Showing papers on "Continental margin published in 1987"

Tertiary oxygen isotope synthesis, sea level history, and continental margin erosion

Abstract: Tertiary benthic and planktonic foraminiferal oxygen isotope records are correlated to a standard geomagnetic polarity time scale, making use of improved chronostratigraphic control and additional Oligocene isotope data Synchronous changes in both benthic and planktonic δ18O values which occurred in the Oligocene to Miocene (36–52 Ma) are interpreted, in part, to represent ice growth and decay The inferred ice growth events correlate with erosion on passive continental margins as interpreted from seismic and chronostratigraphic records This association is consistent with a link between Oligocene to Miocene erosional events and rapid (>15 m/my) glacioeustatic lowerings of about 50 m High benthic foraminiferal δ18O values suggest the presence of continental ice sheets during much of the Oligocene to Recent (36–0 Ma) Substantially ice-free conditions probably existed throughout the Paleocene and Eocene (66–36 Ma) The mechanisms and rates of sea level change apparently were different between the early and late Tertiary, with glacioeustatic changes restricted to the past 36 my Pre-Oligocene erosion on passive continental margins was caused by eustatic lowerings resulting from global spreading rate changes We apply a model which suggests that large areas of the continental shelves were subaerially exposed during such tectonoeustatic lowstands, stimulating slope failure and submarine erosion The different mechanisms and rates of eustatic change may have caused contrasting erosional patterns between the early and late Tertiary on passive continental margins This speculation needs to be confirmed by examination of data from several passive margins

Magmatism at rifted continental margins

Abstract: The intense volcanism and uplift observed on many rifted continental margins, forming basaltic seaward-dipping reflector sequences, is accompanied by the emplacement of a thick igneous section at depth. Partial melting by decompression of passively upwelling asthenosphere that is hotter than normal because it is near a hotspot explains both the thickness of the new igneous crust and the initial elevation of the rifted margins.

REE Variations Across the Peninsular Ranges Batholith: Implications for Batholithic Petrogenesis and Crustal Growth in Magmatic Arcs

Abstract: Rare earth element (REE) patterns of plutonic rocks across the Cretaceous Peninsular Ranges batholith vary systematically west to east, transverse to its long axis and structural trends and generally parallel to asymmetries in petrologic, geochronologic and isotopic properties. The batholith can be divided into three distinct parallel longitudinal regions, each defined by distinct REE pattern types. An abrupt transition occurs between rocks with slightly fractionated REE patterns in the western (coastal) region and rocks with middle to heavy REE fractionated and depleted patterns in the central region. Further to the east a second transition to strongly light REE enriched rocks occurs. The slopes of the REE patterns within each of these regions are largely independent of rock type. The first REE transition is closely coupled to regional discontinuities in other parameters: elimination of negative Eu anomalies, an increase in Sr content, and a marked restriction in petrologic diversity. This transition occurs over a range of initial ^(87)Sr/^(86)Sr ratios and δ^(18)O values, but approximately correlates to a major shift in the emplacement style of the batholith from a stationary arc to a rapidly eastward-migrating (cratonward) arc. The sense of the regionally consistent REE trends cannot be explained by crystallization, assimilation, combined crystallization-assimilation, or mixing processes. The consequences of assimilation and high-level differentiation are not observed generally, despite the sensitivity of the REE to these processes. Geochemical and petrological features argue that the partial melting of mafic source rocks in which plagioclase-rich (gabbroic) residual assemblages abruptly gave way laterally and downward to garnet-bearing (eclogitic) residual assemblages produced all the changes associated with the first REE transition. The change in residual assemblages from gabbroic to eclogitic was superimposed on source regions already zoned in light REE abundances, ^(87)Sr/^(86)Sr and ^(18)O. Temperature and pressure constraints on the source regions place them in a subcrustal location. The calcic nature of the batholith and the dominance of tonalite and low-K_2O granodiorite in all its regions argue that the source materials are broadly basaltic in composition. Experimental studies are consistent with the generation of the abundant tonalitic magmas by the partial melting of basalt under both low and high pressure conditions. Arc basalts such as those commonly erupted in modern island arcs and continental margins are inferred to have provided much of the source material and the heat. Additional high-^(18)O components are needed in the more easterly source regions. These materials must be distributed so as to contribute equally to the range of magmas that occur in a given local region, and must preserve the calcic nature of batholithic sources. Altered basalts of ancient oceanic crust and possibly their associated metasediments, previously incorporated into the lithosphere beneath the continental margin during earlier cycles of subduction, most readily satisfy these constraints. The REE geochemistry of the central and eastern regions of the batholith differs from that of oceanic island arcs in the presence of strongly heavy REE depleted and fractionated magmas. A model is proposed in which arc basalts accumulate beneath a crustal layer. Melting of accumulated material at low pressure produces magmas of the western region. Where thickening of the basaltic underplate is sufficient to form eclogitic assemblages, eclogite-derived magmas of the central and eastern region are produced. The abrupt transition to eclogite-derived magmas that suggests a process driven by a density instability is responsible for their origin. The Peninsular Ranges batholith appears to be representative of a major differentiation process in which mantle-derived basalt is remelted, contributing its more sialic fractions to the continental crust and leaving its mafic to ultramafic residues in the mantle. This process preserves the sialic character of the continental crust and may play a significant role in its growth and evolution. The batholith and the processes that produced it may be a more appropriate basis than immature oceanic island arcs on which to construct models of continental growth and evolution.

