Crustal Thickness Distribution Research Articles

Cretaceous opposite rotations of North China Block and southern Sikhote Alin, northeast China/Russia: Relation to rifting in the petroliferous Songliao Basin

Paleomagnetic data from northeast Asia confirm that the Korea/North China and southern Sikhote Alin blocks rotated in opposite directions from the Berriasian to the Campanian (145–72 Ma). The Songliao Basin evolved between these rotated blocks with synrift sequences dating from the Tithonian or the Berriasian/Hauterivian. Geologic maps and tomographic images demonstrate a curvilinear subduction zone with associated accretionary wedge/magmatic arc stretched from Sikhote Alin, to Japan and southeast Korea near the Cretaceous Tertiary boundary. Arc-related volcanism migrated over 1000 km southeastwards across northeast Asia to the Japan Sea and Sikhote Alin coast from ∼ 140 – 70 Ma. This suggests that opposite microplate rotations resulted from Pacificward retreat of a curved subduction zone from Early to Late Cretaceous. Toroidal or radial flows in the mantle wedge exerting basal drag on the over-riding microplates is a likely driving mechanism. Anisotropic tomography suggesting fossil curved mantle flows which match the forces required to produce opposite rotations and the distribution of crustal thickness and Vp/vs ratios under the Songliao Basin support this mechanism. A major petroliferous basin in China may therefore be the result of double saloon door tectonics occurring during the Cretaceous behind a contemporaneous continental arc.

Crustal Structure Beneath the Arabian Shield Based on the Receiver Function Method

Arabian Shield occupying the western part of the Arabian Peninsula is an area where strong heterogeneities in crustal structures are associated with several factors, such as an ongoing rifting process in the Red Sea basin, massive recent effusive volcanism in several large basaltic fields (harrats), as well as traces of complex tectonic evolution of this area in Precambrian due to the accretion of several terrains. Geophysical studies of the crust give important information to identify the roles of these and other factors. Receiver function is one of the most robust and relatively inexpensive tools to derive the depths of the major interfaces, of which Moho is the most important, as well as mean velocity parameters in the crust. Based on the H-k stacking method, we have determined the Moho depths and the mean Vp/Vs ratios below a number of seismic stations distributed on the Arabian Shield. As in most of previous studies, we have identified a gradual increase of the crustal thickness from 25 km in the coastal areas of the Red Sea to ~40 km in the eastern margin of the shield. The crustal thickness distribution appears to be consistent with seismic velocity anomalies derived at 30 km depth in the tomography model by (El Khrepy, 2021). For the Vp/Vs ratio, we observe strong variations over the entire study area, and in some cases even between stations located close to each other. This is especially clear in areas of recent magmatism, such as in Harrats Lunayyir and Rahat, where stations with high Vp/Vs ratios correspond to zones with fresh monogenic cones and historical magmatic activity.

Self-organization of magma supply controls crustal thickness variation and tectonic pattern along melt-poor mid-ocean ridges

In nature, variations in mantle sources and magmatic processes lead to significant changes of crustal thickness distribution and spreading pattern at mid-ocean ridges. Distributions in crust thicknesses are relatively uniform at fast ridges. On the contrary, great lateral fluctuations with even mantle rocks exhumation are observed at slow and ultraslow ridges. Similarity, the spreading pattern changes from symmetric configuration (at fast ridges) to highly asymmetric plates accretion with detachment faults and oceanic core complexes (at slow ridges). Recent modeling studies suggested that magmatism may play a key role in the variation. Yet, the physical mechanisms controlling spatial-temporal distribution and intensity of the magmatic activity at mid-ocean ridges remain elusive. In this study, using 3D self-consistent magmatic-thermomechanical numerical models, we systematically investigate the effect of mantle potential temperature and spreading rate on the crustal thickness lateral variations and tectonic pattern at spreading ridges of various opening rates. Our numerical results show that along melt-poor slow-ultraslow mid-ocean ridges, there are two fundamentally different types of ridge sections that spontaneously form and alternate. This is due to a tectono-magmatic instability which self-consistently partitions melt supply beneath the ridge. Two geometries form: (1) normal ridge sections (NR) with elevated topography, normal thickness of oceanic crust and a hot thermal structure, and (2) fracture zone sections (FZ) with lowered topography, thin/absent crust, exhumed mantle rocks, and a cold thermal structure. The tectono-magmatic instability along spreading ridges is triggered by the combined effects of a reduced melt supply from the mantle into crustal magma chambers and an increase in brittle layer thickness. Such bimodal structure is then maintained by the combined effects of buoyant melt partitioning along the ridge, localized latent heat release from crustal magma crystallization, and different spreading modes. This results, on one hand, in hotter, thinner and elevated magmatic sections (tectono-magmatic spreading), and, on the other hand, in colder, thicker and subsided amagmatic sections (purely tectonic spreading). This predicted bimodal distribution finds a good agreement with natural observations. This suggests that spontaneous self-organization of magma supply controlled by spreading rate and mantle potential temperature plays a critical role in shaping tectonic and crustal patterns of mid-ocean ridges.

