Local Isostatic Equilibrium Research Articles

Imaging the 2–D crust and upper mantle structure of the Iranian plateau resolved by potential field and seismic data

In this study, the crust and upper mantle density distribution across the Iranian plateau along a ~1500 km profile are investigated using an integrated modeling of potential field data (gravity, geoid and topography) and by assuming local isostatic equilibrium. Our profile is initiated in Arabian platform and running through the main geological structures of Iran including Zagros orogen, Sanandaj – Sirjan Zone (SSZ), Urumieh – Dokhtar Magmatic Arc (UDMA), Central Iran (CI) block, Tabas/Lut blocks, and East Iranian Range (EIR). In the second step, in order to verify the integrated model, the 1–D S–wave absolute velocities for the crust and upper mantle beneath 21 seismic stations which partially covered the density profile are obtained using the joint inversion of P–receiver function (PRF) and surface–wave dispersion data. Our results indicate a thick crust (~60 km) beneath the SSZ and UDMA which thinning toward the east and experienced its minimum value (~40 km) beneath the central Iran, as well as Tabas/Lut blocks. Moreover, a thick lithosphere (>150 km) is observed under the Zagros, SSZ and parts of the UDMA, whereas a thinner lithosphere (~120 km on average in velocity model) is presented under the most of our profile. Both density and velocity models indicate a lithospheric thinning toward the CI block which imply the slab breakoff or lithospheric delamination followed by the upwelling of the asthenosphere in the Neogene. The presence of a relative fast structure in the crust beneath the Tabas block which is coinciding with a shallow high velocity body in the uppermost mantle elucidates a signature of former oceanic crust, which disappeared after the collision of Lut and Afghan continental blocks and E-directed subduction of Neotethys under the Afghan microcontinent either in post-Albian or middle Eocene times.

Isostatic constraint for 2D nonlinear gravity inversion on rifted margins

We have developed a nonlinear gravity inversion for simultaneously estimating the basement and Moho geometries, as well as the depth of the reference Moho along a profile crossing a passive rifted margin. To obtain stable solutions, we impose smoothness on basement and Moho, force them to be close to previously estimated depths along the profile and also impose local isostatic equilibrium. Different from previous methods, we evaluate the information of local isostatic equilibrium by imposing smoothness on the lithostatic stress exerted at depth. Our method delimits regions that deviate and those that can be considered in local isostatic equilibrium by varying the weight of the isostatic constraint along the profile. It also allows controlling the degree of equilibrium along the profile, so that the interpreter can obtain a set of candidate models that fit the observed data and exhibit different degrees of isostatic equilibrium. Our method also differs from earlier studies because it attempts to use isostasy for exploring (but not necessarily reducing) the inherent ambiguity of gravity methods. Tests with synthetic data illustrate the effect of our isostatic constraint on the estimated basement and Moho reliefs, especially at regions with pronounced crustal thinning, which are typical of passive volcanic margins. Results obtained by inverting satellite data over the Pelotas Basin, a passive volcanic margin in southern Brazil, agree with previous interpretations obtained independently by combining gravity, magnetic, and seismic data available to the petroleum industry. These results indicate that combined with a priori information, simple isostatic assumptions can be very useful for interpreting gravity data on passive rifted margins.

Lithospheric structure of the Eastern Iranian plateau from integrated geophysical modeling: A transect from Makran to the Turan platform

We present a 2D profile of density and temperature distribution in the lithosphere across Iran along a more than 1600km long profile extending from the Oman Gulf in the South to the Kopeh-Dagh and the Turan platform in the North. Gravity, geoid, topography and surface heat flow data were used for modeling, assuming local isostatic equilibrium. As much as possible, crustal structure has been constrained by seismic data. Crustal thickening is found under the Taftan-Bazman volcanic-arc (up to 47km), under the Binalud foreland (∼54km) and beneath the Kopeh-Dagh mountains (up to 50km) whereas thin crust has been obtained under the Oman Gulf (20km). Moho depth under the Lut block and the Turan platform is about 40km. The lithospheric thickness is ∼90km under the Oman Gulf and increases slightly until the Jazmourian depression. Then the lithospheric-asthenospheric boundary (LAB) bends significantly and sinks to ∼260km under the Taftan-Bazman volcanic-arc. The LAB depth is about 190km beneath the Lut block. A slight increase of LAB depth under the Binalud foreland towards the North may indicate a suture zone. Under the Turan platform, the LAB depth reaches ∼210km. We also modeled two possible positions of the deep suture zone in NE Iran (along the main Kopeh-Dagh fault or along the Atrak River) and concluded that, when the suture zone is along the Atrak River, we obtained the better fit between calculated and measured data.

