Isostatic Model Research Articles

月の詳細地形(DTM)から推定されたグラーベンの地下構造モデル

月の詳細地形(DTM)から推定されたグラーベンの地下構造モデル

Lithospheric structure of the Gorringe Bank: Insights into its origin and tectonic evolution

[1] The Gorringe Bank is a 5000 m high seamount near the Atlantic coast of Iberia characterized by a 9 m high geoid anomaly and a ∼120 mGal Bouguer anomaly relative to the surrounding abyssal plains. It has been linked to a NW directed thrust carrying exhumed upper mantle rocks and transitional crust on top of flexed-down Eurasian oceanic crust along the Tagus Abyssal Plain. However, estimations of crustal shortening have yielded dissimilar results, and the deep structure of the ridge remains highly unknown. We present a restored cross section and a new model of the lithospheric structure based on gravity, geoid, elevation, and the presence of serpentinized peridotites. At least 20 km of shortening took place along a flat-ramp-flat thrust fault, and the density structure of the lithosphere is consistent with mantle serpentinization varying from 70% at the surface to 20% at 14 km depth and 0% at 40 km. The topographic relief and gravity anomalies are explained by assuming a flexural isostatic model with an elastic thickness Te of ∼30 km. The evolution of the Gorringe Bank since the Late Jurassic is interpreted in relation to Eurasia-Africa-North America plate motion in four stages: (1) transtension between Newfoundland-Iberia and Africa, which generated small oceanic basins and mantle exhumation; (2) opening of the North Atlantic and seafloor spreading at the NW side of the exhumed Gorringe, which produced gabbro intrusions and serpentinization; (3) a quiescent tectonic period dominated by subsidence and sediment accumulation; and (4) a transpressional plate boundary between Eurasia and Africa with NW directed subcrustal thrusting and generation of the present Gorringe relief.

Long-term rift evolution

Rift systems represent one of the most remarkable geomorphological features on Earth. Hydrocarbon resources can be found within deep rift basins, whereas rift flanks form high morphological ridges influencing climate, vegetation, and fauna (Rosendahl 1987; Ziegler 1992; Ruppel 1995; Ziegler and Cloetingh 2004; Sepulchre et al. 2006; Trauth et al. 2007). Rift systems are the perfect laboratory to understand how our planet is changing dynamically through time and how mantle processes, volcanism, rock and surface uplift, erosion and sedimentation, climate, and fauna are ultimately linked. This special issue arises from a session at the EGU 2008 and presents a broad range of papers on long-term rift evolution spanning from the Asthenosphere through the Lithosphere all the way to climate-related processes. Seven papers focus on the evolution of the Albertine Rift System situated in the western branch of the East African Rift System and are the outcome of a DFG (Deutsche Forschungsgemeinschaft) funded Forschergruppe ‘‘RiftLink’’. Another six contributions focus on the Cheb basin, the Brazilian continental margin, the East Barents Sea Basin, the East African Rift System or on rifting processes in general. The contributions consider mantle to climate models: contributions by Semprich et al., Wallner and Schmeling, and Stamps et al. present mantle and lithospheric modelling studies of extensional environments; contributions by Babuska et al., Wolbern et al., and Link et al. treat mantle-crust relations and interactions; contributions by Bauer et al., Franco-Magalhaes et al., Sachau and Koehn, Koehn et al., and Roller et al. study crustal tectonics, rock and surface uplift histories and sedimentary environments; and the contributions by Brachert et al. and Kaspar et al. investigate climate proxies and climate models. Semprich et al. show with a systematic series of density diagrams, which are based on thermodynamic calculations that density changes are crucial for isostatic models in geodynamics. These authors illustrate that buried metapelitic rocks rich in Fe and Mg experience much larger density changes than dry basalt indicating that metamorphism of a hydrous lower crust leads to significant subsidence. The implications of these calculations are discussed in detail for different geodynamic settings. Wallner and Schmeling present a geodynamic modelling study of rift-induced delamination of Mantle Lithosphere (RID) in order to explain the extreme elevation of the Rwenzori Mountains in the western branch of the East African Rift System. These authors argue that the rift systems that surround the mountains trigger the delamination and that they are reducing the viscosity and strength of the lower crust around the horst. In the model, the weakened zone becomes unstable and a block of mantle material and lower crust sinks into the Asthenosphere leading to isostatic uplift of the overlying continental crust. A variation of modelling parameters is presented in order to illustrate under what conditions delamination takes place. Stamps et al. present a modelling study in order to determine if gravitational stresses are large enough to break the continental lithosphere in East Africa. They use a thin sheet approach to compute vertically averaged deviatoric F. U. Bauer U. A. Glasmacher Institute of Earth Sciences, University of Heidelberg, Im Neuenheimber Feld 234, 69120 Heidelberg, Germany e-mail: Friederike.Bauer@geow.uni-heidelberg.de

