Continental Plateau Research Articles

Cybertectonic Earth and Gaia's weak hand: sedimentary geology, sediment cycling and the Earth system

Tectonics, climate and sea level are dominant controls on the nature and distribution of sedimentary environments. By inverse logic, recognition of influences on sedimentation from the stratigraphic record as a result of perturbations in these variables leads to clarification of past environmental and structural change. The succession of sedimentary environments in time and place is determined by a wider combination of variables; namely, sediment flux, biological evolution, atmosphere and water body productivity and chemistry. The feedbacks and interrelationships between all variables cause the complexity seen in sedimentation processes over geological time. The processes of global tectonics that cause widespread mountain belt and continental plateau uplift have produced numerous ‘severe’ events during Earth history: these often random workings-out of the plate tectonic cycle define the state of ‘Cybertectonic Earth’ (from cyber , after the Greek kubernan : to steer or govern). This has worked within a usually zonally arranged series of climatic belts and within the framework provided by biological evolution. At certain times in the geological past, a combination of factors arose as a response to continental uplift and have acted to cause certain Earth surface conditions and variables (notably mean global surface temperature, atmospheric p CO 2 , p O 2 ) to have varied by very large amounts (×2–×10) compared with present values. These large fluctuations, such as those responsible for the Neoproterozoic, Late Palaeozoic and late Tertiary glaciations, lead one to doubt the reality of homeostatic control of surface conditions as proposed in Lovelock's Gaia hypothesis. The Gaian kuberne0te0s (steersman) had a weak hand on the helm, frequently unable to prevent vast areas of the globe experiencing rapid fluctuations in environmental conditions and inimical conditions to life for very long periods during Neoproterozoic and Phanerozoic times. At the same time, biogenic and abiogenic processes have proved capable of returning Earth to states of mean stability, although biogeochemical cycling models seem alarmingly ad hoc and largely untestable as scientific hypotheses in any true geological sense.

Hydrogen Isotopes in Eocene River Gravels and Paleoelevation of the Sierra Nevada

We determine paleoelevation of the Sierra Nevada, California, by tracking the effect of topography on precipitation, as recorded in hydrogen isotopes of kaolinite exposed in gold-bearing river deposits from the Eocene Yuba River. The data, compared with the modern isotopic composition of precipitation, show that about 40 to 50 million years ago the Sierra Nevada stood tall (>/=2200 meters), a result in conflict with proposed young surface uplift by tectonic and climatic forcing but consistent with the Sierra Nevada representing the edge of a pre-Eocene continental plateau.

A variable resolution surface wave dispersion study of Eurasia, North Africa, and surrounding regions

This paper presents the results of a large‐scale study of surface wave dispersion performed across Eurasia and North Africa. Improvements were made to previous surface wave work by enlarging the study region, increasing path density, improving spatial resolution, and expanding the period range. This study expands the coverage area northward and eastward relative to a previous dispersion analysis, which covered only North Africa and the Middle East. We have significantly increased the number of seismograms examined and group velocity measurements made. We have now made good quality dispersion measurements for about 30,000 Rayleigh wave and 20,000 Love wave paths and have incorporated measurements from several other researchers into the study. We have improved the inversion from the previous study by adopting a variable smoothness with the conjugate gradient method for the group velocity tomography. This technique produces higher‐resolution models where the concentration of data allows it without producing artifacts. The current results include both Love and Rayleigh wave inversions across the region for periods from 7 to 100 s on a 1° × 1° grid and at resolutions approaching 1° under some conditions. Short‐period group velocities are sensitive to slow velocities associated with large sedimentary features such as the Caspian Sea, West Siberian Platform, Mediterranean Sea, Bay of Bengal, Tarim Basin, and Persian Gulf. Intermediate periods are sensitive to differences in crustal thickness, such as those between oceanic and continental crust or along orogenic zones and continental plateaus. At longer periods, fast velocities are consistently found beneath cratons, while slow upper mantle velocities occur along rift systems, subduction zones, and collision zones such as the Tethys Belt. We have compared the group velocities at various periods with features such as sediment thickness, topographic height, crustal thickness, proximity to plate boundaries, lithospheric age, and lithospheric thickness, and we find significant correlations. We have developed an empirical relationship between long‐period group velocity and lithospheric thickness that works well for both oceanic and continental lithosphere. We do not find any similar correlation between the longest‐period surface waves and hot spots.

