Further Reading

Ahrens, C. Donald. Meteorology Today. 7th ed. Pacific

Grove, Calif.: Brooks/Cole, 2002.

Proterozoic The Proterozoic refers to the younger of the two Precambrian eras and the erathem of rocks deposited in this era. Divisions of the Proterozoic include the Early or Paleoproterozoic (2.5 Ga-1.6 Ga), Middle or Mesoproterozoic (1.6 Ga-1.3 Ga), and Late or Neoproterozoic (1.3 Ga-0.54 Ga). Proterozoic rocks are widespread on many continents, with large areas preserved especially well in North America, Africa and Saudi Arabia, South America, China, and Antarctica.

Like the Archean, Proterozoic terrains are of three basic types: rocks preserved in cratonic associations, orogens (often called mobile belts in Proterozoic literature), and cratonic cover associations. Wide shear zones, extensive mafic dike swarms, and layered mafic-ultramafic intrusions cut many Pro-terozoic terrains. Proterozoic orogens have long linear belts of arc-like associations, metasedimentary belts, and widespread, well-developed ophiolites.

Many geologists believe that clear records of plate tectonics first appeared in the Proterozoic, although many others have challenged this view, placing the operation of plate tectonics earlier, in the Archean. This later view is supported by the recent recognition of Archean ophiolites (including the Dongwanzi ophiolite) in northern China.

The Proterozoic saw the development of many continental-scale orogenic belts, many of which have been recently recognized to be parts of global-scale systems that reflect the formation, breakup, and reassembly of several supercontinents. Paleoproterozoic orogens include the Wopmay in northern Canada, interpreted to be a continental margin arc that rifted from North America and then collided soon afterwards, closing the young back-arc basin. There are many 1.9-1.6 Ga orogens in many parts of the world, including the Cheyenne belt in the western United States, interpreted as a suture that marks the accretion

~ 700 Ma

AF

Albany-Fraser Belt

AL

Aldan Shield

AS

Anabar Shield

EG

Eastern Ghats Belt

GR

Grenville Orogen

HC

Halls Creek Inlier

IR

Irumide Belt

Kl

Kibaran Belt

LU

Lurian Belt

MA

Madagascar

Ml

Mount Isa Inlier

NN

Namaqua-Natal Belt

PC

Pine Creek Inlier

PO

Patterson Belt

RS

Rondonia-Sunsas Belt

SN

Sveconorwegian Belt

TA

Trans Antarctic Belt

TC

Tennant Creek Inlier

TT

Thelon-Taltson Zone

TX

Texas

WS

Weddell Sea

WO

Wopmay Orogen

Grenvillian belts

Pre-grenvillian proterozoic orogens

Pre-grenvillian cratons

Grenvillian belts

Pre-grenvillian proterozoic orogens

Pre-grenvillian cratons

Pine Creek Inlier

G Infobase Publishing

Plate reconstruction of the continents at 700 million years ago showing the supercontinent of Rodinia, with North America situated in the center of the supercontinent of the Proterozoic arc terrains of the southwestern United States with the Archean Wyoming Province.

The supercontinent Rodinia formed in Meso-proterozoic times by the amalgamation of Laurentia, Siberia, Baltica, Australia, India, Antarctica, and the Congo, Kalahari, West Africa, and Amazonia cratons between 1.1 and 1.0 Ga. The joining of these cratons resulted in the terminal collisional events at convergent margins on many of these cratons, including the ca. 1.1-1.0-Ga Ottawan and Rigolet orogenies in the Grenville Province of Laurentia's southern margin. Globally, these events have become known as the Grenville orogenic period, named after the Grenville orogen of eastern North America. Gren-ville-age orogens are preserved along eastern North America, as the Rodonia-Sunsas belt in Amazonia, the Irumide and Kibaran belts of the Congo craton, the Namaqua-Natal and Lurian belts of the Kalahari craton, the Eastern Ghats of India, and the Albany-Fraser belt of Australia. Many of these belts now preserve deep-crustal metamorphic rocks (granulites) that were tectonically buried to 19-25-mile- (30-40-km-) depth; then the overlying crust was removed by erosion, forcing the deeply buried rocks to the surface. Since 30-40 kilometers of crust still underlies these regions, they might have had double crustal thickness during the peak of metamorphism. Today such thick crust is produced in regions of continent-continent collision and locally in Andean arc settings. The linear quality and wide distribution of the Gren-ville-aged orogens suggest they delineate the sites of continent-continent collisions where the various cratonic components of Rodinia collided between 1.1 and 1.0 Ga.