Forced regressive shoreface sandstone from Himalayan foreland: Implications to early Himalayan tectonic evolution

Sedimentology and sequence stratigraphic analysis of the ∼ 31 Ma old marker White sandstone unit from the Subathu Sub-basin, NW Himalayan foreland, suggest it to be a forced regressive wedge (FRW) formed during the transition from the marine Subathu Formation to the continental Dagshai Formation. The FRW is bounded between the “Surf diastem” below and type 1 unconformity at the top and differs from RSME (regressive surface of marine erosion, occurring below) bounded FRWs described from other classical coastal/foreland settings. Correct identification of bounding surfaces of a FRW has an important implication to the estimation of rate of relative sea-level (RSL) fall. A faster rate of RSL fall, higher than the sedimentation rate, has been postulated for the erosion of the lower shoreface and RSME. Using the logged thickness of the Subathu/Dagshai transition zone including the White sandstone (bounded between the “Surf diastem” and unconformity), available chronology and eustatic sea-level fall (0.023 mm/year at 31 Ma), a higher RSL fall than the sedimentation rate (0.07 mm/year) has been inferred during the deposition of the White sandstone. Petrography of sandstones and their Sr and Nd isotopic compositions indicate a major provenance switch-over from dominant mafic/ultramafic to metamorphic source from White sandstone (∼ 31 Ma) onwards attesting the link between hinterland tectonics, provenance and forced regression. The provenance switch-over at 31 Ma was earlier inferred to be driven by proto-Himalayan thrust propagation in the foreland. Using a simple isostatic model, on the contrary, a mechanism of accelerated surface uplift (at a rate of > 0.10–0.15 mm/year) is suggested for both provenance change and forced regression.

Isostasy of the Eastern Anatolia (Turkey) and Discontinuities of its Crust

This paper presents a probable isostatic model of the East Anatolian Region, which lies in a belt of significant plate movements. Probable locations of the horizontal and vertical discontinuities in the crust structure were determined using the normalized full gradient (NFG) method. For the purpose of explaining the mechanism that supports topography corresponding to the crust thickness in the region, calculations of effective elastic thickness (T e) were carried out initially by utilizing admittance and misfit functions. According to these results, the effective elastic thickness value obtained was less than the crust thickness, even though the isostatic model does not conform with the Airy model. Consequently, it was assumed that there could be problems beneath the crust. Hence, the NFG method was applied on the Bouguer gravity data of the region in order to investigate probable discontinuities in the crust structure. According to the NFG results, vertical structural transitions were observed at a depth ranging between 10 and 30 km, which begin immediately north of the Bitlis Zagros Suture Zone (BZSZ) and continue in a northerly direction. The relationship between the effective elastic thickness (T e; 13 km in average as determined in the last stage), and the seismogenic zone in the region was investigated. If the T e value happens to be less then the crustal thickness, then one can say that there are problems in the crustal structure of the region similar to Eastern Anatolia. Indeed, when NFG results of the study area are examined, numerous vertical and horizontal discontinuities in the crust can be observed. These discontinuities, which correspond to low Bouguer gravity anomalies and shallow focal depth-earthquakes, are probably the source of the factors which rule the tectonic mechanism of the region.

