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Australian passive margins display a large variety of styles, including narrow, wide, volcanic and non-volcanic margins. Their tectonic history has been complicated by tectonic reactivation and anomalous subsidence/uplift, widespread at various times during the post-rift phase. Defining the exact location of the boundary between continental and oceanic crust is of key importance to understanding the structure and evolution of continental passive margins. Here, we review the history of Australian passive margins and the location of the continent-ocean boundary, and we assess rift-related magmatism and anomalous post-rift subsidence based on recently acquired seismic-reflection data, available industry well data, and current tectonic models for margin evolution. Tectonic subsidence and strain-rate analysis of well data from the largely volcanic North West Shelf and the non-volcanic inner South Australian Bight margin result in relatively small lithospheric stretching factors (β

Volcanic rifted margins evolve by a combination of extrusive flood volcanism, intrusive magmatism, extension, uplift, and erosion. The temporal and spatial relationships between these processes are influenced by the plate tectonic regime; the preexisting lithosphere (thickness, composition, geothermal gradient); the upper mantle (temperature and character); the magma production rate; and the prevailing climatic system. Of the Atlantic rifted margins, 75% are believed to be volcanic, the cumulative expression of thermotectonic processes over 200 m.y. Volcanic rifted margins also characterize Ethiopia-Yemen, India-Australia, and Africa-Madagascar. The transition from continental flood volcanism (or formation of a large igneous province) to ocean ridge processes (mid-ocean ridge basalt) is marked by a prerift to synrift transition with formation of a subaerial and/or submarine seaward-dipping reflector series and a significant thickness (to 15 km) of juvenile, high-velocity lower crust seaboard of the continental rifted margin. Herein we outline the similarities and differences between volcanic rifted margins worldwide and list some of their diagnostic features.

The main types of sedimentogenesis appearing under different combinations of environmental factors, composition and properties of sedimentary matter along with sedimentary rock basins, cryogenic facies and the main time relevant features of this process have been discussed above. All this establishes the main cryogenic formations based on the synthesis of all these data.

Namibia is now regarded as one of the type areas for studying the development of a passive volcanic margin. Recent work has concentrated on interpreting seismic data, and little use has been made of existing magnetic or gravity data. New aeromagnetic data that were recently acquired offshore Namibia by NAMCOR have been merged with existing onshore and offshore aeromagnetic surveys, as well as with ship-borne data. In addition, satellite-derived offshore free air gravity data have been merged with onshore Bouguer gravity data. A number of new features pertinent to the evolution of the volcanic margin are apparent from the new combined data sets. The most striking is that the G, M2, and M4 magnetic anomalies are better defined and are seen to swing coastward toward northern Namibia and southern Angola, in contrast to studies by earlier researchers in which these anomalies were interpreted to swing away from the coast. This has important implications for the mechanism and rate of breakup, for the age of the volcanics, and for the age of sedimentation along the coastal margin. Some interpreted seismic lines have also been modeled with coincident magnetic and gravity profiles to test whether the interpretation of the entire package of seaward-dipping reflectors (SDR) as being basaltic flows is correct. The magnetic data indicate that at least part of this package is likely to be filled with sediment, while the gravity data are less conclusive in this respect due to unknown and hence ambiguous density contributions from deeper crustal levels. These SDR packages of interpreted basalts interbedded with sediments are seen as suboutcrops along the prominent edge gravity high, which correlates closely with both the hinge zone and the G magnetic anomaly. Westward (seaward) of the hinge zone the edge gravity anomaly loses amplitude over the underlying basalts. Bearing the sources of ambiguity in mind, forward modeling nevertheless shows that the gravity anomaly can be completely neutralized, in the first order, by the overlying low-density sedimentary pile, which thickens westward. This suboutcrop of the SDR package models well as the source of the dipolar G magnetic anomaly, and contributes significantly to the shelf edge gravity high. No field reversals were required to obtain a good fit in the forward modeling, although their presence cannot be discounted.

Mesozoic and Cenozoic continental flood-basalt provinces, oceanic plateaus, oceanic basin flood basalts, and volcanic passive margins share geologic and geophysical characteristics that indicate an origin distinct from igneous rocks formed at mid-ocean ridges. Such characteristics of mafic large igneous provinces (LIPs) include (1) broad areal extent (>10^5 km^2) of basalts of similar age erupted over ~10^6 yr; (2) lower-crustal bodies characterized by Vp = 7.0–7.6 km·s^–1; (3) some component of intermediate and silicic volcanic rocks; (4) trace element, rare earth element, and isotopic signatures in flood basalts that are distinct from mid-oceanic-ridge basalts (MORBs); (5) thick (10s–100s of meters) individual basalt flows; and (6) long (?750 km) individual basalt flows. In addition, basaltic and gabbroic crustal sections of oceanic LIPs are two to five times thicker than those of “normal” oceanic crust, implying larger magma chambers than at typical mid-ocean ridges and, in the case of some continental flood basalts, resulting in layered intrusive complexes containing chromite. Lastly, some flood-basalt provinces are associated with kimberlites and other ultramafic volcanism. LIPs have formed, on average, every 10 m.y. since 250 Ma. However, despite the lower energy required to obduct relatively high standing oceanic LIPs in contrast to normal oceanic crust, only five obducted oceanic LIPs have been well documented in the Mesozoic and Cenozoic continental and island-arc geologic record. More ophiolite fragments may be obducted sections of volcanic passive margins and oceanic plateaus than we now suppose.

