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Continental margins.

Windows into Earth's History

Continental margins are submerged platforms that surround continents and separate them from the deep ocean. As recently as two decades ago, this zone was geologically termed a "belt of ignorance," because land geologists tended to keep their feet dry and marine geologists typically stayed in deep water. Though it was the study of ocean geology that led to the theory of plate tectonics and revolutionized our understanding of how Earth works, the ocean basins are so effectively destroyed by plate motions that they do not offer a long-term historical record. Today, we know that continental margins preserve the history of continental erosion, oceanic subduction, climate variation, sea-level changes, biological productivity, and numerous other processes. Unlike ocean basins, the remains of continental margins are abundant in Earth's geological record, and knowledge of continental-margin formation provides the tools to understand a good portion of our planet's history.

Fundamentally, continental margins are transition zones. Here land meets water and shallow water meets deep water, less-dense continental crust meets more-dense oceanic crust, terrigenous sediments are interlayered with biogenic deposits of oceanic origin, and vertical crustal motions of continents are juxtaposed with horizontal motions of oceanic crust. The dynamic processes that shape the margins, such as wind, waves, tides, and currents, are often intense and focused. Changes in sea level can dramatically alter the depositional and erosional patterns on continental margins. The result is a region where there are large lateral and vertical variations in the physical and chemical properties of rocks. The sedimentary and volcanic rocks of continental margins contain a composite record of local, regional, and global geological processes. Scientists who study continental margins seek understanding of sea-level fluctuations, oceanic circulation, biological productivity, sediment flux, climatic variation, plate interactions, and mantle dynamics, as well as their relative influences on the margins.

Divergent Margins Form at Spreading Centers

Three types of continental margins are classified according to tectonic origin: divergent, convergent, and transform. Their distribution is shown above. Divergent margins, also called Atlantic or passive margins, result from continents breaking apart or rifting to form new ocean basins. Their history begins with the development of extensional faults, rift basins, and perhaps regional uplift and volcanism. Through processes not yet entirely understood, the rift evolves into an active plate boundary as volcanism increases to the point where seafloor spreading begins. The northern Red Sea and the Salton Trough in Baja California are two of the most thoroughly studied examples of the early phases of continental rifting. As the nascent ocean grows, the spreading center migrates away from the continental edge, leaving the continental margin in the stable interior of the growing plate. In general, the subsequent history of the margin is one of vertical subsidence caused by thermal contraction of the cooling crust and loading by a thickening sedimentary apron. This generalized history is modified by many processes, including changes in sea level and climate, and variations in sediment source.

A divergent margin consists of a gently sloping continental shelf (generally less than 130 meters deep), a steep continental slope (from the shelf edge to depths of 4,000 to 5,000 meters) that is sometimes dissected by submarine canyons, and a continental rise (where the seafloor gradient drops to below 1 meter in 40). The margin off the East Coast of the US illustrates some of the possible variability in morphology: off New Jersey, a well-developed shelf, slope, and rise exist; however, off Florida, the continental slope flattens into the broad, flat Blake Plateau at 800 to 1,000 meters, and then drops precipitously along the Blake Escarpment to 5,000 meters.

Although divergent margins are formed primarily by tension, their origin is thought to be shear between the crust and mantle. There are two models for continental breakup: pure-shear extension, in which the position of stretching of the crust and underlying mantle are coincident, and simple-shear extension, in which the position of crustal stretching is displaced from that of the mantle along a low-angle dipping detachment fault or zone of weakness. The two models have very different implications for the position of volcanism, the angle of faulting, the crust's strength during breakup, the size of rift basins, and the symmetry or asymmetry of the divergent margins.

The basic structural units of divergent margins are the underlying crust, rift basins that form during continental stretching, and overlying sediments. The crust beneath divergent margins consists of, from land to ocean, prerift continental crust 30 to 40 kilometers thick composed primarily of granite, transitional rifted crust, and oceanic crust 6 to 7 kilometers thick that is mostly made up of basalt. Transitional rifted crust is only poorly understood at present. One of the most astonishing discoveries of recent years is the tremendous lava outflow that can accompany the latest rift/earliest drift period at some margins. In these "volcanic passive margins," the transitional crust is composed of as much as 20 kilometers of basaltic intrusive and extrusive rock, and the boundaries between thinned continental crust and normal (6-kilometer thick) oceanic crust are likely to be abrupt. The continental margins of Norway, East Greenland, and the US East Coast are examples of these volcanic margins. The composition and structure of transitional rifted crust beneath nonvolcanic margins, such as off eastern Canada and beneath the Bay of Biscay, are not well documented.