Aerogravity evidence for major crustal thinning under the Pine Island Glacier region (West Antarctica)

The West Antarctic Rift System provides critical geological boundary conditions for the overlying West Antarctic Ice Sheet. Previous geophysical surveys have traced the West Antarctic Rift System and addressed the controls that it exerts on the West Antarctic Ice Sheet in the Ross Sea Embayment. However, much less is known about the rift system under the Amundsen Sea Embayment, a key sector of the West Antarctic Ice Sheet, which is thinning significantly today. New aerogravity data over the Pine Island Glacier region, one of the fastest flowing glaciers within the Amundsen Sea Embayment, sheds new light into the crustal structure under this dynamic part of the West Antarctic Ice Sheet. Three-dimensional (3-D) inversion of terrain-decorrelated free-air and Bouguer gravity anomaly data reveal significant crustal thinning beneath the catchment of Pine Island Glacier. Under the Byrd Subglacial Basin and the newly identified Pine Island Rift, Moho depth is estimated to be 19 ± 1 km. This is the thinnest crust observed beneath the West Antarctic Ice Sheet. Estimates of lithosphere rigidity ( T e ), based on isostatic models, yield a T e of 5 ± 5 km, which is comparable to values from modern rift systems such as the Basin and Range Province. Major crustal thinning, coupled with low lithosphere rigidity, attest to the considerable impact of continental rifting beneath this part of the West Antarctic Ice Sheet. In analogy with the better known Ross Sea segment of the West Antarctic Rift System we suggest that the Amundsen Sea Embayment was affected by distributed Cretaceous rifting, followed by Cenozoic narrow-mode rifting. Narrow-mode rifting within the Pine Island Rift is particularly important as it may serve as a geological template for enhanced glacial flow associated with Pine Island Glacier.

Variable late Neogene exhumation of the central European Alps: Low‐temperature thermochronology from the Aar Massif, Switzerland, and the Lepontine Dome, Italy

Several recent studies proposed an important increase in exhumation rate in the western European Alps since circa 5–4 Ma. In order to assess potential spatial differences in exhumation histories, we present new apatite fission track (AFT) and apatite (U‐Th)/He (AHe) ages from the central Aar Massif (Guttannen area, Switzerland) and the western Lepontine Dome (Formazza area, Italy). Internal U/Th zoning in apatites explains alpha‐ejection‐corrected AHe ages that are older than the corresponding AFT ages in this study. A qualitative interpretation of AFT and AHe age‐elevation relationships suggests a two‐phase (9–7 and 5–3 Ma) exhumation scenario affecting the central Alps, with a stronger expression of the Pliocene signal in the Formazza area. However, a quantitative evaluation of exhumation scenarios using the 3‐D heat equation solver Pecube highlights the existence of several other likely scenarios, casting doubt on the validity of a qualitative interpretation of the age‐elevation relationships. In Formazza, scenarios suggested by quantitative modeling include continuous denudation at a rate of ∼750 m/Ma and a one‐step exhumation rate change from 300 to 1000 m/Ma at 5 Ma. In Guttannen, they include continuous denudation at a rate of ∼400 m/Ma with valley deepening and two periods of higher exhumation rate (increasing from 300 to 700 m/Ma repeatedly at 9–7 and at 5–3 Ma). Contingent upon further flexural isostatic modeling, the magnitude of exhumation recorded in the axial region of the Alps since circa 5 Ma does not appear sufficient to solely explain the denudation recorded in the North Alpine Foreland Basin.