Interpretation of regional gravity, magnetic and seismic data of the offshore region from the platform toward the continental-oceanic crustal boundary in ultradeep water regions of the Campos, Santos and the conjugate basins in the Angola margin indicate that the rift architecture along the South Atlantic sedimentary basins is highly controlled by fracture zones and volcanic centers. Both along the Brazilian and West African margins the crustal limit is a tectonic feature of utmost importance for petroleum exploration in deep waters, and the integration of different geophysical tools suggest new models for the early phase of sedimentation along the divergent margins. Basement-involved structural features probably related to extensional tectonics, as well as salt tectonics structures, were previoulsy interpreted to occur only on continental crust. However, analogies with other sedimentary basins and integrated interpretation of the geophsysical data suggest that these features might be interpreted both on continental and oceanic crust. The characterization of fossil volcanic ridges probably associated with early oceanic spreading centers indicate a mechanism of formation of volcanic rifted margins by progressive oceanic spreading center indentation along rifted continental crust. This mechanism, which shows some similarities to the present-day structures observed along the Afar region, is associated with propagators advancing from the southern (Pelotas) towards the northern (Campos) provinces, with rightlateral steps.

In this chapter we examine continental rifting and associated basins, modern and ancient, for which several lines of evidence imply an origin related to lithosphere thinning linked to mantle plume activity.

A comparison of the reconstructed southeastern margin of the Tethys ocean with the present-day Galicia margin shows that although both margins are of different age and had a different fate, their architectures and tectonic evolutions are very similar. Along both non-volcanic margins the site of rifting shifted from a broad area in the future proximal margins to a localized area in its distal parts, accompanied by a change in the mode of extension. During the initial phase of rifting, extension was accommodated by symmetrically arranged listric faults which soled at midcrustal levels, indicating that deformation in the upper crust was decoupled from deformation in the upper mantle along a hot and weak lower crust. During advanced rifting, extension was dominated by simple shear along low-angle detachment faults with a top-to-the-ocean sense of movement. These shallow crustal structures formed a series of breakaways in the continental crust and cut into mantle rocks, indicating that now deformation in the upper crust and in the upper mantle was no longer decoupled. Cooling and strengthening of the lower crust during an initial stage of rifting apparently led to localization of deformation and a different style of deformation, documenting that the tectonic evolution of nonvolcanic margins is largely controlled by the thermal state of the lithosphere. Seafloor spreading initiated only after exhumation and exposure of the subcontinental mantle on the ocean floor and may have been accompanied by a loss of the yield strength of the upper mantle, due to a combination of simple shear extension, asthenospheric uplift, and increased melt production.

From oblique rifting to a transform margin, what do we really know about that? The integrated analysis of the Brazilian and African equatorial basins provided important insights regarding the process of continental fragmentation through the activation of transform faults. A quite dynamic ruptural process produced an unique segment among the Circum-Atlantic basins, which can not be adequately explained by conventional extensional processes, such as passive or active rifting, or by mechanisms such as pure-shear or simple-shear, typical of divergent margins. Even though shearing signatures and pull apart features are easily recognized throughout the margin, their magnitude and rift architecture vary significantly accordingly with the distance from the main transform faults, generating significant differences in the thermal evolution, basin architecture, tectonic subsidence, facies distribution and uplift history.

The Austroalpine and Upper Penninic nappes in eastern Switzerland represent a passive continental margin and the adjacent ocean of Jurassic-Cretaceous age, imbricated by Late Cretaceous-Tertiary orogenic shortening. Well-preserved, rift-related faults allow reconstruction of the passive margin and ocean-continent transition zone and yield new information on the kinematics of rifting. Rifting evolved from pure-shear stretching to detachment-controlled, asymmetric stretching and resulted in complete exhumation of subcontinental mantle rocks at the sea floor. After precursory normal faulting in the Late Triassic, Jurassic rifting occurred in two phases. During the first rifting phase (Hettangian-Sinemurian), predominantly east-dipping normal faults developed in the upper crust; their dips decreased in the middle to lower crust, where they probably graded into anastomosing shear zones in the lower crust and mantle lithosphere. The resulting overall geometry approximated pure-shear stretching. During the second rifting phase (Toarcian-Middle Jurassic), a system of west-dipping detachment faults formed, penetrating the whole lithosphere and accommodating asymmetric extension. During progressive stretching, subcontinental mantle rocks were tectonically exhumed and exposed at the sea floor in two areas, represented by the Platta and Malenco nappes (Penninic). The intervening Margna and Sella continental nappes are interpreted as an extensional allochthon belonging to the Apulian margin. Finally, a mid-ocean ridge may have formed west of the Margna-Sella allochthon. The Austroalpine realm thus represents the lower-plate margin—and the Brianconnais, the upper-plate margin-of the Piemont-Liguria ocean. This scenario is in qualitative agreement with the subsidence histories of the two margins.

The structural setting of the Northwestern Mediterranean stems from tectonothermal processes which reflect on the nature of the crust. The Oligocene to Present evolution is here analysed with a thermal model which takes into account the significant extension of the continentallithosphere before the onset of sea-floor spread- ing in the bathyal zone. Subsidence data were used to set the boundaries of the oceanic realm which was com- pared with previous reconstructions inferred from other geophysical evidence. The thermal features of the transitional crust that lies between the oceanic crust and the stretched continental margins were also outlined. The Ligurian-Proven~al basin is a marginaI basin, whereas only the continental crust is expected in the Valen- cia trough. An evolutionary sketch of the study area that accounts for the observed subsidence and heat flux is proposed.