Coincident seismic reflection/refraction studies of the continental lithosphere: A global review

Abstract: Nearly 50 coincident seismic reflection/refraction studies to depths of at least the Moho provide an improved understanding of the continental lithosphere. Some conclusions include the following: (1) A transparent upper crust, a common observation on vertical reflection profiles, cannot generally be correlated with velocity gradients or low-velocity zones. Rather, a commonly transparent upper crust may be explained by short-wavelength, steeply dipping features in the brittle upper crust and to a lesser degree by signal contamination from source-generated noise. (2) The reflective lower crust in extensional terranes appears to be characterized by a high average seismic velocity (6.6–7.3 km/s) and to consist of laminated high- and low-velocity layers with typical thicknesses of 100–200 m. (3) Landward dipping reflectors observed in the middle to lower crusts of convergent zones have been identified as paired high- and low-velocity slabs which represent oceanic crust and mantle accreted via underplating to the continental margin. (4) The crust-mantle boundary may differ sufficiently when imaged with vertical incidence and wide-angle data to justify the retention, for the present, of the concept of separate reflection and refraction Mohos. While there is good evidence that these features are coincident within measurement uncertainties in most regions, recently recorded data from the Basin and Range admit the possibility for noncoincidence in that area. (5) Upper mantle reflections which cannot be migrated into the lower crust remain rare, despite isolated unequivocal examples. Thus the upper mantle appears to be relatively homogeneous at seismic reflection wavelengths and to lack the laminations inferred for the lower crust. The wide-angle method will likely provide the most reliable information on the velocity structure and physical state of this portion of the lithosphere for some years to come. (6) There appear to be clear and consistent basic differences between convergent and extensional terranes which have been identified from coincident experiments; these differences may be sufficiently universal to infer the tectonic history of poorly exposed terranes. (7) No truly three-dimensional coincident experiment (i.e., including three-dimensional migration) has been conducted, but some three-dimensional data have been collected using both methods. Measurements of attenuation, Poisson's ratio, and anisotropy within the crust using coincident data sets remain frontiers.