A Geodetic View on Isostatic Models

Before the background of more accurate and denser gravity data it is worthwhile to reassess geodetic isostasy. Currently, in geodesy isostatic models are primarily applied to gravity reduction as needed by geoid and gravity modeling. The selection of the isostatic model is based on four criteria: Isostatically reduced gravity anomalies should be (1) geophysically meaningful, (2) easy to compute, (3) small, smooth and therefore easy to interpolate and (4) the indirect effect, i.e. the change of potential and gravity due to isostatic mass replacement, should be small. In this study we analyze free air anomalies as well as isostatic anomalies based on the Airy-Heiskanen model and on the Pratt-Hayford model in regard to these criteria. Several facts suggest that free air anomalies are the most realistic type of isostatic anomalies. They reflect the actual isostatic compensation, are easy to compute and their indirect effect is negligibly small. However, they are not smooth due to the fact that local topographic loads are only partially compensated. Smoothness can be achieved by introducing either a mathematical low-pass filter or a hydrostatic isostatic model, such as the Airy-Heiskanen or the Pratt-Hayford model. In both cases the resulting isostatically reduced gravity anomalies fulfill all requirements. In order to improve the numerical efficiency, a new mathematical description of the Pratt-Hayford model is formulated. The level of smoothing with respect to free air anomalies is analyzed in global and regional contexts. It turns out that the mechanism of mass compensation in regions of large topographic loads is better described by the Airy-Heiskanen model, whereas the Pratt-Hayford model is more suitable for regions of deep ocean trenches.

Airborne gravity reveals interior of Antarctic volcano

Understanding Antarctic volcanoes is important as they provide a window on magmatic and tectonic processes of the Antarctic plate and contain datable records of ice-sheet changes. We present the results from the first detailed airborne radar and gravity surveys across James Ross Island, northern Antarctic Peninsula, which is dominated by Mt Haddington, an ice-covered Miocene-Recent alkaline stratovolcano. The surveys provide new insights into the subsurface structure of the volcano and hence its development, which are unavailable from the surface geology alone. We show that Mt Haddington is associated with a significant negative Bouguer gravity anomaly (≥26 mGal), which suggests that there has not been significant pooling and solidification of a dense shallow-level mafic magma chamber during the growth of the volcano over at least the past 6 m.y., which is consistent with independent geochemical evidence. Simple flexural isostatic models cannot explain the localised negative Bouguer anomaly. 3D modelling techniques show that the negative anomaly is best explained by a shallow, low-density intra-crustal body with its top close to, or at, the surface. Although comparable gravity anomalies are commonly associated with large (∼20 km) ash-filled calderas, as seen at Yellowstone or Toba, there is no geological evidence on James Ross Island for a similar structure. We therefore propose that the James Ross Island volcanic edifice subsided into the thick underlying pile of relatively soft Jurassic and Cretaceous sediments, which were displaced by low-density hyaloclastite breccia. The type of deformation envisaged is similar to that associated with Concepcioú, or Iwaki volcanoes in South America, although Mt Haddington is much larger.

Glacial isostatic adjustment model constrained by geodetic measurements and relative sea level

The new model of the last glacial isostatic adjustment (GIA) is constrained simutaneously by the data sets from geodetic measurements, relative sea levels and seismic shear tomographic data. The viscosities in mantle for this model are allowed to vary both radially and laterally. Firstly, the scalling factor β is tentatively chosen for which the lateral perturbations in viscosity are computed based on the linear relation with seismic shear wave anomalies. These lateral perturbations in viscosity are then supperposed logarithmically on a laterally homogeneous reference viscosity model to give the total viscosities in the mantle layers. Based on this 3-D viscosity model, the GIA predictions are computed using the coupled-Laplace Finite-Element Method. Such processes will not end untill the predictions agree well with the observed data sets. The main results are as follows. Firstly, the laterally heterogeneous viscosity model (RF3L20(β=0. 4)) is determined from which the obvious lateral heterogeneities and the prominouced effects on GIA predictions are found. It is indicated the lateral heterogeneities are attributable not just to themal effects but also to lateral variations in chemistry and other factors. The RF3L20(β=0.4) model can be used to the modelling in mantle geodynamics. Secondly, the multiple present GIA rates are predicted that can be served as the important corrections for the monitoring of the tectonic motion, land water storage changes, ocean water mass variations and the non-equilibrium of the ice and snow.

