Cross-section

Simplified cross-section through the NW Internal Alps.  Click on the boxes to visit the field sites. ( Matterhorn, Mattmark, Verres )

The Internal Alps represents a kind of tectonic "sandwich". The upper slice of bread is the Austro-alpine (=Italian or Apulian) unit. This forms rocks that are now parts of the Sesia zone, on the SE side of the Alps, and a tectonic outlier (klippe) of an overthrust sheet - called the Dent Blanche nappe. So before erosion the Dent Blanche nappe originally formed a capping roof over the Internal Alps. The mountain of the Matterhorn lies on the eastern edge of the klippe.

Below the Dent Blanche nappe, forming the footwall to the original overthrust, lie rocks that originally formed part of the old ocean floor (Tethys) that separated the continental blocks of Italy (Apulia) and the rest of NW Europe. These ophiolitic units include the Zermatt-Saas Zone (units that have experienced local "ultra-high-pressure" metamorphism - burial to depths in excess of 100km). Parts of the transition between the European continent and this ocean are represented by a metasedimentary sequence called the Combin Zone.

If the Combin and Zermatt-Saas zones collectively form the "jam" of our tectonic sandwich, the lower slice of bread is represented by rocks derived from the old European margin of Tethys. These units are highly complex, representing different parts of this margin and associated sedimentary basins. Old crystalline rocks of this margin include the so-called "internal basement massifs". In our transect the Monte Rosa massif is such a unit, with its thin succession of metasedimentary cover.