The 21-25-km-thick crust of the southern Kerguelen Plateau consists of three units: (1) a ≤ 2.3-km-thick sedimentary cover; (2) a 3-6-km-thick basaltic layer with velocities ranging from 4.5 to 6.2 km/s; and (3) a 15-17-km-thick lower crust with velocities from 6.6 to 6.9 km/s, including a 3-6-km-thick transition zone located at the base of the crust. The low-velocity transition zone has an average velocity of 6.7 km/s and exhibits several internal wide-angle reflections. The velocity-depth structure of the crust differs significantly from that of other hotspot-related oceanic plateaus and suggests that the southern Kerguelen Plateau may be a fragment of a volcanic passive margin composed of a thinned continental crust overlain by basalt flows.

When continental lithosphere is thinned during rift.in g. basall ic melt is generated by decompression of anhydrous mantle if the geotherm intersects the a nhydrous manl le solidus. The qu antity of melt generated depends on four principal factors: the degree of lilhospheric thinning; the potential temperature of the asthenospheric mantle; the thickness of the lithosphere prior to rifting; and t he duration of rifting. Subsidence at rifted continental margins can affected significantly by melt generation during rifting: mantle melting causes reduced subsidence because both the igneous rock added to t he crust and the residual mantle are less dens. then the original mantle. At ‘volcanic’ rifted margins subsidence is also affected by the relative uplift resulting from isostatic compensation of the underlying mantle whose density is reduced hy the thermal anomaly caused by the mantle plume. We present results from a uniform pure-shear lithospheric stretching model for melt generation and for subsidence at continental margins rifted at realistic finite rates. Predict ions from the model are compared with observations of melt generation and Subsidence from the ‘nonvolcanic’ Galicia Bank rifted margin and from the ‘volcanic’ Rockall Platieau rifted margin in the North Atlantic.

We present the results of 44 new heat flow stations which were taken in 1984 and 1989 in profiles across the Goban Spur and Galicia Bank continental margins (NW Atlantic Ocean). Simple extensional models indicate that the heat flow across both these Early Cretaceous rifted margins should increase from values of 45-50 mW/m2 over oceanic crust to 65-80 mW/m2 on the continents. The rate of this increase should help to constrain the mechanism (simple versus pure shear) by which the upper, more radiogenic continental crust is thinned. Measurements across Goban Spur increase from values of 40-45 mW/m2 over oceanic crust to 50-55 mW/m2 near the continental shelf. They follow the predicted trend for pure-shear rifting, but only if the value of upper crustal radiogenic heating is low (1-2-mu-W/m3). Otherwise, they would require the upper crust to thin more rapidly than the total crustal thickness, as with a lower plate, simple-shear margin. Measurements across Galicia Bank show a very different pattern, with similar values over oceanic crust but much lower values (30-35 mW/m2) nearer land. This is difficult to reconcile with any simple, single rifting event but is more compatible with an origin as a pure-shear or lower plate rather than upper plate margin. We also note that oceanic values of heat flow require asthenospheric temperatures 100-degrees-C lower than normal for both margins. This indicates that the triple junction in existence between these margin segments during the breakup of Iberia, Europe, and North America was not the site of a major mantle plume.

The subsidence of rifted margins and of many intracontinental basins is caused by the cooling and thermal contraction of the initially hot lithosphere. Lithospheric cooling takes place in part through increased surface heat flow. Therefore, there is a relationship between the subsidence rate and the heat flow; this relationship depends on the initial and boundary conditions. If the heat flow at the base of the lithosphere is constant, and if horizontal heat transport can be neglected, the excess heat flow at the surface is proportional to the subsidence rate, regardless of initial conditions; it is approximately equal to 2.3mWm−2 when the rate of subsidence is l m M.y.−1. If the temperature is constant at the base of the lithosphere, the ratio of the surface heat flow to the rate of subsidence is always smaller; for short times, it is determined by the initial conditions; for times larger than 0.05 a2/κ (a = lithospheric thickness, κ = thermal diffusivity), the heat flow becomes proportional to half the subsidence rate (i.e. 1.15 mWm−2 per m M.y.−1).

Models of melting accompanying mantle upwelling predict far more melt than is observed at nonvolcanic margins. The discrepancy may be explained if the paradigm of a uniform asthenosphere is incorrect. Work on the velocity structure of the continents has shown that the convecting sublithospheric mantle may have a potential temperature as low as 1200 °C, ∼100 °C cooler than that beneath the oceans. The continental geotherm derived from studies on xenoliths brought up by kimberlites is also compatible with a cool sublithospheric mantle except where perturbed by mantle plumes. Upwelling of such cool mantle during rifting leads to little melt production, even for rapid extension rates, explaining the formation of amagmatic margins away from mantle plumes. However, the transition to seafloor spreading and the development of normal-thickness oceanic crust requires the invasion of the amagmatic rift by hotter oceanic asthenosphere and/or plume material. This influx may cause a transient thermal uplift, recorded as a breakup unconformity. Conversely, at volcanic margins, the onset of seafloor spreading is accompanied by the escape of the hot plume puddle along the mid-ocean ridge system away from the volcanic margin, leading to a pulse of rapid subsidence.

Important advances have been made in recent years regarding our understanding of the processes involved during continental break-up. Much of this knowledge has arisen from deep seismic profiles of the Atlantic margins. The continent-ocean transition (COT) at many locations along the North Atlantic margin consists of a zone of ‘transitional’ crust that lies between thinned continental crust and unequivocal oceanic crust. The origin of this transitional crust is poorly understood, and yet it is likely to provide key information regarding the break-up process.