Seismic Characteristics of the Eastern North American Crust With Ps Converted Waves: Terrane Accretion and Modification of Continental Crust

AbstractThe impact of past tectonic events on the formation and modification of continental lithosphere remains as an open question of fundamental importance. Eastern North America provides a complete record of supercontinent assembly and breakup over the past 1.3 Ga, serving as a natural laboratory for our understanding of continental crust and mantle lithosphere and for integrating geologic and geophysical observations. In this study, we used teleseismic Ps receiver functions to image the detailed distribution of crustal thickness beneath eastern North America. The radial‐component receiver functions were calculated from seismic waveforms recorded by a total of 659 broadband stations during 2010–2017, yielding a high‐resolution image of Moho depth distribution. The depths of the Moho and intracrustal layers vary within and across the major tectonic units. Specifically, there are distinct differences in crustal thickness between the northern and southern Grenville Province. A dipping intracrustal feature can be seen within the central Grenville Province, with the depth increasing eastward from 5 to 27 km. The Moho depth decreases southeastward across the Grenville‐Appalachian boundary, with a sharp Moho offset of up to 12–15 km in the central segment and a more gradual variation to the north and south. The thickness difference between the southern and northern Grenville‐aged crusts suggests different tectonic and/or exhumation histories during and after the Grenville Orogeny. The low‐angle eastward dipping crustal feature is interpreted to be a Grenville‐aged collisional structure. Differences in the steepness of the Moho offset along the strike of Appalachians probably reflect variation of the steepness of the subsurface boundary between Laurentia and accreted terranes with different intensities of postorogenic modification. The observed spatial relation between the geologically defined tectonic boundaries and crustal thickness variations provides new constraints on the depth extent of the tectonic units within the crust.

Lateral heterogeneity of lunar volcanic activity according to volumes of mare basalts in the farside basins

AbstractEstimates for volumes of mare basalts are essential to understand the thermal conditions of the lunar mantle and its lateral heterogeneity. In this study, we estimated the thicknesses and volumes of mare basalts within five farside basins, Apollo, Ingenii, Poincare, Freundlich‐Sharonov, and Mendel‐Rydberg, using premare craters buried by mare basalts and postmare craters that penetrated/nonpenetrated mare basalts employing topographic and multiband image data obtained by SELENE (Kaguya). Furthermore, using the Gravity Recovery and Interior Laboratory crustal thickness model and the mare volumes estimated by this and previous studies, we investigated the relationship between the volumes of the mare basalts and the crustal thicknesses. The results suggest that the minimum crustal thicknesses within the basins were a dominant factor determining whether magma erupted at the surface and that the critical crustal thicknesses for magma eruption were ~10 km on the farside and >20 km on the nearside. The total areas of the regions in which magmas could erupt at the surface are ~10 times larger on the nearside than on the farside. A comparison between the mare volumes within the mare basins on the nearside and the farside shows that magma production in the farside mantle might have been ~20 times smaller than that in the nearside mantle, implying a stronger dichotomy than previously estimated. These results suggest that the mare hemispherical asymmetry should be attributed to both the difference in the crustal thickness distribution and the difference in the quantity of magma production between the nearside and farside mantles.

MeltMigrator: A MATLAB‐based software for modeling three‐dimensional melt migration and crustal thickness variations at mid‐ocean ridges following a rules‐based approach

AbstractMeltMigrator is a MATLAB®‐based melt migration software developed to process three‐dimensional mantle temperature and velocity data from user‐supplied numerical models of mid‐ocean ridges, calculate melt production and melt migration trajectories in the mantle, estimate melt flux along plate boundaries, and predict crustal thickness distribution on the seafloor. MeltMigrator is also capable of calculating compositional evolution depending on the choice of petrologic melting model. Programmed in modules, MeltMigrator is highly customizable and can be expanded to a wide range of applications. We have applied it to complex mid‐ocean ridge model settings, including transform faults, oblique segments, ridge migration, asymmetrical spreading, background mantle flow, and ridge‐plume interaction. In this technical report, we include an example application to a segmented mid‐ocean ridge. MeltMigrator is available as a supplement to this paper, and it is also available from GitHub and the University of Maryland Geodynamics Group website.

