Terrane "I.D." & Activity
 

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 kilometers inland.

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.

Terrane Types- Four types of terranes have been identified: stratified, disrupted, composite, 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"!

Fossil Evidence- 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.

Paleomagnetic Signatures - 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.

Geochemical 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.

Dating 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!


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