Atlantic-type Continental Margins Research Articles

A REGIONAL INTERPRETATION OF THE BROWSE BASIN

The Browse Basin, off the northwest coast of Australia, originated as an intracratonic basin. It resulted from tensional movements and developed into an Atlantic-type continental margin. Sedimentation, which commenced in the Late Carboniferous and extended through the Middle Jurassic, is confined mostly to a linear, northeast-trending depocentre located between the onshore Kimberley Block and the Scott Plateau to the west. This sedimentary series is characterised by rift valley type deposits laid down between successive episodes of tectonism. Tectonism near the end of the Middle Jurassic was associated with breakup of the continent along what developed as the seaward edge of the Scott Plateau. This geological event marks an important change in the depositional environment of the basin from pre-breakup, mainly paralic and fluviodeltaic to post-breakup, transgressive marine conditions. The post-breakup series was formed under relative tectonic quiescence, except for some local structural rejuvenation and regional subsidence of the outer continental margin which eventually resulted in the deposition of Cretaceous and Tertiary sediments across the Scott Plateau. A fully open marine environment, however, was not achieved basinwide until during the Late Cretaceous, when the Scott Plateau had subsided sufficiently to allow unrestricted ocean circulation. Evolution of the basin was completed by a seaward prograding, mainly carbonate Tertiary wedge.Geophysical data show several sub-parallel, structurally high Mesozoic trends, oriented towards the northeast. The Scott Reef gas/condensate discovery lies on one such trend. The structural elements were interpreted as being mostly initiated prior to breakup by episodes of block faulting which produced a horst and graben topography. The horst blocks were subsequently onlapped and draped by the post-breakup sediments. The hydrocarbon accumulation at Scott Reef is restricted to the horst block reservoirs, but hydrocarbon indications elsewhere within the basin in the post-breakup series suggest that accumulations could also exist in this section.

The Pyrenees: Subduction and collision?

This paper presents a new interpretation of the Pyrenees. After Early Cretaceous rifting, two stable Atlantic-type continental margins formed during the Late Cretaceous. Flysch-type sediments were then deposited in a deep east-west basin. During the latest Cretaceous and Early Eocene, this basin underwent subduction towards the south. We propose that the present-day Pyrenees result from collision between a northern stable margin (Aquitaine Basin and Sub-Pyrenean Belt) and an active southern margin (Pyrenees proper). According to this interpretation, the pre-Mesozoic basement of the North Pyrenean Zone may represent the intermediate continental crust of the Cretaceous stable margin.

Observations on the processes of sedimentary basin formation at the margins of Southern Africa

Three variants of Atlantic-type continental margin border Southern Africa. On the west is a rifted margin with a rift phase no more than 50 m.y. in length (180–130 m.y. ago). Sedimentary basin formation was by upbuilding of a sediment terrace during the rift phase and the 30 m.y. following, with outbuilding of the terrace dominant during the Cainozoic. Little downwarping of the oceanic crust occurred but the continent—ocean transition zone appears to be wide. To the south of South Africa is an extensive sheared margin. Basin formation began here in mid-Triassic times with intermontane deposition. Local increase in lower crustal density appears to have accompanied subsidence. Truncation of the basins occurred 130–2100 m.y. ago and in places detrital influx was trapped behind a marginal fracture ridge. No continental rise sedimentary apron and characteristic deep structure were formed in these places. A ‘welding’ of the continental edge appears to have taken place. East of 30° E a complex continental margin with a protracted rift phase exists. From Triassic to Cretaceous times sedimentary basin formation was controlled by an E-W tensional stress regime resulting in N-S horsts and grabens. This was accompanied by vol-canicity and crustal thinning. Other stress systems may have prevailed during continental break-up in the Cretaceous while today the region is seismically active and the tensional stress assumed to be E-W. Following break-up sedimentary basins in Natal Valley and Mozambique Channel encroached southwards.

Flexure of the lithosphere and continental margin basins

The accumulation of sediments at an Atlantic-type continental margin constitutes a load on the lithosphere which simply sags due to its weight. Studies of the geometry of deformation suggests the lithosphere will respond to these loads either by local loading of an Airy-type crust or flexural loading of a strong rigid crust. Sediment loading models of either type cannot, however, explain the substantial thicknesses of shallow-water sediments observed in commercial boreholes from Atlantic-type margins. Other factors such as thermal contraction, gravitational outflow of crustal material or deep crustal metamorphism may contribute to the subsidence. We have used biostratigraphic data from commercial boreholes from the Gulf of Lion and the East Coast U.S.A. to quantitatively determine the contribution of sediment loading to the subsidence. From these data we determined sea-floor and basement depths for sequential time intervals during margin development. Using the sediment loading models the sediment layers at each margin were progressively “backstripped” and the depth basement would have been without the sediment load calculated. The computed basement depths indicate there is a recognizable component of the subsidence of these margins which is caused by processes other than adjustments to the weight of the sediment. The nature of this subsidence is discussed and comparisons are made with that which would be expected from thermal-contraction models.

