The geological story of southern Spain
Chapter 3
collision - zoom in on the Betics
Of course, the structure of the Betic mountains is not quite as simple as shown in the previous section, so one final zoom-in is necessary to complete the picture:
1) Internal Betics The slabs of Chapter 1 and Chapter 2 rocks that were "ploughed down" from their original position in the NE of Spain form the core of the inner arc of the Betic mountains, called the 'Internal Betics'. They consist of three main thrust sheets that have been named. From top to bottom, that is the Maláguide sheet; the Alpujárride sheet; and the Nevada-Filábride sheet. The rocks within the thrust sheets have suffered varying degrees of metamorphism - either from their original involvement in the Hercynian orogeny and/or their involvement in this phase of deformation. For example, the Nevada-Filábride sheet contains Palaeozoic to Cretaceous rocks that show evidence of the most heavy metamorphism - continuing into the Miocene. The Malaguide rocks, by contrast contain Palaeozoic to Oligocene rocks that are barely metamorphosed. The Alpujarride sheet is metamorphosed, but not as heavily as the Nevada-Filabride. The range of metamorphic rocks in these sheets include slate, phyllite, schist, gneiss, quartzite and migmatite. The Alpujarride sheet also includes an unusual rock type called peridotite which is what the upper part of the lithospheric mantle is made of. The exhumation of such a deeply buried, exotic rock type is discussed further in the feature on the Ronda peridotite. 2) External Betics The Chapter 2 rocks that were originally deposited in the warm seas of southern Spain "got ploughed into". They were also thrusted into mountains, although only rocks of Chapter 2 age (the 'green' rocks) were involved. These mountains now form the outside part of the Arc. 3) Miocene to Pleistocene basins As the Betic mountains were rising, they would have initially been islets of rising hills surrounded by water, only later becoming the fully emergent mountains we see today. In-between these rising thrust sheets, were 'basins' - topographic lows - receiving sediments eroded from the rising hills. These basins contained marine sediments (now sedimentary rock) of Miocene and younger age - the 'yellow' rocks of this story. Two particularly prominent sedimentary basins of this age are the Guadalquivir basin to the north of the Betics, on which Sevilla now sits, and the Alboran Basin to the south, in-between the Betics and the Rif in Morocco. Many of these once marine basins have now themselves been uplifted above sea-level as the Betic range fully emerged. The Alboran Basin however continues to receive sediment from the surrounding land and now has over 8km of sediment/sedimentary rock in it. (The Alboran Basin is featured in section three of this website) [Next: Messinian salinity crisis] |