Magmatic development of the outer Vøring margin from seismic data

AbstractThe Vøring Plateau off mid‐Norway is a volcanic passive margin, located north of the East Jan Mayen Fracture Zone (EJMFZ). Large volumes of magmatic rocks were emplaced during Early Eocene margin formation. In 2003, an ocean bottom seismometer survey was acquired over the margin. One profile crosses from the Vøring Plateau to the Vøring Spur, a bathymetric high north of the EJMFZ. The P wave data were ray traced into a 2‐D crustal velocity model. The velocity structure of the Vøring Spur indicates up to 15 km igneous crustal thickness. Magmatic processes can be estimated by comparing seismic velocity (VP) with igneous thickness (H). This and two other profiles show a positive H‐VP correlation at the Vøring Plateau, consistent with elevated mantle temperature at breakup. However, during the first 2 Ma magma production was augmented by a secondary process, possibly small‐scale convection. From ∼51.5 Ma excess melting may be caused by elevated mantle temperature alone. Seismic stratigraphy around the Vøring Spur shows that it was created by at least two uplift events, with the main episode close to the Miocene/Pliocene boundary. Low H‐VP correlation of the spur is consistent with renewed igneous growth by constant, moderate‐degree mantle melting, not related to the breakup magmatism. The admittance function between bathymetry and free‐air gravity shows that the high is near local isostatic equilibrium, precluding that compressional flexure at the EJMFZ uplifted the high. We find a proposed Eocene triple junction model for the margin to be inconsistent with observations.

3D Lithosphere density structure of southern Indian shield from joint inversion of gravity, geoid and topography data

We present the 3D crustal and lithospheric structure and crustal average density distribution of southern Indian shield (south of 18°N), Sri Lanka and adjoining oceans. The model is based on the assumption of local isostatic equilibrium and is derived from joint inversion of free air gravity and geoid anomalies and topography data. The derived crustal thickness of 10–25km in the oceanic region increases to 34–35km along the coast. A crustal thickness of 34–38km is obtained beneath the Eastern Dharwar Craton and 36–45km beneath the Western Dharwar Craton and the Southern Granulite Terrain. Sri Lanka has a thinner crust of 30–35km. The lithosphere–asthenosphere boundary is located at depths of 70–120km under oceanic regions and ∼150–180km below the Dharwar Craton and the Northern block of Southern Granulite Terrain. A notably thinned lithosphere of ∼130km near Bangalore in the Eastern Dharwar Craton, ∼140km beneath the Southern block of Southern Granulite Terrain and ∼130km in Sri Lanka is observed. The thickness of the lithosphere (∼130km) near Bangalore is inferred as the frozen in signature of a small fossil mantle plume and/or tectono-compositional effect of a rifted margin and a suture. Considerable stretching and/or convective removal of pristine lithosphere in the Southern block of Southern Granulite Terrain and adjoining Sri Lanka, before disappearing completely in the Archaean Northern block of Southern Granulite Terrain and Dharwar Craton, is suggested.