Plate tectonics and orogeny: A review

Paleomagnetic pole positions, ocean-floor magnetic anomalies and least-squares fits of continental edges show that most Phanerozoic orogenic belts have formed at or near the borders of continents and island arcs. Large areas of old ocean floor have vanished, probably along former continental margins and islands arcs. Thus most Phanerozoic orogenic belts must include subduction zone effects. Superposed on them may be effects caused by self-heating of thick ‘geosynclinal’ piles of continental margin sediment. It is not yet clear how to discriminate self-heating from slip-zone heating effects. Paleomagnetic data also show that rigid areas of the continental crust have been in relative motion for at least 2500 m.y. These data imply that plates existed and that subduction may have been active since at least the beginning of Proterozoic time. Paleomagnetic data and mapping suggest that some Proterozoic African orogenic belts may not have originated by plate-tectonic processes, though the evidence is inconclusive. There is no geophysical evidence for or against the existence of Archean plates. The causes of Archean orogenesis are speculative. Some plate-tectonic models appear to differ significantly from the actual orogenic belts they purport to describe. In particular, long-continued subduction does not necessarily give rise to continental accretion, but in some cases may actually involve tectonic erosion of the overriding plate by the slip zone. Subduction of continental crust is likely to have taken place in the Alps and probably in many other orogenic belts. Probable continental margin sequences of pre-Mesozoic age are common. Some of these could have been deposited along Atlantic-type continental margins, but in no case has the matching continental fragment been identified. Without such identification, Atlantic-type continental margin sequences cannot be readily distinguished from sequences formed at the borders of new ocean basins next to wide, migrating, active island arcs (‘marginal basins’). Additional evidence suggests that the Cambro-Ordovician sequences of northwest Scotland cannot be deposits formed on an Atlantic-type continental margin, and that the same sequence in northwest Newfoundland may not have formed along such a margin. The plate tectonic significance of most orogenic features is unknown or controversial. How andesites originate is not understood; the reasons for the systematic potash variations with depth to the slip zone in andesite suites are unknown; and the significance of foreland deformation, of slaty cleavage and of regional metamorphism are disputed. Nevertheless, the solutions to all of the above problems are considered attainable, but will require a constant interplay between field, laboratory and theoretical work.

STRUCTURE AND DEVELOPMENT OF THE SOUTHERN MARGIN OF AUSTRALIA

Off the southern coast of Australia, exploration results and deep-water reconnaissance seismic data support the concept of an aseismic Atlantic-type continental margin. Characteristic is a sedimentary wedge which extends from the shelf to the abyssal plains and includes crustal elements of continental and oceanic origin.Oceanward, a continuous level of diffractions ascribed to the top of oceanic crust can be observed on seismic records, steeply dipping beneath the continental rise towards a smooth, flat, often faulted reflector which is correlated with top Precambrian or Palaeozoic continental basement.The sedimentary wedge which overlies the block-faulted and collapsed continental basement is subdivided by unconformities into: (a) a continental Lower Cretaceous unit and a fluviodeltaic unit of Upper Cretaceous-Danian age which are taken to represent rift valley stages of deposition controlled by extensional tectonics and (b) a post-breakup sequence of Tertiary units representing regional collapse and out-building of the shelf. The Upper Cretaceous sequence is missing along much of the continental edge where Tertiary sediments appear to rest directly on the Lower Cretaceous unit.Our interpretation suggests that a prolonged period of uplift took place along the axis of the rift valley prior to continental break-up. On the basis of palaeomagnetic data and biostratigraphic analysis the breakup phase started in the Upper Paleocene.From the continent outward several structural zones can commonly be recognised: (a) a zone of shallow basement with a thin Lower Cretaceous cover normally faulted and overlain by thin gently dipping Tertiary beds, (b) a zone of faulted and landwards tilted basement blocks and Lower Cretaceous sediments overlain (sometimes with clear unconformity) by thick Upper Cretaceous sediments, (c) a zone of thick, moderately deformed Tertiary sediments whose axis of deposition is generally offset to the south of the Upper Cretaceous basinal axis, (d) a zone of rotational faults and associated toe thrusts affecting the Cretaceous sediments and apparently related to the time of margin collapse, (e) an area of little disturbed Cretaceous and Tertiary sediments overlying continental basement. This zone extends into the "magnetic quiet zone" which is therefore believed to be, at least in part, a collapsed portion of the continental margin adjacent to oceanic crust.The interpretation of the geological evolution of the southern Australian margin based on the stratigraphic and structural data presently available can be related to current theoretical models on continental margin development.

