The Alps are a
fold and thrust belt. Folding and thrusting is the expression of
crustal shortening which is caused by the
convergent movements of the European and Adriatic plates.
Breakup of Pangaea At the end of the
Carboniferous period (300
Ma), the Hercynian or Variscan orogeny, in which the supercontinent
Pangaea formed from
Gondwana and
Laurasia, was ended. East of the terranes that now form the Alps was the
Paleo-Tethys Ocean. The effects of
wind and water were able to chemically and mechanically
erode destroy the Hercynic mountain ranges. In the
Permian, the main deposits in Europe were
sandstone and
conglomerate, products of erosion in the Hercynic mountain range. At the same time, crustal extension took place because the mountain range was
isostatically unstable (this is called orogenic collapse). Due to extension, basins formed along the axis of the mountain range and
felsic volcanism occurred. This was the first phase of
rifting between Europe and Africa. Due to the
rising sealevel in the
Triassic period, the eastern margin of Pangaea was flooded. Shallow
shelf seas and
epicontinental seas existed in which
evaporites and
limestones were deposited.
Jurassic In the early
Jurassic period (180
Ma), a narrow ocean began to form between the northern (North America and Eurasia) and southern (Africa and South America) parts of Pangaea. The
oceanic crust that was formed in the process is known as the
Piemont-Liguria Ocean. This ocean is generally regarded as a western extension of the
Tethys Ocean. Although it was not really connected to it, a
peninsular piece of continental crust of the African plate called the
Adriatic plate lay in between the African and European plates and was involved in subdividing the Tethys and early Alps formation. Sometimes the names
Alpine Tethys or
Western Tethys Ocean are used to describe a number of small oceanic basins that formed southwest of the European plate, to distinguish them from the Neo-Tethys Ocean in the east. Because the Jurassic was a time with high sealevels, all these oceans were connected by shallow seas. On the continents, shallow sea deposits (limestones) were formed during the entire Mesozoic. In the late Jurassic the
microcontinent Iberia broke away from the European plate and the
Valais Ocean was formed between the two plates. Both Piemont-Liguria and Valais Oceans were never large oceans such as today's Atlantic Ocean. What they might have been like is the opening below the
Red Sea, continuing down through Africa, forming the
Great Rift Valley. Eventually, a new ocean will cut through east Africa as the rift develops, dividing a large section of land from the main continent. When at the end of the Jurassic the Adriatic plate began to move toward the European plate,
oceanic trenches formed in the eastern Alps. In these, deep marine sediments were deposited, such as
radiolarites and
lutites.
Eo-Alpine phase in the Cretaceous The
divergent movement of the European and African plates was relatively short-lived. When the Atlantic Ocean formed between Africa and South America (about 100
Ma) Africa began moving northeast. As a result of this process, the soft layers of
ocean sediment in the Alpine Tethys Oceans were compressed and folded as they were slowly thrust upwards. Caught in the middle of the merging continents, the area of the Tethys Sea between Africa and Eurasia began to shrink as oceanic crust subducted beneath the Adriatic plate. The tremendous forces at work in the lower continental foundation caused the European base to bend downward into the hot mantle and soften. The southern (African) landmass then continued its northward movement over some 1,000 km (600 mi). The slow folding and pleating of the sediments as they rose up from the depths is believed to have initially formed a series of long east–west volcanic
island arcs.
Volcanic rocks produced in these island arcs are found among the ophiolites of the Penninic nappes. In the
late Cretaceous the first
continental collision took place as the northern part of the Adriatic subplate collided with Europe. This is called the
Eo-Alpine phase, and is sometimes regarded as the first phase of the formation of the Alps. The part of the Adriatic plate that was deformed in this phase is the material that would later form the Austroalpine nappes and the Southern Alps. In some fragments of the Piemont-Liguria Ocean now in the Penninic nappes an Eo-Alpine deformation phase can also be recognized. Apart from the Eo-Alpine fold and thrust belt other regions were still in the marine domain during the Cretaceous. On the southern margins of the European continent shallow seas formed limestone deposits, that would later be (in the Alps) incorporated into the Helvetic nappes. At the same time sedimentation of
anoxic clay took place in the deep-marine realms of the Piemont-Liguria and Valais Oceans. This clay would later become the
Bündner slates from the Penninic nappes.
Paleocene and Eocene When the Piemont-Liguria oceanic crust had completely subducted beneath the
Adriatic plate in the
Paleocene, the
Briançonnais microcontinent, according to some a piece of the
Iberian plate, arrived at the subduction zone. The Briançonnais microcontinent and Valais Ocean (with island arcs) subducted beneath the Adriatic plate. They stayed at around 70 km (45 mi) below the surface during the
Eocene, reaching the
eclogite facies and becoming intruded by
migmatites. This material would later become the Penninic nappes, but a large part of the Briançonnais terrane subducted further into the mantle and was lost. Meanwhile, at the surface the upper crust of the Adriatic plate (the later Austroalpine nappes) was thrust over the European crust. This was the main collisional phase in the formation of the Alps.
Oligocene and Miocene When the subducting
slab broke off (known as slab breakoff,
slab pull) and fell away, the subducted crust began moving up. This led to the uplift of the thickened continental crust which led, in the
Miocene, to
extension. In the case of the Alps, the extension could only take place in a west–east direction because the Adriatic plate was still converging from the south. An enormous thrustzone evolved that would later become the
Periadriatic Seam. The zone also accommodated
dextral shear that resulted from the west–east extension. With the exception of the
allochthon Austroalpine material, this thrust evolved at the boundary of the Adriatic and European plates. The central zones of the Alps rose and were subsequently eroded. Tectonic windows and domes as the
Hohe Tauern window were formed in this way. Meanwhile, the thrust front of the Penninic and Austroalpine nappes moved on, pushing all material in its way northward. Due to this pressure a
decollement developed over which thrusting took place. The thrust material would become the Helvetic nappes. The Adriatic plate started rotating counterclockwise. These parts are still uplifted up to 2.5 mm/year in some areas. It is thought it is mainly due to
rebound after weight loss from melting ice caps after the
last ice age, intensive
erosion during glaciation and some processes in the
lithosphere and
mantle. The Adriatic plate, pushed by the African plate, still rotates counterclockwise around the axis near
Ivrea in northwestern Italy and is subducted in Eastern Alps and causes
tectonic uplift (thrust) there. ==Geomorphology==