Remember "continental drift"
and "the conveyor belt"? For the past 2 billion years, continents
have rifted and drifted, and as many
as twenty oceans have been created and destroyed. Since the break up of
Pangaea, 200 million years ago (mya), that sea floor "conveyor belt"
has brought fault-bounded volcanic island arcs, and bits of microcontinents,
and fragments of ocean crust (more than 200 assorted pieces) all crashing
into North America, building the continent. Geologists call this docking,
or accretion. Each event effectively
moved the now active plate
boundary farther and farther west. Through microplate
tectonics, the average accumulated continental build-up would be 500
The western margin of North America is
a collage of exotic terranes with some commonality. But their differences
outweigh their similarities. San Francisco, California, is made up of
many small terranes - Alcatraz Island is a terrane! Some terranes, made
of displaced continental crust from before the breakup of Pangea, can
be quite large. The Brooks Range of Alaska is a good example. And let's
include India with the large category - it had quite a trip before it
began its crash into Eurasia.
We know exotic terranes are fault-bounded
rock bodies that originated elsewhere - but how can we determine the "elsewhere"?
The internal structure of the
individual terrane, the fossil record, paleomagnetic signatures, and geochemical
markers all contribute to the identification process.
Four types of terranes have been identified: stratified,
and metamorphic. Their
geologic make up and structure can reveal their history, and, along
with their velocity and angle of impact, it will determine how they
will react to a collision. Some will fold up and over, some will shear
along fault lines, some will rotate and migrate along a boundary, some
will fracture, stretch, and disperse. And previous accretions will affect
the scale and reaction of the impacts of "new-comers"!
Most terranes with sedimentary layers hold evidence of their deep-water
origins in the fossil record of the rock. By using microscopic skeletal
remains of radiolarians and
conodonts, workers have been able
to successfully date portions of these terranes. Conodonts are often
found with other fossil assemblages, and they develop different coloring
in response to the different geomorphic events and conditions of the
matrix rock. These facts have been used to successfully corroborate
a time scale for dating rocks with conodonts alone. Tethyan fusulinids
provide evidence for the global movements of some crustal terranes.
Known to have originally lived in the ancient Tethys
Sea, the fossils of these organisms are now found an ocean away
in the terranes of western North America.
- Jones, Cox, Coney and Beck call
paleomagnetic signatures the key to measuring the movement of terranes.
The degree of inclination recorded by the magnetized fraction of the
terranes' rocks establishes distance from the geographic North Pole
at the time of origin; the declination indicates the angle between this
vector and true north. By comparing terrane signatures with those of
the craton's rocks of the same age, the global location of the terrane's
origin can be calculated. These signatures can also help to determine
terrane rotation before or after impact, direction of movement before
docking and after impact, and the direction of ancient sea currents.
Markers - Geologists use calculated ratios of strontium
87 and strontium 86 isotopes found in rocks to help delineate boundaries
between ancient (Precambrian) continental crust and ocean crust rocks.
Samples with high strontium 87 to strontium 86 ratios are identified
as continental in origin; low ratios indicate exotic ocean crust. These
results can be combined with other dating techniques for accurate identification.
the rocks in a terrane is important to determining time of docking. Simply
put, it is a matter of logic. The time of docking must be no later than
the age of the rock which sutures the accretion nor earlier than the youngest
rock in the accreted terrane. That makes sense!