Rift basins occur shoreward of transitional rifted crust, and deposits in these basins record the early events that accompanied extension and continental breakup. The largest active continental rifts on Earth include the East African Rift, the Baikal Rift in Siberia, and the West Antarctic Rift system. Within divergent margins, most of the rift deposits lie deeply buried beneath a wedge of overlying sediments, but those still exposed provide a record of this stage in margin evolution. Along eastern North America, for example, there are outcrops from Nova Scotia to Florida of the Newark series of rocks formed from lake and river sediments and volcanic flows. These are the only directly observable rocks formed during the rifting event that separated America and Africa.

The most familiar and observable part of a divergent margin is the wide continental shelf and slope underlain by thick sediments. Some of the thickest sediment accumulations on Earth fill basins along divergent continental margins. The postrift sedimentary rock off New Jersey, for example, is approximately 15 kilometers thick in the Baltimore Canyon Trough. These sediments alone are slightly less than half the thickness of normal continental crust (40 kilometers) and more than twice the thickness of normal oceanic crust (6 to 7 kilometers). The sedimentary wedge that covers most divergent margins develops over millions of years. Its geometry and composition record variations in original breakup geometry, sediment source, sediment type, sediment supply, climate, ocean circulation, sea level, and dynamic sedimentary processes such as salt diapirism, compaction, burial diagenesis, landslides, and other mass-wasting features.

The US East Coast began forming approximately 225 million years ago when rifting from Africa first began. Numerous northeast trending detrital basins containing lakes and river systems developed in the rugged relief of the Appalachian-Mauritanides mountain chains now located in the US and Africa. The climate was mostly equatorial. As rifting progressed and the continents shifted northward, a more arid climate evolved. Large basins formed at sea level and filled with evaporites near the future axis of seafloor spreading. The spreading began 190 to 180 million years ago. The early postrift deposition was characterized by rapid subsidence and high accumulation rates caused by terrigenous detritus from the Appalachians. From around 160 to 135 million years ago, sedimentation rates dropped and a huge barrier reef, similar to the Great Barrier Reef off Australia, grew along the East Coast and acted as a sediment dam. This reef formed the shelf edge for millions of years, is the single largest sedimentary feature beneath the Atlantic margin, and has been the target for much hydrocarbon exploration and drilling. A long period of generally high sea level and low sedimentation rates followed the reef's drowning until about 16 million years ago. With the onset of global cooling and glaciation at that time, sea level fell, oceanic circulation intensified, and sedimentation rates increased to the largest rate at any time in the margin's history. Most modern deposition bypasses the continental shelf and slope to form a thick wedge on the continental rise. Two examples of contrasting structural and depositional geometries are shown at left.

Convergent Margins Form Where Plates Collide

Convergent margins, also called Pacific or active margins, result from the collision and interaction of plates along a continent's edge. The basic plate-tectonic configuration involves subduction of an oceanic plate beneath an overriding plate carrying a continent. The resulting margin is a product of the two plates' histories and their relative motions. Convergent margin morphology is considerably more complicated than that of divergent margins: The shelf may be narrow and contain numerous islands, the slope may change frequently from steep to moderate dips, and a trough or trench often occurs at the base of the slope. Continental rises are rarely present. Also, sedimentary basins rarely fill with more than a few kilometers of deposits before being modified or destroyed by plate motion.

Rupture and plate subduction may begin along preexisting weaknesses in the crust or mantle, such as along boundaries between different crustal types, along oceanic fracture zones, or along fossil plate boundaries. After subduction begins, the margin develops characteristic domains that are, from ocean to continent (see figure overleaf):

(1) the active trench, where the downgoing plate first interacts with the material on the overriding plate as it begins to descend into the mantle;

(2) the subduction complex, or accretionary wedge, where deformed rocks adhere as a result of subduction;

(3) the forearc basin on the landward part of the accretionary wedge containing less-deformed sediments;

(4) the frontal arc, a zone of uplift and deformation immediately between the accretionary wedge and the volcanic chain;

(5) the volcanic chain, a zone of active igneous activity: and

(6) the backarc region behind the volcanic chain that may contain marginal basins or inactive ancient arcs.

Together, these domains comprise what is generally called a volcanic arc, and contain some of the most dynamic tectonic environments on Earth. The trench, the subduction complex, and the forearc basin are commonly submerged and make up the continental margin.

