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Why Does Italy Look Like a Boot?
The Formation of Italy and the Tyrrhenian Sea
Ryan C. Wallace
Department of Earth and Planetary Sciences, Washington University, St. Louis, Missouri
Introduction
The Mediterranean Sea is a very complex region, geologically speaking. It is a place where two huge continents, Africa and Europe, collide. In the midst of this rather epic meeting a debated number of smaller microplates are trapped, squeezed, subducted or otherwise abused. This complexity of plates and forces gives the area many interesting geologic features. Some of the most interesting of these features are the deep ocean basins in the western Mediterranean, and one of the more interesting among these is the Tyrrhenian basin, or, to be very specific, the two basins that make up the Tyrrhenian, the Vavilov and Marsili basins (Rosenbaum and Lister, 2004). Part of the same process that made the Tyrrhenian Basin is thought to have torn Corsica and Sardinia away from southern France and rotated them to a position almost orthogonal to their original orientation (Rosenbaum et al., 2002).
This paper will deal with
An Introduction to the Area
The major plates interacting in the Western Mediterranean system are the African and European plates, and the Adria, Tyrrhenian and Ionian microplates, which are arranged approximately as shown in Figure (Gvirtzman and Nur, 2001). Europe and Africa are moving towards each other in a basically north-south direction at a rate of a few millimeters per year (Faccenna et al., 2001). The microplates are twisted and deformed between them at various rates. The Ionian plate, which is made up primarily of oceanic crust, is currently subducting in a northwesterly sort of way under the Tyrrhenian plate, and the Tyrrhenian plate is moving into the Adria plate.
The Adria microplate underlies the Adriatic Sea and the eastern side of the Italian peninsula. The west side of Italy and the islands of Sicily, Corsica and Sardinia are on the Tyrrhenian plate, though the border between the two is not very clear. In many places the Tyrrhenian plate itself is on top of the Adria plate, which makes drawing a definite line difficult (Rosenbaum and Lister, 2004).
The Adria plate is almost if not all continental crust, and Tyrrhenian plate is continental in all but its basins. The bottoms of the Tyrrhenian and Liguro-Provençal basins (the Liguro-Provençal is a large basin to the west of Sardinia, which was formed by the same process as the Tyrrhenian, but is outside the scope of this paper) are formed from oceanic type crust (Faccenna et al. 2001). The European and African plates are, of course, a delightful combination of ocean and continent, but most of the oceanic crust material has been subducted away in the Mediterranean (Malinverno and Ryan, 1986).
The geologic aspects of the Italian peninsula that are important to know for this paper are the various islands and pieces around and attached to southern Italy and the Apennines. In Figure (Rosenbaum), Sardinia and Corsica are the two islands on the far left side. Corsica in the north, and Sardinia is just south of it. Sicily is the roughly triangular island in the southernmost part of the map, just at the toe of the Italian “boot.” The toe itself is Calabria. The Apennines are a chain of mountains that run along the central length of Italy. They were formed when the Tyrrhenian microplate was thrust on top of the Adria. Finally, not shown in the figure, are the Aeolian Islands. These volcanic islands are in the Southern Tyrrhenian, and were they on the map, they would form a rough semi-circle around the south-southeast end of the Marsili Basin (Selli, 1981)
Early Mediterranean Evolution
Although the tectonic processes that formed the Tyrrhenian basin and shaped the Italian peninsula have occurred in the last 30 million years, it is important to have an understanding of what the Mediterranean area was like before then, and how it got that way. A complete history should probably involve details of Pangea and the Tethys Ocean, but that would be beyond the scope of this paper. To sum the important part of that up, around 250 Ma, the European plate was attached to the North American plate, and the North American plate was attached to the African plate, and they all stayed pretty stationary relative to each other (Figure ,). In the wedge between them was the Tethys Ocean. In the Jurassic age, rifting began between North America and Africa, and Africa and Europe started their long march towards each other. They continued rotating towards each other at the rate of a few millimeters per year, slowly closing up the Tethys, as shown in Figure (Schettino and Scotese, 2002, Faccenna et al., 2001).
Finally, around 70-80 Ma, something vaguely interesting happened. The oceanic lithosphere of the Tethys began subducting off the coast of the Iberian Peninsula (Spain and Southern France). Subduction was very slow, at an average speed of 0.8cm/yr for the first several million years (Faccenna et al., 2001). A main factor in this was that the trench ran approximately parallel to the direction of Africa’s motion, so there was little force pushing the subducting plate into the trench (Faccenna et al., 2001). This subduction continued slowly and uneventfully until around 30 Ma, when subduction sped up and all the interesting things explained in the later sections began to happen (Faccenna et al., 2001; Rosenbaum et al., 2002).
The Tyrrhenian Sea
Past Models
There has been quite an evolution of thought over the past 40 years as to the formation of the Tyrrhenian basin. The first models, generally from the 1960s to the 1980s, focused on two primary hypotheses: the sea is the result of uplift and collapse, or a back arc basin (Selli, 1981; Malinverno and Ryan, 1986).
The first group emphasized vertical motion as the prime factor in the formation. They proposed that the current lowest areas of the basin had been lifted up during the Miocene by a hot magmatic uprising, and that the sedimentary layers that had covered them slid off to the sides. The exposed crust was then dramatically thinned by erosion, and eventually the hot area beneath cooled and the crust settled back down, only was now much lower and thinner than before (Malinverno and Ryan, 1986).
This theory has two major problems, however. One is that there is nowhere near enough sediment around the current basin to account for the amount that would have to be stripped off to thin the crust as dramatically as in the Tyrrhenian (Malinverno and Ryan, 1986). The other is that if the basin were a remnant of a Miocene uplift of the crust, then it would need to have been subsiding at about 2 mm/yr. Unfortunately, that rate is nearly ten times faster than could be expected from cooling of the lithosphere (Malinverno and Ryan, 1986).
The second theory is that the Tyrrhenian Sea is the back-arc basin part of a trench-arc-back arc system. This has been the most accepted theory since the discovery of plate tectonics. The trench part of the system in this theory is the trench off the southern coasts of Calabria (the toe of Italy’s “boot”) and Sicily, as seen in the present day part of Figure , where the crust under the Ionian is being subducted under the Tyrrhenian plate. The arc part is the Aeolian Arc, a string of volcanic islands and seamounts off the northern coasts of Calabria and Sicily (Selli, 1981).
The process of back-arc basin formation will be discussed in the next section of this paper, as in 1986, Malinverno and Ryan published their paper “Extension in the Tyrrhenian Sea and shortening in the Apennines as result of arc migration driven by sinking of the lithosphere.” The hypothesis they advanced provided a variation on the back-arc basin model that continues to remain the most widely accepted theory today.
The Current Model
In the most recent models, the Mediterranean basins formed when the subduction in the trench that formed off the coast of Iberia increased its speed rather dramatically, from 0.8 cm/yr to up to 6 cm/yr (Faccenna et al., 2001). This increase was due to the beginning of slab rollback.
A Brief Explanation of Slab Rollback
When two plates come together and one begins subducting under the other, one of three very general things may happen. If the plates are moving relative to each other at a speed such that the trench does not move relative to the overriding plate, there is no tension or compression in the system, and the arc is neutral. If the trench tends to want to move towards the overriding plate, the overriding plate is compressed, and if the trench moves away from the overriding plate, the back-arc extension takes place (See Figure ) (Malinverno and Ryan, 1986). The back-arc extension case is the one that operates in the Mediterranean. Basically, the subducting plate moves down faster than it can move forward, and the overriding plate is pulled forward, and extensional features, like basins, can form (Malinverno and Ryan, 1986).
Back to the Model
So, about 30 Ma, the previously slow subducting ocean crust suddenly sped up. The slab began accelerating in part because it had, by slow subduction, gotten enough crust down the trench to begin pulling it down more quickly (Faccenna et al., 2001). Part of the reason that the slab was able to eventually reach the subduction speeds that it did is that it was fairly old, from the Miocene, and so therefore very cold and dense. This coldness and density of the plate give it negative buoyancy, so it is inclined to sink into the mantle (Gvirtzman and Nur, 2001).
When the crust is pulled by the moving
The second stage of subduction, which lasted from 30 Ma to 15 Ma, opened up the Liguro-Provençal basin. It also pulled Corsica, Sardinia, and Around 15
The Formation of Italy and the Tyrrhenian Sea
Ryan C. Wallace
Department of Earth and Planetary Sciences, Washington University, St. Louis, Missouri
Introduction
The Mediterranean Sea is a very complex region, geologically speaking. It is a place where two huge continents, Africa and Europe, collide. In the midst of this rather epic meeting a debated number of smaller microplates are trapped, squeezed, subducted or otherwise abused. This complexity of plates and forces gives the area many interesting geologic features. Some of the most interesting of these features are the deep ocean basins in the western Mediterranean, and one of the more interesting among these is the Tyrrhenian basin, or, to be very specific, the two basins that make up the Tyrrhenian, the Vavilov and Marsili basins (Rosenbaum and Lister, 2004). Part of the same process that made the Tyrrhenian Basin is thought to have torn Corsica and Sardinia away from southern France and rotated them to a position almost orthogonal to their original orientation (Rosenbaum et al., 2002).
This paper will deal with
An Introduction to the Area
The major plates interacting in the Western Mediterranean system are the African and European plates, and the Adria, Tyrrhenian and Ionian microplates, which are arranged approximately as shown in Figure (Gvirtzman and Nur, 2001). Europe and Africa are moving towards each other in a basically north-south direction at a rate of a few millimeters per year (Faccenna et al., 2001). The microplates are twisted and deformed between them at various rates. The Ionian plate, which is made up primarily of oceanic crust, is currently subducting in a northwesterly sort of way under the Tyrrhenian plate, and the Tyrrhenian plate is moving into the Adria plate.
The Adria microplate underlies the Adriatic Sea and the eastern side of the Italian peninsula. The west side of Italy and the islands of Sicily, Corsica and Sardinia are on the Tyrrhenian plate, though the border between the two is not very clear. In many places the Tyrrhenian plate itself is on top of the Adria plate, which makes drawing a definite line difficult (Rosenbaum and Lister, 2004).
The Adria plate is almost if not all continental crust, and Tyrrhenian plate is continental in all but its basins. The bottoms of the Tyrrhenian and Liguro-Provençal basins (the Liguro-Provençal is a large basin to the west of Sardinia, which was formed by the same process as the Tyrrhenian, but is outside the scope of this paper) are formed from oceanic type crust (Faccenna et al. 2001). The European and African plates are, of course, a delightful combination of ocean and continent, but most of the oceanic crust material has been subducted away in the Mediterranean (Malinverno and Ryan, 1986).
The geologic aspects of the Italian peninsula that are important to know for this paper are the various islands and pieces around and attached to southern Italy and the Apennines. In Figure (Rosenbaum), Sardinia and Corsica are the two islands on the far left side. Corsica in the north, and Sardinia is just south of it. Sicily is the roughly triangular island in the southernmost part of the map, just at the toe of the Italian “boot.” The toe itself is Calabria. The Apennines are a chain of mountains that run along the central length of Italy. They were formed when the Tyrrhenian microplate was thrust on top of the Adria. Finally, not shown in the figure, are the Aeolian Islands. These volcanic islands are in the Southern Tyrrhenian, and were they on the map, they would form a rough semi-circle around the south-southeast end of the Marsili Basin (Selli, 1981)
Early Mediterranean Evolution
Although the tectonic processes that formed the Tyrrhenian basin and shaped the Italian peninsula have occurred in the last 30 million years, it is important to have an understanding of what the Mediterranean area was like before then, and how it got that way. A complete history should probably involve details of Pangea and the Tethys Ocean, but that would be beyond the scope of this paper. To sum the important part of that up, around 250 Ma, the European plate was attached to the North American plate, and the North American plate was attached to the African plate, and they all stayed pretty stationary relative to each other (Figure ,). In the wedge between them was the Tethys Ocean. In the Jurassic age, rifting began between North America and Africa, and Africa and Europe started their long march towards each other. They continued rotating towards each other at the rate of a few millimeters per year, slowly closing up the Tethys, as shown in Figure (Schettino and Scotese, 2002, Faccenna et al., 2001).
Finally, around 70-80 Ma, something vaguely interesting happened. The oceanic lithosphere of the Tethys began subducting off the coast of the Iberian Peninsula (Spain and Southern France). Subduction was very slow, at an average speed of 0.8cm/yr for the first several million years (Faccenna et al., 2001). A main factor in this was that the trench ran approximately parallel to the direction of Africa’s motion, so there was little force pushing the subducting plate into the trench (Faccenna et al., 2001). This subduction continued slowly and uneventfully until around 30 Ma, when subduction sped up and all the interesting things explained in the later sections began to happen (Faccenna et al., 2001; Rosenbaum et al., 2002).
The Tyrrhenian Sea
Past Models
There has been quite an evolution of thought over the past 40 years as to the formation of the Tyrrhenian basin. The first models, generally from the 1960s to the 1980s, focused on two primary hypotheses: the sea is the result of uplift and collapse, or a back arc basin (Selli, 1981; Malinverno and Ryan, 1986).
The first group emphasized vertical motion as the prime factor in the formation. They proposed that the current lowest areas of the basin had been lifted up during the Miocene by a hot magmatic uprising, and that the sedimentary layers that had covered them slid off to the sides. The exposed crust was then dramatically thinned by erosion, and eventually the hot area beneath cooled and the crust settled back down, only was now much lower and thinner than before (Malinverno and Ryan, 1986).
This theory has two major problems, however. One is that there is nowhere near enough sediment around the current basin to account for the amount that would have to be stripped off to thin the crust as dramatically as in the Tyrrhenian (Malinverno and Ryan, 1986). The other is that if the basin were a remnant of a Miocene uplift of the crust, then it would need to have been subsiding at about 2 mm/yr. Unfortunately, that rate is nearly ten times faster than could be expected from cooling of the lithosphere (Malinverno and Ryan, 1986).
The second theory is that the Tyrrhenian Sea is the back-arc basin part of a trench-arc-back arc system. This has been the most accepted theory since the discovery of plate tectonics. The trench part of the system in this theory is the trench off the southern coasts of Calabria (the toe of Italy’s “boot”) and Sicily, as seen in the present day part of Figure , where the crust under the Ionian is being subducted under the Tyrrhenian plate. The arc part is the Aeolian Arc, a string of volcanic islands and seamounts off the northern coasts of Calabria and Sicily (Selli, 1981).
The process of back-arc basin formation will be discussed in the next section of this paper, as in 1986, Malinverno and Ryan published their paper “Extension in the Tyrrhenian Sea and shortening in the Apennines as result of arc migration driven by sinking of the lithosphere.” The hypothesis they advanced provided a variation on the back-arc basin model that continues to remain the most widely accepted theory today.
The Current Model
In the most recent models, the Mediterranean basins formed when the subduction in the trench that formed off the coast of Iberia increased its speed rather dramatically, from 0.8 cm/yr to up to 6 cm/yr (Faccenna et al., 2001). This increase was due to the beginning of slab rollback.
A Brief Explanation of Slab Rollback
When two plates come together and one begins subducting under the other, one of three very general things may happen. If the plates are moving relative to each other at a speed such that the trench does not move relative to the overriding plate, there is no tension or compression in the system, and the arc is neutral. If the trench tends to want to move towards the overriding plate, the overriding plate is compressed, and if the trench moves away from the overriding plate, the back-arc extension takes place (See Figure ) (Malinverno and Ryan, 1986). The back-arc extension case is the one that operates in the Mediterranean. Basically, the subducting plate moves down faster than it can move forward, and the overriding plate is pulled forward, and extensional features, like basins, can form (Malinverno and Ryan, 1986).
Back to the Model
So, about 30 Ma, the previously slow subducting ocean crust suddenly sped up. The slab began accelerating in part because it had, by slow subduction, gotten enough crust down the trench to begin pulling it down more quickly (Faccenna et al., 2001). Part of the reason that the slab was able to eventually reach the subduction speeds that it did is that it was fairly old, from the Miocene, and so therefore very cold and dense. This coldness and density of the plate give it negative buoyancy, so it is inclined to sink into the mantle (Gvirtzman and Nur, 2001).
When the crust is pulled by the moving
The second stage of subduction, which lasted from 30 Ma to 15 Ma, opened up the Liguro-Provençal basin. It also pulled Corsica, Sardinia, and Around 15
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