Sesia unit
Encyclopedia
The Sesia unit or Sesia nappe, also called the Sesia-Dent Blanche unit is a tectonic
unit or terrane
in the Swiss
and Italian
Alps
. The zone crops out in the Pennine Alps
and in the southeastern part of the Aosta Valley. It is widely seen as part of the Austroalpine nappes
and correlated with the Dent Blanche nappe that crops out further to the northwest.
, which is geologically part of the Southern Alps
. The Sesia zone is (just as the correlated Dent Blanche nappe) tectonostratigraphically
on top of (ophiolitic) Zermatt-Saas zone
of the Penninic nappes
.
The Sesia zone has a penetrative foliation
because it has seen a large amount of shear
during exhumation. It is therefore hard to reconstruct any older structures. It can however structurally be divided in three parts: an external, an intermediate and an internal zone. The internal zone is structurally the highest unit and is next to the Insubric line.
and blueschist relicts. Most of it, esecially the external and intermediate zones, is in the greenschist facies though. This greenschist metamorphic grade is seen as a late (Meso-Alpine) overprint, most researchers think the whole Sesia zone or at least part of it has been in eclogite or blueschist conditions during Paleogene
subduction. Because clear evidence for high pressure metamorphism is restricted to the internal zone, it is not clear weather the other two zones have also subducted to great depth.
or a semi-independent microcontinent that was situated just north of Apulia. When the continents Europe
and Africa
were divided by a rift
zone in the Jurassic
period, Apulia and the Austroalpine are supposed to have rifted apart from Africa (just like for example the present day British Isles
, which are separated from the rest of Europe by the North Sea
basin). When the plates converged
again in the Paleogene
, many small pieces of continental crust
, like the Austroalpine microcontinent, became incorporated in the nappe stacks of the Alps.
Tectonics
Tectonics is a field of study within geology concerned generally with the structures within the lithosphere of the Earth and particularly with the forces and movements that have operated in a region to create these structures.Tectonics is concerned with the orogenies and tectonic development of...
unit or terrane
Terrane
A terrane in geology is short-hand term for a tectonostratigraphic terrane, which is a fragment of crustal material formed on, or broken off from, one tectonic plate and accreted or "sutured" to crust lying on another plate...
in the Swiss
Switzerland
Switzerland name of one of the Swiss cantons. ; ; ; or ), in its full name the Swiss Confederation , is a federal republic consisting of 26 cantons, with Bern as the seat of the federal authorities. The country is situated in Western Europe,Or Central Europe depending on the definition....
and Italian
Italy
Italy , officially the Italian Republic languages]] under the European Charter for Regional or Minority Languages. In each of these, Italy's official name is as follows:;;;;;;;;), is a unitary parliamentary republic in South-Central Europe. To the north it borders France, Switzerland, Austria and...
Alps
Alps
The Alps is one of the great mountain range systems of Europe, stretching from Austria and Slovenia in the east through Italy, Switzerland, Liechtenstein and Germany to France in the west....
. The zone crops out in the Pennine Alps
Pennine Alps
The Pennine Alps are a mountain range in the western part of the Alps. They are located in Switzerland and Italy...
and in the southeastern part of the Aosta Valley. It is widely seen as part of the Austroalpine nappes
Austroalpine nappes
The Austroalpine nappes are a geological nappe stack in the European Alps. The Alps contain three such stacks, of which the Austroalpine nappes are structurally on top of the other two...
and correlated with the Dent Blanche nappe that crops out further to the northwest.
Structural geology and lithology
The outcrop in the southern part of the Aosta Valley is bounded to the south by the Insubric line. On the other side of this steeply dipping fault zone is the Ivrea zoneIvrea zone
The Ivrea zone is a tectonic terrane in the Italian Alps, that consists of a steeply dipping piece of the Earth’s lower crust of the Apulian plate. The zone is named after the Italian city of Ivrea....
, which is geologically part of the Southern Alps
Southern Alps (geology)
The Southern Alps are a geological subdivision of the European Alps. The Southern Alps are the part of the Alps that are found south of the Periadriatic Seam, a major geological faultzone across the Alps...