Geology and resource potential of the continental margin of Western North America and Adjacent Ocean Basins: Beaufort Sea to Baja California. Circum-Pacific council for energy and mineral resources earth science series, Volume 6

Abstract: This collection of papers summarizes the geologic framework and resource potential of offshore western North America from the Arctic Ocean to Baja California, including ocean regions of the United States Exclusive Economic Zone (EEZ). The idea that these papers should be prepared and brought together in a single volume arose late in 1982. At that time the US Geological Survey`s program of marine studies, which began in the early 1960s, completed its regional reconnaissance of the stratigraphy and structure of almost the entire margin of western North America and adjacent sea floor. It was therefore deemed timely and appropriate to summarize the important results of this effort and provide a systematic, region-by-region description of the framework geology and resource potential of this extensive, submerged region. It also appeared--as economic conditions ultimately become more favorable for the commercial development of marine georesources--that such a summary would be useful to the petroleum and mineral industries, and to public agencies and officials concerned with management of the offshore domains of the United States. Each paper has been processed separately for inclusion in the Energy Science and Technology Database.

Continent-ocean transition at the western Barents Sea/Svalbard continental margin

Abstract: The change in crustal type at the western Barents Sea/Svalbard margin takes place over a narrow zone related to primary rift and shear structures reflecting the stepwise opening of the Greenland Sea. Regionally, the margin is composed of two large shear zones and a central rifted-margin segment. Local transtension and transpression at the plate boundary caused the early Cenozoic tectonism in Svalbard and the western Barents Sea, and might explain the prominent marginal gravity and velocity anomalies.

Least squares multiple tracer analysis of water mass composition

Abstract: We present an inverse method that uses measurements of the concentration of several tracer variables to find the mixture of source water types that best describes (in a weighted least squares sense) the composition of the water sample. The solution includes two physically realistic constraints: first, that all sources together must sum to 100% and, second, that no source is present in negative amounts. These constraints improve the statistical stability of the solution. The method is particularly valuable in regions involving strong mixing among a relatively large number of source water types. In contrast to conventional TS analysis, more water types can be considered, the assumption of mixing along isopycnals is unnecessary, and sensitivity to errors in individual tracer measurements is reduced by averaging the influence of a larger number of tracers. We demonstrate the method using data from two oceanographic regions. Analysis of summer data from the continental margin off Vancouver Island (British Columbia, Canada) confirms and quantifies the importance of deep upwelling of California Undercurrent water onto the continental shelf. It also identifies localized outer shelf filaments of low-salinity water originating from the Vancouver Island Coastal Current. Finally, we reanalyze the North African data of Tomczak and confirm his interpretation of diapycnal mixing at the frontal boundary between North Atlantic Central Water and South Atlantic Central Water masses.

Evidence for transform margin evolution from the Ivory Coast–Ghana continental margin

Abstract: The relative motions between lithospheric plates can be reduced to three major types: convergence, divergence and transform motion. Convergence leads to an active margin (and eventual collision); divergence results in the construction of a rifted (passive) margin; transform motion generates a transform (or sheared) margin. Although passive margins have been extensively studied1,2, and many models have been proposed for their origin and subsequent evolution3–5, little is known about transform margins, with the exception of a few studies of their morphologies6,7, shallow structures or crustal sections8,9. Here we present the main conclusions derived from a recent study of the northern Gulf of Guinea margins, particularly off the eastern Ivory Coast and Ghana, where the continental margin is one of the best-preserved examples of an extinct transform margin10–12. Our observations support a four-stage model for transform margin evolution. Technically active transform contacts, first between normal continental crusts and then between thinned margins, induce characteristic structures such as pull-apart grabens and shear folds. The next stage, in which thermal exchange between oceanic and continental lithos-pheres controls a complex subsidence, is followed by the transition to a true intra-oceanic fracture zone.

A sequential back-arc and foreland basin thrust duplex model for the Southern Uplands of Scotland