There are high-stress and low-stress subduction zones. High-stress regime characteristics include a large accretionary prism, large shallow earthquakes, a wide range in the composition of the igneous rocks, and a shallow-dipping subducting slab. In a high-stress subduction zone, typified by the South American West Coast along the Peru-Chile Trench, the subducting oceanic crust is generally young (and therefore relatively thin and "hot") and the two plates are considered well-coupled. A low-stress subduction zone exhibits a small accretionary wedge, few large earthquakes, igneous rocks with a narrow basaltic compositional range, a steeply dipping down-going slab, and a well-developed backarc basin. Low-stress subduction zones generally occur where oceanic crust is subducting beneath oceanic crust, such as the Mariana Arc in the Pacific, and the subducting crust is generally old (and therefore relatively thick and "cold").

Accretionary wedges have been extensively studied using seismic techniques and drilling, and these studies identify four major accretionary processes. Accretion occurs where oceanic sediments are scraped off a subducting plate and added to a large wedge on the edge of the overriding continental plate. "Kneading" describes large-scale, complicated folding and faulting ("mixing") of forearc-basin and accreting-wedge sediments. Subduction erosion occurs where the front of the accretionary wedge migrates landward, presumably because of mechanical abrasion by the down-going plate. Sediment subduction occurs where sediment is carried down with the subducting slab. Variability in subduction, accretion, and volcanic processes has at different times been attributed to relative motions, ages, and temperatures of the descending and overriding plates.

The North American West Coast illustrates some of the complex geology that develops along convergent active margins and the differences between divergent and convergent margins. After millions of years as a divergent margin, around 200 million years ago the West Coast became a convergent margin and the site of active volcanic arcs (this was about the same time that the American-African supercontinent was rifting to form the Atlantic Ocean). An east-dipping subduction zone formed and North America became an overriding plate, a configuration that has persisted nearly to the present. After subduction began, a long history of accretionary events followed. Early on, a large continental fragment, Stikinia, arrived near North America on the subducting plate; it then collided with and became attached to the continent's western edge. Subsequently, at least three island arcs were pushed from their locations west of North America onto the continental edge. These are now identified as the Koyukuk Terrane (part of Alaska), Alexander Terrane (part of British Columbia), and Guerrero Terrane (Baja California). One of the largest continental fragments to be added to North America, Wrangellia (parts of Alaska and British Columbia), arrived by about 65 million years ago. The collage of terranes that forms western North America was mostly in place with the collision of Wrangellia. At that time, a trench-arc system was essentially continuous from Alaska to Mexico.

Modern tectonics of western North America have been dominated by the arrival and subduction of two mid-ocean ridge sections, one now off British Columbia, the other now off central California. As these ridge sections have been subducted, extensive strike-slip faults developed and fragmented the multiple components of the margin. The only intact portions of the previously extensive convergent margin are the Aleutian (off Alaska) and Cascadia (off Washington and Oregon) trench-arc systems.

The accretionary and tectonic events that shaped western North America continuously changed the shape and position of the continental margin. Through time, the margin shifted westward from the interior western mountains as new terranes and arcs became accreted. These tectonic events are the primary forces shaping the geometry of the margin, and other processes, such as climate, oceanic circulation, and sea level have been only secondary forces. The modern active margin is but the most recent evolutionary stage.

Plates Slide By One Another at Transform Margins

Transform margins form where two plates slide by each other or have slid by each other in the past. They can be associated with either divergent or convergent tectonic settings. The south-facing margin of West Africa began as a translational margin when South America and Africa first rifted apart. Likewise, the margin along southwest Newfoundland originated as a transform when Iberia rifted away. After the continents separated, these margins became passive in the sense that the transforms were inactive, but they are characterized by narrow shelves and narrow ocean-continent transition zones.

Convergent transform margins are numerous, and are generally characterized by a long and complicated history of translational and compressional movement. Parts of the North American West Coast became transform margins in the latest evolutionary stage. The San Andreas Fault, the largest of many strike-slip faults in a wide zone of deformation that includes the continental shelf, represents this kind of margin in California. To the north in western Canada, a transform fault connects the Aleutian and Cascadia trenches, and is the site where the Pacific plate slides past the North American plate. These margins are characterized by a shelf that can be as narrow as a few kilometers and a complex amalgamation of islands, banks, and basins that form in response to the geometry of the strike-slip fault zone. In transform convergent settings, most of the terrigenous sediment is trapped in the nearshore sedimentary basins, leaving the offshore basins to contain thinner layers of mostly biogenic sedimentation. Sediment thicknesses rarely exceed a few kilometers.

