This week we're going to talk about rock deformation, mountains and plate tectonics. So what I would like to do is sort of connect these three by talking about the structure of mountain belts using the Swiss Alps as an example. Now I trust that you understand that mountains form at collisional boundaries, and of course there are various types of collisional boundaries, so there are various types of mountains. But what we're going to focus on today are mountain belts that are formed by collision of two continents, and the quintessential example is shown here of the collision of India. The Indian subcontinent as it's moving toward Asia several million years ago and ultimately India collides with Urasia to form the Himalayan Mountains. And this is an ongoing process, the Himalayas are still being formed. So let's look at the lower slide in a little more detail because I want to show you some features associated with these mountain belts. The first, is that the continental crust of India is actually under thrusting the crust of Eurasia, and the reason is that the continental crust is, is far less dense than the mantle below, so the continental crust doesn't subduct. The result is that that crustal sec, section is very thick in the case of the Himalayas, it's something like 80 kilometers or so thick. And the result of that is that this buoyancy causes extensive uplift, that's why the Himalayas are so high. In turn, the result of that extensive uplift is very rapid erosion rate and this exposes the deep roots of the mountain belt so that we can study the mountain belts and understand how they form. In addition to that, we see thick, sedimentary sequences in these mountain belts, here represented by this tan wedge. These are sedimentary rocks, they are once sediments that were deposited in the ocean basin between the two continents as they were converging and they got caught up in the collision. We also see fragments of that oceanic crust that are typically found sort of in the boundary between the different continental blocks also caught up in that collision. Again, these things are very deep in the mountain belts and they're exposed by erosion. So that's typically what we see in these. They're the other examples include the Appalachians which are old and deeply erroded and partly covered so they're low. and the Swiss Alps and I want to talk about the Swiss ALps because well, for two reasons. First of all because the Swiss Alps have been studied for more than two centuries now by geologists, so we know a lot about them. And also because they display classic structures that we see in, in, in all of these kinds of mountain belts. So here is a satellite image of the Swiss Alps, extending from France on the west to Austria on the east, and of course the southern part of the Swiss Alps is, in Italy. These, this part of the Alpine chain which actually extends from Gibraltar all the way to the Middle East, this part of the mountains, formed mainly in the Miocene, in about 17 million years ago. over a relatively short time period of maybe 5 to 6 million years when the main mountain building activity occurred. That mountain building activity involved something like 300 kilometers of of crustal shortening, so there's compression and shortening of the crust by an enormous amount. We've unraveled the tectonic history of the Alps mainly by two sets of observations. First of all, by looking at the rocks themselves, for example the sedimentary rocks tell us this, the environment of deposition. And the metamorphic rocks tell us, where the rocks had been metamorphosed, the depth from which they originally came. And the second set of observations concern the structures of, that we see, these, in the mountains, the structure of the mountains, which tells us about how they were deformed, so these two sets of observations give us a tectonic history. Now, I'm not going to talk about the tectonic history per say, but what I would rather do is talk about the growth structure of the mountain belt and to do so we're going to look at a cross section in this area. So here's a characteristic structural profile of the Swiss Alps, and I know it's really complicated so I just want to talk about some of its gross features. To the far northwest, we find what's known as the Foreland, which is not actually part of the mountain belt per say, but in France, the rocks in the Foreland are relatively flat lines of sedimentary rocks overlying a crystal embasement. And moving now into the flanks of, the distant flank of the mountain belt, we move into what's known as the Molasse Trough. These are just local names, by the way, not things that you really should need to remember. In this particular region, the sedimentary rocks are not significantly deformed. They're basically flat lined, but they've been displaced many, many tens of kilometers northward along relatively flat line thrust faults, shallow, maybe several, you know, 10 kilometers, 5 kilometers, deep thrust faults. Then we move into the main Alps themselves, the Helvetic, the Brianconnais, the Piemont zones, and these particular