Crustal structure beneath Discovery Bank in the Scotia Sea from group velocity tomography and seismic reflection data

The Bruce, Discovery, Herdman and Jane banks, all located along the central-eastern part of the South Scotia Ridge, represent isolated topographic highs, surrounded by young oceanic crust (∼5–23 Ma), whose petrological and structural nature is still the subject of speculation due to the lack of relevant data. Surface wave tomography in and around the Scotia Sea region, performed using eight broadband seismic stations and 300 events, shows that the central-eastern part of the South Scotia Ridge is characterized by negative surface wave group velocity anomalies as large as 6% in the period range from 15 s to 50 s. The spatial resolution of our data set (∼300 km) makes it possible to study a specific area (centred at 61°S and 36°W) that includes Discovery Bank and appears to show dispersion characteristics similar to those found beneath the northern tip of the Antarctic Peninsula and southern South America. Surface wave dispersion curves are inverted to obtain 1-D isotropic shear wave velocity profiles that suggest a continental nature of Discovery Bank. Crustal thickness is in the range 23–28 km with a sub-Moho velocity of 4.1–4.2 km s−1. The boundaries of the negative group velocity anomalies are marked by a high level of seismic activity. The depth of the events and their large seismic moment suggest the presence of continental lithosphere. The continental-type crust of this topographic relief is supported by our interpretation of multichannel seismic reflection profiles acquired across this rise, where the observed seismic structures are interpreted as thinned and faulted continental plateau.

Partial melting and flow of orogens

A comparison of large orogenic belts formed at convergent plate boundaries indicates that orogenic evolution comprises a period of crustal thickening followed by the formation of a continental plateau and commonly syn- to post-convergence extension and thinning of the previously thickened crust. The instantaneous and finite strain patterns recorded in orogenic belts do not appear to be linked in a simple way to plate kinematics. These patterns suggest that deformation of continental crust is not solely controlled by horizontal compression due to plate convergence and that gravity-driven flow is likely to play a significant role on the deformation of thickened crust. Geophysical data in southern Tibet, an actively convergent orogen, and geologic data of the exhumed mid-crustal core of eroded orogens reveals that orogenic crust is affected by widespread partial melting. Partial melting of thermally mature zones of thickened crust results in the generation of a layer of low-viscosity rocks that may affect the behaviour of the orogenic belt. In particular, the presence of this layer favours mechanical decoupling between the subducting plate and the overlying thickened orogenic crust, resulting in flow of the crust driven by contrasts in gravitational potential energy.

Reviews

Internal-tide and internal-wave deposits are a new and important type of deep-water traction-current deposits. Based on an analysis of documented internal-tide and internal-wave deposits, five facies types can be recognized: ( i ) bidirectional cross-laminated sandstone fades, ( ii ) unidirectional cross-bedded and cross-laminated sandstone facies, ( Ⅲ ) rhythmic alternating sandstone facies and mudstone, (iv ) flaser, wavy and lenticular bedded foraminiferal limestone facies, (v ) sandy oolitic limestone facies with flaser, wavy and lenticular bedding and epsilon cross bedding, which formed in submarine channel, unchannelized continental slope and plateau environments respectively. Accoding to the available data and knowledge, three sedimentation models are built; some discriminating characteristics of internal-tide and internal-wave deposits, and the distinction between them and contourites and turbidites are summed up.