Abstract: Thrust imbrication of Ordovician and Silurian submarine fan sequences overlapping pelagic deposits in the Southern Uplands has been interpreted in terms of an accretionary prism formed above a NW-directed subduction zone. Structural features invoked to support accretion are not definitive and could be explained in terms of a thin-skinned thrusting model. New palaeocurrent and compositional evidence from Llandeilo to mid-Llandovery age turbidites in the northern part of the Southern Uplands proves interdigitation of sediments with strongly contrasting petrography. Turbidites derived from the south contain significant quantities of fresh andesitic detritus whereas those from the north form more mature quartz-rich formations. This implies a back-arc situation; the turbidites being deposited in a basin with a relatively mature continental landmass to the north and a rifted continental fragment containing an active volcanic arc to the south. Oblique collision of the opposing continental margins of the Iapetus Ocean during the Llandovery caused the cessation of subduction. Underthrusting of the southern margin initiated a SE-propagating thrust stack which deformed the back-arc basin sequence and may eventually have ramped over the eroded and faulted remains of the volcanic arc. A southward-migrating foreland basin formed ahead of the rising thrust stack and is now represented by the late Llandovery Hawick Group and Wenlock sequences. Mid- to end-Silurian sinistral strike-slip resulted from oblique collision and produced a transpressional regime during which reactivation of deep-seated structures allowed the intrusion of lamprophyre dykes and granites.

The transition from a passive margin to an Upper Cretaceous foreland basin related to ophiolite emplacement in the Oman Mountains

Abstract: The Upper Cretaceous Muti Formation in the Oman Mountains exemplifies the transition from a passive continental margin to a foreland basin related to thrusting and ophiolite emplacement. Field data largely substantiate theoretical models that depict a flexural forebulge moving continentward of an advancing thrust-load, followed by flexural downwarping and collapse as nappes override a continental margin. Five units of the Upper Cretaceous (? late Cenomanian–late Campanian) Muti Formation depositionally, or structurally, overlie Mesozoic passive-margin paleoenvironments ranging from abyssal plain to continental rise, slope, and carbonate platform. The Arabian continental margin and Tethys ocean rifted in Permian and Triassic time, giving rise to a mature passive margin in the Jurassic and Lower Cretaceous. At the end of Lower Cretaceous time, northeastward subduction apparently began, and the Semail ophiolite formed, probably in a marginal basin setting. As subduction continued, the trench migrated toward the Arabian continent preceded by a flexural fore-bulge that soon reached the adjacent Arabian continent (Cenomanian-Turonian; ca. 90−88.5 Ma), halting deposition regionally (“Wasia-Aruma break”). Flexural uplift was greatest near the shelf edge, with removal of as much as 600 m of the platform edge succession, and redeposition onto the base of slope as slump sheets, debris flows, and lithoclastic turbidites. As the advancing thrust-load impinged on the edge of the Tethys ocean, the crust was downflexed (late Coniacian–Campanian time; 88.5−73 Ma), creating a foredeep that partly filled with terrigenous clastics, olistostromes, detached blocks of rift-related carbonate build-ups, seamount volcanics and continental basement rocks. The basin depocenter migrated inboard with time, collapsing, in turn, Mesozoic abyssal plain, base of slope, and slope settings to establish a foreland basin on the foundered shelf. Terrigenous sediments accumulated in deep water below the carbonate compensation depth (CCD), together with lithoclastic limestones and detached blocks shed from submarine fault scarps. Successive foreland basins were overridden by the advancing nappes. During the later stages of thrusting onto the continent, Mesozoic sediment decollement nappes were bulldozed ahead of the Semail nappe, ploughing into the existing foreland basin. In conjunction with the final emplacement of the Semail ophiolite, the foreland basin migrated to its most continentward location and filled with clastics derived by subaerial erosion of the Semail nappe and other units. The nappes were submerged again in late Maastrichtian time, possibly in response to flexural relaxation, followed by re-establishment of a carbonate platform. Further compression and thrusting in the Paleocene-Eocene gave rise to a successor foreland basin adjacent to the northern Oman Mountains front.