groups of zones are characterized by, huge Nappe structures. so what do I mean by a Nappe structure? Basically a Nappe is a large incumbent fold commonly with a thrust fault. This is a thrust fault, on it's lower limb, so it forms as one group of rocks is thrust over another, get a large fault like, a large fold like this, again, it's a flat line fold, with a thrust, on its sole, if you will. I'm going to show you a photograph of one of the, most spectacular Nappes in Switzerland, the Glarus Nappe and, the, the thrust actually, the photograph is of the thrust fault. Beneath the grat, the, the, the Glarus Nappe, and the photograph that you'll see now, is in, this position, so here's the Nappe. you can follow the, contact between, the, the dark rocks above and the light rocks below. Right here and it's also shown by this break in the snow because the lower rocks form much steeper cliffs and the snow doesn't accumulate there. what's interesting about the Glarus Thrust and for that matter almost most all of these Nappe uh,structures, is, the rocks above namely, these rocks here, are actually older than these rocks there. And the reason we know that, is, if we go elsewhere in the Alps we find, the rocks below on top of, the rocks above here and, and that's, again it's, it's, it's characteristic of Nappe structures they bring older rocks above younger rocks. And, and the way to understand that is, actually, we should probably redraw this, thrust to cross at a low angle the, stratigraphy. So that's how you get the, older rocks thrust above, the younger rocks. Let me now show you some more detailed cross sections and, these are extremely artistic. they're, based on, geological maps made by Obrehouser, and Albert, and Arnold Heim, early in the 20th century again, these aren't, these aren't figments of imagination. They are interpretations, but they're interpretations based on really detailed mapping and what you see in these cross sections are, you know, rocks such as this group of brown rocks that have been thrust far, far, far to the northwest on. On some kind of a thrust like that you know, the root of these rocks, the origin, or, or the location where these rocks actually existed first, is far, far to the northeast, to the right on this cross section. similarly in the lower one, here's a group of of, of brownish looking rocks and you know, the origin of these rocks is far, far to the south east, again to the right. And they've been transported on, apparently on a gigantic thrust, many, many tens of kilometers. So that's the sort of structures you see in the Alps you can see in the, you know. On the, on the steep mountainsides these beautifully exposed recumbent folds such as you see here in this cross section. Look at the note the scale here, this is ten kilometers, so you can see the scales of these features of these cross sections. They're really quite spectacular, returning to this cross section. So you get Nappe structures now throughout the Alps but the Nappes are more extensive and more deformed and even larger, the further to the southeast that you go. Also what you find here are these fragments of oceanic crust that are rendered in black here. And then you get into the what's known as the Sesia Lanzo zone, which is basically a zone of highly deformed, multiply metamorphose rocks, with intercolated slivers of oceanic crust, granitic plutons. And even chunks of the African plate welded onto them. Further to the south east, these rocks are actually covered by younger sedimentary rocks that are not exposed. So this is the real core of the mountain belt. So we see here as we go from the flank of the mountain belt in toward the core of the mountain belt the deformation becomes more intense. The metamorphism becomes more intense and the uplift in erosion is greater. So the rocks, say here, were probably buried maybe, 15 kilometers, when they were originally deformed. Whereas the rocks here may have been buried more than 60 kilometers and again, because of the extensive uplift in erosion, that's why these rocks are exposed. And that's why we have this more intense deformation and metamorphism as we go toward the core of the mountain belt. So what have we learned? Well, first of all, we made the point that mountain belts display clear evidence of extensive crustal shortening. And secondly, although mountain belts are to some extent unique, they display characteristic structures, Nappes are amongst those characteristic structures that develop during or at genesis. I should add that the deformation of these rocks is a consequence of their ductility, this is a property of rocks, in other words, rocks can flow. they deform plastically, under high temperature and pressures and this is something that we're going to talk about in more detail this week. So that's a very important characteristic of rocks that and, and the evidence for that is in these large folds that you see in the mountain belts. and finally mountain belts, and this should be obvious, or explicable by plate tectonics. So thank you very much and I hope you find this to be an interesting week.
