Modern passive margins bound the oceans formed by the spreading involved in the break-up of the Paleozoic supercontinent which includes: the North and South Atlantic oceans, the Indian Ocean, the Arctic and Norwegian seas, and the margins of Antarctica. The initial rifting was preceded by regional uplift and vulcanism over a rising convection current cell or plume with the rifting area either above or below sea level after an initial period of crustal thinning and erosion. If rifting occurred within a continental block, magma penetrates the rift, and begins the accretion of oceanic crust at the edges of the separated blocks.
The shape of a passive margin is inherited from the trend and character of the initial rift, but it also reflects subsequent modification. The present form of many continental slopes results from prograding sedimentation over the initial tectonic slope . The configuration and internal characteristics of the sedimentary sequences are controlled by tectonically driven subsidence (isostasy), and by the character and influx of available terrigenous sediment supply. Christie-Blick, 1990 The margins may also be modified by in situ carbonate sediment production.
The North American continent is growing along the Atlantic slope and the shelf break advances seaward as sediments accumulate over older strata. The sequence in the growth of the Atlantic margin is a model for divergent margin development. Following the rifting and initial accretion of oceanic crust, the Atlantic grew to over 1,000 kilometers wide during the Jurassic, but this early ocean was shallow because it was over the spreading center. Heezen, 1974 The rifted margins subsided, forming basins in which continental and marine sediments accumulated with evaporites forming in restricted basins. Sills, dikes and lava flows were injected into predominantly non-marine sediments during the early (Triassic) history. The rate of subsidence slowed with time as the margin cooled, but the accumulating sediment contributed to the sink rate.
During the late Cretaceous and Cenozoic, the controls on shelf sedimentation were regional warping and faulting and eustatic sea level changes. Subsidence with sediment influx and accumulation of sediments continued on the shelf and a continental rise was developed by turbidity current deposits from the shelf and slope.
Modifications of the continental margin have led to certain distinct types of shelf and slope:
The difference between a flexured margin and a subsiding constructional margin is that there is much less influx of sediment and therefore restricted accumulation. This type of shelf is generally narrow. Flexured margins are common in the western Mediterranean Sea and parts of the west coasts of the Americas where terrigenous sediment influx has been low.
Isostatic adjustment can play a major role in the development of continental margins. Loading of the crust under the continental rise and on the continental shelf is a factor in subsidence. Internal mantle variability, tectonic movements and thermal cooling are structural controls that allow the sediment accumulation. Fossil and rock data from Florida and the Bahama Islands indicate up to 5,000 meters of subsidence since early Cretaceous and the slope prograded five to ten kilometers seaward. These shelf deposits were formed in shallow water similar to depths of the present continental shelf and upper slope, indicating a general rate of margin subsidence and deposition of one centimeter per thousand years. Spences, 1967 Other Atlantic margins show a history of subsidence and sediment accumulation on the margins since the Cretaceous.
Margin dams are a part of both constructional-subsiding passive shelves and active margins, but they are an additional element. When these barriers bound a subsiding shelf, the sediment accumulation can be much thicker than on an open shelf because seaward transport was blocked. They may be diapirs, tectonic features, or reef growth along the outer edge or the interior of the shelf.
The basic barrier is a basement ridge. This feature occurs at the top of the continental slope between Nova Scotia and New Jersey where the ridge is two to three kilometers higher than the continental shelf basement, and it separates a sediment-filled basin over the shelf from the continental slope. A similar basement ridge off California crops out as quartz diorite in the Farallon Islands. Similar ridges have been mapped in other areas.
Fold belts create structural highs to form barriers off West Africa, the Bering Sea and the East China Sea. These occur as a series of parallel arcs of uplift and folding that baffle and trap sediment.
Diapiric intrusions in salt dome regions can form an effective margin barrier . The hummocky topography of the shelf and slope off east Texas and Louisiana results from both slope failure and diapiric intrusions of salt. The outer slope is marked by an abrupt change in inclination at the Sigsbee Scarp that coincides with the leading edge of the salt beds. This is a margin with multiple diapiric dams. Wilhelm & Ewing, 1972 Along the Mexican shelf and slope, flow in the plastic salt layer has produced a series of folds that trap sediments .
Both the west Florida shelf and the Blake Plateau at the eastern side of Florida have a reef type structure at the outer margin. South of Miami, the shelf and slope are part of a carbonate platform and we have an older constructional, subsiding margin of Cretaceous age with a coral reef shelf margin dam. After lagoonal sediments were deposited, additional sedimentation prograded across the fill and built a fan beyond the barrier. Uchupi & Emery, 1967 Similar structures are found in many places. As the margin sinks, lines of barrier reefs build upward, allowing deposition of thick sediment sequences on the landward side. Subsidence with continued reef accretion can result in a reef barrier several kilometers thick. In many cases, the reef developed on top of a tectonic dam and the rift faulting pattern of the initial separation can be seen below the Jurassic sediments.
