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Geology of the Pacific Northwest

Basics -- Accreted Terranes

Introduction-What are terranes and why are they important in geology?

Geologists working on the geology of western North America were among the first in the world to demonstrate that plate tectonics added rocks created elsewhere to the edges of continents. These scientists were puzzled by seemingly out of place areas of rock they called terranes. A terrane is a group of related rocks that formed together in one area, do not show any relationship to the other rocks around them, and are separated from the rocks around them by faults. Terranes range in size from a few square miles to thousands of square miles. Plate tectonics explains how terranes can be moved across an ocean and added to a continent. Because terranes come from a distant location they are often referred to as exotic terranes.

Terrane accretion is most common at convergent plate boundaries, but it may be possible for a terrane to be brought from an exotic location along a transform plate boundary. It is also possible for a new divergent plate boundary to develop that rifts a continent apart. Part of the continent can then drift away on a moving plate to become an accreted terrane on another continent.

Terranes were a mystery to geologists until plate tectonic theory allowed them to see how pieces of the crust could be moved and added to a continent far from where they had originated. Now terrane accretion is seen as one of the main processes by which the continents have formed and grown larger over the course of geologic history. The Pacific Northwest is an example of a place where terrane accretion is happening today, and has been for the last 200 million yeas or so.

Terrane terminology

Accreted terranes may be called exotic terranes, suspect terranes, or tectonostratigraphic terranes. The terms all mean the same thing. The word terrane, with its distinctive spelling, is technically a tectonostratigraphic terrane, a piece of the earth's crust or lithosphere that has been accreted to the edge of one tectonic plate from another tectonic plate.

A group of terranes that accreted together with each other prior to arriving at their final accretion destination is called a superterrane.

The term tectonostratigraphic derives from how terranes originate. A terrane has a geological history, as recorded in its stratigraphy, which is different from the geologic history of rocks adjacent to the terrane. A terrane is bounded, separated from the geologic surroundings into which it was accreted, by faults, which result from the tectonic processes leading to the accretion.

What are the different types of terranes and what are their characteristics?

Oceanic crust, island arc crust, and continental crust can make up accreted terranes. Each type of crust tells its own story about how it formed and where it came from. For example, terranes made of crust that originated in an oceanic setting but are now part of continents definitely point to accretion and are a key piece of evidence for plate tectonic theory.

To get an idea of the types of rock likely to have accreted to North America in the past, take look at what is carried on the plates in today’s Pacific Ocean basin. The oceanic crust includes pillow basalts and seafloor sediments. Oceanic plateaus contain thick layers of basalt along with limestone and other marine sediments. Ocean islands are basaltic islands, some of which formed at hotspots like Hawaii. Island arcs form at ocean-ocean subduction zones and are built of intermediate rocks such as andesite and granodiorite. Island arcs also have graywackes formed from erosion of the igneous rocks. Island arcs in tropical settings commonly have limestone, from reefs that fringe the islands. Continental crust is characterized by quartz-rich rocks such as granite and arenite.

What are ophiolites and why are they special as terranes?

Sometimes a piece of oceanic lithosphere, including ultramafic rock from the upper mantle, accretes to the edge of a continent. A section of accreted oceanic lithosphere is called an ophiolite.

Ophiolites are stratified sequences of rock that include, in their complete form, from the top down:

The oceanic sediments may include deep-ocean layers of chert or shale and shallower-ocean layers of limestone.

Peridotite and dunite are the two most common types of rock in the upper mantle. Serpentinite is produced from metamorphism of ultramafic rocks such as peridotite and dunite. Much of the world's serpentinite is found in ophiolites.

For a long time the origin of ophiolites was something of a mystery to geologists, because although they clearly had oceanic affinities, they were commonly found in mountain ranges such as the Alps in Europe or the Coast Ranges in northern California.

With the establishment of plate tectonics theory in the 1960s, ophiolites were explained.

Most ophiolites form at divergent plate boundaries. Beneath a mid-ocean spreading ridge, the upwelling asthenosphere melts to produce bodies of mafic magma. Crystals settle out of the magma at the base of the magma chambers, forming layered gabbro, peridotite, and dunite.

The top of the magma body forces molten rock up through cracks in the rifting crust, forming sheeted dikes as the crust keeps cracking and spreading apart and new dikes keep intruding. On the floor of the ocean the lava erupts from fissures fed by the dikes. The lava cools quickly, quenching against the cold ocean water. This produces piles of basalt blobs on the ocean floor known as pillow basalt.

