The Sesia unit or Sesia nappe, also called the Sesia-Dent Blanche unit is a

tectonic Tectonics ( via Latin ) are the processes that result in the structure and properties of the Earth's crust and its evolution through time. The field of ''planetary tectonics'' extends the concept to other planets and moons. These processes ...

unit or

terrane In geology, a terrane (; in full, a tectonostratigraphic terrane) is a crust fragment formed on a tectonic plate (or broken off from it) and accreted or " sutured" to crust lying on another plate. The crustal block or fragment preserves its d ...

in the

Swiss Swiss most commonly refers to: * the adjectival form of Switzerland * Swiss people Swiss may also refer to: Places * Swiss, Missouri * Swiss, North Carolina * Swiss, West Virginia * Swiss, Wisconsin Other uses * Swiss Café, an old café located ...

and

Italian Italian(s) may refer to: * Anything of, from, or related to the people of Italy over the centuries ** Italians, a Romance ethnic group related to or simply a citizen of the Italian Republic or Italian Kingdom ** Italian language, a Romance languag ...

Alps The Alps () are some of the highest and most extensive mountain ranges in Europe, stretching approximately across eight Alpine countries (from west to east): Monaco, France, Switzerland, Italy, Liechtenstein, Germany, Austria and Slovenia. ...

. The zone crops out in the

Pennine Alps The Pennine Alps (, , , ), sometimes referred to as the Valais Alps (which are just the Northern Swiss part of the Pennine Alps), are a mountain range in the western part of the Alps. They are located in Italy (the Aosta Valley and Piedmont) an ...

and in the southeastern part of the

Aosta Valley The Aosta Valley ( ; ; ; or ), officially the Autonomous Region of Aosta Valley, is a mountainous Regions of Italy#Autonomous regions with special statute, autonomous region in northwestern Italy. It is bordered by Auvergne-Rhône-Alpes, Fr ...

. It is widely seen as part of the

Austroalpine nappes The Austroalpine nappes are a geological nappe stack system in the European Alps. They structurally on top of the Penninic (meaning they were thrust over them). The prefix Austro in this term refers to Austria, rather than south/southern. This ...

and correlated with the

Dent Blanche nappe The Dent Blanche nappe or Dent Blanche klippe is a geologic nappe and klippe that crops out in the Pennine Alps. The nappe is tectonostratigraphically on top of the Penninic nappes and by most researchers seen as Austroalpine. The nappe is n ...

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 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. Geologically the Ivrea zone is considere ...

, which is geologically part of the

Southern Alps The Southern Alps (; officially Southern Alps / Kā Tiritiri o te Moana) are a mountain range extending along much of the length of New Zealand, New Zealand's South Island, reaching its greatest elevations near the range's western side. The n ...

. The Sesia zone is (just as the correlated Dent Blanche nappe) tectonostratigraphically on top of (

ophiolitic An ophiolite is a section of Earth's oceanic crust and the underlying upper mantle that has been uplifted and exposed, and often emplaced onto continental crustal rocks. The Greek word ὄφις, ''ophis'' (''snake'') is found in the name of o ...

)

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. The Zermatt-Saas zone is tectonostratigraphically the highest part of the Penninic nappes and lies d ...

of the

Penninic nappes The Penninic nappes or the Penninicum, commonly abbreviated as Penninic, 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 ...

. The Sesia zone has a penetrative

foliation In mathematics (differential geometry), a foliation is an equivalence relation on an topological manifold, ''n''-manifold, the equivalence classes being connected, injective function, injectively immersed submanifolds, all of the same dimension ...

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.

