December 6. Alpine-Himalayan Orogeny

You recall that the supercontinent Pangaea began to have some cracks in it almost as soon as it was assembled back in the Carboniferous and Permian. The break-up really got underway in the Jurassic and Cretaceous, and in particular, India separated from Africa and from Australia-Antarctica. India began to move northward, opening what is now the Indian Ocean and closing the most recent incarnation of the Tethys Ocean, sometimes called the Neotethys, in Late Cretaceous time. India’s northward drift from the old East Gondwana was remarkably fast, at rates close to 20 centimeters, 8 inches, per year. That’s not quite ten times the more typical rate of sea-floor spreading. 

Sea-floor spreading rates are affected by the geometry of mid-ocean ridges and by the chemistry of the basaltic material brought up – sometimes it is somewhat denser than normal, or less dense. It may have something to do with the strength of a mantle heat plume, and spreading rates are definitely not uniform all along a spreading center at a mid-ocean ridge. In the case of India, differential rates meant that as the Indian block moved, it rotated a bit. I don’t think we have a clear idea of why India moved so quickly. But we do have a pretty clear idea of what happened when it reached another strong continent. The Himalayas happened. 


India probably began to collide with Eurasia about 55 million years ago, about the boundary between the Paleocene and Eocene epochs. One line of evidence for this is the change in the spreading rate – it slowed dramatically from almost 20 cm per year to more like 4 or 5 cm per year. The collision may have slowed it down. The collision is still going on, 55 million years later. 

from USGS

When continents collide, their relatively low density, compared to oceanic crust, means that they do not subduct beneath each other, at least not much. But the crust does shorten, as the margins of the two colliding continents together with whatever was between them get scrunched together into huge folds and uplifts – and there is some pushing down, too, not truly subduction, but the crust on one continent may at least flex down under the weight of all the squeezed-together stuff. In India, that downward flexing created the Ganges Plain in northern India, just south of the Himalayan Mountain front. The India-Eurasia collision is so intense that the force is breaking the continent at great distances from the point of impact. Much of southeast Asia is being pushed out from between the two colliding continents, and 1000 kilometers or more inland, tension has broken the crust of Asia to form the Lake Baikal rift in Siberia.

The high mountain relief of the Himalayas is the result of this squeezing. Present-day measurements of the rate of uplift, the highest on earth at about 10 cm per year in places, show that the collision is still continuing, and the earthquakes in the region are of course evidence for it as well. The Himalaya also has one of the highest erosion rates on earth, no surprise, at 5 to 10 cm per year. So erosion is pretty close to keeping pace with the uplift.

Map of Alpine Orogeny elements by Woudlouper, used under Creative Commons license.

India may have been the speediest part of Gondwana to charge headlong into Eurasia, but it wasn’t the only part. The northeastern prong of Africa, today’s Arabian Peninsula, was toward the narrow, western end of the Tethys Ocean. Tethys had already been narrowed somewhat by the movement of the Cimmeride continental blocks across it – we talked about that back in the Permian and again in the Triassic.  The Cimmerian blocks had amalgamated to the southern margin of Eurasia to form much of what is Iran, Afghanistan, and Pakistan today. Some other Cimmerian blocks were probably caught in the big squeeze between India and Asia, and are now somewhere within the Himalayas. The parts further west, Iran and vicinity, were collided with by the Arabian edge of Africa, and that collision is still happening too. This part of the complex Alpine-Himalayan Orogeny began more or less in Miocene time, about 20 million years ago and continuing to the present. The Zagros Mountains of Iran are the result of this collision, and the Red Sea, on the other side of Arabia, is the evidence of the pull-apart that sent Arabia into Iran and Iraq. The Red Sea is part of the East Africa Rift System that began probably during Eocene time and became pretty active during the Miocene. We’ll talk a little more about that later this month.

Even further west, Africa was pushing up toward Europe. The Africa-Europe collision seems like it has been an on-again off-again kind of action, with both collisions and pull-aparts at different times. The Tethys Ocean hasn’t been completely consumed or caught up in the mountain building as it was in the Himalayas. Parts of the Mediterranean, Black Sea, and southern Caspian Sea are bits of old oceanic crust that are still hanging around.

The northern coast of the African part of Gondwana had a few semi-independent continental blocks. One of them, today’s Italian Peninsula, is being pushed by Africa like a giant rigid finger into the soft underbelly of Europe, raising up the Alps. Other aspects of this Alpine Orogeny, pretty much the results of interactions among various small continental blocks but also with the main mass of Africa itself, include the Pyrenees, the Rif and Atlas in Morocco, Algeria, and Tunisia, the Betic ranges in southeastern Spain, the mountains of the Balkans, Greece, and Turkey, where the Anatolian Plate is another Cimmerian block caught between the pincers of the African and European continental blocks, as well as the Caucasus. All these mountains are mostly Cenozoic in terms of their uplift time, although there were initial reactions to the collisions in places back in the Cretaceous. And it’s still happening. The complex interaction of small oceanic plates, small and large continental plates, subduction zones and more give rise to the volcanoes in Italy and Greece and the earthquakes that occur all over this region. That’s not going to end any time soon, geologically speaking.

—Richard I. Gibson

LINKS
Cenozoic sea-floor spreading rates
India-Antarctica separation

Map by Woudlouper, used under Creative Commons license.