If deposition does not keep pace, a continental slope plateau may develop during subsidence. Slope plateaus are terraces forming steps below the normal level of the continental shelf that are large features covering more than 100,000 square kilometers. They seem to be downwarped sections of the continental shelf resulting from general subsidence without sufficient sediment accumulation to build up the surface; similar to a flexured margin. Very little deposition has occurred on the deep Blake Plateau since Miocene and the plateau is almost a kilometer below normal shelf depth. Locally Eocene and upper Cretaceous outcrops may be found on the plateau surface. The surface of the continental slope and the slope from the shelf break to the plateau is generally smooth, but the plateau surface is irregular, probably due to scour and erosion by the Gulf Stream. Indications of bottom currents can be seen in photographs of ripples and rock outcrops. During the six million years that the margin was subsiding and sediments were accumulating on the inner shelf, the Gulf Stream prevented deposition on the plateau. Drake, et al., 1968
Faulting can create a borderland margin with a unique irregular topography and complicated structure. Often there is a block faulted basin and range province like the borderland off southern California, where basins more than 3,000 meters deep are separated by submarine ridges, islands, and banks . These shelves are similar to a faulted basin and range continental system; faulting and local structural complications have developed a series of depressions and ridges on a grand scale. The Sahul Shelf off Australia is similar. The shelf off north central Venezuela is separated into two parts by the Cariaco Basin, and is broken by a series of faults with modification by sediment fill.
Margin cross sections -
A Grand Bank
The overlying Tertiary is a thin sediment layer of about 300 meters on the continental shelf. It thickens beneath the continental rise to about five kilometers of prograding slope forming a constructional, subsiding margin. Drake, et al., 1968 The sediments are underlain by continental crustal rock under the shelf and by oceanic rock beyond the base of the continental rise. The Grand Bank section shows a truncation of the prograded beds of the slope and erosional effects .
B Nova Scotia
The Nova Scotia-Georges Bank section has less well-developed fault blocks and no salt or evaporite deposition. The seismic section is characteristic of a faulted margin with deposition on a sinking crust with a margin dam of oceanic basement. Schlee & Klitgord, 1988
C New Jersey
Off New Jersey, the ancient continental slope (Cretaceous to Eocene) thickens at the slope-to-rise transition in response to a decreased seaward slope. This shelf had a reef margin dam. A landward shift of the shelf break occurred after early Eocene, and the Eocene surface is exposed along the lower slope. Large deltas of Miocene and Pliocene age migrated over the shelf and across part of the slope to form a different type of continental shelf locally. Poag & Mountain, 1987
Delta margins are a significant type of subsiding and prograding margin and deltaic sequences have been seen in Tertiary strata of the continental margin in many studies. Modern deltas such as the Mississippi Delta have produced a modification of the basic subsidence model by extending across the shelf and slope, and depressing the shelf several kilometers.
The same basic rift pattern is present in the Florida section, but Jurassic carbonates lie directly over the rift zone and a reef margin dam persisted from Jurassic through Cretaceous. The development of a major dam and limestone deposition controlled the relief, but deeper structural control probably dominated an earlier stage. The reef dam is situated over a buried ridge forming the edge of the continent.
The western boundaries of North and South America are marked by deep trenches that are the sites of subduction. The fault scarps have been modified by slumping, and in some cases by erosion or prograding. The accretionary prism of sediments and micro continents from the subducting plate is added to the continental margin block which grows in width as successive increments are added by underthrusting at the trench. With continued subduction, older accreted material is uplifted and older thrusts are rotated landward by the addition of younger wedges . Hayes 1966 The margin trench and negative gravity anomaly are features of the active margin just as the subsidence and a continental rise mark a divergent margin.
Volcanoes are common along the leading edge of the convergent continent, but they are inland from the margin. The western edge of the North American plate and the eastern edge of the Pacific plate meet in California with the Cascades and Mount St. Helens being part of a volcanic arc that results from subduction of the Pacific Ocean floor. The active volcanic belt ranges from single chains to zones of volcanic activity more than 200 kilometers wide. Tectonic activity and the volcanism marking a convergent margin may show episodicity through geological time because of variations in the rate of plate convergence.
One of the major factors in developing active margin shelves is availability of sediments. Influx of an adequate sediment supply from adjacent continental areas will widen and add to the accretionary prism. When volcanic or folded coastal mountain chains are developed as part of a convergent margin, the drainage system may be reduced, thereby cutting down on available sediment supply to the shelf and reducing the amount that the shelf can prograde.