Metamorphism

The unit shows traces of high-grade

metamorphism Metamorphism is the transformation of existing Rock (geology), rock (the protolith) to rock with a different mineral composition or Texture (geology), texture. Metamorphism takes place at temperatures in excess of , and often also at elevated ...

in the form of

eclogite Eclogite () is a metamorphic rock containing garnet ( almandine- pyrope) hosted in a matrix of sodium-rich pyroxene ( omphacite). Accessory minerals include kyanite, rutile, quartz, lawsonite, coesite, amphibole, phengite, paragonite, zoisit ...

and

blueschist Blueschist (), also called glaucophane schist, is a metavolcanic rock that forms by the metamorphism of basalt and similar rocks at relatively low temperatures () but very high overburden pressure, pressure corresponding to a depth of . The b ...

relicts. Most of it, especially the external and intermediate zones, is in the

greenschist facies Greenschists are metamorphic rocks that formed under the lowest temperatures and pressures usually produced by regional metamorphism, typically and 2–10 kilobars (). Greenschists commonly have an abundance of green minerals such as chlorite, ...

though. This greenschist metamorphic grade is seen as a late ( Meso-Alpine)

overprint An overprint is an additional layer of text or graphics added to the face of a Postage stamp, postage or revenue stamp, postal stationery, banknote or Ticket (admission), ticket after it has been Printing, printed. Post offices most often use ...

, most researchers think the whole Sesia zone or at least part of it has been in eclogite or blueschist conditions during

Paleogene The Paleogene Period ( ; also spelled Palaeogene or Palæogene) is a geologic period and system that spans 43 million years from the end of the Cretaceous Period Ma (million years ago) to the beginning of the Neogene Period Ma. It is the fir ...

subduction. Because clear evidence for high pressure metamorphism is restricted to the internal zone, it is not clear whether 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

Apulia Apulia ( ), also known by its Italian language, Italian name Puglia (), is a Regions of Italy, region of Italy, located in the Southern Italy, southern peninsular section of the country, bordering the Adriatic Sea to the east, the Strait of Ot ...

or a semi-independent

microcontinent Continental crustal fragments, partly synonymous with microcontinents, are pieces of continents that have broken off from main continental masses to form distinct islands that are often several hundred kilometers from their place of origin. Caus ...

that was situated just north of Apulia. When the continents

Europe Europe is a continent located entirely in the Northern Hemisphere and mostly in the Eastern Hemisphere. It is bordered by the Arctic Ocean to the north, the Atlantic Ocean to the west, the Mediterranean Sea to the south, and Asia to the east ...

and

Africa Africa is the world's second-largest and second-most populous continent after Asia. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 20% of Earth's land area and 6% of its total surfac ...

were divided by a

rift In geology, a rift is a linear zone where the lithosphere is being pulled apart and is an example of extensional tectonics. Typical rift features are a central linear downfaulted depression, called a graben, or more commonly a half-graben ...

zone in the

Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately 143.1 Mya. ...

period, Apulia and the Austroalpine are supposed to have rifted apart from Africa (just like for example the present day

British Isles The British Isles are an archipelago in the Atlantic Ocean, North Atlantic Ocean off the north-western coast of continental Europe, consisting of the islands of Great Britain, Ireland, the Isle of Man, the Inner Hebrides, Inner and Outer Hebr ...

, which are separated from the rest of Europe by the

North Sea The North Sea lies between Great Britain, Denmark, Norway, Germany, the Netherlands, Belgium, and France. A sea on the European continental shelf, it connects to the Atlantic Ocean through the English Channel in the south and the Norwegian Se ...

basin). When the plates converged again in the

, many small pieces of

continental crust Continental crust is the layer of igneous, metamorphic, and sedimentary rocks that forms the geological continents and the areas of shallow seabed close to their shores, known as '' continental shelves''. This layer is sometimes called '' si ...

, like the Austroalpine microcontinent, became incorporated in the

nappe stack In geology, a nappe or thrust sheet is a large sheetlike body of rock that has been moved more than or above a thrust fault from its original position. Nappes form in compressional tectonic settings like continental collision zones or on the ...

s of the Alps. Geology of the Alps Structural geology Geology of Italy Geology of Switzerland {{struct-geology-stub