[1] Boutilier and Keen [1999] showed that small scale convection may produce the observed thick igneous crust along volcanic passive margins provided that a non-linear rheology is adopted and the viscosity of the upper mantle is increased suddenly after 132 km of continental crust extension. The sudden viscosity increase is justified by the melt induced dewatering of the upper mantle rocks. Through this comparative study, it is shown that inclusion of the pressure of the newly formed igneous crust into the melting model, and/or adopting a more realistic viscosity law, significantly reduces the thickness of igneous crust produced by Boutilier and Keen's models. Furthermore, it is shown that melt migration would narrow the width of the thick igneous crust. Results of this study show that small scale convection may not play a significant role in the formation of the observed thick igneous crust along volcanic passive margins.

The conditions under which continental rifting may occur without an accompanying protracted period of synextensional magmatism are explored with a series of finite element models that dynamically simulate continental extension. The models demonstrate that 5–7 km of new oceanic crust can be generated at the time of breakup while restricting synextensional magmatism to less than 2 m.y. prior to the end of the rifting episode if the extension rate exceeds ∼15 mm/yr and the crust contains only minor lateral variations in strength. Under these conditions, extensional strain in the mantle is localized during the early stages of rifting, preventing diffuse lithospheric thinning. Melting does not begin until lithospheric necking is well established, resulting in a narrow zone of melt production that limits magmatism to within ∼30 km of the ocean-continent boundary. Once melting begins, rifting progresses rapidly to breakup, restricting the period of synextensional magmatism to as little as 1.6 m.y. prior to the onset of sea-floor spreading. Because the models do not require either a cool mantle prior to rifting or pronounced conductive cooling of the mantle during rifting, they are able to account for the generation of new oceanic crust at the time of breakup. The model predictions are consistent with the structure and magmatic history of nonvolcanic rifted margins in the North Atlantic Ocean.

This review discusses results from recent seismic experiments at continental margins conducted by U.S. investigators as well as those by non-U. S. investigators necessary to place the results of U.S. workers into context. Continental margins may be characterized as either rifted, transform, or convergent. Rifted (or passive) margins form where continents are torn apart during continental drift. Transform margins accommodate transform plate motions along strike-slip faults such as the San Andreas fault. At convergent margins oceanic crust is being thrust beneath (or subducted under) the continental crust. All three types of margins are found in North America.

The crust across each margin of the Labrador Sea, a conjugate rift margin pair, consists of three well-defined zones: thinned continental crust, 70–80-km-wide transitional crust, and oceanic crust. The transition zone is characterized by a low-velocity upper crust (4–5 km/s) underlain by a 6.4–7.7 km/s layer. We propose that the lower layer is serpentinized upper mantle; it is less likely to be gabbroic igneous lower crust. The low-velocity upper crust may be either an oceanic basaltic layer or very thin (

Large igneous provinces (LIPs) are a continuum of voluminous iron and magnesium rich rock emplacements which include continental flood basalts and associated intrusive rocks, volcanic passive margins, oceanic plateaus, submarine ridges, seamount groups, and ocean basin flood basalts. Such provinces do not originate at “normal” seafloor spreading centers. We compile all known in situ LIPs younger than 250 Ma and analyze dimensions, crustal structures, ages, and emplacement rates of representatives of the three major LIP categories: Ontong Java and Kerguelen-Broken Ridge oceanic plateaus, North Atlantic volcanic passive margins, and Deccan and Columbia River continental flood basalts. Crustal thicknesses range from 20 to 40 km, and the lower crust is characterized by high (7.0-7.6 km s?1) compressional wave velocities. Volumes and emplacement rates derived for the two giant oceanic plateaus, Ontong Java and Kerguelen, reveal short-lived pulses of increased global production; Ontong Java’s rate of emplacement may have exceeded the contemporaneous global production rate of the entire mid-ocean ridge system. The major part of the North Atlantic volcanic province lies offshore and demonstrates that volcanic passive margins belong in the global LIP inventory. Deep crustal intrusive companions to continental flood volcanism represent volumetrically significant contributions to the crust. We envision a complex mantle circulation which must account for a variety of LIP sizes, the largest originating in the lower mantle and smaller ones developing in the upper mantle. This circulation coexists with convection associated with plate tectonics, a complicated thermal structure, and at least four distinct geochemical/isotopic reservoirs. LIPs episodically alter ocean basin, continental margin, and continental geometries and affect the chemistry and physics of the oceans and atmosphere with enormous potential environmental impact. Despite the importance of LIPs in studies of mantle dynamics and global environment, scarce age and deep crustal data necessitate intensified efforts in seismic imaging and scientific drilling in a range of such features.

When continents break apart to create new ocean basins, rifts of remarkable diversity form at the continental edges or margins. Seismic studies show that at one extreme, the 25'30-km-thick continental crust is stretched like gum until it finally breaks to allow a new oceanic spreading center to form. This is a cold rift with little accompanying volcanism. At the other extreme is a hot rift, created when the stretching and rifting of some continental margins produce huge volumes of molten rock. The rock pours out of the rift or becomes trapped and solidifies at the base of the crust.

During the Cretaceous and early Tertiary a series of sedimentary basins opened by rifting on the northwest margin of Europe. None of these developed into a fully-fledged ocean until the northern North Atlantic opened at 55 Ma between NW Europe and Greenland. The subsidence, magmatism, and thermal history of these basins and of the continental margin depended critically on the amount and style of stretching and on the temperature of the underlying asthenospheric mantle. Small variations in the mantle temperature can have dramatic effects on both the magmatism produced during rifting and on the subsidence of the basin. An increase of as little as 10% in the mantle temperature can cause uplift above sea-level rather than the more common subsidence when continental crust undergoes large amounts of extension. The initiation of the Iceland mantle plume at about 62 Ma caused abnormally elevated mantle temperatures across a 2000 km diameter region and had a dramatic effect on the subsidence and rifting history of sedimentary basins west of Britain. Their thermal history can be documented from the subsidence and magmatic record in the sedimentary basins and the continental margin. Using normal incidence and wide-angle seismic data and drilling results from the continental margin and from basins of NW Europe, their subsidence and thermal history can be modelled as a function of the amount of stretching and the temperature of the underlying mantle. The Rockall Trough itself is underlain by 5 km of sediments intruded by Tertiary volcanics. The basement imaged on seismic . . .

