November 29. Verkhoyansk fold belt

A handful of people have asked about donations to be sponsors for the podcast. That’s really not on the list; the budget for the whole production is about $39- $35 for the digital voice recorder, plus $4 for replacement AAA batteries. I was trying to make it through the whole year on the first set of batteries, but the originals died the other day. Otherwise, there’s no cost other than my time. I hate commercial announcements enough, even on public broadcasting, that I’m not interested in accepting sponsors for this podcast. I’m not rich, but I can afford to do this, and it’s really a labor of love. Thanks for your support.

Likewise, I’m not in it for the clicks. I’m glad to see my statistics showing 4,000 pageviews per month on the blog, and as near as I can tell, about 1,000 downloads a day of the podcast, but I’d keep doing it if those numbers were 100 people I don’t know. I don’t do much to promote the podcast other than getting it listed on iTunes and other providers, so it is what it is. Clearly a lot of people find it through search engines. Again, thanks for your support and interest.

To repeat my disclaimer, I’m not an expert in everything – I like to think of myself as a geological generalist, although of course I do have specialties, strengths, ranging from kidney stone mineralogy to gravity and magnetics in oil exploration. Please take these podcasts or the blog as summaries of the topics. I try to be even handed and unbiased, but I don’t guarantee that. If something I talk about strikes your fancy, by all means, investigate more! There’s a vast amount of information out there on everything I talk about. Or feel free to ask me questions. I’m happy to say “I don’t know,” but like most geologists, that won’t stop me from expounding on something – but I’ll try to make it clear that I may be approaching the topic from a point of general knowledge but specific ignorance. 

* * *

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

September 27. West Siberian Rift

Today I’m calling on your memory again, to think back on the vast flood basalts that erupted in Siberia just about at the end of the Permian. They’ve been implicated in the extinction event then, the most devastating extinction the earth has seen. 

Today, the basalt flows are exposed over extensive areas of Siberia, but west of those exposures, in the relative lowlands occupied by two huge river systems, the Yenisey and the Ob, the basalt flows are found in the subsurface of what’s called the West Siberian Basin. 

W. Siberian Rifts (green) from USGS (Ulmishek)

A few million years after the start of the Triassic and probably continuing for millions of years into the Triassic, a rift began to form in this West Siberian area. It was perhaps something like the rifts, the grabens, that were forming in eastern United States, but it was unlike them in that the whole system ultimately failed – no ocean was created by the rifting here, whereas the Atlantic formed from the rifting between North America and Africa. The West Siberian rifts were pretty big, though – one extends continuously for at least 1,800 kilometers.

Now visualize something like Nevada’s basin and range – long linear rift valleys adjacent to long linear mountain ranges. I’ve mentioned repeatedly that when you have such a situation, the erosion begins to fill those valleys – but this time, let’s focus on the mountains that are being eroded. The Permian and early Triassic flood basalts covered this region in vast, continuous sheets. When the rifting began, the sheets were broken – some segments were dropped down into the grabens, while other segments of the sheets were left on top of the mountain ranges, which are technically called horsts, a German word for heap or pile, something standing higher than the surrounding land.

The basalt on the tops of the mountain ranges got eroded away, leaving the basalt in the down-dropped basins. An alternative interpretation is that the basalts were erupting as the basins and ranges were forming, so the lavas flowed into the low-lying basins and were never deposited on the mountain tops, but I think the preferred interpretation is that the basalt flows were eroded off the mountains. In any case, what’s left, deep in the subsurface, is linear basins full of basalt and linear uplifts barren of basalt. The whole thing subsided even more, millions of years later, so all that complex structure, basins and ranges, is completely buried by later sediments, and the surface of the West Siberian Basin today is fundamentally a flat marshy plain.

Cross-section by Richard Gibson - source

So how do we know the basalt flows are down there? One way of course is to drill into the subsurface and get samples. But that’s expensive. Another way is to recognize that basalt is a dark, iron-rich rock, and a good bit of that iron is in the form of magnetite – an iron oxide mineral that is highly magnetic. We can measure the earth’s magnetic field at a distance from the magnetic rocks, from an airborne magnetometer, so that we can infer the distribution of magnetite-rich rocks in the subsurface. When Soviet geophysicists did that in the 1950s and 1960s, they revealed long linear magnetic highs – representing zones where there was a lot of magnetite – alternating with long linear magnetic lows, where magnetite was absent or less abundant. You’ve probably guessed that what they were defining were the grabens filled with basalt and the buried uplifts where the basalt had been eroded away.