Magmatism and tectonic processes in Area A hydrothermal vent on the Southwest Indian Ridge

The hydrothermal vent in Area A (37.78°S, 49.65°E) is the first active hydrothermal vent discovered on the Southwest Indian Ridge (SWIR). Heat source and adequate bulk permeability are two necessary factors for the formation of a hydrothermal vent. Along the SWIR 49.3°E to 51.2°E, the gravity-derived crustal thickness is up to 9.0 km, much thicker than the average thickness of the global oceanic crust. This characteristic indicates that the magma supply in this area is robust, which is possibly affected by a hotspot. The large-scale residual mantle Bouguer anomalies (RMBA) reveal prominent negative-gravity anomalies between the first-order ridge segment (from Indomed to Gallieni, 46.0°E to 52.0°E) and the Marion-Del Cano-Crozet region. These anomalies indicate the channel of the hotspot-ridge interaction. The tomography data corrected with theoretical thermal model indicate that the low-velocity anomalies corresponding to this channel can reach the base of the lithosphere. Near the hydrothermal vent area, the topography and crustal thickness at the off-axis area are extremely asymmetrical. South of the SWIR, the high topography corresponds to the thinning crustal thickness. The residual isostatic topography anomalies indicate that Area A is a deviation from the local isostatic equilibrium, similar to the characteristics of the transform fault inside corner. The forward profiles of the magnetic data indicate that the thinning magnetic layer at the south side of Area A corresponds to the shallow, high-velocity area revealed by the OBS, which is the result of tectonic extension of a detachment fault. The active tectonic processes in Area A can provide sufficient crustal permeability to the hydrothermal circulation and may form massive sulfide deposits.

3D Crustal and Lithospheric Structures in the Southeastern Mediterranean and Northeastern Egypt

Crustal and lithospheric thicknesses of the southeastern Mediterranean Basin region were determined using 3D Bouguer and elevation data analysis. The model is based on the assumption of local isostatic equilibrium. The calculated regional and residual Bouguer anomaly maps were employed for highlighting both deep and shallow structures. Generally, the regional field in the area under study is considered to be mainly influenced by the density contrast between the crust and upper mantle. Use of the gravity and topographic data with earthquake focal depths has improved both the geometry and the density distribution in the 3-D calculated profiles. The oceanic-continental boundary, the basement relief, Moho depth and lithosphere-asthenosphere boundary maps were estimated. The results point to the occurrence of thick continental crust areas with a thickness of approximately 32 km in northern Egypt. Below the coastal regions, the thickness of crust decreases abruptly (transition zone). An inverse correlation between sediment and crustal thicknesses shows up from the study. Furthermore, our density model reveals the existence of a continental crustal zone below the Eratosthenes Seamount block. Nevertheless, the crustal type beneath the Levantine basin is typically oceanic; this is covered by sedimentary sequences more than 14 km thick. The modeled Moho map shows a depth of 28–30 km below Cyprus and a depth of 26–28 km beneath the south Florence Rise in the northern west. However, the Moho lies at a constant shallow depth of 22–24 km below the Levantine Basin, which indicates thinning of the crust beneath this region. The Moho map reveals also a maximum depth of about 33–35 km beneath both the northern Egypt and northern Sinai, both of which are of the continental crust. The resulting mantle density anomalies suggest important variations of the lithosphere-asthenosphere boundary (LAB) topography, indicating prominent lithospheric mantle thinning beneath south Cyprus (LAB ~90 km depth), followed by thickening beneath the Eratosthenes seamount, Florence Rise, Levantine Basin and reaching to maximum thickness below Cyprian Arc (LAB ~115–120 km depth), and further followed by thinning in the north African margin plate and north Sinai subplate (LAB ~90–95 km depth). According to our density model profiles, we find that almost all earthquakes in the study area occurred along the western and central segments of the Cyprian arc while they almost disappear along the eastern segment. The active subduction zone in the Cyprian Arc is associated with large negative anomalies due to its low velocity upper mantle zone, which might be an indication of a serpentinized mantle. This means that collision between Cyprus and the Eratosthenes Seamount block is marked by seismic activity. Additionally, this block is in the process of dynamically subsiding, breaking-up and being underthrusted beneath Cyprus to the north and thrusted onto the Levantine Basin to the south.