Nature and distribution of deformation across the Banda Arc–Australian collision zone at Timor

Abstract: Recently acquired seismic-reflection and SeaMARC II (side-scan and swath bathymetry) profiles near Timor show that the Banda Arc–Australia collision zone has a tectonic framework similar to that of a typical oceanic subduction system. Deformation is occurring, at present, most intensely at the foot of the inner slope of the Timor Trough. This deformation front is discontinuously advancing southward as new thrust slices develop within the subducted Australian margin strata. In contrast, present deformation is apparently negligible in the Savu Basin, the complex fore-arc basin north of Timor. A possible significant exception is a postulated right-lateral, northeast-trending fault zone offsetting the outer-arc high between Savu and Roti. Although back-arc thrusting has been documented north of the volcanic arc, this component of convergence is minor compared with the scale of ongoing deformation in the Timor Trough. The detailed nature of these surveys has also led to the recognition of along-strike variations in deformation in the Timor Trough and in the Savu Basin. These variations may be related to the variable degree of involvement of the Australian continental margin along the arc.

Early Cenozoic crust at the Norwegian continental margin and the conjugate Jan Mayen Ridge

Abstract: Multichannel seismic profiles at the Voring Plateau Margin off Norway and the northern Jan Mayen Ridge have provided a framework for the early Tertiary evolutionary history of these areas which were adjacent prior to the opening of the Norwegian Sea. We propose that their Paleogene evolution is quite similar and that the two acoustic basement reflectors, EE and JO, were formed in the early Eocene by extrusion of flow basalts. From the time of opening to about anomaly 23 time, oceanic seafloor was generated by Icelandic-type spreading. During that time, the Central Jan Mayen Fracture was the main transform. The seaward dipping reflector sequences are flow basalts generated by the Icelandic spreading. The existence of less well developed dipping sequences seaward of the primary ones probably reflects that these regions submerged later because of higher initial elevation. During the early Eocene subaerial spreading episode, which is related to the North Atlantic Volcanic Province, basalt flows also covered the adjacent thinned and heavily intruded continental crust. It is proposed that the continent-ocean boundary is located just landward of anomaly 24B, a short distance seaward of the Voring Plateau Escarpment. Reflector K is only recognized landward of this boundary in a region of “transitional crust.” The escarpment experienced syndepositional faulting, and the most elevated parts of the Voring Marginal High did not subside below shallow water depths before Oligocene/Miocene time. We suggest that the Voring Margin and the Jan Mayen Ridge represent “volcanic” passive margins. The evolution of this margin type may relate to initial uplift due to rifting in previously thinned crust.

Evolution of a Lower Paleozoic Continental-Margin Carbonate Platform, Northern Canadian Appalachians

Abstract: The western margin of the northern Appalachian orogene and adjacent craton is composed of a lower Paleozoic, low-latitude carbonate platform which originally lay along the northern margin of Iapetus Ocean. Parts of the platform interior are now exposed in Quebec, much lies beneath the Gulf of St. Lawrence, and the outer shelf and deep-water deposits crop out in western Newfoundland. The shelf break and upper slope are nowhere exposed, but their nature has been determined from numerous clasts in sediment gravity flows redeposited on the lower slope and now stacked in allochthonous thrust complexes. Four separate phases recording platform growth and demise can be differentiated, reflecting the interplay between tectonics, eustasy and the evolving lower Paleozoic biota. Phase 1, Preplatform Shelf, reflects initial siliciclastic deposition on rifted crystalline basement followed by a short phase of carbonate sedimentation dominated by archeocyathan buildups and ooid shoals that was terminated by offlap of thick quartzarenites. Phase 2, Narrow, High-Energy Platform, is characterized by mixed siliciclastic-carbonate peritidal sedimentation throughout, manifest as three grand cycles. Contemporaneous deep-water sediments comprise basal welded conglomerates overlain by quartzose carbonate turbidites. Phase 3, Wide Low-Energy Platform is an onlap package of muddy fossiliferous subtidal and peritidal carbonates arranged inmore » the form of two unconformity-bounded megacycles. The adjacent deep-water slope was a narrow belt of debris flows and wide carbonate-shale apron, deposited in the lower part of an oxygen minimum zone. Phase 4, Foundered Platform, documents the initial uplift, faulting, subsidence, and fragmentation of the platform in a sequence of peritidal to subtidal to deep-water carbonates, reflecting the initial stages of the Taconic orogeny.« less

