without references and starting in http://www.angelfire.com/nc/isoptera/roach.html . The following end Permian extinctions are discussed in http://www.angelfire.com/nc/isoptera/permian.html If you see any errors in them or possible additions, please let me know
    Sincerely, Charles Weber

    DID the WOOD ROACH CAUSE the PERMIAN ARIDITY, RED BEDS, and CONIFER RISE?
    by Charles Weber
    Cellulose digestion by wood roaches may have removed enough mulch (detritus) to have caused hiatus of coal, aridity, and some of the temperature rise, as well as increasing conifers in late Permian. The largest part of the temperature rise may have been from overturn of an anoxic ocean by a huge comet.
    ABSTRACT
    It is suggested that the symbiosis of cellulose digesting microbes with the cockroach, probably in the Permian, caused fundamental ecological changes which lowered soil organic matter, created aridity, helped increase atmospheric carbon dioxide, helped eliminate glaciers, and favored conifers with their inert interior and wood poisons. In the form of prototermites with a soldier caste, it is suggested that they spread the conifers in early Triassic, caused the early Triassic coal hiatus, and possibly contributed to extinctions at the close of the Permian when dropping sea levels permitted them to spread around the world, the last possibly from the indirect effects of a comet impact coupled to filling of below sea level depressions.

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    Replies
    1. Thanks for this information - interesting!

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