Lithospheric structure of southern Indian shield and adjoining oceans: integrated modelling of topography, gravity, geoid and heat flow data

For the present 2-D lithospheric density modelling, we selected three geotransects of more than 1000 km in length each crossing the southern Indian shield, south of 16°N, in N–S and E–W directions. The model is based on the assumption of local isostatic equilibrium and is constrained by the topography, gravity and geoid anomalies, by geothermal data, and where available by seismic data. Our integrated modelling approach reveals a crustal configuration with the Moho depth varying from ∼40 km beneath the Dharwar Craton, and ∼39 km beneath the Southern Granulite Terrane to about 15–20 km beneath the adjoining oceans. The lithospheric thickness varies significantly along the three profiles from ∼70–100 km under the adjoining oceans to ∼130–135 km under the southern block of Southern Granulite Terrane including Sri Lanka and increasing gradually to ∼165–180 km beneath the northern block of Southern Granulite Terrane and the Dharwar Craton. This step-like lithosphere–asthenosphere boundary (LAB) structure indicates a normal lithospheric thickness beneath the adjoining oceans, the northern block of Southern Granulite Terrane and the Dharwar Craton. The thin lithosphere below the southern block of Southern Granulite Terrane including Sri Lanka is, however, atypical considering its age. Our results suggest that the southern Indian shield as a whole cannot be supported isostatically only by thickened crust; a thin and hot lithosphere beneath the southern block of Southern Granulite Terrane including Sri Lanka is required to explain the high topography, gravity, geoid and crustal temperatures. The widespread thermal perturbation during Pan-African (550 Ma) metamorphism and the breakup of Gondwana during late Cretaceous are proposed as twin cause mechanism for the stretching and/or convective removal of the lower part of lithospheric mantle and its replacement by hotter and lighter asthenosphere in the southern block of Southern Granulite Terrane including Sri Lanka. Unusually thinned LAB beneath the region near Bangalore apparently indicates the preserved tectonocompositional effect of late Proterozoic rifted margin of Dharwar Craton.

What compensates the topography of southern Norway? Insights from thermo-isostatic modeling

The origin of the high topography of the Norwegian Mountains is currently much debated. Several geophysical studies show that the uppermost mantle below southern Norway has anomalously low velocities as compared to other parts of the Baltic Shield. This study aims to shed lights on the structure of the lithospheric mantle below southern Norway by adapting and further refining a method based on isostatic and thermal equilibrium to compute temperature, temperature-related density and synthetic S-wave velocity in stable continental domains. The one-dimensional steady-state heat equation is used with topographic, Moho depth, crustal density and surface heat flow data. A condition of local isostasy is assumed and geoid undulations are used to constrain the range of possible lithosphere models.Results derived from this method suggest a thickening of the thermal lithosphere below southern Norway from west to east. The western part is found to have higher temperatures, lower densities and lower synthetic S-wave velocities than the eastern part, compatible with results from a recent P-wave travel time residual study. Comparison of the synthetic shear-velocity profiles beneath southwestern Norway with velocity profiles inverted from Rayleigh wave dispersion data suggests that the higher temperatures associated with a thinner lithosphere can explain parts of the seismic low-velocity anomaly.The inferred lithospheric structure is sensitive to uncertainties in the crustal input model, but the main features remain undisturbed by changes in the input data. The results show that the lithosphere of southwestern Norway can be in local isostatic equilibrium, if it is thinner and warmer than the lithosphere of eastern Norway. The present-day high topography may therefore be partially sustained by lower densities in the mantle lithosphere.