Deep crustal seismic observations from northwest Australia demonstrate that the Cuvier margin is a volcanic passive margin that formed as Greater India rifted away from Australia in the Early Cretaceous. Formation of the Cuvier Basin and initial sea-floor spreading resulted in the emplacement of exceptionally thick oceanic crust, while contemporaneous spreading off the adjacent Exmouth Plateau formed normal-thickness oceanic crust. Crust of normal thickness was generated in the Cuvier Basin within 3-4 m.y. after spreading began. There is no evidence for hot-spot activity with the appropriate timing or in the appropriate spatial pattern to account for the difference in volcanism along the margin. We believe that rapid rifting of the Cuvier margin created strong lateral temperature gradients in the subjacent upper mantle at the time of breakup. This induced small-scale convection in the upper mantle that increased the rate of upwelling and, thus, the volume of melt generated by decompression partial melting. In turn, this resulted in a larger volume of magma at the initiation of sea-floor spreading, and thicker oceanic crust was initially emplaced. Rifting on the Exmouth Plateau happened over a much longer period of time, and no such convective partial melting occurred. The volume of melt associated with rifting was therefore entirely the result of passive upwelling, and a normal thickness of oceanic crust was emplaced immediately adjacent to the Exmouth Plateau.

Several large cratons and numerous smaller blocks, all with Precambrian sialic crust, are seen clearly in the present-day structure of Asia. The outer parts of larger Precambrian terranes and entire smaller ones are covered with Vendian-early Paleozoic shallow-water carbonate sedimentary rocks coupled with the so-called "old" (late Vendian-Early Cambrian) phosphorites in Siberia, southern China, India, Kazakhstan, and Mongolia. These sedimentary rocks are interpreted to represent a global belt of near-equatorial sedimentation on passive continental margins of numerous Vendian-early Paleozoic continents with free access to the world ocean. Later Precambrian rifting is recorded clearly in many Precambrian terranes of Asia; rifting was repeated during Cenozoic time, with the same pattern and in the same places. It was preceded and accompanied by various magmatic activity from 750 to 600 Ma. Tillites filling rifts are about 650 Ma, and are believed to be a feature of a single Proterozoic super-continent. Rift-to-drift transition, as attested by the beginning of thick and laterally continuous carbonate sedimentation on passive margins, seems to be diachronous; it started in early Vendian time or even earlier in some places, but not earlier than Cambrian in others.

New deep-penetration multichannel seismic reflection data, combined with refraction results and magnetics modeling, support a hypothesis that the Carolina trough is a Mesozoic volcanic passive margin exhibiting a seaward-dipping wedge and associated underplating. The structure of Carolina platform continental crust is consistent with the late Paleozoic continental collision that produced the Appalachians, but imbrication has had no obvious effect on shallower structures produced by Mesozoic extension and volcanism. The origin of prominent magnetic anomalies crossing the Southeast Georgia embayment can be explained by processes attending Mesozoic separation of Africa and North America, and is not related to a Paleozoic continental suture, as previously postulated.

The geology of continents is to a large degree the geology of continental margins. The margins mark the site where materials that form major components of the continental crust are distilled from Earth's mantle and where continents grow through plate interactions that progressively incorporate these margins into the continental mass. Much of the continents consists of the remnants of ancient margins that offer an extensive and detailed record of continental evolution over time and space. Knowledge of this record is essential for understanding processes of global change, their diversity, and long-term history. Unraveling this record is one of the major challenges in modern Earth sciences.

Summary. The stretching and thinning of the continental crust, which occurs during the formation of passive continental margins, may cause important changes in the velocity structure of such crust. Further, crust attenuated to a few kilometres’ thickness can be found underlying ‘oceanic’ water depths. This paper poses the question of whether thinned continental crust can be distinguished seismically from normal oceanic crust of about the same thickness. A single seismic refraction line shot over thinned continental crust as part of the North Biscay margin transect in 1979 was studied in detail. Tau-p inversion suggested that there are differences between oceanic and continental crust in the lower crustal structure. This was confirmed when synthetic seismograms were calculated. The thinned continental crust (p> 7.0) exhibits a two-gradient structure in the non-sedimentary crust with velocities between 5.9 and 7.4 km s-l; an upper 0.8 s-l layer overlies a 0.4 s-l layer. No layer comparable to oceanic layer 3 was detected. The uppermost mantle also contains a low-velocity zone.

The evolution and geophysical features of the continental margins of eastern and western Canada are reviewed in light of recent plate-tectonic concepts. The two margins are very different in age, structure, and origin. The eastern margins were formed either by rifting or by transform motion during the latest separation of the continents around the Atlantic that occurred from Jurassic to Tertiary times. Studies of these margins centre around a reconstruction of plate motions, the inception of which occurred over 70 Ma ago, and on subsequent processes such as subsidence and sedimentation. The subsidence of the margin is explicable in terms of cooling of the lithosphere and sediment loading. Deep crustal features are inferred from seismic, gravity, and magnetic data. The recognition of the ocean–continent boundary at these margins involves consideration of edge effects, magnetic quiet zones and rifting mechanisms. The western Canadian margins are present active plate boundaries. Recent geophysical studies of these margins centre around the detailed definition of the present plate boundaries and relative plate motions, and those of the recent past (about the past 10 Ma), and involve spreading ridges, transform faults, and subduction zones. The plate convergence predicted by offshore geophysical data has a pronounced effect on the continental crust and upper mantle extending several hundred kilometres inland from the coast. In southwestern Canada patterns characteristic of subduction zones are seen in seismic structure, the gravitational and magnetic fields, heat flow, and deep electrical structure.

