Geology of Pyrenees -

Introduction -
 The Pyrenees orogen is an east-west trending, bivergent mountain belt with two associated foreland basins: the Ebro basin to the south and the Aquitaine basin to the north (Fig 1).  The orogen's northern section is characterized by north vergent thrusts composed of Mesozoic sediments.  The southern part of the orogen is characterized by south vergent thrusts involving Tertiary synorogenic sediments and an anticlinal stack of Hercynian basement rocks (Munoz, 1992; Coney et al, 1996; Teixell, 1998).  Significant structural variations are observed laterally through the range (Teixell, 1998).

East Central Pyrenees-
    In the Eastern Central Pyrenean zone, four main structural units are recognized (Fig 2).  The southern most is the South Pyrenean Central Unit.  This zone is predominantly composed of upper thrust sheets of Mesozoic platform rocks.  These thrust sheets have been imbricated southwards and overlie the Ebro basin (Munoz, 1992).
    The next structural unit, the Hercynian basement thrust sheets, lies just north of the South Pyrenean Central Unit.  Here Hercynian basement thrusts are arranged in an antiformal stack known as the Axial Zone.  Steeply dipping faults deform the exposed basement rocks in this area (Munoz, 1992).
    The third structural zone is the North Pyrenean fault zone (NPF).  This region north of the Axial Zone antiformal stack is characterized by steeply dipping faults within Jurassic and Lower Cretaceous rocks deformed during Mid Cretaceous time.  The North Pyrenean Fault is considered to be the axis of the collisional belt and present boundary between the Iberian and European plates in this region (Munoz, 1992).
    The final structural zone in the Eastern Central Pyrenees is the North Pyrenean thrust sheets.  This region is a large-scale anticline that overlies the Aquitaine basin north of the NPF.  This anticline has north-directed thrusts composed of basement rocks and Mesozoic sediments.  The basement rocks, known as the North Pyrenean Massifs, are predominantly granulitic rocks bounded by Early Cretaceous normal faults (Munoz, 1992).  These blocks have been ejected up through the sediments after their initial emplacement (Souriau and Granet, 1995).

Western Central Pyrenees -
    Farther to the west, the observed structure is somewhat different (Fig 1 and Fig3).  The South Pyrenean Thrust Belt in this region still includes south vergent thrust sheets composed of Mesozoic and Tertiary sediments and basement involved rocks within the Axial Zone.  However, the Axial Zone in this region is not an antiformal stack because the thrust sheets here overlap less than the thrusts to the east implying less shortening was taken up in the western part of the orogen.
    In the northern part of the range, the North Pyrenean Thrust Belt is characterized by north vergent thrust sheets composed of Mesozoic sediments much like in the Eastern Central Pyrenees.  However, the boundary between the South and North Pyrenean Thrust Belts in this region is the divergence axis separating the north and south vergent thrust sheets.  The North Pyrenean Fault dies out as it heads west and no longer represents the plate boundary in this region (Teixell, 1998).
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Author: Christina M. Butzer
Spring 2001 Orogenic Systems Project
Last Updated: May 9, 2001