The geological story of southern Spain
Chapter 3
collision - zoom in on the western Med
According to the subduction rollback model, the main African-Eurasian Plate collision that created the Alpine orogeny also triggered a period of crustal extrusion that swept southwestwards, scooping up parts of existing crustal rocks as it did so, and creating an arcuate subduction zone at its leading edge.
A helpful mental image for this, is one of a snow plough, gouging through crustal rocks, piling layers of rock over on top of each other to the front and sides ("thrusts"), with the crust ahead of the plough (oceanic crust) being buried deep beneath the advancing front; - except we're talking here about a 10-30km thickness of solid rock that's being ploughed rather than pliant snow !! The key features of this process which began in earnest around 30 my in the Oligocene are:
These features are illustrated schematically in cross-section opposite which takes the 6my map above and conceptualises a roughly North-South cross-section running from the Betics to the Rif. The imagery is one of a "bow wave" of rocks being thrusted up ahead and to the side of an advancing plough. The internal arc of the Betics is composed of Palaeozoic to pre-Miocene rocks (Chapter 1 and 2, blue and green rocks) that have been "ploughed down" from their original position to the NE. As these rocks "ploughed into" the Chapter 2 (green) rocks already in place along southern Spain, they too became involved in the thrusting and now form the outside, or external, arc of the Betic mountains. In between the compressive forces creating the Betics and Rif, there was extension which thinned the crust*, and created a topographic low which acted as a 'basin' to receive sediments being eroded from the mountains to either side. (* just to give you the sense of scales here - the thickness of the Crust today beneath the Betic mountains is about 35km, whilst it is about 15km below the Alboran Sea). From the West to East perspective, you see the oceanic crust of the Mediterranean, and later the Atlantic, being subducted beneath the "bow wave" and the hinge of this subduction zone, migrating progressively westwards ('rolling back'), helping pull the Arc towards it. Above the subducting slab, Crust and Mantle partially melts creating magma that then erupts as volcanic lava into the Alboran Sea. At the front of the Arc, for all its journey westwards, has been an accretionary wedge - a chaotic jumble of material scraped off the subducting Plate and off the front of the overriding Plate creating an assemblage of sediments called Flysch. [Next: Zoom in on the Betics] |
The "subduction rollback" model
- after Rosenbaum et al |