Nd–Sr Isotopic composition of present-day sediments from the Gironde Estuary, its draining basins and the WestGironde mud patch (SW France)

The isotopic composition of silty clay sediments in the Gironde Estuary, which were sampled between 1986 and 1995, shows homogeneity. The sediments from the Garonne and the Dordogne Rivers, including the tributary Isle River, are well mixed within the turbidity maximum zone. The composition of the clay solid load of the upstream Garonne River was also homogeneous between 1985 and 1990, even though there is a large diversity and temporal variabilities in the floods of the many tributaries of this river. Isotopic data were used to calculate the silty clay contribution of the two main rivers to the total estuarine stock: 70% of the total material is brought in by the Garonne River and 30% by the Dordogne River, including its tributary (the Isle River). The same isotopic signature can be observed in the present-day sediments and suspended materials of the WestGironde mud patch, on the adjacent Aquitaine continental plateau.

Controls on Late Quaternary sedimentation of the South Tasman Rise

The South Tasman Rise is a submarine continental plateau of about 1000–3000 m depth, in the Southern Ocean south of and separated from Tasmania by a saddle more than 3000 m deep. Its palaeoceanographic importance is that it lies between the subtropical convergence and the polar front and that latitudinal migration of these oceanographic features should be recorded in the calcareous oozes deposited on the rise. Late Quaternary sediment cores collected from the South Tasman Rise record climatically and current‐controlled changes in sedimentation. Accelerator mass spectrometry 14C dating of down‐core variations in the carbonate concentration demonstrates that the core‐top carbonate maxima are Holocene and down‐core minima are successive glacial intervals. During glaciations the percentage of carbonate decreases, whereas the percentage of detrital minerals (terrestrial input) increases. The magnitude of climatically related signatures expressed in the sediments is a function both of proximity to terrestrial input from the north and of overlying water depth. Cores from the saddle separating the South Tasman Rise from Tasmania have large terrestrial input and high variations in carbonate concentration. Shallower water cores from the rise are consistently and strongly influenced by winnowing in both glacial and interglacial intervals, whereas deeper cores appear to be more influenced by carbonate dissolution during glaciations.

Study of the simultaneous physical retrieval method for meteorological parameters over the continental plateau of China

It is common knowledge that continental retrieval especially for Qinghai-Xizang Plateau has not been solved to date. In order to explore applicable inverse model and method for continent including the plateau, in this study authors use an improved simultaneous physical retrieval method hereafter referred to as the ISPRM, for computing meteorological parameters from NOAA-10 satellite TOVS data. The retrieval results verified by nearby radiosondes show that the ISPRM is more applicable for the continental plateau.

Continental extension of the Pacific Plate at the southern termination of the Hikurangi subduction zone: The North Mernoo Fault Zone, offshore New Zealand

A complex transition from subduction at the Hikurangi margin to transpressive, continental collision across the Southern Alps occurs in the NE South Island region of New Zealand. Offshore, the southern limit of oceanic subduction is constrained by the Chatham Rise, a submerged continental plateau on the Pacific Plate, which is presumably too buoyant for normal subduction. In the NW corner of the rise is the North Mernoo Fault Zone (NMFZ), a 100 by 300 km region of basement‐involved, active normal faulting of late Miocene (circa 8–6 Ma) to Recent age. The NMFZ evolved contemporaneously with the rotation and development of the adjacent plate boundary zone. Growth faulting is characteristic within the sedimentary cover, which is generally less than 2 km thick, but the distribution of faulting has varied temporally. The amount of mid Pliocene‐Recent extension is estimated to be not more than a few percent. The NMFZ consists of an array of overlapping, south dipping normal faults, which are typically 2–5 km apart and which trend roughly E–W at a high angle to the plate boundary zone. Late Quaternary surface traces are widely distributed on the mid to upper continental slope, but many surface scarps are poorly preserved due to extensive erosion of the seafloor. The western end of the fault zone crosses submarine canyons at the southern end of the Hikurangi Trough and almost abuts two fold and thrust fault systems on the NE South Island continental margin. Although few data constrain the kinematics of extension, a single MW 6.1 earth quake at the western end of the NMFZ suggests planar rotating faults with a component of left‐lateral oblique slip. Two possible models of regional extension are considered; (1) lateral buckling of the upper crust, in which faulted slivers of crust are peeled off as the Chatham Rise slides past the transpressive Marlborough fault system toward the Southern Alps and (2) flexure of the edge of continental Pacific Plate as the Chatham Rise is bent downward into the southern end of the Hikurangi subduction zone. Improved seismological data and deep crustal seismic reflection profiles may help to constrain the kinematics of deformation.