Fold nappes as asymmetric mantled gneiss domes and ensialic orogeny

Descriptions are presented of the development of asymmetric gravity-driven structures rising from wedge-shaped bodies of light fluids overlain by denser fluids. Two programmes of experiments are described: 1. (1) the stabilization of wedges of oil overlain by glucose syrup in the field of gravity 2. (2) the stabilization while spinning in a centrifuge of wedge-shaped bodies of a light stiff silicone putty beneath denser and less viscous silicone putties. The development in these experiments of structures analogous to either basement cored domes or fold nappes typical of many orogenic belts is found to depend mainly on the inclination of the interface between the light and the overlying heavier fluids. Simple dome-like structures rise from this interface if it is horizontal or very shallow, whereas nappe-like structures alone develop if the interface is steeper than about 25° to the horizontal. A range of inclinations exist from which complex combinations of “domes” and “nappes” interfere to produce what would appear to be episodic deformations on small scales. It is argued that these experiments demonstrate that basement cored fold nappes are merely asymmetric mantled gneiss domes and that an Atlantic-type continental margin is, with an increase in the geothermal gradient, analogous to the unstable configuration of fluids at the start of the experiments. The conceptual model of ensialic orogeny due to Ramberg (1967) based on experiments like these is described and its implications discussed.

THE DEVELOPMENT OF CONTINENTAL MARGINS IN PLATE TECTONIC THEORY

Atlantic-type continental margins are formed by rifting and subsequent breakup of continents by sea-floor spreading. Large scale horizontal displacements of continental blocks on lithospheric plates are well-described by current plate tectonic theory. However, the rifting process itself entails vertical tectonics which operate prior to breakup, and such processes are not well understood within the framework of plate tectonic theory.The qualitative models, at present described in the literature involve a sequence of events illustrated by type examples: the East African Rift; the ethiopian-Afar Rift; the Red Sea Rift; and the Gulf of Aden. The thinning and subsidence of the continental crust during rifting have been considered to be the result of crustal stretching. However the writer considers the evidence ambiguous and the theory inadequate. The model presented here involves the response of the crust and the lithosphere to thermal processes implicit in plate tectonic theory. Firstly, uplift is caused by thermal expansion and phase-boundary migration in the lithosphere. which leads to crustal thinning by erosion. Secondly, subsidence is caused by metamorphism in the deep crust. These events occur out of phase and result in two cycles of uplift/erosion/and subsidence during continental margin formation.From this generalised model, theoretical time-stratigraphic and structural cross sections may be constructed to suit the different continental margins. These cross sections are composed of different litho-tectonic elements, namely: pre-rift, rift valley, and post breakup, which are in general separated by two angular unconformities. These features may be readily identified on Atlantic continental margins with only a minimum of seismic and well control.

Plate Tectonics Origin of the Caribbean Mountain System of Northern South America: Discussion and Proposal

A brief summary of the petrotectonic assemblages involved in the Cretaceous to early Tertiary orogeny of the Caribbean Mountain System is presented in order to delineate those geological constraints pertinent to a plate tectonic interpretation of the area. In particular, it is concluded that the Mesozoic sediments caught up in the orogeny were originally deposited along a quiescent Atlantic-type continental margin or “tensile open series geosyncline” (Wang, 1972) and not in association with an active subduction zone. Moreover, paired metamorphic belts are not found, and all metamorphic rocks, whose grade increases from south to north across the orogenic belt, are of medium- to high-pressure type. Thus a collision-type model appears to be more appropriate as the major mountain-building mechanism than the cordilleran-type mechanism proposed for the Caribbean Mountain System by various authors. An alternate tectonic model which incorporates the following main stages in tectonic development is suggested: (1) formation of an Atlantic-type continental margin along northern South America by continental rifting in middle Mesozoic time or earlier; (2) movement of an oceanic plate with a leading edge characterized by a subduction-zone–island-arc complex toward this continental margin; (3) collision in Late Cretaceous time; (4) overriding of the island arc over the continental margin, resulting in widespread gravity sliding and thrusting, and high p/T metamorphism of the shelf and rise sediments; and (5) isostatic readjustment due to reorganized plate movement in the early Tertiary. Ephemeral subduction zones active during this sequence of events explain the singular pattern of sporadic volcanism observed during this time span. The direct applicability of this model to areas outside of the region of interest is limited. Necessary restrictions on timing and plate movement in the Caribbean area, however, suggest that a probable feature of the most likely model for the tectonic evolution of this area will be the insertion of an oceanic plate from the Pacific between North and South America in late Mesozoic time.