Deep seismic reflection characteristics of the continental crust

Abstract: Four types of seismic reflection “fabrics” appear to characterize the continental crust: (1) a cratonic fabric dominated by diffractions and dipping reflections but lacking a pronounced reflection Moho; (2) regions of thin-skinned foreland deformation with abundant shallow but sparse deep reflections; (3) a unidirectional fabric that dips toward the hinterland and may represent a crustal-scale ramp zone; and (4) zones of highly layered lower crust and high-amplitude reflection Moho. The first three types are commonly observed in transects across single mountain belts, but the fourth appears to correspond to regions of elevated lower crustal temperatures such as those encountered in rift provinces and perhaps elsewhere.

Timing the breakup of a Proterozoic supercontinent: Evidence from Australian intracratonic basins

Abstract: In the Late Proterozoic, several broad, shallow, intracratonic depressions appeared across a vast area of central Australia. The basins, which all contain shallow marine to fluvial successions, appear to have been tenuously interconnected through much of their history. Analysis of their fill by means of tectonic-subsidence curves suggests that they are the product of two separate and distinct periods of crustal extension, one at about 900 Ma and a second at about 600 Ma. These extensional episodes were probably the result of failed rifting events that almost fragmented the Australian continent during the Late Proterozoic. The second period of extension almost certainly relates to the breakup of a Proterozoic supercontinent. The results suggest that sediments preserved in the relatively protected environment of interior basins may provide a more subtle record of major tectonic events than continental margin sequences that were exposed to major continental interactions.

Restoration of the Caledonian Baltoscandian margin from balanced cross-sections: the problem of excess continental crust

Abstract: Consideration of six balanced cross-sections through parts of the Finnmark Caledonides, N Norway indicates that shortening varies between 25% and 75%. A restored long cross-section across the width of the orogen, constructed with the aid of a branch line map, demonstrates a foreland propagating thrust system, with earlier formed more internal metamorphic nappes thrust SE 330 km under ductile conditions and then carried piggyback ESE a further 296 km on later brittle thrust sheets. Total shortening is 78·7% with a translation of the most internal thrust sheet of 626 km.The restored section suggests that: (1) the rate of propagation of deformation from hinterland to foreland is c. 2·27 cm y−1; (2) incorporation of basement into the nappes resulted from inversion of extensional faults formed during Iapetus rifting; (3) during rifting a Finnmark basement ridge separated a 220 km wide southeasterly Gaissa basin from the passive Iapetus continental margin which was at least 423 km wide; (4) the Finnmark Caledonides resulted from a continent-microcontinent collision which obducted continental crust at least 600 km across the Baltic margin; and (5) the Caledonian Baltoscandian margin prior to Iapetus suturing extended at least 400 km W of the Norwegian coast. On a Bullard reconstruction this overlaps with Laurentian rocks in Greenland. The excess continental crust is accounted for by shortening of the Baltoscandian margin during collision.

Extensional structures on the western UK continental shelf: a review of evidence from deep seismic profiling

Abstract: Summary BIRPS deep seismic reflection data collected on the western United Kingdom continental shelf show the existence of a wide variety of sedimentary basins, most of which originated during Palaeozoic-Mesozoic crustal extension. Symmetrical interior-fracture basins are numerous, but more complex basins are also common and show the importance of fault reactivation and the influence of pre-existing structures on basin development. All of the major basement-penetrating faults are interpreted as having been reactivated. They do not visibly cut through the entire crust and into the upper mantle. The crystalline crust thins dramatically beneath the basins indicating local crustal extensions of up to 60%, although regional extension is less than 30%. The typical BIRPS profile shows a highly reflective lower crust sandwiched between an unreflective upper crust and upper mantle. This pattern of reflectivity appears to be characteristic of deep seismic data collected from within extended regions. The relatively small amount of extension which has affected this region suggests that the highly reflective lower crust is more likely to be due to lithological variation formed by mafic igneous intrusion and underplating during crustal extension than to-extensional ductile fabrics within the lower crust.