The Massif Central is one of the two large basement massifs in France, the other being the Armorican Massif. The Massif Central's geological evolution started in the late Neoproterozoic and continues to this day. It has been shaped mainly by the Caledonian orogeny and the Variscan orogeny. The Alpine orogeny has also left its imprints, probably causing the important Cenozoic volcanism. The Massif Central has a very long geological history, underlined by zircon ages dating back into the Archaean 3billion years ago. Structurally it consists mainly of stacked metamorphic basement nappes.
Introduction
The basement outcrops of the Massif Central have roughly the outline of a triangle standing on its tip. Because of its size – 500 kilometers long and 340 kilometers wide – the Massif Central partakes in several tectono-metamorphic zones formed during the Variscan orogeny. The bulk of the massif belongs to the Ligero-Arvernian Zone, sometimes also called the microcontinent Ligeria. With its northeastern tip, the Morvan, it reaches into the Morvano-Vosgian Zone which becomes the Moldanubian Zone farther east. All these zones constitute the interior core of the Variscan orogen in Europe which is characterized by the following traits: It contains remnants of oceanic crust that were subducted during the Silurian and the Devonian. The approach of Gondwana from the South onto the cadomian microcontinent Armorica (and farther east onto the Saxothuringian Mid-German Crystalline Rise) induced a continental collision slicing the basement into several large-scale nappes and thrusting them into southerly directions. After the thrusting the orogen was exhumed diachronously. Exhumation started in the western and northern Massif Central already in the Upper Devonian (Frasnian) 380 million years ago whereas the southern part was elevated much later (in the Tournaisian, 350 MA BP). The Vosges farther east were raised even later – at the end of the Viséan at 330 MA BP.
In the far south the Massif Central forms part of the Montagne Noire Zone. This zone constitutes together with the Pyrenees the microcontinent Aquitania; it is no longer made up of basement nappes, but contains low-grade Paleozoic sedimentary nappes having gravitationally slid off to the south from the rising Neoproterozoic basement.
Geography
The Massif Central is crossed by major fault zones dividing it into several spatial domains.
The most important fault line is probably the NNE-SSW-striking Sillon Houiller, a 250 kilometer long normal fault with a strong sinistral wrenching component. The Sillon Houiller separates the nonvolcanic western section from the volcanic central and eastern section. Farther south it becomes the Toulouse fault.
The Oligocene Limagne graben penetrates nearly 150 kilometers into the Massiv Central from the North and almost manages to cut through towards the Grands Causses.
The narrow central section west of this graben system carries stratovolcanoes like the Cantal – Europe's highest shield volcano – and the Monts Dore (including the highest elevation in the massif, the Puy de Sancy), but also maar and explosion craters of the Chaîne des Puys farther north.
The eastern section extends from the Morvan in the Northeast to the Cévennes in the South. It is bounded in the East by the Bresse graben and its extension into the Bas Dauphiné. The change in altitude towards the grabens is quite drastic. The graben structures along the southeastern edge already form part of the oceanic Liguro-Provençal Basin. The eastern section is further subdivided by the Roanne graben and its southern continuation the Plaine du Forez. It is also cut by the NE-trending Permian strike-slip basin of Blanzy – Le Creusot which separates the Morvan from the main massif.
An important ESE-WNW-striking division is situated near Figeac and Decazeville separating the Rouergue and the Montagne Noire in the South almost completely from the main basement outcrops.
In general the Massif Central is an asymmetric basement plate elevated at its southern margin by the Pyrenees orogeny and along its eastern margin by the Alpine orogeny. Along these margins it descends very abruptly to the surrounding grabens. These margins also show the highest elevations, the plate being gently inclined to the Northwest where the basement rocks disappear under the Mesozoic cover of the Aquitaine basin and the Paris Basin. This somewhat simplistic model is locally disturbed by fault lines and graben structures – for instance the massif's highest elevation is positioned in the central section (Puy de Sancy culminating at 1886 meters) as already mentioned.