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
- Form at mid-oceanic ridges.
- As plates spread, magma chambers filled with mafic crystal mush form from
the fractional melting of peridotite, an ultramafic.
dikes extend upward, parallel to boundary, like a deck of cards.
- When magma cools in the dikes, it forms gabbro. When it erupts, pillow
- Through it all hydrothermal metamorphism occurs.
- As it sits on ocean floor, a thin sediment drape forms.
- All of this rests on mantle peridotite. At mid-oceanic ridges, mantle if
very close to surface, but still not directly observable.
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
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.
- Continent is stretched, causing massive normal faulting.
- A normal fault-bounded rift valley appears.
- Basaltic eruptions occur in the rift valley
- At the same time, continental sedimentation fills the rift valley (E.G. East
African rift Valleys)
- If rift widens, it is flooded by the ocean. New oceanic crust forms. On
either side, the continental crust is warm and stands topographically high.
- As continental crust thins, it insulates less heat -->contracts-->
floods, forming continental shelf. This may be fringed with carbonate
platforms. (E.G. East
Coast of North and South America).
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
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.
- Subducting slab releases water.
- The water induces melting of peridotite, yielding ultramafic magma rises
- Fractional crystallization and assimilation of crustal rock yields mafic
to intermediate magma.
- Intrusions and volcanic eruptions occur, forming and island arc
(E>G> Aleutians. Japan)
- Where the slab subducts, it is topographically expressed as a trench.
- Overlying sediments from subducting slab are sheared off of the subducting
slab to form an accretionary wedge which shows up as a topographic
- The accretionary ridge and Island arc form a forearc basin that
accumulates sediment 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.
- Subducting slab releases water.
- Induces melting of peridotite - ultramafic magma rises
- Fractional crystallization and assimilation of continental crustal rock
--> mafic to felsic magma.
- Large intrusions and volcanic eruptions --> Volcanic - magmatic
mountain belt (E.G modern Cascades) Accompanied by high temperature low
pressure metamorphism. (Hornfels facies)
- Copious sediment from continent fill the trench.
- These thick continental sediments from subducting slab sheared off to form
subduction melanges, which show low temp high pressure (blueschist
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.
- 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.
- 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. In ancient rocks, Urals, Appalachians, Atlas.)
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.
- No significant deformation. Abrupt contrast of rocks with differing
histories is the only clue. Modern example: Southern
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).
- 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, reveALIGN the oldest history will be located in
the interior and
- that rocks currently or recently undergoing deformation will be on the
Peripheral rocks - Orogenic belts: Record the history of
continent collisions and microplate terrane collisions along a belt that can be
hundreds of km wide.
- Shields: Regions in which basement is broadly exposed. E.G.
Canadian Shield, dominated by pre-Phanerozoic metamorphic rocks and granite.
- Platforms: Parts of cratons covered by more recent (i.e.
Phanerozoic) sediment. North American platform encompasses plains and midwest,
western Canada east of Rockies.
- Basins: Regions of subsidence and thicker sedimentary deposition.
- When collisions happen, the continental crust shortens, absorbing energy
of collision in two ways:
- Thrust faults push continent into overlapping fault blocks.
- Intense folding
- Crust also thickens:
- Stacking of thrust sheets
- Microplate accretion
- Intrusion of batholiths.