. The Sesia zone is (just as the correlated Dent Blanche nappe) tectonostratigraphically
Tectonostratigraphy
In geology, tectonostratigraphy refers either to rock sequences in which large-scale layering is caused by the stacking of thrust sheets or nappes in areas of thrust tectonics or the effects of tectonics on lithostratigraphy.-Tectonically formed stratigraphy:...
on top of (ophiolitic) Zermatt-Saas zone
Zermatt-Saas zone
The Zermatt-Saas zone is a tectonic unit in the western part of the Alps. Its lithology is mainly ophiolite but there are some pelitic zones too....
of the Penninic nappes
Penninic nappes
The Penninic nappes or the Penninicum are one of three nappe stacks and geological zones in which the Alps can be divided. In the western Alps the Penninic nappes are more obviously present than in the eastern Alps , where they crop out as a narrow band...
.
The Sesia zone has a penetrative foliation
Foliation
In mathematics, a foliation is a geometric device used to study manifolds, consisting of an integrable subbundle of the tangent bundle. A foliation looks locally like a decomposition of the manifold as a union of parallel submanifolds of smaller dimension....
because it has seen a large amount of shear
Shear strength (soil)
Shear strength is a term used in soil mechanics to describe the magnitude of the shear stress that a soil can sustain. The shear resistance of soil is a result of friction and interlocking of particles, and possibly cementation or bonding at particle contacts. Due to interlocking, particulate...
during exhumation. It is therefore hard to reconstruct any older structures. It can however structurally be divided in three parts: an external, an intermediate and an internal zone. The internal zone is structurally the highest unit and is next to the Insubric line.
Metamorphism
The unit shows traces of high-grade metamorphism in the form of eclogiteEclogite
Eclogite is a mafic metamorphic rock. Eclogite is of special interest for at least two reasons. First, it forms at pressures greater than those typical of the crust of the Earth...
and blueschist relicts. Most of it, esecially the external and intermediate zones, is in the greenschist facies though. This greenschist metamorphic grade is seen as a late (Meso-Alpine) overprint, most researchers think the whole Sesia zone or at least part of it has been in eclogite or blueschist conditions during Paleogene
Paleogene
The Paleogene is a geologic period and system that began 65.5 ± 0.3 and ended 23.03 ± 0.05 million years ago and comprises the first part of the Cenozoic Era...
subduction. Because clear evidence for high pressure metamorphism is restricted to the internal zone, it is not clear weather the other two zones have also subducted to great depth.
Paleogeography
The Sesia zone is, like the rest of the Austroalpine nappes, considered to have been a northern piece of the microcontinent ApuliaApulian Plate
The Adriatic or Apulian Plate is a small tectonic plate carrying primarily continental crust that broke away from the African plate along a large transform fault in the Cretaceous period. The name Adriatic Plate is usually used when referring to the northern part of the plate...
or a semi-independent microcontinent that was situated just north of Apulia. When the continents Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
and Africa
Africa
Africa is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area...
were divided by a rift
Rift
In geology, a rift or chasm is a place where the Earth's crust and lithosphere are being pulled apart and is an example of extensional tectonics....
zone in the Jurassic
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...
period, Apulia and the Austroalpine are supposed to have rifted apart from Africa (just like for example the present day British Isles
British Isles
The British Isles are a group of islands off the northwest coast of continental Europe that include the islands of Great Britain and Ireland and over six thousand smaller isles. There are two sovereign states located on the islands: the United Kingdom of Great Britain and Northern Ireland and...
, which are separated from the rest of Europe by the North Sea
North Sea
In the southwest, beyond the Straits of Dover, the North Sea becomes the English Channel connecting to the Atlantic Ocean. In the east, it connects to the Baltic Sea via the Skagerrak and Kattegat, narrow straits that separate Denmark from Norway and Sweden respectively...
basin). When the plates converged
Convergent boundary
In plate tectonics, a convergent boundary, also known as a destructive plate boundary , is an actively deforming region where two tectonic plates or fragments of lithosphere move toward one another and collide...
again in the Paleogene
Paleogene
The Paleogene is a geologic period and system that began 65.5 ± 0.3 and ended 23.03 ± 0.05 million years ago and comprises the first part of the Cenozoic Era...
, many small pieces of continental crust
Continental crust
The continental crust is the layer of igneous, sedimentary, and metamorphic rocks which form the continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called sial due to more felsic, or granitic, bulk composition, which lies in...
, like the Austroalpine microcontinent, became incorporated in the nappe stacks of the Alps.