Plume-Generated Triple Junctions: Key Indicators in Applying Plate Tectonics to Old Rocks

Continental lithosphere-especially where stationary with respect to mantle plumes-is marked by plume-generated uplifts typically crested by volcanoes that rupture in three rifts at angles of about 120° to each other, perhaps because this configuration requires the least work. It is proposed that since the plate tectonic regime began, about $$2 \times 10^{9}$$ years B.P., divergent plate motion has commonly begun at axial dikes emplaced in rifts formed in this way. A normal course of events is that two of the rifts meeting at a junction to open by plate accretion while the third rift becomes inactive as a failed arm. The evolution of 45 selected junctions, with ages ranging back to $$2 \times 10^{9}$$ years B.P., illustrates a variety of ways in which triple junctions may develop. Bends in rifted Atlantic-type continental margins reflect the distribution of triple junctions at the time continents parted and plume traces on ocean floors lead away from these former triple junctions. Where oceans have closed by continental collision, rifts (failed arms) (aulacogens of Soviet authors), striking at high angles into orogenic belts, mark the location of former triple junctions. Reactivation of old rifts is common and new rifts have frequently developed on the sutures along which oceans have closed. Base metal mineralization, especially in the form of syngenetic copper ores, is a feature of some failed arms (Montana, Zambia, Coppermine) and others, which contain up to 10 km of marine sediment, possess some of the world's major petroleum deposits (Northern North Sea, Niger Delta, Gippsland Basin, Gulf of Suez, and Gulf of Sirte). Many of the world's great rivers flow down failed arms (Mississippi, Amazon, Niger, Zambezi, Limpopo, Rhine).

Mountain belts and the new global tectonics

Analysis of the sedimentary, volcanic, structural, and metamorphic chronology in mountain belts, and consideration of the implications of the new global tectonics (plate tectonics), strongly indicate that mountain belts are a consequence of plate evolution. It is proposed that mountain belts develop by the deformation and metamorphism of the sedimentary and volcanic assemblages of Atlantic-type continental margins. These assemblages result from the events associated with the rupture of continents and the expansion of oceans by lithosphere plate generation at oceanic ridges. The earliest assemblages thus developed are volcanic rocks and coarse clastic sediments deposited in fault-bounded troughs on a distending and segmenting continental crust, subsequently split apart and carried away from the ridge on essentially aseismic continental margins. As the continental margins move away from the ridge, nonvolcanic continental shelf and rise assemblages of orthoquartzite-carbonate, and lutite (shelf), and lutite, slump deposits, and turbidites (rise) accumulate. This kind of continental margin is transformed into an orogenic belt in one of two ways. If a trench develops near, or at, the continenal margin to consume lithosphere from the oceanic side, a mountain belt (cordilleran type) grows by dominantly thermal mechanisms related to the rise of calc-alkaline and basaltic magmas. Cordilleran-type mountain belts are characterized by paired metamorphic belts (blueschist on the oceanic side and high temperature on the continental side) and divergent thrusting and synorogenic sediment transport from the high-temperature volcanic axis. If the continental margin collides with an island arc, or with another continent, a collision-type mountain belt develops by dominantly mechanical processes. Where a continent/island arc collision occurs, the resulting mountains will be small (e.g., the Tertiary fold belt of northern New Guinea), and a new trench will develop on the oceanic side of the arc. Where a continent/continent collision occurs, the mountains will be large (e.g., the Himalayas), and the single trench zone of plate consumption is replaced by a wide zone of deformation. Collision-type mountain belts do not have paired metamorphic belts; they are characterized by a single dominant direction of thrusting and synorogenic sediment transport, away from the site of the trench over the underthrust plate. Stratigraphic sequences of mountain belts (geosynclinal sequences) match those asciated with present-day oceans, island arcs, and continental margins.

Continental Margins, Geosynclines, and Ocean Floor Spreading

On Atlantic-type continental margins there is no differential movement between ocean floor and continent. On Andean-type margins spreading ocean floor descends beneath a submarine trench and continental mountain arc, and on island arc-type margins ocean floor descends beneath a trench and island arc bordering a small ocean basin of Japan Sea type. The corresponding modern geosynclines of Atlantic. Andean, island arc, and Japan Sea type each have distinct associations of sediments and volcanics. Geosynclines of Mediterranean type occur in small ocean basins between continents. Orogeny of Andean and island arc type occurs during development of the respective geosynclines. Himalayan-type orogeny results from collision of migrating continents. Development of the types of continental margin and associated geosynclines, and changes in type of geosyncline, are related to the oscillation of continents between ocean rises. The sequence pre-flysch flysch molasse in many ancient geosynclines can be interpreted in terms of modern geosynclinal deposits and continental margins.