Deep temperature distributions on six seismic cross sections on the passive continental margins of eastern Canada have been calculated. Each of the considered profiles of the crustal structure includes sections with continental and oceanic crusts, and also a zone of ocean-continent transition. Present calculations show that on each profile the crust and upper mantle beneath the ocean have temperatures considerably higher than those beneath the continent. The difference between sub-oceanic and sub-continental temperatures at the same depth amounts to 100–150 °K. At the depth of 50 km the temperature in the continental and oceanic regions reaches 500°–700°C and 700°–900°C respectively. In the region with continental crust, the crustal radioactivity and the outflow of heat from the mantle constitute approximately equal heat sources in producing the crustal temperatures. In the area with oceanic crust the mantle heat flow is the dominant source of heating; on these sections of the profiles more than about 75% of the surface heat flow has its origin in the mantle. The thickness of the lithosphere across the margins decreases seawards; in the region with oceanic crust its thickness reduces to 70–75 km, typical of the North Atlantic.

A tectonic and sedimentary facies model is proposed to explain progressive evolution of the late Proterozoic to early Paleozoic Adelaide Rift (Geosyncline) of southern Australia. Tectonic and stratigraphic similarities are noted between the Adelaide Rift and many post Permian rifts and passive continental margins. Also the time span of the pre oceaniccrust accretion stage of the rifting process may be of the same order of magnitude, both in the Adelaide Rift and in post-Permian passive margins. These observations suggest that the underlying cause of the rifting process and the resultant crustal response have not changed significantly since late Precambrian times. More specifically the so-called “breakup unconformity”, observed in stratigraphic sequences beneath many present day passive continental margins, has been shown by various authors to correlate in time with earliest oceanic crust accretion, and it often separates underlying non-marine or paralic from fully marine shelf strata. In the Adelaide Rift, the unconformable Precambrian—Cambrian boundary is proposed as the analogue of this breakup unconformity, thereby explaining the apparently sudden influx of largely marine metazoans in Cambrian strata immediately above this unconformity.

Summary. A model for the formation and evolution of rifted continental margins based on lithospheric extension during rifting and its thermal and mechanical consequences is proposed. Model predictions are then compared with geological and geophysical observations from a transect of the ∼185 Ma old rifted margin off Nova Scotia through the Scotian Basin. Three kinematic models of the rifting process are discussed. These are: (1) the uniform extension model, in which the amount of extension is uniform with depth but varies with position across the margin; (2) the uniform extension and melt segregation model which has similar properties, but also provides an explanation for the properties of the extended continental crust and its transition to oceanic crust by postulating that basaltic melt segregates from the asthenosphere and migrates to the crust, and; (3) the depth-dependent extension model in which the first-order consequences of rapidly changing rheological properties with depth in the lithosphere are included by decoupling the lithosphere into two zones with depth, each of which undergoes differing amounts of extension. These rift models predict the form of crustal and lithospheric thinning, subsidence and temperature change once the amount of extension has been determined. This is estimated from seismic measurements of present crustal thickness on the assumption that the crust had a uniform thickness, equal to that currently measured in the adjacent continental region, before rifting occurred. Rifting is also considered to occur instantaneously. A time-stepping thermo-mechanical model is used to predict the cooling of rifted margin, additional thermal contraction subsidence and its amplification by water and sediment loading. Thermal aspects are calculated using a finite difference model of time-dependent conductive heat transport, whereas regional isostatic response to loading is calculated by a finite element model. The models are coupled because the temperature distribution is used to define a rheological lithosphere (an elastic region with thickness that varies in time and space as the model evolves) for flexural calculations in the mechanical model. Secondary coupling occurs through perturbations to the temperature field by sedimentary thermal blanketing and advection of heat during isostatic adjustment. The model predictions of: (1) sedimentary basin stratigraphy, (2) Moho position, (3) free air gravity anomaly, (4) age—depth relations for deep exploratory wells and (5) subsidence and temperature histories agree well with observations from the Scotian Basin. Additional effects due to sedimentary and crustal radiogenic heat production, lateral heat transport and the possible existence of a near surface brittle listric faulted region created during rifting are also considered. It is concluded that a model in which rifting occurred by depth-dependent extension, and which includes a finite thickness for the rheological lithosphere, radiogenic heat production in the sediments and crust, and a brittle listric faulted crustal layer affords an accurate description of the first-order processes that occur during rifting and evolution of this margin.

Elongated sedimentary basins are found along continental margins of the Atlantictype. The evolution of these basins is seen to be related to continental break-up and the associated fault system, cooling of the spreading lithosphere, sedimentary loading, and metamorphic processes. In this study we assume that the transformation of metastable phases of the lower crust beneath the continental margin will dominate the mature phase of the development of the margins. Further on this process is assumed to be involved in the possible development of lithospheric instabilities at continental margins of the Atlantic type. By means of the finite element technique, assuming non-linear rheology, we calculated a number of models on the dynamics of passive continental margins. We studied in detail the effect of metastable phase transition, sedimentary loading, active faulting at the continental oceanic transition as well as horizontal loading of the lithosphere on the subsidence of Atlantic-type continental margins. These mechanisms may be responsible for the late development of sedimentary basins along passive continental margins. The numerical results support the idea that the late development of a passive continental margin will be dominated by loading at the continental rise.