Interpretation of geoid anomalies in the contact zone between the East European Craton and the Palaeozoic Platform-II: Modelling of density in the lithospheric mantle

SUMMARY We present analysis of lateral variations of density in the upper mantle in the area of contact of the precambrian East European Craton (EEC) and the Palaeozoic Platform (PP) in Poland, obtained by analysis of the gravimetric geoid undulations. A precise 3-D density model of the crust in the study area down to a depth of 50 km, discussed in the first part of this paper (Majdanski et al., in press) did not explain all features of the observed geoid. This suggests that these features can be due to density inhomogeneities in the upper mantle. To estimate them, we performed inversion of a residual between the observed geoid and undulations caused by the 3-D density distribution in the crust. Basing on the assumption of local isostatic compensation and Pratt–Hayford isostasy model, the density distribution in the upper mantle was parametrized as a 40-km-thick layer located above the assumed compensation depth of 140 km and subdivided into irregular blocks. The boundaries of the blocks were defined according to boundaries of major tectonic units in the study area and position and shape of the most pronounced anomalies in the residual geoid. A series of sensitivity tests calculated for such density heterogeneities in the upper mantle showed that they can produce geoid undulations of the order of several metres. The density values in each unit were taken as model parameters for the inversion procedure, and inverse problem was solved using global optimization with constraints. The density variations in the upper mantle in the final model correlate well with the surface heat flow. This suggests that these variations can be due to diversity in mantle temperature. The Trans-European Suture Zone (TESZ), which is a major suture separating the EEC from the PP, is not observed as a distinct unit in the mantle. Instead, our study suggests continuation of the lithosphere of the EEC beneath the PP and confirms subdivision of the TESZ into terranes with distinctly different evolution. A simple calculation shows that condition of local isostatic equilibrium is almost achieved at the assumed compensation depth, with the exception of several small-scale units. However, different loads and mass deficiencies in the lithosphere are compensated at different depth levels. This proves that the lithosphere is alternation of mechanically strong and weak layers, both in the crust and upper mantle, and that different loads can be compensated at different depth. However, the total compensation of all lithospheric loads occurs in the upper mantle.

Tectonic evolution of the North Sea basin: crustal stretching and subsidence

Summary. The lithospheric stretching model for the formation of sedimentary basins was tested in the central North Sea by a combined study of crustal thinning and basement subsidence patterns. A profile of crustal structure was obtained by shooting a long-range seismic experiment across the Central Graben, the main axis of subsidence. A seabed array of 12 seismometers in the graben was used to record shots fired in a line 530 km long across the basin. The data collected during the experiment were interpreted by modelling synthetic seismograms from a laterally varying structure, and the final model showed substantial crustal thinning beneath the graben. Subsidence data from 19 exploration wells were analysed to obtain subsidence patterns in the central North Sea since Jurassic times. Changes in water depth were quantified using foraminiferal assemblages where possible, and observed basement subsidence paths were corrected for sediment loading, compaction and changes in water depth through time. The seismic model is shown to be compatible with the observed gravity field, and the small size of observed gravity anomalies is used to argue that the basin is in local isostatic equilibrium. Both crustal thinning and basement subsidence studies indicate about 70 km of stretching across the Central Graben during the mid-Jurassic to early Cretaceous extensional event. This extension appears to have occurred over crust already slightly thinned beneath the graben, and the seismic data suggest that total extension since the early Permian may have been more than 100km. The data presented here may all be explained using a simple model of uniform extension of the lithosphere.

Lithosphere structure underneath the Tibetan Plateau inferred from elevation, gravity and geoid anomalies

The Tibetan Plateau is the product of crustal thickening caused by collision between India and Asia. Plate tectonic reconstructions suggest continuous northward movement of the Indian plate relative to stable Eurasia at nearly 50 mm/yr for the last 50 My. The plateau is now at ~5 km elevation with steep topographic gradients across the southern and northern margins. These steep topographic gradients are also related to large lateral variations in the geoid and gravity anomalies. In a SSW to NNE cross section, the Bouguer gravity anomaly decreases over a distance of 500 km from about 0 mGal in the India plate to ~−500 mGal on the Tibetan Plateau. The geoid anomaly also presents steep gradients on both the Himalayan front and the northern margin, reaching values between 20–30 m on the plateau, suggesting a pronounced thinning of the lithospheric mantle. Uplift late in the tectonic evolution of the plateau, the widespread extension, and the associated magmatism have been attributed to convective removal of the lower part of lithospheric mantle and its replacement by hotter and lighter asthenosphere. Here we present a two-dimensional lithospheric thermal and density model along a transect from the Indian plate to Asia, crossing the Himalaya front and the Tibetan Plateau. The model is based on the assumption of local isostatic equilibrium, and is constrained by the topography, gravity and geoid anomalies and by thermal data within the crust. Our results suggest that the height of the Tibetan Plateau is compensated by thick crust in the south and by hot upper mantle to the north. The Tibetan Plateau as a whole cannot be supported isostatically only by thickened crust; a thin and hot lithosphere beneath the northern plateau is required to explain the high topography, gravity, geoid and crustal temperatures. The lithosphere reaches a maximum depth of ~260 km beneath the southern Plateau, and thins abruptly northward to ~100 km under the central and northern Plateau. The lithosphere depth increases again beneath the Qaidam basin and the Qilian Shan to ~160 km.