Modeling sea level changes as the Atlantic replaces the Pacific: Submergent versus emergent observers

We evaluate the sea level changes caused by changing the age/area relationship of the seafloor by altering the thermal structure and hence the elevation of the world seafloor. We do this by having a passive‐margined (and therefore continuously aging) “Atlantic‐type” ocean open at a uniform rate at the expense of an active‐margined (and therefore constant age) “Pacific‐type” ocean. This run of the model was specifically constructed to answer the question of how much sea level would change if the Atlantic opened at a uniform rate. Therefore it specifically excludes the other major processes that change sea level. These would include changes in ice volume, other changes in plate generation/consumption rates, continental orogeny and epeirogeny, lithospheric stretching, lithospheric flexure, and thermally induced subsidence at passive margins and amplification thereof caused by water and sediment loading. The isostatic effects of continental flooding and oceanic unloading are accommodated by application of balanced global isostasy to an Earth model consisting of 121 independent columns. The lithospheric portions of the columns are linked to the hypsographic curve. Sea level changes are viewed from two different observational positions: (1) geologically stable and continually emerged continental plateau and (2) geologically stable and continually submerged shelf break. The modeled sea level that results from changing the age/area relationship of the seafloor since 160 Ma peaks at ∼85 Ma and is ∼30% less for the emerged observer (34 m) than for the submerged observer (48 m). Furthermore, the peak is in phase with, but only ∼1/4; (48 m versus 200+ m) of that portrayed by either the “eustatic” curves inferred from seismic stratigraphy or the isostatically readjusted positions of marine strandlines of the stable platform of North America. We therefore conclude that the other 3/4 of sea level rise that accompanies the opening of the Atlantic must be partitioned among the other causes suggested above.

Geologic Framework of Oil and Gas Genesis in Main Sedimentary Basins from Romania Oprea Dicea

Oil and gas fields located in Moldavic nappes are encompassed in Oligocene and lower Miocene formations, mostly in the marginal folds nappe, where Kliwa Sandstone sequences have high porosity, and in the Black Sea Plateau. The origin of the hydrocarbon accumulations from the Carpathian foredeep seems to be connected to the Oligocene-lower Miocene bituminous formations of the marginal folds and sub-Carpathian nappes. In the Gethic depression, the hydrocarbon accumulations originate in Oligocene and Miocene source rocks and host in structural, stratigraphical, and lithological traps. The accumulations connected with tectonic lines that outline the areal extension of the Oligocene, Miocene, and Pliocene formations are in the underthrusted Moesian platform. The hydrocarbon accumulations related to the Carpathian foreland represent about 40% of all known accumulations in Romania. Most of them are located in the Moesian platform. In this unit, the oil and gas fields present a vertical distribution at different stratigraphic levels, from paleozoic to Neogene, and in all types of reservoirs, suggesting multicycles of oleogenesis, migration, accumulation, and sealing conditions. The hydrocarbon deposits known so far on the Black Sea continental plateau are confined in the Albian, Cenomanian, Turonian-Senonian, and Eocene formations. The traps are of complex type structural, lithologic, andmore » stratigraphic. The reservoirs are sandstones, calcareous sandstones, limestones, and sands. The hydrocarbon source rocks are pelitic and siltic Oligocene formations. Other older source rocks are probably Cretaceous.« less