EVEREST: ARCHETYPAL ENVIRONMENTAL MODEL FOR ETIOLOGY OF BRAIN PRIMEVAL DISORDERS

Bouguer gravity anomalies in the region of the Western Himalayas, Karakoram and Tien Shan show large negative values, but classical isostatic models are insufficient to account for the detailed pattern of the observed anomalies. In the past years gravimetric surveys in the Karakoram (Marussi, Caputo and others in 1954) have been extended and densified. The full body of available gravimetric data, including the pendulum observations by De Filippi in 1913–14 and Hedin in 1929–33 have been re-analyzed. Terrain corrections have been computed systematically for all available data using an algorithm and Digital Terrain Model. The isostatic anomalies along a profile from the Indo-Gangetic foredeep, across the Karakoram Range and terminating in the Tarim Basin show the oscillating values already noticed by Marussi. This oscillatory pattern can be explained by a model in which the convergent boundaries of the Indian and Tarim plates deform by elastic flexure, besides isostasy. The gravity data are used to constrain the numerical values of the model parameters, particularly the flexural rigidity of the plates. For the Indian Plate it is found that the best fitting value of the flexural rigidity is D=5×1024 N m, a value very similar to those reported in the Central Himalayas. The flexural rigidity of the Tarim Plate turns out to be considerably larger, D=7×1025 N m, which makes the Tarim more rigid than the neighboring Central Tibet. Both plates are loaded by an estimated shear force of 7 1012 N m−1 located in a region corresponding to the Nanga Parbat Haramosh syntaxis. It is concluded that the Indo–Asian continental collision in Western Himalaya and Karakoram results in the development of flexural basins on both sides, unlike in Central Himalaya where the collision produces a flexural basin, the Ganga Basin, to the South and, to the North, the indentation of an isostatically supported Tibetan Block with possible rheological layering and eastward lateral extrusion.

Deep structure of the western South African passive margin — Results of a combined approach of seismic, gravity and isostatic investigations

The passive margin of the South Atlantic shows typical features of a rifted volcanic continental margin, encompassing seaward dipping reflectors, continental flood basalts and high-velocity/density lower crust at the continent–ocean transition, probably emplaced during initial seafloor spreading in the Early Cretaceous. The Springbok profile offshore western South Africa is a combined transect of reflection and refraction seismic data. This paper addresses the analysis of the seismic velocity structure in combination with gravity modelling and isostatic modelling to unravel the crustal structure of the passive continental margin from different perspectives. The velocity modelling revealed a segmentation of the margin into three distinct parts of continental, transitional and oceanic crust. As observed at many volcanic margins, the lower crust is characterised by a zone of high velocities with up to 7.4 km/s. The conjunction with gravity modelling affirms the existence of this body and at the same time substantiated its high densities, found to be 3100 kg/m 3. Both approaches identified the body to have a thickness of about 10 km. Yet, the gravity modelling predicted the transition between the high-density body towards less dense material farther west than initially anticipated from velocity modelling and confirmed this density gradient to be a prerequisite to reproduce the observed gravity signal. Finally, isostatic modelling was applied to predict average crustal densities if the margin was isostatically balanced. The results imply isostatic equilibrium over large parts of the profile, smaller deviations are supposed to be compensated regionally. The calculated load distribution along the profile implies that all pressures are hydrostatic beneath a depth of 45 km.