Crustal thickness and VP/ VS variations in the Grenville orogen (Ontario, Canada) from analysis of teleseismic receiver functions

We have developed a simple semblance-weighted stacking technique to estimate crustal thickness and average V P/ V S ratio using teleseismic receiver functions. We have applied our method to data from 32 broadband seismograph stations that cover a 700 × 400 km 2 region of the Grenville orogen, a 1.2–0.98 Ga Himalayan-scale collisional belt in eastern North America. Our seismograph network partly overlaps with L ithoprobe and other crustal refraction surveys. In 8 out of 9 cases where a crustal-refraction profile passes within 30 km of a seismograph station, the two independent crustal thickness estimates agree to within 7%. Our regional crustal-thickness model, constructed using both teleseismic and refraction observations, ranges between 34.0 and 52.4 km. Crustal-thickness trends show a strong correlation with geological belts, but do not correlate with surface topography and are far in excess of relief required to maintain local isostatic equilibrium. The thickest crust (52.4 ± 1.7 km) was found at a station located within the 1.1 Ga mid-continent (failed) rift. The Central Gneiss Belt, which contains rocks exhumed from deep levels of the crust, is characterized by V P/ V S ranging from 1.78 to 1.85. In other parts of the Grenville orogen, V P/ V S is found to be generally less than 1.80. The thinnest crust (34.5–37.0 km) occurs northeast of the 0.7 Ga Ottawa–Bonnechere graben and correlates with areas of high intraplate seismicity.

New gravity maps of the Eastern Alps and significance for the crustal structures

The deep seismic profile T ransalp crosses, from north to south, Germany, Austria and Italy. The gravity measurements for each country were made by national agencies with different reference systems and data reduction methods. Within the frame of the T ransalp-project a comprehensive database of the Eastern Alps was compiled covering an area of 3.5° by 4° in longitude and latitude (275 by 445 km), respectively. To increase the data coverage in the south Alpine area two gravity surveys were carried out, resulting in 469 areally distributed new stations, of which 215 have been measured with the intent to improve the geoid in the area of the planned Brenner Basistunnel (BBT). The resulting gravity database is the best in terms of resolution and data quality presently available for the Eastern Alps. Here the free air, Bouguer and isostatic gravity fields are critically discussed. The spatial density of existing gravity stations in the three countries is discussed. On the Italian side of the Alps the spatial density is rather sparse compared to the Austrian side. The Bouguer-gravity field varies between − 190 * 10 − 5 m/s 2 and + 25 * 10 − 5 m/s 2, with the minimum located along the Alpine high topographic chain, but with a small offset (a few tens of km) to the greatest topographic elevation, showing that the Airy-type local isostatic equilibrium does not fully apply here. The maximum of the Bouguer anomaly has an elongated shape of 100 by 50 km located between the towns of Verona and Vicenza and covers the Venetian Tertiary Volcanic Province (VTVP), a feature not directly related to the plate collision in the Eastern Alps. The gravity high is only partly explainable by high-density magmatic rocks and requires also a deeper source, like a shallowing of the Moho. The isostatic residual anomalies (Airy model) are in the range ± 50 * 10 − 5 m/s 2, with the greatest positive anomaly corresponding to the location of the VTVP, indicating here under-compensation of masses. At last a discussion of a 2D density model based on reflection seismic data and receiver functions is made.