Crustal structure and continental dynamics of Central China: A receiver function study and implications for ultrahigh-pressure metamorphism

The Qinling–Tongbai–Hong'an–Dabie–Sulu orogenic belt records the tectonic history of Paleozoic convergence between the South China and North China Blocks. In this study, the distribution of crustal thickness and P - and S -wave velocity ratio ( Vp/Vs ) is obtained by using the H – k stacking technique from the Dabie–Sulu belt in central China. Our results show marked differences in the crustal structure between the Dabie and Sulu segments of the ultrahigh-pressure (UHP) orogen. The lower crust in the Dabie orogenic belt is dominantly of felsic-intermediate composition, whereas the crust beneath the Sulu segment is largely intermediate-mafic. The crust of the Dabie orogenic belt is thicker by ca. 3–5 km as compared to that of the surrounding region with the presence of an ‘orogenic root’. The crustal thickness is nearly uniform in the Dabie orogenic belt with a generally smooth crust–mantle boundary. A symmetrically thickened crust in the absence of any deep-structural features similar to that of the Yangtze block suggests no supportive evidence for the proposed northward subduction of the Yangtze continental block beneath the North China Block. We propose that the collision between the Yangtze and North China Blocks and extrusion caused crustal shortening and thickening, as well as delamination of the lower crust, resulting in asthenospheric upwelling and lower crustal UHP metamorphism along the Dabie Orogen. Our results also reveal the presence of a SE to NW dipping Moho in the North China Block (beneath the Tran-North China Orogen and Eastern Block), suggesting the fossil architecture of the northwestward subduction of the Kula plate. • The Dabie orogenic belt preserves a symmetrically thickened crust. • No supportive evidence from the crustal structure for deep subduction • Seismic evidence for delamination beneath the Dabie orogenic belt • Fossil architecture of the northwestward subduction of the Kula plate

Structure of the crust and the lithosphere beneath the southern Puna plateau from teleseismic receiver functions

We present a teleseismic P and S receiver function study using data from a temporary passive-source seismic array in the southernmost Puna plateau and adjacent regions. The P receiver function images show the distribution of crustal thickness and Vp/Vs ratio for this area. Over much of the southern Puna plateau, the crustal thickness is 50–55 km, whereas to the west a thicker crust (∼60 to 75 km) is observed beneath much of the Andean volcanic arc region. From the Puna southward, there is little obvious change in the crustal thickness across the border of the plateau (south of 28°S). The crust is seen to progressively thin towards the east in the Pampean Ranges where it is 35–40 km thick. The southern Puna plateau is characterized overall by a low crustal Vp/Vs ratio (less than 1.70), implying a felsic crustal composition. An anomalously high Vp/Vs ratio of 1.87 is observed beneath the Cerro Galan volcanic center, in the region where a prominent crustal low-velocity zone identified below ∼10 km depth probably extends into the lower crust. The crustal thickness determined under the Cerro Galan area (59 km) is close to that of the rest of the southern Puna. The prominent high Vp/Vs ratio and low-velocity zone beneath the Galan region implies the presence of a zone of partial melt or a magma chamber is consistent with hypothesis calling for lithospheric delamination beneath the Galan caldera. A widespread crustal low-velocity layer observed beneath much of the southern Puna, correlates well with crustal low-velocity anomalies observed by teleseismic tomography. The lithosphere–asthenosphere boundary beneath the array can be clearly observed by both P and S receiver functions at depths of 70–90 km in agreement with previous studies suggesting a thin lithosphere beneath the high elevated plateau. The mantle transition zone discontinuities appear at expected depths.

Crustal thickness and support of topography on Venus

The topography of a terrestrial planet can be supported by several mechanisms: (1) crustal thickness variations, (2) density variations in the crust and mantle, (3) dynamic support, and (4) lithospheric stresses. Each of these mechanisms could play a role in compensating topography on Venus, and we distinguish between these mechanisms in part by calculating geoid‐to‐topography ratios and apparent depths of compensation. By simultaneously inverting for mass anomalies at two depths, we solve for the spatial distribution of crustal thickness and a similar map of mass anomalies in the mantle, thus separating the effects of shallow and deep compensation mechanisms on the geoid. The roughly circular regions of mantle mass deficit coincide with the locations of what are commonly interpreted to be buoyant mantle plumes. Additionally, there is a significant geographic correlation between patches of thickened crust and mass deficits in the mantle, especially for spherical harmonic degree l < 40. These mass deficits may be interpreted either as lateral thermal variations or as Mg‐rich melt residuum. The magnitudes of mass deficits under the crustal highlands are roughly consistent with a paradigm in which highland crust is produced by melting of upwelling plumes. The mean thickness of the crust is constrained to a range of 8–25 km, somewhat lower than previous estimates. The best two‐layered inversion of gravity incorporates a dynamic mantle load at a depth of 250 km.

