4.2 Revealing past plate tectonic events
In Section 3 we referred to the Caledonian and Variscan Orogenic Belts. These are interpreted as representing past destructive plate margins or more strictly speaking, representing the final phases of ocean closure that resulted in continental collision. Section 4 explores the extent to which it is possible to detect different stages throughout the cycle of ocean basin formation and closure; in other words it examines how geologists can identify:
the initial rifting and separation of continents;
the former presence of a wide ocean;
the closure of an ocean by subduction; and
the collision of two continents once an ocean has closed.
When doing this, it is important to remember that individual features such as andesitic lavas, granite plutons, intense folding or high-grade metamorphism should never by themselves be taken as unequivocal evidence for the occurrence of a particular type of plate margin. It is essential to consider all the geological evidence, as well as how all of the evidence fits together on a regional basis. (This approach is sometimes called the principle of consilience.) Even if such detective work is successful, it cannot indicate how wide a past ocean was, even though the suture zone can often be detected marking where its closure is located. To estimate the width of past oceans, additional information such as palaeomagnetic data or fossil evidence needs to be used.