The geological story of southern Spain
Chapter 3
collision - the big picture
As we left Chapter 2, about 40-50 million years ago, the continental lithosphere of Africa, Arabia and India were just arriving at the southern margin of Eurasia. Let's take a look at what typically happens when continents like this collide.
Continent-Continent collision happens when the leading oceanic edge of one Plate, with continental crust trailing behind (e.g. Plate A opposite in the stylised schematic), is subducted beneath the continental crust of another Plate (e.g. Plate B opposite), eventually bringing the two continental crusts into contact. This is the mechanism by which India travelled northwards to eventually collide with Eurasia beginning about 50 my. We noted in the last chapter that since they first touched, India had travelled over 2500km into Eurasia. How is that amount of penetration achievable? Basically, the shortening is achieved along three axes - (i) by folding and thrusting of sheets of continental material from each Plate over each other (ii) by vertical uplift; and finally (iii) by sideways extrusion! In the India-Himalaya collision, amazingly the sideways extrusion component accounts for about half of the space accommodation, with much of adjacent Tibet and China formed that way. This 'continental escape' as it's called, is particularly characteristic of early-stage continental plate margin collision where the thick continental crust of each plate has not yet come into uniform full contact along all the margin. Continental crustal material is ejected from a region of first contact, sideways into the area where the continental margins have not yet fully met, and still consists of oceanic crust. (Imagine a gigantic tube of toothpaste being squeezed where the Plates meet, except that the material being squeezed out is a slow extrusion of hundreds of cubic kilometres of rock over tens of millions of years, rather than a rapid squirt of soft paste !!) At the leading edge of the extruding continental crust, it is also characteristic for an arcuate subduction zone to develop, with thrusting/shearing along the top and bottom edges (schematic opposite). This arcuate subduction zone progressively retreats (or "rolls back") in a manner called 'subduction rollback' , in the process helping pull the advancing continental crustal front forward. And indeed, this series of complex interactions seems to be essentially what happened at the western end of the old Tethys ocean (now the modern day Mediterranean) as it closed and Africa met Eurasia.... [Next: Zoom in on the Western Mediterranean] |