Tesis Doctoral: Estudio geológico de la vertiente sur del Pirineo oriental y central Evolución cinemática en 3D




Geological study of the southeastem and southcentral side of the Pyrenees. 3D kinematic evolution.

The oblique Segre ramp zone between the eastern and central
Pyrenees represented one of the major unsolved problems on the southern side of the Pyrenees. Here, the understanding of its geometry and evolution not only explains the eastern boundary of the South Central Unit but also represents an advanced step for the comprehension of the whole evolution of the southern Pyrenees (Fig. 1). A network of 15 seismically supported perpendicular cross-sections is presented in this study. Some of them have been balanced and restored, and thus allow shortening calculations. Cross-cutting relationships between structures and syntectonic sediments along with the interna1 geometries of these sediments have been extensively determined to constrain the timing of thrusting.

The final versions of these 15 cross-sections are the result of a
double loop of testing based on: a) comparison of thrust geometries, rotation, amount of shortening and ages of thrusting between different cross-sections, and b) correspondence between restored cross-sections and maps (palinspastic maps), simultaneously completed. The comparison between transects results in a network of cross-sections that forms an interpretation that is geometrically homogeneous and geologically consistent. The correspondence between restored cross-sections and maps results in a 3D restoration.

The network of cross-sections shows the along-strike varying
structure of the southern Pyrenean cover thrust sheets. The study covers more than 150 km, from the easternmost cross-section J-1 to the westernmost cross-section 5-13, The evolution, shortening and rates of shortening for the southern Pyrenean structural units can be bnefly summarized as follows:

The positive inversion of the Mesozoic extensional basins and further southward motion restrained the irregular geometry of upper and older Pyrenean thrust sheets (Pedraforca and South Pyrenean Unit) and their limits (Segre thrust). The Vallfogona thrust branched eastwards with the Segre thrust (South Central unit) carrying the previously emplaced and blocked Pedraforca thrust sheets on its hanging wall, after 47 Ma. The eastern margin of the South Central Unit (Segre thrust zone)
was emergent to the synorogenic surface and exposed to erosion throughout its evolution. The western margin was blind and accretionary during Middle-Upper Eocene. These two contrasting geometries were controlled by the original disposiiion of the Keuper décollement level.

Cross-section 5-3 documents the most complete geological record to calculate shortening and time control. According to its restoration, the total amount of shortening is -70 km for the cover thrust sheets. Shortening including al1 south-directed structures south of the Nortb Pyrenean Fault is 88.5 km.

During the uppermost Cretaceous-Paleocene (83-55 Ma), rates of shortening were extremely low (<OS rnmlyr.). From Early Ilerdian (55 Ma) to Middle Lutetian (47 Ma) rates of shortening increased to 4.4 rnmlyr., mainly during the second half of this period (Fig. 2). Slowest rates of shortening (1.3-2.6 mmlyr.) characterized the period ranging from Middle Lutetian (47 Ma) to Early Miocene. The end of thrusting migrated from 34.4 Ma (J-1 section) in the East to 23 Ma (Riglos section) and younger to the West with rates of 11 mmlyr. in the eastern Pyrenees (34.4-29 Ma) and 21 mrnlyr. in the central Pyrenees (29-23
Ma and younger).