I’m sure I’ve mentioned Siberia from time to time. It was a discrete continental block, like Baltica or Europe, out there kind of doing its own thing. By the late Pennsylvanian, or late Carboniferous as it would be called in Europe and Asia, Siberia was beginning to reach the edge of Europe. But it wasn’t alone.
There’s a smaller – but still large – continental block occupied by much of Kazakhstan today. It contains Precambrian rocks, like Siberia and Baltica and North America, but it probably formed by a complex assemblage of smaller Precambrian microcontinents, island arcs, and oceanic terranes. It may not have been assembled into something we’d call a continent until about Ordovician time. It’s such a complex mess, geologically, that the details, and even the general aspects of its history are still being worked out.
By late Carboniferous, about 300 million years ago, the Kazakh Block was beginning to impact the southern part of Baltica, the core of Europe. The ocean between the two was closing, and the typical continent-continent collision was starting to create a mountain range – the Ural Mountains. At least the southern and probably central part of today’s Ural Mountains.
|Tectonic Map of Soviet Union (above) from “Atlas SSSR,” Moscow 1984. Annotated by Gibson. KAZ = Kazakh Craton|
Boundaries are highly generalized.
The collision was between a passive margin – like the present-day east coast of North America – on the Baltica side and an active subducting margin, like western South America today, on the Kazakh side. Both sides were deformed, and there’s a 2,000-kilometer-long fault, called the Main Ural Fault, which is taken to represent the suture, the line of joining, between the two continental blocks. Parts of the oceanic crust that had been between the two got caught up in the collision as well, and some were even pushed up and over the rocks of the Baltica craton. The term for oceanic crustal rocks that are found on the continents today is ophiolite, and they are fairly common in these sorts of collision zones.
Depending on which papers you read, the Kazakh-Baltica collision might have begun as long ago as early Carboniferous, perhaps 340 million years ago, but I believe the consensus is that it culminated with the two blocks combining near the end of the Carboniferous.
The other side of the Kazakh Craton, where today it is attached to Siberia, is even less clear. The interaction between the Kazakh Craton and the Siberian Craton appears to have been long – many millions of years – and complex, ranging from a typical subduction interaction to multiple long strike-slip faults like the San Andreas today, and the fault zone along the Queen Charlotte Islands off western British Columbia. Most evidence seems to indicate that the Kazakh Block and the Siberian Craton were assembled at least a bit later than the Kazakh-Baltica collision, probably during the Permian or later, and that the northern Ural Mountains formed when the combined Kazakh-Siberia continent finally collided with Baltica. But I’m definitely not sure about that, even though I spent a year working on an interpretation of the magnetic map of the Soviet Union, to create a tectonic framework for oil exploration back in 1990. It’s complicated, for sure.
* * *
July 28, 1840, was the birthday of Edward Drinker Cope in Philadelphia. Cope was a paleontologist and fossil collector, focusing extensively on dinosaur and giant mammal fossils of the western U.S. His personal feud with O.C. Marsh is called the “Bone Wars” today.
Today is also the anniversary of the Tangshan Earthquake, in eastern China in 1976. The estimated death toll, 242,000, probably makes it the most deadly earthquake of the 20th century. The site in northeastern China is near the tectonic boundaries of several smaller blocks that make up China today. These blocks are still moving relative to each other, but the ultimate push that probably caused the Tangshan Earthquake is most likely the collision between India and Eurasia. Even though that collision is taking place thousands of kilometers from Tangshan, its impacts are felt across eastern Eurasia as old weak zones and boundaries are exploited to relieve the compressive stress caused by the collision of India.
—Richard I. Gibson
Geodynamics of East Kazakhstan
Tectonic History of the Urals
Tectonic Map of Soviet Union (above) from “Atlas SSSR,” Moscow 1984. Annotated by Gibson. KAZ = Kazakh Craton