- Introduction--What are terranes and why are they important in geology?
- What are the different types of terranes and what are their characteristics?
- What are ophiolites and why are they special as terranes?
- What is the evidence for exotic terranes?
- Open Source Web Links
Introduction-What are terranes and why are they important in geology?
Geologists working in the Pacific Northwest first developed the idea that plate tectonics added rocks created elsewhere to the edge of a continent. 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 tens to a few 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.
What are the different types of terranes and what are their characteristics?
A look at what is carried on the plates in today's Pacific Ocean basin gives some idea of the types of rock most likely to have accreted to North America in the past. The two most likely types of terranes are sections oceanic crust and entire island arcs. Also likely are oceanic plateaus (broad zones of thick oceanic crust in shallow water), ocean islands (such as the islands of Hawaii), and groups of island arcs that have already merged together. It is also possible for a rifted piece of a continent to be removed from its homeland and become part of a moving oceanic plate. Any of these types of crust can become an accreted terrane.
What are ophiolites and why are they special as terranes?
When a terrane, especially a large terrane, accretes at a subduction zone, a piece of oceanic lithosphere can be wedged up onto the edge of the continent as part of the accretion process. This occurs at the boundary between the large accreted terrane and the pre-existing continental edge, which is known as the "suture zone." The section of oceanic lithosphere that gets thrust into the suture zone is called an ophiolite--a name that comes from a Greek word for snake and refers to the green, slippery serpentinite rocks that are usually part of an ophiolite. Ophiolites mark many of the world's suture zones, past and present.
An ophiolite, from top to bottom, consists of:
- sedimentary rock from the ocean floor such as ribbon chert and shale.
- pillow basalts from mafic magma erupted onto the ocean floor.
- dikes that acted as feeders of magma from the gabbroic magma chambers to the ocean floor
- gabbro from deep in the oceanic crust
- layered mafic and ultramafic intrusions from the top of the mantle and the base of the oceanic crust
- ultramafic rocks from the uppermost mantle, which is commonly altered by metamorphism into a green, slick-sided rock called serpentinite
What is the evidence for exotic 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 plants and animals in the world today do not exist on all continents or in all parts of the oceans-they have a limited geographic range. The same is true for species of the past. Therefore, terranes containing fossils that do not match the continent's fossil sequence and do match the sequence of another place probably came from that other place.
Paleomagnetism
The direction of the earth's magnetic field changes over time. 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 latitude at which a magnetized rock first formed--its position relative to the poles and the equator. 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.
Isotopes
The isotope strontium-87 provides additional clues about terrane accretion. Strontium-87 values are much higher in older continental crust than in oceanic and island arc crust. Careful collection, mapping and laboratory analysis of the isotopic composition of rocks enables geologists to distinguish original continent from accreted terranes of oceanic or island arc crust. In some places igneous rocks have intruded or covered up pre-existing crust. These rocks provide information about the isotopic composition of the crust they have intruded or erupted onto. Magma picks up the higher values of the isotope strontium-87 when it passes through older continental crust.
Open Source Web Links
The National Park Service-U.S. Geological Survey have produce a site 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
Basics--Exotic Terranes
Created by Ralph L. Dawes, Ph.D. and Cheryl D. Dawes, including figures unless otherwise noted
updated: 7/10/11
Unless otherwise specified, this work by Washington State Colleges is licensed under a Creative Commons Attribution 3.0 United States License.
