6. Rotational kinematics of the Boltaña anticline - Instituto Geológico ...

Boltaña kinematics Mochales, 2011
Figure 9. Tectonic model of the South Pyrenean Zone (Cámara and Klimowitz, 1985). This model describes a
footwall sequence of cover thrusting propagated from E to W.
2.5. The South Pyrenenan Central Unit (SPCU)
The SPCU is characterised by a set of allochthonous imbricate thrust sheets and associated
south verging folds. They were emplaced over the autochthonous rocks of the Ebro foreland basin
with thrusting propagating southwards in a piggyback sequence (Séguret, 1972; Garrido-Megías,
1973; Cámara and Klimowitz, 1985; Puigdefàbregas et al., 1991; Muñoz, 1992). These thrust sheets
are detached along Triassic evaporites and incorporate Mesozoic and Tertiary cover materials. From
Early Cuisian, the emplacement of the South Pyrenean central Unit (SPCU) produced the
fragmentation of the South Pyrenean Basin and conditioned subsequent sedimentation. The geometry
of these imbricate thrusts is strongly conditioned by the structural inversion of the Cretaceous
extensional basin.
In the SPCU, the units of Bòixols, Cotiella, Montsec and Marginal Sierras can be
distinguished (Séguret, 1972). Originally, Séguret (1972) included the Pedraforca sheet, which in the
present description is included in the Eastern South-Pyrenean Zone. The Bòixols-Turbón Unit has a
thick Mesozoic of about 5000 m, including Triassic, Jurassic and Lower Cretaceous rocks and a thin
Paleocene and Lower Eocene strata with local Upper Eocene-Oligocene conglomerate cover (García-
Senz, 2002), unconformably overlying the previous marine sequence, and subsequent to the main
folding stage. Compressive deformation is here recorded from Late Santonian (Late Cretaceous) as a
consequence of the inversion of Cretaceous extensional basins (Garrido-Megías, 1973; Simó 1985;
Bond and McClay, 1995; García-Senz, 2002). This unit was passively transported southwards on the
Montsec thrust sheet, which was active from Ilerdian to Cuisian times (Garrido-Megías, 1972b;
Nijman and Nio, 1975; Farrell et al., 1987; Vergés and Muñoz, 1990; Teixell and Muñoz, 2000). The
Cotiella Unit present an Eocene footwall with marked NNW-SSE trending cleavage, also present in
the Cretaceous marls of the hangingwall (Séguret, 1972). It is detached in the Keuper facies and its
origin is likely associated with an oblique ramp (Martínez-Peña, 1991). Several authors consider the
emplacement of the Bòixols-Cotiella nappe isochronous during the Late Cretaceous as a consequence
23