The Neoproterozoic breakup of Rodinia and the formation of Gondwana at the end of the Precam-brian and the dawn of the Phanerozoic represents one of the most fundamental problems being studied in earth sciences today. There have been numerous and rapid changes in the understanding of events related to the assembly of Gondwana. one of the most fundamental and most poorly understood aspects of the formation of Gondwana is the timing and geometry of closure of the oceanic basins, separating the continental fragments that amassed to form the Late Neopro-terozoic supercontinent. The final collision between East and West Gondwana most likely followed the closure of the Mozambique ocean, forming the East African orogen, which encompasses the Arabian-Nubian shield in the north and the Mozambique Belt in the south. These and several other orogenic belts are commonly referred to as Pan-African belts, recognizing that many distinct belts in Africa and other continents experienced deformation, metamor-phism, and magmatic activity spanning the period of 800-450 Ma. Pan-African tectonothermal activity in the Mozambique Belt was broadly contemporaneous with magmatism, metamorphism, and deformation in the Arabian-Nubian shield. Geologists attribute the difference in lithology and metamorphic grade between the two belts to the difference in the level of exposure, with the Mozambican rocks interpreted as lower crustal equivalents of the juvenile rocks in the Arabian-Nubian shield. Recent geochronologic data indicate the presence of two major "Pan-African" tectonic events in East Africa. The East African Orogeny (800-650 Ma) represents a distinct series of events within the Pan-African of central Gondwana, responsible for the assembly of greater Gondwana. Collectively, paleomagnetic and age data indicate that another later event at 550 Ma (Kuunga orogeny) represents the final suturing of the Australian and Antarctic segments of the Gondwana continent. The Arabian-Nubian shield in the northern part of the East African orogen preserves many complete ophio-lite complexes, making it one of the oldest orogens with abundant Penrose-style ophiolites, with crustal thicknesses similar to those of Phanerozoic orogens.

The Proterozoic record preserves several continental-scale rift systems. Rift systems with associated mafic dike swarms cut across the North China craton at 2.4 and 1.8 billion years ago, as well as in many other cratons. One of the best-known of Proterozoic rifts is the 1.2-1.0-Ga Keweenawan rift, a 950-mile-(1,500-km-) long 95-mile- (150-km-) wide trough that stretches from Lake Superior to Kansas in North America. This trough, like many Proterozoic rifts, is filled with a mixture of basalts, rhyolites, arkose, conglomerate, and other, locally red, immature sedimentary rocks, all intruded by granite and syenite. Some of the basalt flows in the Keweenawan rift are 1-4 miles (2-7 km) thick.

Massive Proterozoic diabase dike swarms cut straight across many continents and may be related to some of the Proterozoic rift systems or to mantle plume activity. Some of the dike swarms are more than 1,865 miles (3,000 km) long, hundreds of kilometers wide, and consist of thousands of individual dikes ranging from less than three feet to more than 1,640 feet (1-500 m). The 1.267 Ga Mackenzie swarm of North America and others show radial patterns and point to a source near the Coppermine River basalts in northern Canada. Other dike swarms are more linear and parallel failed or successful rift arms. The direction of magma flow in the dikes is generally parallel to the surface, except in the central 300-650 miles (500-1,000 km) of the swarms, suggesting that magma may have fed upward from a plume that initiated a triple-armed rift system, and then the magma flowed away from the plume head. In some cases, such as the Mackenzie swarm, one of the rift arms succeeded in forming an ocean basin.