Neotectonic deformation of the southern Puna Plateau, northwestern Argentina

Continental plateaus have been likened to pressure gauges that record the mechanical state of the lithosphere involved in mountain building. We outline the kinematic state of the southern Puna Plateau and bounding foreland provinces of the NW Argentine Andes based on surficial fault slip data and crustal seismicity. Kinematic analysis of about 350 faults, measured in a transect from the northern Sierras Pampeanas into the southern Puna, document a relatively uniform late Tertiary history of thrust faulting and vertical extension, followed by a more variable Quaternary history. During the latter, the plateau has undergone horizontal extension on strike-slip and normal faults whereas the thick-skinned foreland continues to experience thrust kinematics. The extension in the plateau, however, is not perpendicular to the trend of the Andes, but sub-parallel to it. A regional survey of all available kinematic data in the Central Andes between 12° and 32°S shows a similar style of kinematics characterizes the northern termination of the Altiplano in southern Peru and northernmost Bolivia. Although kinematic data do not exist for most of the Bolivian Altiplano, the lack of crustal earthquakes and scarcity of modern fault scarps tentatively suggest that the neotectonic activity is concentrated at both ends of the plateau. We interpret this apparent concentration in terms of an edge effect, related to along-strike narrowing topography and diminishing foreland shortening, which in turn are probably related to variations in the underlying geometry of the subducted Nazca Plate.

Concluding remarks

The focus of this discussion meeting is strongly connected to that of the history of continental crust. I was reminded of G. K. Gilbert (1893), who said that 'the permanence of the continental plateau, though highly probable, is not yet fully established; and the doctrine of continental growth, though generally accepted, has not been placed beyond the field of profitable discussion’. Recently, Kerr (1985) remarked that we increasingly see continents as a ‘collage of wandering fragments’, and this present discussion has been most concerned with processes associated with this model. I think we are sometimes confused by what is formed, when we observe what is preserved. Continental metamorphism presents us with a great puzzle. Given the present heat flow, stable continental regions have temperatures little above 400 °C at Moho depths; the continents should be dominated by facies of the lowest grades. Yet continental rocks show an amazing diversity of P—T regimes, far exceeding any normal range. Verhoogen (1980) wrote ‘as deformation and orogeny are commonly associated with regional metamorphism, orogeny should perhaps be described as a thermal disturbance, rather than a mechanical one’. Modern metamorphic studies involve the central theme of tracking the convective style of the earth through time. We are now entering a new period of observation, where deep continental structure is being refined by seismic and electrical methods, while actual plate motions can be observed by satellites. Many of the metamorphic terranes discussed at this meeting involve subduction and collision-related phenomena. Recent studies of the subduction process (Uyeda 1983; Yorath et al. 1985; Kaiko Staff 1985) are beginning to elucidate the critical problems of the mechanics of subduction and the materials involved (even serpentine and diamonds; see, for example, Schulz 1986; Ozima et al. 1985). Such studies feed back to the complex paths now being revealed by the metamorphic record. Rocks show us the range of physical conditions, and place some constraints on time, while modern geophysics can show the mechanisms. We are reaching the point where petrology and geophysics are joining to produce sound models of dynamics and thermal history.

Some tests of precision for a finite element model of ocean tides

A finite element method is presented to solve the problem of ocean tides. A spectral (in time) model is implemented by deriving a second order partial differential equation from the classical shallow-water equations. The influence of several key factors on the precision of the method are investigated: density of triangles, degree of approximation and numerical integration, computer costs. Two analytical solutions are used as a reference: a damped Kelvin amphidrome in a channel of constant depth over a rotating earth, and a tidal wave propagating from the deep ocean over a continental plateau. The efficiency of the P2-Lagrange approximation is clearly demonstrated in terms of precision and computer costs. A criterion is established to guarantee a given precision, relating the basic grid size of the triangulation to be used to the typical wavelength of the tidal wave. An application of the model to the solution of a problem including a variable topography, rotation, nonlinear bottom friction and a tide-generating potential forcing is presented as a final complex test.

Volcanic and geochemical stratigraphy of the Nussir Group of Arctic Norway—an early Proterozoic greenstone suite

