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 a weekly schedule with diverse topics, and the Facebook Page showcases photos on Mineral Monday and Fossil Friday. Thanks for your interest!

Saturday, November 29, 2014

November 29. Verkhoyansk fold belt

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So on to today’s Cretaceous topic – the Verkhoyansk Fold-Thrust Belt. We talked a lot about the collision on the west coast of North America that formed much of the basis for the modern geography of the Rocky Mountains, including such details as oil fields. Similar things were happening in other parts of the world during the Cretaceous too.

The tectonic framework of far eastern Asia is really a mess. There were microcontinents, island arcs, entrapped oceanic crust, and more, all becoming amalgamated to what we think of today as the far eastern part of Eurasia. One aspect of it that is moderately well documented is the collision between a small, almost circular microcontinent called the Kolyma Block and the eastern side of the Siberian Craton.

Tectonic Map of far eastern Siberia (Khain, 1973)

The Siberian Craton is one of the really old, fundamental continental blocks, like the Superior Craton in North America or the Baltica Craton in Europe. By the Cretaceous, it was firmly attached to Europe and the Kazakhstan Craton, so the concept of Eurasia was a reasonable one at that time. The details were still in progress, especially on the eastern and southern margins of the young Eurasia.

What I’m saying about this collision and fold belt is based in part on my own work on the magnetic map of the former Soviet Union. The Kolyma Block, that I’m calling a microcontinent, has a magnetic character that’s a lot like Kansas or Texas – magnetic character that says it’s probably Precambrian crust, but the Kolyma Block was a relatively small, independent block, not part of a major continental terrane until the Cretaceous.

This eastern margin of Siberia was for millions of years a passive margin – like the Atlantic coast of North America today, with lots of various sedimentary units accumulating. The rocks are as old as Cambrian, but most of the sedimentary pile is Carboniferous to Jurassic in age. Beginning in late Jurassic time and continuing into the Cretaceous, the Kolyma block began to collide with Siberia, pushing the older passive-margin rocks against the strong buttress formed by the Siberian craton. The collision was intense enough to have some subduction that produced granites, but the main expression today is the 400-kilometer-wide Verkhoyansk Fold Belt.

Present-day plate-tectonic context of
Verkhoyansk fold belt (base from USGS)
The collision was pretty much done by around 90 million years ago, the early part of the late Cretaceous. The rest of eastern Siberia was added even later. Today, most of this area and the region to the east is actually tectonically a part of North America. The boundary between Eurasia and North America is along a rift zone in the Arctic Ocean, called the Nansen Cordillera, an extension of the Mid-Atlantic Ridge. The boundary is not well-defined as it continues into Asia about where the Lena River enters the Arctic Ocean, but it extends on south along Sakhalin Island and to the northern part of Japan. The great earthquake offshore Japan, March 11, 2011, was located pretty close to the intersections of the boundaries of the North American, Eurasian, and Pacific Ocean tectonic plates. So even though the boundary is pretty much inactive where it goes through the Verkhoyansk Fold Belt, that Cretaceous heritage is still affecting tectonic activity in the region today.
—Richard I. Gibson

Reference: Defining the eastern boundary of the North Asian craton from structural and subsidence history studies of the Verkhoyansk fold-and-thrust belt, by A.K. Khudoley and A.V. Prokopiev, in Whence the Mountains?: Inquiries Into the Evolution of Orogenic Systems : a Volume in Honor of Raymond A. Price, edited by James W. Sears, Tekla A. Harms, C. A. Evenchick. Geological Society of America Special Paper 433, 2007.

Reference: U.S. Geological Survey Professional Paper 1626, 2000: Phanerozoic Tectonic Evolution of the Circum-North Pacific, by Warren J. Nokleberg, Leonid M. Parfenov, James W.H. Monger, Ian O. Norton, Alexander I. Khanchuk, David B. Stone, Christopher R. Scotese, David W. Scholl, and Kazuya Fujita

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