(Note: Do not confuse the term terranewith the word terrain,which means
landform.) Terranes are usually bounded by faults and are distinct from their
geologic surroundings.The boundary between two or more terranes is called
a suture zone. The composition of terranes generally resembles that of an
oceanic island or plateau. Others are composed of a consolidated conglomer-
ate of pebbles, sand, and silt that accumulated in an ocean basin between col-
liding crustal fragments.
Terranes, which are as much as 1 billion years old, are dated by analyz-
ing entrained fossil radiolarians (Fig. 73), marine protozoans that lived in deep
water and were abundant from about 500 million to 160 million years ago.
Different species also defined specific regions of the ocean where the terranes
originated. Many terranes traveled great distances before finally adhering to a
continental margin. Some North American terranes have a western Pacific
origin and were displaced thousands of miles to the east.
From the Cambrian to the end of the Paleozoic, the western edge of
North America ended near present-day Salt Lake City. Over the last 200 mil-
lion years, North America has expanded by more than 25 percent during a
major pulse of crustal growth. Much of western North America was assem-
bled from oceanic island arcs and other crustal debris skimmed off the Pacific
plate as the North American plate headed westward after the breakup of the
supercontinent Pangaea.
Terranes exist in a variety of shapes and sizes, ranging from small slivers
to subcontinents such as India, itself a single great terrane. Most terranes are
elongated bodies that deform when colliding with a continent. Terranes cre-
Figure 72 Distribution
of 2-billion-year-old
terranes.ORDOVICIAN VERTEBRATES