Continental margins, the border zones between continents and oceans, are preferred sites for hydrocarbon accumulations throughout geologic history. Passive Atlantic-type margins are found worldwide. Active Pacific-type margins exist together with passive margins where endogenous vertical crustal mobility took place. Vertical mobility is preferred over lateral mobility. Plate-tectonic hypotheses are doubted. Exploration for gas and oil must be based on sound data.

As the Laurasian Plate tracked north over the New England hotspots in the LateTriassic, the heated and stretched crust failed along re-activated basement structures including micro-plate sutures, and continental extensions of transforms. This created the rifted passive margins of the Atlantic and established the tectonic and climatic setting of wrench-generated coastal ranges and detrital basins bordering vast salt flats that were overlain with waters from the Tethys Sea. In tracking north from an equatorial position in the Late Triassic to a subtropical latitude in the Middle Jurassic, the plate transgressed first humid, then savanna and finally arid climatic zones, which were then bordered by a transgressing epeiric Tethyan Sea. Within these climatic zones, monsoonal circulation profoundly affected patterns of sedimentation as tropical air masses cooled and warmed adiabatically as they crossed the coastal ranges and broad salt flats. Where the basement had been pulled apart as in the Newark-Gettysburg Basin or the Argana Basin of Morocco, plutons intruded the axis of the basin in the form of dikes, lava flows and subaqueous fissure flows. Differential horizontal shear along strike-slip faults created assymetric basins with an upthrown leading plate and a subsiding trailing plate. Strata within the basins record a history of recurrent, but alternating, transtentional and transpressional episodes in an overall wrench-tectonic regime. While the borderfault facies is marked by complex unconformities, young basin sediment, volcanics, en-echelon folds, fanglomerates, turbidites and deep-water lacustrine deposits with organic-rich black shale, sediments on the trailing plate are marked by an older suite of gently inclined fluvialdeltaic sands that rest with profound unconformity on the Hercynian — Variscan basement. Where shallow marine waters of the Tethys Ocean transgressed sagged pull-apart basins (as in the Khemisset and Berrichid Basins of Morocco) or where the basement was faulted by straignt, non-branching transforms (as in Grand Banks), vast salt flats occurred forming thick, deposits of halite and potash salt. The extent of Tethyan transgression and concomitant subsidence of these basins is marked by salt diapirs in the Baltimore Canyon Trough and in the Aaiun Basin of Africa.

Haworth, R.T. and Keen, C.E., 1979. The Canadian Atlantic margin: a passive continental margin encompassing an active past. In: C.E. Keen (Editor), Crustal Properties across Passive Margins. Tectonophysics, 59: 83–126. The continental margins of Atlantic Canada described in this paper show the effects of plate tectonic motions since Precambrian time and thus represent an ideal natural laboratory for geophysical studies and comparisons of ancient and modern margins. The Grenville Province shows vestiges of Helikian sedimentation on a pre-existing continental block beneath which there may have been southeastward late-Helikian subduction resulting in collision between the Grenville block and the continental block comprised of the older shield provinces to the north. The Grenville block was subsequently split in Hadrynian time along an irregular line so that the southeastern edge of the Grenville exhibited a series of promontories and re-entrants similar to those seen at the present Atlantic continental margin of North America. That margin, which had a passive margin history perhaps comparable with that of the present Atlantic margin, was separated by the Iapetus ocean from the Avalon zone whose Precambrian volcanism has been attributed both to that associated with an island arc and with intra-cratonic rifting. However, the Iapetus ocean appears to have been subducted in early Paleozoic time with a southeastward dip beneath the Avalon zone, leaving exposures of oceanic rocks in place as in Notre Dame Bay, or transported onto Grenville basement as at Bay of Islands. Plate motions proposed for Devonian and Carboniferous time are numerous, but resulted in the welding of the Meguma block to the Avalon zone of New Brunswick and northern Nova Scotia, extensive faulting within Atlantic Canada which can be correlated with contemporaneous European faulting, and extensive terrestrial sedimentation within the fault zones. Graben formation, continental sedimentation and basaltic intrusion in the Triassic represent the tensional prelude to the Jurassic opening of the present Atlantic Ocean. This Jurassic opening produced a rifted margin adjacent to Nova Scotia and a transform margin along the southern Grand Banks. The width of the ocean—continent transition across the transform margin (approx. 50 km) is narrower than for the rifted margin (approx. 100 km). The eastern part of the transform margin is associated with a complex Cretaceous (?) volcanic province of seamounts and basement ridges showing evidence of subsidence. The western portion of the transform margin is non-volcanic, adjacent to which lies the 350 km wide Quiet Magnetic Zone floored by oceanic crust. Development of the margin east of Newfoundland was more complicated with continental fragments separated from the shelf by deep water basins underlain by foundered and atypically thin continental crust. Although thin, the crust appears unmodified, the similarities between the crustal sections of the narrow Flemish Pass and the wide Orphan Basin suggesting that the thinning is not simply due to stretching. The Newfoundland Basin shows evidence for two-stage rifting between the Grand Banks and Iberia with both lateral separation and rotation of Spain, leaving a wide zone of transitional crust in the south. The overall pattern of variations in crustal section for the margin east of Newfoundland is comparable with that of the British margin against which it is located on paleogeographical reconstructions. The major sedimentary unconformities on the shelves (such as the Early Cretaceous unconformity on the Grand Banks) reflect uplift accompanying rifting. Tracing of the sedimentary horizons across the shelf edge is complicated by paleocontinental slopes, which separate miogeocline and eugeocline depositional environments. The subsidence of the rifted margins is primarily due to cooling of the lithosphere and to sediment loading. The subsidence due to cooling has been shown to vary linearly with (time)1/2, similar to the depth—age behaviour of oceanic crust. The consequent thermal history of the sediments is favourable for hydrocarbon generation where other factors do not preclude it.

