Continental deformation II
Continental - Continental convergences: :
Himalayas, Mt. Everest in center, from Wikipedia
Now imagine we take the situation described, and slam a continent into it.
In ancient rocks, we recognize continent - continent convergences through parallel juxtaposed volcanic-magmatic belts, suture zones, and fold and thrust belts with the occasional ophiolite. Ancient examples include the Urals, Appalachians, Atlas Mts.
- When a continent moves into a subduction zone, it does not subduct and subduction is eventually shut down. Three deformational zones result:
- The mountain belt of the overriding plate, which preserves remnants of the magmatism that characterized it before the arrival of the second continent and shut-down of the subduction zone.
- A suture consisting of greatly deformed subduction melange and sometimes ophiolites.
- The arriving continent is compressed and thickened, experiencing widespread thrust faulting. (E.g India and Asia.)
6. Transform boundaries:
7. Microplate terranes: Continental margins often include remnants of continental or island arc crust that are substantially different from surrounding rock. Thought to be results of continental collisions with small continents and arcs that were not big enough to shut down subduction. Indeed, much of western North America is an incompletely sutured moasic of microplates.
- No significant deformation. Abrupt contrast of rocks with differing histories is the only clue. Modern example: Southern California.
Regional tectonic structures:
Interior rocks - Cratons: Thick, extensive, topographically flat, tectonically stable interiors of continents whose basement rocks are old (typically Archean or early Proterozoic) and crystalline (metamorphic and igneous).
- On the surface, regions present two types;
- Undeformed sedimentary rocks: Thin layer of undeformed sedimentary rocks deposited in an orderly fashion
- Deformed rocks: That have been subjected to intense geologic forces in the past.
- Continents grow by accretion of microplate terranes at their active margins. It follows that:
- the most ancient rocks, revealing the oldest history will be located in the interior and
- that rocks currently or recently undergoing deformation will be on the active edges