5 NORTH AMERICA: A HALF BILLION YEARS AGO

䊳 20.5 NORTH AMERICA: A HALF BILLION YEARS AGO

THE APPALACHIAN MOUNTAINS Figure 20–3c shows that eastern North America contin-

Figure 20–5 Eastern North America collided with South ued to separate from South America as the intervening

America in late Devonian time, about 374 million years ago, ocean widened through Middle Cambrian time. Then the

forming great thrust faults and folds such as these in Nova two continents began converging again and collided in

Scotia. (Geological Survey of Canada)

360 CHAPTER 20 T H E G E O L O G I C A L E VO L U T I O N O F N O RT H A M E R I C A

All of these events, beginning with subduction in and flooding low-lying portions of continents. When mid-Ordovician time followed by two collisions with

Pangea II rifted apart in late Precambrian time, a very South America and finally one with Africa, are collec-

long and wide mid-oceanic ridge system formed among tively called the Appalachian orogeny. They built the

the separating fragments of the supercontinent. The new Appalachian mountain chain as well as the Ouachita and

ridge raised sea level for much of the time from the Marathon mountains in Arkansas, Oklahoma, and east-

Cambrian through the Pennsylvanian periods, flooding ern Texas. Related events built the Innuitian mountain

the low, central craton of North America. The sea did not chain along the northern continental margin (Fig. 20–1).

cover the craton continuously, however. Seas advanced Note that the Appalachian orogeny was similar to

and withdrew several times as sea level rose and fell. the events that built the Himalayas—subduction of

Each time sea level rose, the shoreline migrated inland, oceanic crust beneath the continent, followed by a con-

spreading a blanket of marine sand, mud, and carbonate tinental collision. At one time, the Appalachians must

sediment across the craton. As a result, a veneer of sand- have been immense mountains similar to the Himalayas.

stone, shale, and limestone ranging in age from Cambrian Today, however, erosion has worn the Appalachians down

through Pennsylvanian now covers much of the central to maximum elevations of less than 2000 meters.

part of North America. These rocks are less than a few As the Appalachians rose over a period of more than

hundred meters thick in most places and are called plat- 200 million years, all other continents joined the grow-

form sedimentary rocks (Fig. 20–7). They blanket large ing landmass. Thus, Pangea III had assembled by about

areas of the Precambrian igneous and metamorphic rocks 265 million years ago (Fig. 20–3g). North America

of the craton.

formed the northwestern portion of Pangea III. This latest supercontinent was Alfred Wegener’s Pangea, described in Chapter 2.

䊳 20.6 BREAKUP OF PANGEA III

FLOODING OF NORTH AMERICA: PLATFORM OPENING OF THE MODERN SEDIMENTARY ROCKS

ATLANTIC OCEAN

Recall that a long, rapidly spreading mid-oceanic ridge Pangea III remained intact from about 300 to 180 mil- displaces a large volume of seawater, raising sea level

lion years ago and then began to rift apart. As North and

265 MILLION YEARS AGO

A composite map showing the movements of North America from 750 million years ago

550

to 265 million years ago. The other continents are shown in their posi-

530 tions of 260 million years ago. (Redrawn from Ian W. D. Dalziel, “Earth

750

Before Pangea,” Scientific American, January 1995, p. 58)

South America separated from Eurasia and Africa, the modern Atlantic Ocean began to open. The Atlantic Ocean was born, and continues to grow today, as a result of sea-floor spreading along the mid-Atlantic ridge. Passive continental margins developed on both sides of the newly opening ocean basin. Tectonic activity on the eastern margin of North America ceased, and the lofty Appalachians began to wear away.

The new eastern margin of North America devel- oped in about the same place it had been before Pangea

III assembled. Thus, rifting followed the sutures where continents had welded together 120 million years previ- ously (Fig. 20–8). Suture zones may be lines of weak- ness within a supercontinent, like the perforations in tear-out advertisements bound into magazines. The rift- ing did not perfectly follow the old sutures, however. As the supercontinent broke up, small pieces of Europe and Africa remained stuck to the east coast of North America, and parts of North America rode off with Africa and Europe.