Lithospheric structure under the western African‐European plate boundary: A transect across the Atlas Mountains and the Gulf of Cadiz

We present a two‐dimensional lithospheric thermal and density model along a transect running from the southwestern Iberian Peninsula to the northwestern Sahara. The main goal is to investigate the lithosphere structure underneath the Gulf of Cadiz and the Atlas Mountains. The model is based on the assumption of topography in local isostatic equilibrium and is constrained by surface heat flow, gravity anomalies, geoid, and topography data. The crustal structure has been constrained by seismic and geological data where available. Mantle density is supposed to vary linearly with temperature, providing the link between thermal and density‐related data. The lithospheric thickness varies strongly along the profile, going from near 100 km under the Iberian Peninsula to at least 160–190 km under the Gulf of Cadiz and the Gharb foreland basin in Morocco and to 70 km underneath the Atlas Mountains, coinciding with a region of Neogene volcanism. The thickening of the lithosphere is interpreted as a SW trending lithospheric slab extending from the western Betics to the Gulf of Cadiz and the Gharb Basin, whereas the thin lithosphere underneath the Atlas may be interpreted as plume‐like asthenospheric upwelling similar to those observed in the west European Alpine foreland or as a side effect of a slab penetrating the less viscous asthenosphere.

Accretional curvature of lithosphere at magmatic spreading centers and the flexural support of axial highs

Fluid magma should tend to rise up to the level of local isostatic equilibrium at an axis of plate spreading. The magma and underlying partial melt accretes to the side of the separating plates and gets denser as it cools and freezes. This load bends down the lithosphere and additional accretion forms a plate with concave upward curvature. The topography resulting from a curved plate passively moving off axis can be described analytically for accretion on the vertical side of uniform lithosphere when its flexural response is that of a thin elastic plate. Model predictions are consistent with the shape of the axial highs at fast spreading centers (typically ∼400m high and ∼20 km wide) as well as related gravity anomalies. This model requires at least several kilometers of lithospheric thickening within a few kilometers of the ridge axis. Seismic data appear consistent with this lithospheric structure. The magnitude and lateral distribution of faulting on the flanks of axial highs is crudely consistent with formation during plate unbending as lithosphere moves away from the axis. According to this model abandoned fast spreading centers should retain an elastically supported axial high, while for previous models abandoned highs should collapse.

Crustal isostasy indicates anomalous densities beneath Iceland

Recent seismological determinations of crustal thickness in Iceland, made mainly through identification of wide‐angle PmP and SmS reflections, indicate that the depth to the Moho deepens from 11 km beneath the low‐lying Reykjanes Penninsula to 39 km beneath the central highlands. By compiling all available crustal thickness measurements, we show that this trend holds generally for Iceland, with elevation (above m.s.l.) increasing at a rate of 35.54±6.4 (1 σ) meters for each kilometer of Moho depth (below m.s.l.). If one assumes that the topography and Moho are in local isostatic equilibrium with thickness changes in a lower crustal layer, while an upper crust maintains a constant thickness, with the part above sea level having a density of of 2500 kg/m³, then the rate of increase of depth with topographic elevation implies a density jump of 89±12 kg/m³ across the Moho. Petrological and seismological evidence indicate that the lower‐crustal densities probably do not exceed about 3060±50 kg/m³. Mantle densities must therefore be low, 3150±60 kg/m³. The combined effects of chemical depletion, thermal expansion and melt entrainment in the mantle can account for some, but not all, of the anomaly.