Arabian Nubian Shield

Arabian-Nubian shield in eastern Sudan (Earth Sciences and Image Analysis Laboratory, NASA Johnson Space Center)

Cratonic cover sequences are well preserved from the Proterozoic in many parts of the world. In China, the Mesoproterozoic Changcheng Series consists of several-kilometer-thick accumulations of quartzite, conglomerate, carbonate, and shale. In North America, the Paleoproterozoic Huronian Supergroup of southern Canada consists of up to 7.5 miles (12 km) of coarse clastic rocks dominated by clean beach and fluvial sandstones, interbedded with carbonates and shales. Thick sequences of continentally derived clastic rocks interbedded with marine carbonates and shales represent deposition on passive continental margins, rifted margins of back arc basins, and cratonic cover sequences from epicontinental seas. Many parts of the world have similar cratonic cover sequences, showing that continents were stable by the Proterozoic, that they were at a similar height with respect to sea level (freeboard), and that the volume of continental crust at the beginning of the Proterozoic was at least 60 percent of the present volume of continental crust.

One of the more unusual rock associations from the Proterozoic record is the 1.75-1.00 Ga gran-ite-anorthosite association. The anorthosites (rocks consisting essentially of all plagioclase) have chemi cal characteristics indicating that they were derived as cumulate rocks from fractional crystallization of a basaltic magma extracted from the mantle, whereas partial melting of the lower crustal rocks produced the granites. The origin of these rocks is not clearly understood—some geologists suggest they were produced on the continental side of a convergent margin, others suggest an extensional origin, and still others suggest an anorogenic association.

Proterozoic life began with very simple organisms similar to those of the Archean, and 2.0 Ga planktonic algae and stromatolitic mounds with pro-karyotic filaments and spherical forms are found in many cherts and carbonates. The stromatolites formed by cyanobacteria exhibit a wide variety of morphologies, including columns, branching columns, mounds, cones, and cauliflower type forms. In the 1960s, many geologists, particularly from the Russian academies, attempted to correlate different Precambrian strata based on the morphology of the stromatolites they contained, but this line of research proved futile as all forms are found in rocks of all ages. The diversity and abundance of stromatolites peaked about 750 million years ago, and declined rapidly after that time period, probably due to the

Stromatolites in the Helena Formation along Highline Trail, Glacier National Park, Montana (USGS)

sudden appearance of grazing multicellular metazo-ans, such as worms, at this same time. Eukaryotic cells (with membrane-bound nuclei and other distinct organelles) are preserved in sedimentary rocks from as early as 1.8 Ga, reflecting increased oxygen in the atmosphere and ocean. The acritarchs are spherical fossils of single-celled, photosynthetic marine plankton found in a wide variety of rock types. Around 750 million years ago some of the prokaryotes experienced a sudden decline, as eukaryotic life-forms adapted to fill their niches. This dramatic change is not understood, but its timing is coincident with the breakup of Rodinia and the formation of Gondwana is notable; thus tectonic changes could have induced atmospheric and environmental changes that favored one type of organism over the other.

A wide range of metazoans, complex multicel-lular organisms, are recognized from the geological record by 1.0 Ga, and probably evolved along several different lines before the record was well established. A few metazoans up to 1.7 Ga have been recognized from North China, but the fossil record from this interval is poorly preserved since most animals were soft-bodied. The transition from the Proterozoic fauna to the Paleozoic is marked by a remarkable group of fossils known as the Ediacaran fauna, first described from the Ediacara Hills in the Flinders Ranges of southern Australia. These 550-540 million-year-old fauna represent an extremely diverse group of multicellular, complex metazoans including jellyfishlike forms, flatwormlike forms, soft-bodied arthropods, echinoderms, and many other species. The ages of these fauna overlap slightly with the sudden appearance and explosion of shelly fauna in Cambrian strata at 540 million years ago, showing the remarkable change in life coincident with the formation of Gondwana at the end of the Proterozoic.

See also Archean; Gondwana, Gondwana-land; Precambrian; supercontinent cycles.

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