Divergent plate boundaries are like factories which keep producing these rocks that spread away from the mid-ocean ridge as if on conveyor belts. As the newly-formed oceanic lithosphere moves away from the spreading center and ages, oceanic sediments accumulate on top of it, completing the ophiolite sequence.

What is the evidence for accreted terranes?

Terranes are separated from their surroundings by faults, most commonly thrust faults or reverse faults. Each terrane records its own geologic history in its stratigraphy, geologic structures, rock types, and fossils. The geologic history of an accreted terrane is different from the geologic history of nearby rocks that are native to the continent, indicating that it is exotic.

Fossils

Fossils in some terranes provide strong evidence of an exotic origin. Most species of plants, animals, and other organisms in the world do not exist on all continents or in all parts of the oceans - they have a limited geographic range. Many terranes contain fossils that do not match the continent's fossil sequence but do match the sequence of other places. This is taken to indicate that the terranes came from other places.

Paleomagnetism

The direction of the earth's magnetic field changes over time. In addition, if you use a three-dimensional compass, you find that the earth's magnetic field at a given time is oriented nearly horizontal near the equator and vertically at the magnetic poles.

When rocks that contain magnetized minerals first form, the magnetized minerals "lock in" the direction of the earth's magnetic field at that time by aligning toward the earth's magnetic poles. Once the rock is solid, the alignment of the magnetized minerals remains. This characteristic, called paleomagnetism, allows determination of the approximate latitude - its position relative to the equator and poles -at which a magnetized rock first formed. For a paleomagnetic measurement to be useful, the original horizontal position of the rocks must be known. That is why paleomagnetic measurements work best on lava flows or sedimentary beds that show in their bedding which way was originally horizontal. Paleomagnetic measurement indicates only original latitude, not longitude. Therefore the distance that a plate may have moved in the east-west direction cannot be determined by paleomagnetism.

There are several other ways the past motions of tectonic plates and terranes can be detected from paleomagnetism. For example, paleomagnetism can tell if a terrane has rotated, because the paleomagnetic north direction in its rocks is also rotated. Terranes can be strongly rotated if they are caught in the shearing between two plates that are moving side-by-side in opposite directions.

Isotopes

Isotopes such as strontium-87 provide additional clues about terrane accretion in places where rocks have been buried deeply in the crust, metamorphosed, or melted and recrystallized as granite. Strontium-87 values are much higher in older continental crust than in oceanic and island arc crust. Isotope analyses of rocks from western North America show an abrupt change that marks the edge of the craton - the older part of the continent that existed by the end of Precambrian time. The low strontium-87 values west of the Rocky Mountains and Basin and Range indicate crust added to continent by accretion since the Precambrian.

Overlap formations and stitching plutons

One of the distinguishing features of a terrane is that the contacts with its geological surroundings are faults. However, the faults that separate a terrane from its surroundings can only be seen if they are exposed at the earth's surface. In some places, a contact of a terrane with adjacent bedrock is not exposed because the contact has been buried beneath younger sedimentary deposits or intruded by igneous plutons. A sedimentary deposit that buries the contact of the terrane with adjacent rock is called an overlap formation. A pluton that has intruded and obscured the contact of a terrane with adjacent rock is called a stitching pluton.

Overlap formations and stitching plutons can be used to help estimate how long ago a terrane was accreted. Accretion must have taken place prior to the deposition of an overlap formation or the intrusion of a stitching pluton. Accretion must have occurred after the youngest rocks within the terrane had formed. In sum, a terrane must have accreted after its youngest rocks had been formed and before its bordering faults were buried or intruded.

Web Links

The National Park Service and United States Geological Survey combined to produce a web page that describes some ways to tell whether an accreted terrane was originally oceanic, island arc, or continental crust:
http://www.nature.nps.gov/geology/usgsnps/noca/sb7terorig.html

Glossary terms that appear on this page: terrane; fault; exotic terrane; convergent plate boundary; transform plate boundary; divergent plate boundary; accreted terrane; plate tectonic theory; island arc; limestone; graywacke; arenite; lithosphere; ophiolite; ultramafic; serpentinite; mafic; mantle; oceanic crust; gabbro; dike; magma; pillow basalt; sedimentary rock; ribbon chert; shale; stratigraphy; paleomagnetism; isotope; overlap formation; stitching pluton


Geology of the Pacific Northwest
Basics--Exotic Terranes
© 2001 Ralph L. Dawes, Ph.D. and Cheryl D. Dawes
updated: 7/17/13