Earth's History is Written in Continental Margins

Continental margins provide a means to understand continental evolution, and then to reconstruct Earth's evolution. In a simple view, continental margins grow from both recycled continental material (sediments) and new mantle material (volcanic rocks). Through time, margins are successively incorporated into continental mass by plate collisions. Evidence for this includes the numerous remnants of divergent and convergent continental margins found in old mountain belts, and the progressive decrease in the age of rocks toward coastal areas. In North America, the oldest Precambrian rocks (1 to 4 billion years old) are found in the "interior shield" or mid-continent regions; Paleozoic sedimentary and volcanic rocks (several hundred-million years old) surround the shield--the Appalachian Mountains and parts of interior western Cordillera are examples; Mesozoic and younger sedimentary and volcanic rocks (less than 200 million years old) underlie the coastal borderlands.

The identification of continental-margin deposits within mountain belts is an observation related to the Wilson Cycle, a theory about how oceans are born through rifting, mature through drifting, and eventually die by subduction and continental collision. Within the Appalachian Mountains, fossil passive continental margin deposits have been used to infer that a proto-Atlantic Ocean, known as the Iapetus Ocean, preceded the Atlantic Ocean. This ocean formed during a rifting event more than 600 million years ago, and the Iapetus passive margin lasted about 150 million years before becoming involved in oceanic subduction. The Iapetus Ocean may have disappeared by 400 million years ago, followed by several pulses of compressional tectonics that culminated with continental collision between North America and Africa, producing a mountain range whose remnants are the Appalachians in North America and the Mauritanides in Africa. Subsequent rifting started about 225 million years ago and led to the development of the present Atlantic Ocean. The Atlantic rift followed the general trend of the preexisting collision zone, although blocks of Africa became stranded in what are now known as Nova Scotia and Florida.

The oldest evidence for widespread development of continental margins comes from the proliferation of continental rifts about 2 billion years ago. The Circum-Ungava system of rocks in Labrador provides evidence for one of the oldest rifts, 2.0 to 2.3 billion years ago. These rocks are interpreted to represent a rift that opened into a small ocean basin that was later deformed, metamorphosed, and thrust back toward the continent. Prior to 2 billion years ago, the geological record is complicated by severe deformation and metamorphism, making interpretations of original rock successions and their tectonic environments equivocal. However, recent advances in dating techniques have produced ages for detrital zircons that suggest continental sedimentary rocks may have formed more than 4 billion years ago in Australia. In North America, the oldest known continental crust, located in northwest Canada, is about 4 billion years old. These data suggest that chemical differentiation of the Earth's mantle into continents and oceans may have begun shortly after Earth was born, but the record for these ancient fossil continents (and their surrounding margins) has mostly been obliterated by later meteor bombardment, erosion, and subsequent deformation. Therefore, the geological record of the widespread development of continents, continental margins, and tectonics indicative of plate interactions is so far only available to us for the second half of Earth's 4.5 billion year age.

In many respects, the study of continental margins has matured in the past few decades. No longer are these regions of transition complete zones of ignorance. They are now recognized as "tape recorders" for many aspects of global evolution. Margins also hold promise as natural laboratories for studying dynamic processes affecting Earth's chemistry, physics, and biology. These processes include how igneous rocks contribute to crustal growth, crustal recycling, and heat and material transfer from the mantle; how faults can move catastrophically or by aseismic creep; or how fluids affect material, chemical, and heat transfer within the sedimentary column. If the past has revealed the mysteries of the morphology, architecture, and composition of continental margins, then the challenge of the future will be to define the deeper and more elusive secrets of global dynamics as we develop new technologies to measure and model the processes that construct and destroy these fundamental building blocks of the continents.

Debbie Hutchinson was introduced to geology through lab exercises in cow pastures of Vermont and through field trips across very old rocks of the Canadian shield of Ontario. After joining the US Geological Survey (USGS) in Woods Hole in 1974, she pursued studies on the tectonics of large inland lakes and of the Atlantic continental margin. With a degree from the University of Rhode Island, she is now firmly entrenched in marine studies of rifts and divergent continental margins, and currently heads the Framework Studies group for the Branch of Atlantic Marine Geology at USGS.
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Author:Hutchinson, Deborah R.
Date:Dec 22, 1992
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