Transform tectonics and thermal rejuvenation on the Côte d’Ivoire-Ghana margin, west Africa

Formation of a pronounced basement ridge along many transform continental margins has been attributed to a variety of processes during continental break-up, including transpressional crustal thickening, and thermal rejuvenation and igneous underplating during passage of a spreading ridge. ODP drill holes on the Côte d’Ivoire-Ghana margin now provide the first opportunity to quantify the vertical motions along this type of margin. Apatite fission-track dating of detrital sands suggests that large amounts of erosion occurred on the flanks of an intra-continental wrench zone that predated margin formation. Rapid cooling of >120°C at 120–115 Ma corresponds to erosion of 3.5–5 km along the conjugate Brazilian margin, reflecting c. 1 km of tectonically driven uplift, subaerial erosion, and isostatic uplift due to unloading. Following rift initiation at 120 Ma (Aptian), an oceanic spreading axis passed adjacent to this part of the margin at 90 Ma (Cenomanian). Maximum uplift during the ridge-transform intersection was 390 m, considerably less than the 2000+ m predicted by heat conduction models in local isostatic equilibrium. The modern ridge is partially the product of thicker crust (22 km) underlying the ridge than the adjacent Deep Ivorian Basin (19 km), and partially related to flexural unloading of the transform ridge between the end of intra-continental wrenching and ridge-transform intersection. Flexural coupling between the continental and oceanic plates since ridge-transform intersection has caused a progressive depression of the offshore margin, estimated at about 650 m in the study area.

Cenozoic deposition in the Nansen Basin, a first-order estimate based on present-day bathymetry

Considering the age of the underlying oceanic crust, the Eurasia Basin in anomalously shallow, especially the southern Nansen sub-Basin. Particularly large bathymetric anomalies are found outside major erosional troughs on the Barents Sea Shelf, indicating sediment infill from the adjacent shelf as a major cause of the shallowing. To obtain a first order estimate of the sediment isopachs in the Eurasia Basin, the standard age-depth curve for oceanic crust in the North-Atlantic was used as a reference to define a bathymetric anomaly which was then inverted, assuming local isostatic equilibrium. The isopachs reveal a huge delta outside the St. Anna and Voronin troughs. Conservatively estimated this “twin delta” covers ≈ 75,000 km 2 and has a maximum sediment thickness ≈ 7 km. Similar, but smaller, deltas are found both east and west of this delta. The deltas in the Nansen Basin appear analogous to the Bjørnøyrenna and Storfjordrenna deltas, which are located off major submarine troughs in the Western Barents Sea and are comparable to the St. Anna/Voronin Delta in size. The latter deltas contains huge amounts of young, glacigenic sediments, and this is also likely to be the case for the deltas in the Nansen Basin. The huge sediment accumulations off the northeastern Barents Sea, the anomalously large depth of the Barents Shelf, and its trough dominated morphology, suggest that extensive erosion similar to that documented in the Western Barents Sea (0.5–1.5 km) affected the entire Barents Shelf.

Heat flow and regional uplift at the north-eastern border of the Ebro basin,NE Spain

Summary The north-eastern border of the Ebro foreland basin is characterized by relatively high elevation. the presence of Neogene to Present volcanism, and a high thermal gradient. These facts suggest that the area is affected not only by the topographic load of the Pyrenees but also by the opening of the Valencia trough. To examine this hypothesis, we first present the results of a surface heat-flow survey carried out in the Osona depression. After selecting those wells which are essentially affected by conductive heat transport and applying topographic and palaeoclimatic corrections, we obtain a corrected surface heat-flow density of 88 ± 16m Wm−2. Assuming this value as representative of the regional conductive heat flow, we relate the resulting lithospheric thermal structure to the topographic elevation assuming local isostatic equilibrium and considering a plausible range of crustal thickness and crustal density. The results obtained together with geothermometric and geobarometric data from xenoliths sampled in the nearby Olot volcanic zone indicate a regionally thinned lithosphere about 60–70 km thick. This thinning is related to the Neogene opening of the Valencia trough whose effects extend beyond the rift shoulder (Catalan Coastal Ranges) affecting a broad area of the Ebro basin. Two different uplift mechanisms are proposed as having taken place in the study area: a mechanical uplift related to Eocene compression, followed by a thermal uplift phase associated with asynchronous differential stretching produced during Neogene extension. The combination of these two mechanisms accounts for the observed vertical movements and the migration of the Middle Miocene and Plio-Quaternary volcanic activity.