Ups and downs of the Mississippi Delta

During the last glacial period, when sea level was low, meltwater discharge drove incision of the lower Mississippi valley, with valley filling and delta construction during Holocene sea-level rise. Isostatic modeling shows that sediment volumes removed and replaced were sufficient to induce uplift of >9 m along valley margins followed by subsidence of the same magnitude, with effects dissipating only over distances of >100–150 km along the Gulf of Mexico coast. Recognition of cyclical uplift and subsidence refutes recent interpretations of delta stability, and suggests that late Holocene relative sea-level curves from the delta region are instead a record of subsidence of the pre-Holocene depocenter. More broadly, incised valley cutting and filling is a common fluvial response to glacioeustasy, and cyclical uplift and subsidence should be common to large alluvial-deltaic systems elsewhere.

Cordilleran Intermontane thermotectonic history and implications for neotectonic structure and petroleum systems, British Columbia, Canada

Heat flow increases northward along Intermontane Belt in the western Canadian Cordillera, as shown by geothermal differences between Bowser and Nechako sedimentary basins, where geothermal gradients and heat flows are ∼30 mK/m and ∼90 mW/m2 compared to ∼32 mK/m and 70 –80 mW/m2, respectively. Sparse temperature profile data from these two sedimenatary basins are consistent with an isostatic model of elevation and crustal parameters, which indicate that Bowser basin heat flow should be ∼20 mW/m2 greater than Nechako basin heat flow. Paleothermometric indicators record a significant northward increasing Eocene or older erosional denudation, up to ∼7 km. None of the heat generation, tectonic reorganization at the plate margin, or erosional denudation produce thermal effects of the type or magnitude that explain the north–south heat flow differences between Nechako and Bowser basins. The more southerly Nechako basin, where heat flow is lower, has lower mean elevation, is less deeply eroded, and lies opposite the active plate margin. In contrast, Bowser basin, where heat flow is higher, has higher mean elevation, is more deeply eroded, and sits opposite a transform margin that succeeded the active margin ∼40 Ma. Differences between Bowser and Nechako basins contrast with the tectonic history and erosion impacts on thermal state. Tectonic history and eroded sedimentary thickness suggest that Bowser basin lithosphere is cooling and contracting relative to Nechako basin lithosphere. This effect has reduced Bowser basin heat flow by ∼10–20 mW/m2 since ∼40 Ma. Neither can heat generation differences explain the northerly increasing Intermontane Belt heat flow. A lack of extensional structures in the Bowser basin precludes basin and range-like extension. Therefore, another, yet an unspecified mechanism perhaps associated with the Northern Cordilleran Volcanic Province, contributes additional heat. Bowser basin’s paleogeothermal gradients were higher, ∼36 mK/m, before the Eocene and this might affect petroleum and metallogenic systems.

Effects of topographic–isostatic masses on gravitational functionals at the Earth’s surface and at airborne and satellite altitudes

Topographic–isostatic masses represent an important source of gravity field information, especially in the high-frequency band, even if the detailed mass-density distribution inside the topographic masses is unknown. If this information is used within a remove-restore procedure, then the instability problems in downward continuation of gravity observations from aircraft or satellite altitudes can be reduced. In this article, integral formulae are derived for determination of gravitational effects of topographic–isostatic masses on the first- and second-order derivatives of the gravitational potential for three topographic–isostatic models. The application of these formulas is useful for airborne gravimetry/gradiometry and satellite gravity gradiometry. The formulas are presented in spherical approximation by separating the 3D integration in an analytical integration in the radial direction and 2D integration over the mean sphere. Therefore, spherical volume elements can be considered as being approximated by mass-lines located at the centre of the discretization compartments (the mass of the tesseroid is condensed mathematically along its vertical axis). The errors of this approximation are investigated for the second-order derivatives of the topographic–isostatic gravitational potential in the vicinity of the Earth’s surface. The formulas are then applied to various scenarios of airborne gravimetry/gradiometry and satellite gradiometry. The components of the gravitational vector at aircraft altitudes of 4 and 10 km have been determined, as well as the gravitational tensor components at a satellite altitude of 250 km envisaged for the forthcoming GOCE (gravity field and steady-state ocean-circulation explorer) mission. The numerical computations are based on digital elevation models with a 5-arc-minute resolution for satellite gravity gradiometry and 1-arc-minute resolution for airborne gravity/gradiometry.