Passive margins form by continental splitting which may follow an early pre-split graben stage of development. Such margins are divisible into rifted and sheared types. After formation the margins develop by predominantly vertical tectonics as the intervening ocean widens by seafloor spreading. The plate splitting mechanism is not yet understood, but evidence from aseismic ridges and associated continental volcanism is relevant. Location of the original continent-ocean contact is also problematical in many regions, especially where quiet magnetic zones occur above crust of uncertain status. Rifted margins are notable for great subsidence resulting from early graben formation and later flexural subsidence of the shelf and slope regions. Graben formation is attributed to wedge subsidence of the upper crust in response to crustal stretching, possibly associated with doming before splitting. Four factors have been recognized as contributing to flexural subsidence: gravity loading, thermal subsidence of the adjacent oceanic lithosphere, possible thermal subsidence of the continental lithosphere following heating and erosion at the time of break-up, and thinning of the continental crust by seaward creep of lower crustal material. The relative importance of these four mechanisms needs clarification.

JOIDES drilling results provide new evidence concerning facies patterns on evolving passive margins that strengthens and extends hypotheses constructed from studies of morphology, seismic reflexion data and shallow samples on modern margins, and from field geologic studies of uplifted ancient margins. On the slopes and rise, gravity-controlled mechanisms - turbidity currents, debris flows, slides and the like - play the dominant role in sediment transport over the long term, but when clastic supplies are reduced, as for example during rapid transgressions, then oceanic sedimentation and the effects of thermohaline circulation become important. Sedimentary facies models used as the basis of unravelling tectonic complexities of some deformed margins, for example in the Mesozoic Tethys, may be too simplistic in the light of available data from modern continental margins.

In this paper we obtain two solutions relating to the alternative active and passive hypotheses for continental rifting. Active rifting assumes the impingement of a mantle plume on the base of the continental lithosphere. We determine the rate of lithosphere thinning due to this impingement. We conclude that it takes 50–75 million years to thin the continental lithosphere to the base of the crust; the long length of time required appears to rule out this lithospheric erosion mechanism as a viable mechanism for complete lithospheric thinning. We suggest that diapiric penetration is the mechanism by which hot mantle rock reaches the base of the continental crust at a rift. We have also obtained values for the tensional stresses generated by the diapiric penetration of hot asthenospheric rock into the lithosphere. The stress levels are obtained as functions of lithospheric and crustal thinning factors. These stresses may lead to rift propagation and the splitting of a continent.

A normal oceanic crust cannot be generated after continental crust splitting of rifts filled by a thick pile of young sediments. The magma released by the rising mantle is trapped mainly in these sediments as basalts sills and plugs. The sediments are themselves metamorphosed by the heat flux from the underlying mantle. A metasedimentary crust is formed in direct contact with an abnormal mantle (Vp ≃ 7.5 km s−l) produced by partial serpentinization.

Geophysical studies suggest that the thin crust characteristic of the Basin and Range Province extends eastward beneath the west margin of the Colorado Plateau and Rocky Mountain regions. In Arizona and Utah, zones perhaps over 100 km wide may be defined, bounded on the west by the east limit of upper crustal normal faults that account for more that 10% extension and on the east by the east limit of thinning beneath the Colorado Plateau. A discrepancy exists within these zones between the negligible extension measurable in the upper crust and the substantial extension apparent from crustal thinning, assuming the "discrepant zone" crust was as thick as or thicker than the Colorado Plateau – Rocky Mountain crust prior to extensional tectonism.If various theories appealing to crustal erosion are dismissed, mass balance problems evident in the discrepant zones are most easily resolved by down-to-the-east normal simple shear of the crust, moving lower and middle crustal rocks that initially were within the zones up-and-to-the-west to where they now are locally exposed in the Basin and Range Province. West of the discrepant zones in both Arizona and Utah, east-directed extensional allochthons with large displacement are exposed. These geophysical and geological observations complement one another if it is accepted that the entire crust in both Arizona and Utah failed during extension on gently east-dipping, east-directed, low-angle normal faults and shear zones over a region several hundred kilometres wide.Large-scale, uniform-sense normal simple shear of the crust suggests the entire lithosphere may do the same. Such a hypothesis predicts major lithospheric thinning without crustal thinning will occur in plateau areas in the direction of crustal shear. In the case of the Arizona, Utah, and Red Sea extensional systems, and possibly the Death Valley extensional terrain, a broad topographic arch, typically 1500–2000 m higher than the extended terrain, is present, suggesting lithospheric thinning in areas predicted by the hypothesis.

During the past 200 Ma (1 Ma = 10 6 years) the arrangement of continents and ocean basins has been reorganized from a pattern of one supercontinent, with mainly plate edge, subduction, or active continental margins bordering one essentially contiguous ocean basin, to the present configuration of dispersed continents and several oceans. Most of the world’s present continental margins which were formed during that 200 Ma period are ‘passive’ margins lying within the interiors of lithospheric plates. Several models of rifting and evolution of these passive margins have been proposed. The objectives of IPOD include testing of these models by learning as much as we can about the history of rifting of passive continental margins, their internal structure, distribution of facies, subsidence history, and the nature of the transition and modification of the crust at the margin. These objectives cannot be attained by drilling alone, and geophysical surveying and analysis of samples from the drilling are essential parts of the overall programme.