GEOL100
10-29-03
Continental deformation

Reconstructing past plate movements:

  • The sea floor record: Inferring the recent movements of plates and continents is relatively easy. Just "rewind" the paleomagnetic record of sea floor spreading until all of the continents are together in Pangaea. Piecing together the Plate movements that led to the assembly of Pangaea, by comparison, is difficult and relies on much speculation because we don't have a clear sea-floor basalt record to go by.

  • Rodinia and Pannotia: And yet, geologists have reconstructed the general outline of two supercontinents prior to Pangaea. How? Obviously they didn't use the sea floor record. To do this, they matched up continental rock assemblages and interpreted the continental paleomagnetic record.

  • Rock assemblages: Nevertheless, under favorable circumstances, ancient plate boundaries leave tangible records in identifiable regional assemblages of specific types of rocks.

    In the following lecture, we review seven major types of rock assemblages that might be found on continents.

    1. Ophiolite suites - Rocks from the sea floor: Ophiolites are pieces of oceanic crust that have been thrust onto the continents. Because all sea floor rocks are near a sea-floor spreading zone at some point in their existence, they have usually been altered by hydrothermal metamorphism and are important sources of metal ores. (Remember, Cyprus gets its name from the metal mined in its extensive ophiolites.) The typical structure of ophiolites is a function of the sequence of their formation:

    Ophiolites are recognized by characteristic sequence of hydrothermally altered pillow basalts and parallel gabbro dikes, Gabbro. In some cases, maybe even deeper layers of mantle peridotite get stranded. In this case, it is possible for a geologist to walk across the MOHO.

    2. Intracontinental rifts: In this case, a continent is stretched apart, with a new sea floor spreading center and new oceanic crust ultimately forming between the resulting pieces. Our concern here is what happens to the continental crust.

    Usually, as a continent thins, more than one rift valley forms, but at most one of these is destined to become a new divergent boundary. The remainder is preserved as remnant in series of elongate sedimentary bodies that formed in rift valley basins, roughly paralleling continental margin.

    In ancient rocks, we recognize intracontinental rifts by continental-shelf deposits and adjacent parallel rift valley deposits. EG. Newark Supergroup of Eastern North America. Newark Supergroup outcrop

    3. Oceanic - Oceanic convergences: These bear the mark of the ancient subduction of one plate beneath another.

    In ancient rocks, we recognize oceanic - oceanic convergences by the parallel juxtaposition of mafic volcanism, deformed marine sediment and forearc sediment derived from the island arc.

    4. Oceanic - Continental convergences: In this case, the situation is complicated by interactions between rising ultramafic magma and rocks of the thick continental crust and by the much greater amount of sediment being shed from the continent into the trench.

    Modern example: The west coast of South America.

    In ancient rocks we recognize parallel remnants of volcanic-magmatic mountain belts and subduction melanges. E.G. Jurassic - Cretaceous Sierras parallel the subduction melange of the Coast ranges in California.

    5. Continental - Continental convergences: 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.

    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.

    Regional tectonic structures:

    Interior rocks - Cratons: extensive, topographically flat, tectonically stable interiors of continents whose basement rocks are old (typically Archean or Proterozoic) and crystalline (metamorphic and igneous).

    Peripheral rocks - Orogenic belts: Record the history of continent collisions and microplate terrane collisions along a belt that can be hundreds of km wide.