Development of a software package for visualization of three-dimensional mass distributions and forward gravity modelling

This paper describes the development of a software package, called GEOCOM, for the visualization of three-dimensional mass distributions and forward gravity modelling. The developed software package is intended for use in geophysical and geodetic fields and represents a powerful tool for scientific research within the scope of forward gravity modelling. The development process was based on the high-level programming language MATLAB®. In order to guarantee an efficient handling of numerous functions, the whole software program is based on several graphical user interfaces (GUIs), which provide an interactive environment for data visualization, three-dimensional mass modelling and numeric gravity computation. The density application enables user-defined topographical as well as isostatic density models based on a layer-oriented input scheme. The subdivision of the density model into volumes of discrete elements (right rectangular prisms) permits the usage of an analytical solution with respect to the calculation of the gravitational attraction of each element. Focusing on several numerical case studies, which demonstrate topographic, as well as topographic-isostatic density models, this paper shows possible software applications using the developed software package GEOCOM.

Compilation of a Map of Recent Vertical Crustal Movements in Eastern Canada Using Geographic Information System

In this paper, we use geographic information system as a useful tool in compiling vertical crustal movement (VCM) over Eastern Canada. For modeling linear VCM, we employ tide gauge records and precise releveled segments, which reflect the glacial isostatic submergence of the coasts, as well as other global, regional, and local effects. The area of study is divided into two zones: Maritimes and South St. Lawrence zones. Piecewise polynomial surfaces of different orders are calculated by the method of least squares for each region to obtain representations of the recent vertical movements. This work provides the framework for multidisciplinary studies of geodynamics in the region, allowing different selection of zones for vertical crustal movements and simplifying the incorporation of more data in future investigations. We use ArcGIS to visualize the theoretical approaches, and to integrate and analyze different spatially distributed data. This powerful tool helps us to produce a more physically meaningful VCM map for the region. The map reflects the most significant geophysical phenomenon in the region, postglacial rebound, and it is in good agreement with the 2004 glacial isostatic adjustment model of Peltier [ICE-5G(VM2)].

Paleo sea levels reconsidered from direct observation of paleoshoreline position during Glacial Maxima (for the last 500,000 yr)

The drastic climatic changes which characterise the cooling trend of the last few million years of Earth history led to variations in eustatic sea level that had tremendous impact on the geology and ecology of continental margins. Reconstructing a sea-level curve back in time is not an easy task. Observations of shoreline positions are always a local measurement of Relative Sea Level that needs to be corrected from the effect of tectonic and thermal subsidence, sediment loading, compaction and glacio-hydro isostasy. Extensive studies have been done for the last deglaciation and for the last 100,000 yr cycle. But very few studies deal with position of sea level during earlier cycles, simply because conditions are very rarely favourable for the preservation of such witnesses. The shelf of the Golfe du Lion (Western Mediterranean) reveals a unique record of shoreline paleopositions during glacial maxima of at least the last five circa 100 kyr glacial/interglacial cycles. In fact it is the entire glacial deltaic lobe of up to 50 m thick (from delta front or shoreface to prodelta) that has been preserved in place and which provides direct and independent constraints for relative sea-level minima. We measure a relative sea level of: −112m, −128, −134, −246 and −262 m for MIS 2, 6, 8, 10 and 12 respectively. After corrections taking into account postdepositional movement of strata (subsidence), we find, that sea level dropped to a depth of −102±6 m during the last three glaciations (MIS2, MIS6, MIS8) but reached exceptionally low values of more than −150±10 m during the preceding glaciations MIS10 and MIS 12 at about 340 and 434 kyr BP. This general time framework and sedimentological interpretation has been confirmed by preliminary results from two deep drillings during the PROMESS cruise (july 2004), which validate our methodology. However, no detailed and absolute datings of such witnesses are available so far, so that we cannot prove that these levels are the lowest ever reached during each glacials, but they correspond undoubtedly to the last preserved shoreface before rapid sea-level rise. We also suggest that the abrupt change in sea-level maxima might be the overprint of 400 kyr orbital periodicity cycles. Last but not least, these results prove that the Golfe du Lion is indeed a unique laboratory to study paleoclimates and sea-level variations on a larger time scale. Further work is needed for a complete glacio-hydro-sedimento isostatic modelling of each sequence and each glacial to further constraint local sea level versus global sea level and quantify, in particular the relative effect of glacio-hydro isostatic effect (which differ according to ice sheet extend) but also of erosion–sedimentation isostatic effect (erosion on land and deposition on the outer shelf and slope).

