Reconstructing past plate movements:

  • The sea floor record: Inferring the recent movements of plates and continents is relatively easy. Just "rewind" the paleomagnetic and geochronological record of sea floor spreading until all of the continents are together in Pangea.
    Piecing together the Plate movements prior to Pangaea, by comparison, is difficult and relies on much speculation because we don't have a clear record of sea-floor basalt to go by. And yet, geologists have reconstructed the general outline of two supercontinents, Rodinia (right) and Pannotia, prior to Pangaea. How? Obviously they didn't use the sea floor record. No sea floor is that old. 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 continental rocks. For example, the rocks of the Grenville Orogeny (orange) record similar conditions at roughly the same time during the assembly of the supercontinent Rodinia.

    Clearly, the identification of such rock assemblages is a powerful tool in reconstructing the ancient Earth, but these assemblages don't just jump up and lick you in the face. In the following lecture, we review the major types of rock assemblages that might be found on continents.

    Ancient Divergent Boundaries

    Ophiolite suites - Oceanic divergent plate boundaries:

    Every piece of oceanic crust is a record of a divergent plate boundary because all sea-floor bedrock was formed at such a boundary. Ophiolites are pieces of oceanic crust that have been thrust onto the continents. 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.)

    Ophiolites preserve the features we see in mid-ocean ridges, and this is how we recognize them:

    Intracontinental rifts - Continental divergent boundaries:

    Divergent boundaries, by definition, separate plates of oceanic lithosphere, however they can originally form as a continent is stretched and rifted apart. In this case, 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. Typically, as a continent begins to stretch prior to rifting, "stretch marks," clusters of roughly parallel rift valleys (like the Tanzanian Rift Valley at right) form. Only one will eventually become a new sea-floor spreading zone. The rest - the failed rifts - will be preserved as elongate troughs that eventually fill with continental sediment.

    In ancient rocks, we recognize failed intracontinental rifts by ancient parallel rift valley deposits. EG. Newark Supergroup of Eastern North America. Newark Supergroup outcrop

    Ancient Convergent Boundaries

    Oceanic - Oceanic convergences:

    The image at right is of Mt. Redoubt, one of the four major island arc volcanoes of the Alaska Peninsula. (We still call them part of a volcanic island arc, even though they slightly overlap the continental mainland, because they are cause by subduction melting in which magma rises through oceanic crust.) In the modern world, such active arcs are easy to recognize. Millions of years from now, after this subdiction zone shuts down, they will remain recognizable as the traces of an oceanic - oceanic convergence.

    What's new here:

    What's new here: In ancient rocks, we recognize oceanic - oceanic convergences by the parallel juxtaposition, in map view, of:

    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 forearc basin. The scene above is from the Cascade Range, a volcanic - magmatic mountain belt (as opposed to a volcanic island arc) formed by the subduction of the Juan de Fuca Plate beneath North America. As before, the traces of such convergences are preserved long after the subduction zone ceases to be active.

  • Review:

    What's new here:

    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.

    Transform boundaries:

    Microplate terrains:

    What if a large continent collides with a very small on, so that subduction is not shut down? Continental margins often include microplate terrains - remnants of small pieces of continental or island arc crust that are substantially different from surrounding rock.

    Indeed, much of western North America is an incompletely sutured moasic of microplates.
    Regional tectonic structures:

    Cratons: The processes that formed continental crust in the early Earth really were different. Ancient continental crust, especially from the Archean Eon, tends to be very thick and mechanically stable. Such thick ancinet continental crust masses are called cratons. They are thick, extensive, topographically flat, tectonically stable interiors of continents whose basement rocks are old (typically Archean or early Proterozoic) and crystalline (metamorphic and igneous).

    They come in two forms:

    Peripheral rocks - Orogenic belts: More recent rocks record the history of subduction, continent collisions, and microplate terrane collisions along peripheral belts that can be hundreds of km wide.

    Key concepts and vocabulary: