Atlantic-type Continental Margins Research Articles

Stratigraphy of the Late Palaeoproterozoic (∼2.03 Ga) Wooly Dolomite, Ashburton Province, Western Australia: A carbonate platform developed in a failed rift basin

This paper describes the sequence stratigraphy of the ∼2.03Ga Wooly Dolomite, the uppermost stratigraphic unit of the Horseshoe Basin, strata of which unconformably overlie the 2772–2410Ma Hamersley Province, unconformably truncate folded ∼2210Ma dolerite sills and post-date the 2195–2145Ma Ophthalmia Orogeny. The Horseshoe Basin was an intracontinental rift that initiated at ≤2.05Ga with deposition of ≤360m of fluvial and shallow-marine siliciclastic sandstones (Beasley River Quartzite), followed by eruption of ∼2.7km of flood basalt (Cheela Springs Basalt), and culminated with deposition of ≥325m of platformal carbonates and associated volcaniclastic and siliciclastic sediments (Wooly Dolomite). The three formations of the Horseshoe Basin stack conformably, with tectonically driven unconformities developed only at a late stage, within the Wooly Dolomite. Depositional systems of the Wooly Dolomite appear to have been compartmentalised, but connected to an ocean. Five depositional sequences are identified. Depositional Sequence 1, which developed conformably on the Cheela Springs Basalt, records the establishment of a carbonate platform coeval with volcaniclastic sedimentation, but it was developed only in the southeastern-most sector of the basin. Resedimented tuffs (volcaniclastic siltstones) are present in all of the depositional sequences, but are most abundant in DS1. Depositional Sequence 2 cuts deeply into the Cheela Springs Basalt, and has a lower section of siliciclastic sedimentary rocks that is overlain by carbonate-platform deposits. The Wyloo Dome, at an interpreted rift triple-junction, was an uplifted area until the late stage of DS2, implying that accommodation generation was also fault compartmentalised. Depositional Sequences 3, 4 and 5 have predominantly carbonate-platform deposits, and are also unconformity bound, whereas DS4 and DS5 preserve a shelf-slope transition. Subsidence in the Horseshoe Basin ended with basin inversion during the D1 (Panhandle) deformation event, which predated emplacement of an ∼2008Ma dolerite dyke swarm. A subsequent rifting event, recorded by the McGrath Basin, led to a rift-drift transition that finally initiated a west-facing Atlantic-type continental margin.

Jurassic ophiolite formation and emplacement as backstop to a subduction-accretion complex in northeast Turkey, the Refahiye ophiolite, and relation to the Balkan ophiolites

The eastern Mediterranean region within the Tethyan realm shows a high concentration of ophiolites with contrasting times of formation and emplacement along the belt: In the Balkans, the ophiolites formed during the early to medial Jurassic, and were obducted during the late Jurassic, whereas in Turkey and farther east, structurally intact Jurassic ophiolites are rare and Jurassic ophiolite obduction is unknown. Here we report a structurally intact, large ophiolite body of early Jurassic age from NE Turkey, the Refahiye ophiolite, located close to the suture zone between the Eastern Pontides and the Menderes-Taurus block. The Refahiye ophiolite forms an outcrop belt, 175 km long and 20 km wide, and is tectonically bound by the late Cretaceous ophiolitic melange to the south, and by the North Anatolian Transform Fault against the Triassic low-grade metamorphic rocks to the north. Early to medial Jurassic very low- to low-grade metamorphic rocks, interpreted as intraoceanic subduction-accretion complexes, occur either beneath the ophiolite or as thrust slices within it. The ophiolite body within the studied section is made up of mantle peridotite (clinopyroxene-bearing harzburgite and minor dunite) crosscut by up to 20 cm thick veins of clinopyroxenite and later dikes/pods/stocks of gabbro ranging in size from 2 m to several hundreds of meters. The gabbro is represented by two distinct types: (i) cumulate gabbro, and (ii) non-cumulate gabbro with locally well-developed igneous foliation. Within the non-cumulate gabbro or enclosing peridotite, there are up to 5 m and 50 cm-thick veins of trondhjemite and pegmatitic gabbro, respectively. LA-ICP-MS dating on zircons from two trondhjemite samples yielded weighted mean ages of ∼184 ± 4 Ma and 178 ± 4 Ma (2σ), respectively, suggesting formation during early Jurassic time. Formation in a suprasubduction-zone forearc setting is inferred from (i) wide-ranging pyroxene and spinel compositions in the peridotites as documented in most suprasubduction-zone ophiolites, (ii) arc tholeiitic signature of the non-cumulate gabbros, and (iii) association of the ophiolite with the coeval subduction-accretion complexes. Emplacement of a trapped forearc ophiolite above its own subduction-accretion complex as a backstop is proposed based on a series of field relationships such as (i) intimate association of the unsubducted suprasubduction-zone ophiolite with coeval accretionary complexes, (ii) absence of unambiguous relationship to the southern Atlantic-type continental margin, and (iii) absence of any stratigraphic indications for the ophiolite obduction in the southern Atlantic-type continental margin during Jurassic time. This is a clear difference from the Jurassic ophiolites in the Balkans that were obducted over the Atlantic-type continental margin. This difference in mode of emplacement is most probably related to the greater distance of the intra-oceanic subduction zone to the Atlantic-type continental margin than it was in the Balkans, which is commensurate with the greater width of the Tethys in the east during Jurassic time.

Gravity anomalies and segmentation at the East Coast, USA continental margin

SUMMARY The free-air gravity ‘edge effect’ anomaly at rifted continental margins has generally been attributed to the transition between thick continental and thin oceanic crust. While crustal thinning is a major contributor, recent studies suggest that sediment loading and magmatism may significantly modify the edge effect anomaly and cause it, at some margins, to be highly segmented along their strike. In this paper, we use a combined 3-D flexural backstripping and gravity anomaly modelling technique to determine the role that sediment loading has played in controlling the segmentation of Atlantic-type continental margins. We focus on the East Coast, USA since a substantial amount of high-quality seismic reflection and refraction, gravity, and magnetic data already exists for this margin. By calculating the gravity anomaly associated with rifting and sediment loading and iteratively comparing it to the observed free-air anomaly, we have determined the ‘best-fit’ elastic thickness, Te, structure of the margin. We show that 0 10 5 a) integrated strength of the lithosphere, these results imply that weak regions abut strong ones at the East Coast, USA margin. Te generally decreases with increase in the amounts of crustal thinning, β, and the flexed basement curvature, K, suggesting it is controlled, at least in part, by the mechanical structure of the pre-rift lithosphere and yielding due to flexural loading. However, there is considerable scatter, suggesting other factors such as along-strike changes in crustal composition. Irrespective, we show that an isostatic anomaly that takes into account the ‘best-fit’ Te distribution is significantly reduced in spectral power compared with one which is computed assuming only Airy compensation (i.e. Te = 0 km). This is not to imply that rifting and sediment loading completely accounts for the anomalies along-strike and across-strike the East Coast, USA margin. To the contrary, significant isostatic anomaly highs and lows persist, especially in inner and middle shelf regions. One of the most prominent is an arcuate, 670 km long, high with flanking lows offshore Carolina that we attribute to magmatism during the initial stages of continental break-up.

MIDDLE JURASSIC-LOWER CRETACEOUS BIOSTRATIGRAPHY IN THE CENTRAL PONTIDES (TURKEY): REMARKS ON PALEOGEOGRAPHY AND TECTONIC EVOLUTION

The deposition of Jurassic-Lower Cretaceous carbonates in the Pontides was controlled mainly by the evolution of an Atlantic-type continental margin in the Tethys. The study of several stratigraphic sections from allochthonous slices and blocks of the North Anatolian Ophiolitic Melange provided insight into the Middle Jurassic-Early Cretaceous paleogeographic evolution of the Central Pontide Belt. The Callovian-Aptian successions span the Globuligerina gr. oxfordiana, Clypeina jurassica (equivalent of the Tubiphytes morronensis zone), Protopeneroplis ultragranulata (with the Haplophragmoides joukowskyi subzone), Montsalevia salevensis , Hedbergella delrioensis - Hedbergella planispira - Leupoldina - Globigerinelloides and Globigerinelloides algerianus biozones. Two major stratigraphic gaps corresponding to the pre-Callovian and Hauterivian-Early Aptian ages are recognised within the successions. Lithostratigraphic and biostratigraphic studies indicate strong similarities in the evolution of the successions in the Amasya region (Central Pontides) and Biga-Bursa-Bilecik (BBB ) Platform (North-western Anatolia).

Sequence stratigraphy and architecture on a ramp-type continental shelf: the Belgian Palaeogene

Abstract In Palaeogene times, the ‘Southern Bight’ of the North Sea functioned as an intracratonic, shallow-marine, siliciclastic basin and accumulated a few hundred metres of gently dipping sediment packages. A fine-scale seismic-stratigraphical model for the Palaeogene was formulated on the basis of a dense, high-resolution reflection seismic grid. In total 13 major seismic-stratigraphical units were defined, based on geometry and seismic facies characteristics. The seismic stratigraphy has been complemented with the results of four cored wells near the Belgian coast, containing a nearly continuous, 200 m thick sediment succession of Eocene age. Facies analyses of these cores suggest that part of these sediments were deposited on a muddy shelf and part in adelta environment. Evidence from relevant onshore outcrops has been used to complete the geological history of the Palaeogene, with special emphasis on the Eocene. A sedimentation model for the Eocene is presented, and relative sea-level changes, regional tectonic events and changes in sediment input are discussed. Genetic interpretation of the various lithological units and the large-scale architecture of the ramp-type margin enable evaluation of sequence-stratigraphical concepts, initially defined for a typical shelf-slope-basin section along an Atlantic-type continental margin.

A physical explanation of the relation between flank uplifts and the breakup unconformity at rifted continental margins

Rift-flank uplifts and the breakup or postrift unconformity are characteristic features of many rifted, passive, or Atlantic-type continental margins, but are not predicted as primary features of simple lithospheric stretching models. The explanations that have been proposed for their origin remain controversial, either because they call upon special circumstances or because they are difficult to test. Plane-strain finite element models are used in this paper to explore the dynamics of lithospheric necking during rifting and rupture. The results agree with the conceptual interpretation that uplift of the rift boundaries to form flank mountains and uplift of the basin responsible for the breakup unconformity are related consequences of regional isostatic compensation of mass that is redistributed during the necking and rupture phases. Although the amplitudes of these uplifts depend on the model parameter values, the relations between and relative signs of these two effects appear to be fundamental. The explanation we propose may have been missed in recent studies because there has been a tendency to concentrate on kinematic stretching models, which assume local isostatic equilibrium through out the rifting process. Such an approach is predicated on the assumption that the lithosphere has an insignificant strength or flexural rigidity during extension, which is not true if our explanation is correct.

Timing of opening of the Black Sea basin

A detailed study of the Mesozoic palaeogeographic evolution of the Rhodope-Pontide fragment and the region surrounding the Black Sea as a whole shows that the Black Sea basin began opening as a back-arc basin by the rifting of a young continental margin magmatic arc during Aptian-Cenomanian time. During the early Cretaceous the present Black Sea region was occupied by an extensive, generally south-sloping, shallow, carbonate-dominated shelf developed in the late Jurassic along the south-facing Atlantic-type continental margin of Eurasia north of the Neo-Tethys. Following the onset of subduction-related volcanism on the Rhodope-Pontide fragment in the Aptian-Albian, disintegration of this shelf started in the Western Pontides and in the Moesic platform. Block-faulting and accompanying subsidence accelerated in the Cenomanian and in a number of places (e.g. the Rhodope-Pontide fragment, Pre-Caucasus, Crimea, Moldavia) caused the formation of large depressions or basins on the subsided blocks with deposition of deeper water sediments locally intercalated with volcanics. This Cenomanian disintegration, which began tearing the Rhodope-Pontide fragment from the main Eurasian continental block along the young volcanic axis of the south-facing Neo-Tethyan magmatic arc, is here interpreted as the initial rifting and opening of the Black Sea basin, which underwent a uniform, probably thermally-induced, subsidence later in the Senonian. Data from the circum-Black Sea region show that the Black Sea basin is unrelated to the basin opening and closing events in the Caucasus and in the Balkans that occurred in the Jurassic to early Cretaceous.

Numerical models for the thermo-mechanical evolution of Atlantic-type continental margins : Int J Num Meth EngngV24, N1, Jan 1987, P141–157

Numerical models for the thermo-mechanical evolution of Atlantic-type continental margins : Int J Num Meth EngngV24, N1, Jan 1987, P141–157

Earthquakes recorded stratigraphically on carbonate platforms

The longest records of earthquake activity1 may be preserved near the edges of carbonate platforms where reefs have bordered active faults. Because the carbonate sediments build up almost to low tide level, because their character and lithification are very sensitive to deviations from that datum, and because they commonly lithify soon after deposition2, strata can precisely record changes in lithospheric flexure3, which in turn record the buildup and release of elastic strain energy during stick–slip faulting. The approximate stratigraphy is predictable for a given history of faulting. Comparison of predictions with observations leads to an explanation for some of the hemicyclic sedimentary sequences common4–6 on carbonate platforms. These records are important for characterizing the seismicity of Atlantic-type continental margins in general. After rifting, faults evidently remain active7, but major earthquakes are so rare that their recurrence times cannot be established from direct observations.

Structure and Evolution of Some Continental Rifts--Consequences for Their Thermomechanical Behavior: ABSTRACT

Continental rifts are attractive basins for hydrocarbon exploration, because they presently furnish 10% of the world hydrocarbon production, yet cover only 5% of the Earth's surface. The evolution of the rifts where volcanism is of reduced importance will be emphasized through present examples such as the Suez rift (Egypt), the rifts beneath the Bay of Biscay, and other Atlantic-type continental margins. The continental rifts, typically 50-100 km (30-60 mi) in width, are tectonically characterized by the extension and thinning of the continental crust together with subsidence in the central part of the rift. The superficial structure consists mainly of tilted blocks bounded by listric normal faults. Continental extension may reach 50% of the crust's original horizontal length, and even more depending on the amount of dike intrusions from the mantle. The thinning of the crust may reach much higher rates, perhaps 300% or more where oceanic floor is formed in the central part of the rift. The subsidence in the rift trough is characterized by the alternation of rapid downward movements over a period of a few million years and periods of relative stagnation. Meanwhile, the shoulders of the rift may uplift after the beginning of normal faulting in the main troughs. Heat flow is higher generally in the rifts than in the surrounding areas. This evolution is the consequence of an initial thermal perturbation in the deep asthenosphere. The evolution of the lithosphere-asthenosphere system is studied by means of a thermomechanical model with the hypothesis of a non-newtonian rheological behavior of the lithosphere as a function of the depth. The vertical movements of the continental crust, the thinning of the lithosphere, the stresses, and the gravity anomalies in the thermal regime are given as a function of time during rifting. End_of_Article - Last_Page 793------------

Continental Margin of Eastern Canada--Geologic Framework and Petroleum Potential: ABSTRACT

The Atlantic-type continental margin of eastern Canada is underlain by a series of Mesozoic-Cenozoic sedimentary basins separated by basement highs or areas of thinner sediments. Regional and/or salt tectonics have structured the Mesozoic sequence, which is masked by a less-deformed wedge of prograding uppermost Cretaceous and Cenozoic sediments. The basins have been targets of active hydrocarbon exploration for over 2 decades. Data from 138 exploratory wells and over 680,000 km (420,000 mi) of multichannel seismic coverage have indicated four major geologic/geochemical regions: Scotian Shelf, southern Grand Banks, northeastern Grand Banks, and Labrador-Southeast Baffin Shelf. On the Scotian Shelf, 13 significant gas/condensate discoveries have been made out of 62 wildcats drilled since 1967. Five of the discoveries, including the Venture field, are in an overpressured zone that has been explored only since 1979. No commercial hydrocarbon accumulations have been found in the southern Grand Banks where 28 wildcats were drilled between 1966 and 1975. The northeastern Grand Banks region has been actively explored since 1971. The 22 wildcat wells drilled through late 1983 have yielded six significant light oil discoveries, including the giant Hibernia oil field. Labrador-Southeast Baffin Shelf exploration has yielded six gas/condensate discoveries in 26 exploratory wells drilled since 1971. The Geological Survey of Canada has developed hydrocarbon-generation models to explain the regional variation in oil and gas occurrence and to assess future potential in terms of the nature and thermal maturity of the source rocks, type of organic material, and time of trap formation. These factors are related to the geologic history of the margin, which is characterized regionally by diachronism in major basin inception and in the resultant stratigraphic record. We predict an exciting future for this vast petroleum province. End_of_Article - Last_Page 1203------------

Geology and Regional Setting of Kuparuk Oil Field, Alaska

The Kuparuk oil field is located on the Alaskan Arctic plain in the Colville-Prudhoe basin, 10 to 30 mi (16 to 48 km) west of the Prudhoe Bay field. The 24° API crude is similar in type to that in the Permo-Triassic reservoirs in the Prudhoe Bay field; however, it is from the Lower Cretaceous Kuparuk Formation. This reservoir is located in a basin between the Colville and Prudhoe highs. The origin of the oil is believed to be predominantly Lower sequence formations with migration occurring possibly via the Prudhoe Bay field. The dominant trapping mechanism is stratigraphic pinch-out and truncation of the reservoir at a local unconformity along the southern and western flanks of a southeast-plunging antiform. Structural dip closure exists along the northern and eastern flanks. The reservoir sandstones occur within sequences which become cleaner and coarser upward, and are thought to be shallow marine in origin with a provenance to the northeast. They are interpreted to be infrarift sediments on what is now a passive, Atlantic-type continental margin. Two of the four major lithostratigraphic units mapped within the Kuparuk Formation exhibit good reservoir characteristics and extend over an area in excess of 200 mi2 (518 km2). The cumulative net pay in the Kuparuk field ranges up to 90 ft (27 m), and the estimate of movable oil-in-place is 4.4 billion stock tank bbl. There is no gas cap. The field exhibits a variable oil-water contact ranging from -6,530 ft (-1,990 m) in the southeast to -6,700 ft (-2,042 m) in the north. After secondary waterflooding, the potential recoverable reserves are estimated to be about 1.0 to 1.5 billion stock tank bbl. Kuparuk field, therefore, ranks as one of the largest oil fields in the United States.

Model for a Passive to Active Continental Margin Transition: Implications for Hydrocarbon Exploration

This paper proposes a model for the transition of a passive, Atlantic-type continental margin to an active one with reverse polarity, prior to its ultimate collision with an island arc or continent. Significant morphological and stratigraphic patterns which are believed to characterize this transition include: (1) reactivation as reverse faults of preexisting listric, normal faults (not Gulf Coast-type growth faults, but those affecting basement as well) that were established during the margin's most recent period of extension; (2) landward migration with time of this fault rejuvenation; (3) development of deeper water-shallower water-deeper water or deeper water-unconformity-deeper water stratal assemblages on the margin; (4) regional flexure of the continental margin, a in to that known for the outer rise area of oceanic plates, landward of the subduction zone; and (5) landward migration of major unconformities coeval with continental margin flexure. This model creates implications for hydrocarbon exploration because: (1) it predicts the presence or absence of unconformities and thus potential stratigraphic traps among units of a particular basin; (2) provides spatial and temporal constraints for secondary-porosity development in potential reservoirs; (3) allows more accurate interpretations to be made of thermal or burial-history diagrams constructed for the purpose of determining relative thermal maturities of potential source units; and (4) helps distinguish between basement reactivation and decollement or Gulf Coast-type growth faulting. The orogenic phase of the margin's development, however, may later overprint or conceal these morphological and stratigraphic patterns to the extent that they might be unrecognizable. Because they have implications from both academic and economic perspectives, the role of these patterns in the margin's evolution should not be overlooked.

Variations in the degree of crustal extension during formation of a back-arc basin

Ophiolite complexes in southern Chile represent the remnants of the mafic portion of the floor of a Cretaceous back-arc basin which widened markedly from north to south over a length of 600 km. Detailed field and geochemical studies of ophiolites in the northern (Sarmiento complex) and southern (Tortuga complex) extremities of the originally wedge-shaped back-arc basin floor, indicate significant north—south differences in the mode of emplacement of basaltic magmas into the pre-existing continental crust, during the formation of the basin. In the northern narrow extremity of the original basin, mafic melts intruded into the continental crust over a diffuse zone causing extensive remobilization and reconstitution of the sialic continental crust. In the southern wider part of the original basin, mafic magmas appear to have been emplaced at a localized oceanic-type spreading centre. The observed north—south variations resulted in formation of back-arc floor with crustal characteristics ranging from intermediate between continental and oceanic to typically oceanic. These variations are interpreted as representing different stages of evolution of a back-arc basin which formed due to a subtle interplay between subduction induced back-arc mantle convection and the release of stress across the convergent plate boundary, possibly related to ridge subduction. Prior to the release of stress, heat transferred from mantle diapirs to the base of crust caused widespread silicic volcanism in South America. With the release of stress, mantle derived melts erupted to the surface along structural pathways resulting in extensive basaltic volcanism in a linear belt behind the island arc and the cessation of silicic volcanism. Initially, basaltic magmas intruded the continental crust over a diffuse region causing reconstitution of sialic crustal rocks. Progressive localization of the zone of intrusion of mafic magmas from the mantle eventually resulted in the development of an oceanic-type spreading centre. Observations in southern Chile and elsewhere suggest that variability in horizontal stress across a convergent plate boundary may be the overriding factor in determining the regional response of continental crust to subduction induced back-arc convection, and hence the mechanism of emplacement into the crust of mafic mantle melts. The various lithologies observed in southern Chile could also be expected to form during the opening phase of major ocean basins and to currently underlie Atlantic-type continental margins.

The Gulf of Lion: subsidence of a young continental margin

The origin of the subsidence of Atlantic-type continental margins is of much interest to geology and geophysics. The main problem at these margins is in explaining the substantial thicknesses of mainly shallow-water sediments which accumulate soon after rifting and plate separation. We have now used biostratigraphic data from commercial wells to study the subsidence history of a young Atlantic-type continental margin. Backstripping the sediment load reveals that the margin has been subsiding at nearly mid-ocean ridge rates. Geological data indicate that lithospheric stretching alone cannot account for this amount of subsidence and other processes must be involved.

Appalachian Orogen in Canada

The Appalachian Orogen is divided into five broad zones based on stratigraphic and structural contrasts between Cambrian–Ordovician and older rocks. From west to east, these are the Humber, Dunnage, Gander, Avalon, and Meguma Zones.The westerly three zones fit present models for the development of the orogen through the generation and destruction of a late Precambrian – Early Paleozoic Iapetus Ocean. Thus, the Humber Zone records the development and destruction on an Atlantic-type continental margin, i.e., the ancient continental margin of Eastern North America that lay to the west of Iapetus; the Dunnage Zone represents vestiges of Iapetus with island arc sequences and mélanges built upon oceanic crust; and the Gander Zone records the development and destruction of a continental margin, at least in places of Andean type, that lay to the east of Iapetus.The Precambrian development of the Avalon Zone relates either to rifting and the initiation of Iapetus or to subduction and a cycle that preceded the opening of Iapetus. During the Cambrian Period, the Avalon Zone was a stable platform or marine shelf.Cambrian–Ordovician rocks of the Meguma Zone represent either a remnant of the continental embankment of ancient Northwest Africa or the marine fill of a graben developed within the Avalon Zone.Silurian and younger rocks of the Appalachian Orogen are mixed marine and terrestrial deposits that are unrelated to the earlier Paleozoic zonation of the system. Silurian and later development of the orogen is viewed as the history of deposition and deformation in successor basins that formed across the already destroyed margins and oceanic tract of Iapetus.

Mechanisms of Basin Subsidence: ABSTRACT

The accumulation of sediments in interior and continental-margin basins constitutes a load on the lithosphere which sags because of the sediment weight. Studies of the geometry of deformation suggest the lithosphere responds to these loads either by local compensation of an Airy-type crust or regional flexure of a strong rigid crust. Sediment-loading models of either type cannot, however, explain the substantial thicknesses of shallow-water sediments observed in deep boreholes in these basins. Other factors, such as thermal cooling of the lithosphere, crustal stretching or necking, or deep crustal metamorphism, must therefore contribute to the subsidence. By quantitatively accounting for the subsidence due to surface loads, the subsidence caused by these other factors c n be isolated. We have used biostratigraphic data from commercial boreholes in the Mississippi embayment, the North Sea, and the Atlantic-type continental margin off the East Coast of the United States to examine the origin of the subsidence of these basins. From these data we determined the depths to stratigraphic horizons during basin development. Using the sediment-loading models the sediment layers at each basin were progressively backstripped and the depth at which the basement would have been in the absence of sediment laods was calculated. Corrections for the effects of compaction, water depth, and eustatic changes of sea level were included. The backstripped basement depths indicate there is a recognizable component of the subsidence of these basins which is caused by processes other than adjustments to the weight of the sediments. End_of_Article - Last_Page 702------------

Late stage development of mature atlantic-type continental margins

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.

Metastable phase transition models and their bearing on the development of atlantic-type geosynclines

The transformation of metastable basalt to stable garnet granulite or eclogite is assumed to be a possible mechanism involved in the development of sedimentary basins adjacent to passive continental margins. It might play a significant role for both the subsidence of those regions, as recorded by the sedimentary fill, as well as for the crustal structure with respect to the observed gradational crust-mantle boundary indicated by intermediate seismic velocities in those regions. In order to investigate the phase transition beneath a sedimentary layer, we combine a one-dimensional thermal model of the crust with different diffusion models for the reacting grains. The first model describes the movement of the boundary between the metastable and the stable phase as a function of the activation energy for diffusion, the thermal properties of the crust and the time function of sedimentation. The second model gives the degree of transformation of the grains in metastable state to the stable state, and shows the existence of an extended reaction zone. The results of the numerical calculations encourage one to draw some conclusions which support the above assumption. Thus, the reaction rate and the sedimentation rate appear to be nearly equal, but it seems to be likely that subsidence and sedimentation occur step by step. The beginning of the reaction requires a minimum thickness of sedimentary deposits: between 3 and 6 km for the given parameters. Finally, the derived reaction zone has an extension of several kilometers depending on the activation energy. This zone might give an explanation for the intermediate seismic velocities found at Atlantic-type continental margins.

The Siberian Connection: A case for Precambrian separation of the North American and Siberian cratons

The hypothesis that the Cordilleran geosyncline originated as an Atlantic-type continental margin by the rifting of an older Precambrian continental mass and the opening of a new ocean basin leads to the question of where the counterpart of the North American Precambrian craton may be. The Siberian platform—a large, discrete, older Precambrian craton, lodged in northeast Asia and surrounded by younger fold belts—is a likely candidate for the missing Precambrian continental fragment. The outline of its northeast margin fits the southwest margin of the North American Precambrian craton to produce a congruence of tectonic grain and age provinces. Both margins are overlapped by sediments of the same general type that began to accumulate about 1,500 m.y. ago, after the initiation of the separation.