The Nussir Group of West Finnmark is a suite of metabasaltic lavas and tuffs about 1.8 km thick, associated with continental and shallow marine sediments. This assemblage is typical of Karelian (Early Proterozoic) supracrustal sequences throughout northern Norway and northernmost Sweden. Primary textures are well preserved and permit the establishment of a volcanic stratigraphy for the Nussir Group. The Krokvatn Formation is dominated by mafic aquagene tuffs while the overlying Svartfjell Formation is composed mainly of submarine pillow basalt. Petrographic evidence for the petrogenesis of the metabasalts is poor because of greenschist facies metamorphism during the Svecokarelian Orogeny c . 1840 Ma. The earliest lavas have the most primitive compositions, but subsequent flows exhibit strong enrichment in Fe, Ti and incompatible elements as a consequence of extensive crystal fractionation. The tholeiitic nature of the suite is confirmed by the trace element data. The later lavas are more uniform and less evolved: and are considered to represent steady-state compositions produced by a periodically replenished/tapped open-system magma chamber. The field relationships, lithological and geochemical evidence suggest that the Nussir Group lavas were erupted in a submarine rift basin within continental crust. They form part of an extensive tholeiitic magmatic province which occupied much of northern Norway, Sweden and Finland in the early Proterozoic. The tectonic environment of this province is poorly constrained by the geochemical data. Continental plateau basalt provinces such as the Columbia River and Karoo provide the closest Phanerozoic geochemical analogues.

Accretionary processes at subduction zones in the eastern mediterranean

The tectonic history of the eastern Mediterranean as well as the present tectonic style are affected by the collision and accretion of oceanic plateaus at subduction zones. Small but distinct oceanic plateaus exist today in the eastern Mediterranean away from the plate boundary. Many other oceanic plateaus crowd the plate boundary, particularly in the Hellenic arc area. Their presence results in the unusual complexity of this subduction zone, as is evidenced by the seismicity and bathymetry patterns. The present two arcs system in this region, the Hellenic and Cyprean arcs, is probably the consequence of the collision of a large continental plateau with an older arc. This plateau has been thrusted over the continental mass of Anatolia and pieces of it are to be found in the region between the two arcs. Active subduction in the Hellenic arc takes place in the outer trenches, particularly in the Ionian and Strabo trenches. Subduction of Mediterranean lithosphere in the Pliny and Ptolemy trenches is limited to their western parts. Inner segments of these trenches served as the main subduction trenches before the subduction zones migrated outward following the collision of oceanic plateaus.

Gas Hydrates Beneath Continental Slope off Northern California: ABSTRACT

The continental slope and plateau off northern California are underlain by a well-defined and extensive acoustic reflector that crosses other reflectors and mimics the surface of the sea floor. This bottom-simulating reflector (BSR) lies at an average subsurface depth of 250 m and is laterally continuous beneath the Klamath Plateau (water depths of 800-1,200 m) off northernmost California. Limited data show that it extends northward into offshore Oregon and seaward at least to the base of the slope (3,000 m water depth). The BSR has been mapped along the continental margin for a distance of over 130 km and underlies an area of at least 3,000 km2. The water depths and sediment depths of the BSR, as well as its pervasiveness, all suggest that it represents the base of a natural-gas hydrate. Using standard phase-boundary diagrams for hydrate stability, we estimate that the geothermal gradient in this area is about 50°/km. Such a gradient is higher than found along most subduction margins, possibly because the subducting oceanic crust in this region is young. This acoustically inferred gas hydrate is the first mapped along the western United States conterminous continental margin. End_of_Article - Last_Page 255------------

Termite Mounds on the Continental Plateau in the SE Region in Zaire

Termite Mounds on the Continental Plateau in the SE Region in Zaire

A transect study of forests and woodlands on dolerite in the Eastern Tiers, Tasmania

A transect study of variation in eucalypt-dominated vegetation on the dolerite of the Eastern Tiers at 42°S revealed two sets of species; one set being largely confined to the high altitude stands on the relatively continental plateau and the other being largely confined to the seaward slopes of the Tiers. Within each of these sets the eucalypts have overlapping distributions that relate to moisture availability, except for E. barberi and E. ovata which occupy sites with impeded drainage. The density of the large size-class trees increases with increasing inferred precipitation. However, recent fire and grazing history affects the density of the smaller size classes. A floristic classification of the transect sites proved similar to a classification based on dominance. However, there is no exact correspondence between understorey communities and the eucalypt dominants. Treeless vegetation and closed-forest dominated by Atherosperma moschatum also occur in the Eastern Tiers.