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

tectonic Tectonics (; ) are the processes that control the structure and properties of the Earth's crust and its evolution through time. These include the processes of mountain building, the growth and behavior of the strong, old cores of continents ...

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 ow ...

in the

Swiss Swiss may refer to: * the adjectival form of Switzerland * Swiss people Places * Swiss, Missouri *Swiss, North Carolina * Swiss, West Virginia * Swiss, Wisconsin Other uses * Swiss-system tournament, in various games and sports *Swiss Internati ...

and

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

Alps The Alps () ; german: Alpen ; it, Alpi ; rm, Alps ; sl, Alpe . are the highest and most extensive mountain range system that lies entirely in Europe, stretching approximately across seven Alpine countries (from west to east): France, ...

. The zone crops out in the

Pennine Alps The Pennine Alps (german: Walliser Alpen, french: Alpes valaisannes, it, Alpi Pennine, la, Alpes Poeninae), also known as the Valais Alps, are a mountain range in the western part of the Alps. They are located in Switzerland (Valais) and Italy ...

and in the southeastern part of the

Aosta Valley , Valdostan or Valdotainian it, Valdostano (man) it, Valdostana (woman)french: Valdôtain (man)french: Valdôtaine (woman) , population_note = , population_blank1_title = Official languages , population_blank1 = Italian French ...

. It is widely seen as part of the

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 (meaning they were thrust over the other two). The name A ...

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 The Periadriatic Seam (or fault) is a distinct geologic fault in Southern Europe, running ''S-shaped'' about from the Tyrrhenian Sea through the whole Southern Alps as far as Hungary. It forms the division between the Adriatic plate and the Eur ...

. 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 consider ...

, which is geologically part of the

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

. 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 above sea level and often emplaced onto continental crustal rocks. The Greek word ὄφις, ''ophis'' (''snake'') is found ...

)

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 di ...

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 ''n''-manifold, the equivalence classes being connected, injectively immersed submanifolds, all of the same dimension ''p'', modeled on the decomposition ...

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 (the protolith) to rock with a different mineral composition or texture. Metamorphism takes place at temperatures in excess of , and often also at elevated pressure or in the presence of ...

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, ...

and

blueschist Blueschist (), also called glaucophane schist, is a metavolcanic rock that forms by the metamorphism of basalt and rocks with similar composition at high pressures and low temperatures (), approximately corresponding to a depth of . The bl ...

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 or revenue stamp, postal stationery, banknote or ticket after it has been printed. Post offices most often use overprints for internal administrative pur ...

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

Paleogene The Paleogene ( ; also spelled Palaeogene or Palæogene; informally Lower Tertiary or Early Tertiary) is a geologic period and system that spans 43 million years from the end of the Cretaceous Period million years ago (Mya) to the beginning of ...

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 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 large peninsula conventionally considered a continent in its own right because of its great physical size and the weight of its history and traditions. Europe is also considered a subcontinent of Eurasia and it is located enti ...

and

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

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-grabe ...

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 Mya. The J ...

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 a group of islands in the 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 and Outer Hebrides, the Northern Isles (O ...

, which are separated from the rest of Europe by the

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

basin). When the plates converged again in the

, many small pieces of

continental crust Continental crust is the layer of igneous, sedimentary, and metamorphic 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 '' sial' ...

, 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 o ...

s of the Alps. {{DEFAULTSORT:Sesia Zone Geology of the Alps Structural geology Geology of Italy Geology of Switzerland