The impact of lateral density variation model in the determination of precise gravimetric geoid in mountainous areas: a case study of Iran

The existence of topography above the geoid violates the basic assumption of Stokes’ formula for the determination of the geoid. Usually a constant density of 2.67 g cm−3 is used in the determination of the geoid. However, we know that the density of the topographical mass departs by about 10–20 per cent from the actual mean value of the crustal density. Iran has one of the largest relief and density variations in the world and the geoid computation in this area is affected the most by topographical density effects. The influence on the geoid height coming from the actual and Pratt-Hayford's isostatic models are studied. Numerical results show that the differences in the geoid height due to actual and isostatic density models can reach up to 0.22 and 0.29 m, respectively, which is not negligible in a precise geoid determination with centimetre accuracy. Our results suggest that the effect of topographical density lateral variations is significant enough and ought to be taken into account specially in mountainous regions in the determination of a precise geoid model.

The structure, isostasy and gravity field of the Levant continental margin and the southeast Mediterranean area

A 3-D layered structure of the Levant and the southeastern Mediterranean lithospheric plates was constructed using interpretations of seismic measurements and borehole data. Structural maps of three principal interfaces, elevation, top basement and the Moho, were constructed for the area studied. This area includes the African, Sinai and Arabian plates, the Herodotus and the Levant marine basins and the Nile sedimentary cone. In addition, an isopach map of the Pliocene sediments, as well as the contemporaneous amount of denuded rock units, was prepared to enable setting up the structural map of the base Pliocene sediment. Variable density distributions are suggested for the sedimentary succession in accord with its composition and compaction. The spatial density distribution in the crystalline crust was calculated by weighting the thicknesses of the lower mafic and the upper felsic crustal layers, with densities of 2.9 g/cm 3 and 2.77 g/cm 3, respectively. Results of the local (Airy) isostatic modeling with compensation on the Moho interface show significant deviations from the local isostasy and require variable density distribution in the upper mantle. Moving the compensation level to the base of the lithosphere (∼ 100 km depth) and adopting density variations in the mantle lithosphere yielded isostatic compensation (± 200 m) over most of the area studied. The spatial pattern obtained of a density distribution with a range of ± 0.05 g/cm 3 is supported by a regional heat flux. Simulations of the flexure (Vening Meinesz) isostasy related to the Pliocene to Recent sedimentary loading and unloading revealed concentric oscillatory negative and positive anomalies mostly related to the Nile sedimentary cone. Such anomalies may explain the rapid subsidence in the Levant Basin and the arching in central Israel, northern Sinai and Egypt during Pliocene–Recent times. Comparison between the observed (Bouguer) gravity and the calculated gravity for the constructed 3-D lithospheric structure, which has variable density distributions, provided a good match and an independent constraint for the large-scale structure suggested and confirmed an oceanic nature for the Levant Basin lithosphere.