So what? Well, those uplifts are buried beneath Jurassic and Cretaceous rocks that include some rich hydrocarbon source rocks and excellent reservoirs. The sedimentary rocks draped over the deep Triassic uplifts contain some of the largest natural gas fields in the world. And the gas fields coincide almost perfectly with the low values in the magnetic data, the uplifts where basalt is absent. Many trillions of cubic feet of natural gas from the West Siberian Basin have heated homes in Russia and much of Europe for decades. Back in 1990 I did an extensive analysis of the magnetic map of the Soviet Union for oil exploration, and in the process discovered this correlation between magnetic lows and gas fields. The Soviets had known it for decades, of course, but it was pretty satisfying to unravel the details of the relationships myself.

I have a link below to a nice paper from the USGS if you are interested in more about the oil and gas in the West Siberian Basin. Or email me.

—Richard I. Gibson

Links:
Siberian basalt flows 
West Siberian Basin Oil  (PDF - source of Map)
Cross-section by Richard I. Gibson
Urengoy gas field

August 30. Siberian basalts

Original extent of Siberian traps (see below for credit)

At just about the end of the Permian Period, one of the most voluminous volcanic eruptions of the past 500 million years occurred, in what is now Siberia. The rocks are largely basalts, relatively iron-rich, dark, fine-grained igneous rocks that erupted from many vents and perhaps some fissures scattered through the area. The eruptions spanned about a million years, 251 to 250 million years ago, and ultimately covered as much as 7 million square kilometers, about 2,700,000 square miles – about the area of the conterminous United States excluding Texas and California. Probably less than a third of the original extent is still preserved; the rest has been eroded away.

Flow after flow was erupted, along with explosive eruption of ash and other volcanic products. The flows are usually called “flood basalts” because they flooded out of the vents and fissures to cover vast areas. And the whole package is often referred to as the Siberian Traps – the word “trap” in geology comes from a Swedish word meaning “step,” because the piles of flows typically create a stair-step landscape.

There have been several huge eruptions of flood basalts through earth history, including 11 of various sizes in the past 250 million years. The Columbia Plateau in Washington and Oregon, the Deccan in India, and the Parana basalts in Brazil, are all huge, but the Siberian traps were the largest volcanic flows in the past half billion years.

What caused it? The ultimate cause of all the flood basalts is not completely certain. Ideas include active rifting, pulling apart of the crust, under special circumstances such as unusually thin crust. A mantle plume, a hot spot like those beneath Yellowstone and Iceland today, could have made it happen, especially if associated with rifting. It’s been suggested that an impact might have blasted the crust to the point that vast volumes of magma would erupt through the crater, but there isn’t much support for that idea.

The age of the eruptions has been pinned down quite accurately. The eruptions do have a moderately wide range, taken together, from about 256 to 246 million years ago, but the age dates really do strongly cluster at 251 to 250 million years ago. This is right at the Permian-Triassic boundary, the end of the Permian, when the greatest mass extinction in the history of life on earth occurred. It’s pretty much inescapable, given the extremely close correlation in time between the vast Siberian eruptions and the mass extinction event, that there was probably a connection between the two. But there’s more to it than a simple one-for-one correlation, and we’ll explore that in more depth tomorrow.

* * *

Today is the birthday of William Stephens Twenhofel, August 30, 1918, in Madison, Wisconsin. His father, William Henry Twenhofel, has been called the patriarch of sedimentary geology: he pioneered the study of sedimentation as a subdivision of geologic science. The son, William Stephens, was a geologist with the U.S. Geological Survey who worked on ore deposits in Alaska, among many other things.

—Richard I. Gibson

Links and references:
Siberian Traps (Cowan)  
Siberian Traps ages 
Siberian Flood Basalts 
Multiple eruptions – not one
Extinction connection

Map by Jo Weber, under Creative Commons license.