Bulk crustal properties in NE Tibet and their implications for deformation model

The crust beneath the northeastern (NE) Tibetan Plateau records far field effects of collision and convergence occurring between the Indian and Eurasian plates. A better structural understanding of the crust beneath NE Tibet can improve our understanding of Cenozoic deformation resulting from the India–Eurasia collision. Taking advantage of the relatively dense coverage in most areas in NE Tibet except for the Qaidam basin by regional seismic networks of Gansu and Qinghai Provinces, we isolate receiver functions from the teleseismic P wave data recorded from 2007 to 2009 and resolve the spatial distribution of crustal thickness and Vp/Vs ratio beneath NE Tibet from H–κ scanning. Our results can be summarized as: (1) NE Tibet is characterized by ~60-km-thick crust beneath the Nan Shan, Qilian Shan thrust belts and the Anyemaqen Shan, and 45–50km-thick crust beneath the Tarim basin, the Alashan depression and the Ordos basin; the crust thins gradually from west to east in addition to the previously observed pronounced thinning from south to north; (2) the crust of NE Tibet exhibits a relatively lower Vp/Vs ratio of 1.72 than the north China block and a decrease in average crustal Vp/Vs ratio with increasing crustal thickness; and (3) the crustal thicknesses are less than the values predicted by the simple isostatic model of the whole Tibetan plateau wherein the elevation is larger than 3.0km. Our observations can be explained by the hypothesis that deformation occurring in NE Tibet is predominated by upper-crustal thickening or lower-crust extrusion.

Towards self-consistent modeling of the martian dichotomy: The influence of one-ridge convection on crustal thickness distribution

In order to find an explanation for the origin of the martian crustal dichotomy, a number of recent papers have examined the effect of layered viscosity on the evolution of a degree-1 mantle convection, e.g. Roberts and Zhong [Roberts, J.H., Zhong, S., 2006. J. Geophys. Res. 111. E06013] and Yoshida and Kageyama [Yoshida, M., Kageyama, A., 2006. J. Geophys. Res. 111, doi:10.1029/2005JB003905. B03412]. It was found that a mid-mantle viscosity jump, combined with highly temperature- and depth-dependent rheology, are effective in developing a degree-1 convection within a short timescale. Such a layered viscosity profile could be justified by martian mineralogy. However, the effect of a degree-1 convective planform on the crustal thickness distribution has not yet been demonstrated. It is not obvious whether a thinner crust, due to sublithospheric erosion and crustal thinning, or a thicker crust, due to enhanced crustal production, would form above the hemisphere of mantle upwelling. Also, the general shape of the dichotomy, which is not strictly hemispherical, has not yet been fully investigated. Here we propose a model of the crustal patterns produced by numerical simulations of martian mantle convection, using the finite-volume multigrid code StagYY [Tackley, P.J., 2008. Phys. Earth Planet. Int. 107, 7–18, doi:10.1016/j.pepi.2008.08.005] A self-consistent treatment of melting, crustal formation and chemical differentiation has been added to models of three-dimensional thermal convection. This allows us to obtain global maps of the crustal thickness distribution as it evolves with time. The obtained results demonstrate that it is indeed possible to form a crustal dichotomy as a consequence of near degree-1 mantle convection early in Mars' history. We find that some of the observed patterns show intriguing first order similarities to the elliptical shape of the martian dichotomy. In all models, the region of thick crust is located over the region of mantle upwelling, which itself is a ridge-like structure spread over roughly one half of the planet, a planform we describe as “one-ridge convection.”

The early martian evolution—Constraints from basin formation ages

Impact basin formation ages give insight into the early evolution of a planet. The martian basins Hellas, Isidis and Argyre provide an important time-marker for the cessation of the magnetic dynamo and the crustal thickness distribution, both established before 4 Ga ago. No martian surfaces are older than 4.15 Ga based on crater count statistics, and all are younger than the oldest lunar ones. I show that the heavy bombardment period on the Moon and Mars evolved similarly, but endogenic processes have removed the oldest martian basin record. The basin-forming projectile population appears to be different from the impactor population observed today in the inner Solar System. It is yet uncertain whether the heavy bombardment period is cataclysmic or characterized by the decaying flux of planetary formation.

Crustal evaluation of the northern Red Sea rift and Gulf of Suez, Egypt from geophysical data: 3-dimensional modeling

Combined 3-D interpretations of gravity and aeromagnetic data have been used in addition to continental and marine seismic profiles, well logs and geological cross-sections. The combination of gravity and magnetic data has improved the geometry and the density distribution in the 3-D calculated profiles. Results of the current work reveal possible crustal thickness and density distribution between the sedimentary cover and the upper mantle including the Moho discontinuity. The Moho depth map of the region, as obtained from the 3-D modeling exhibits various crustal thickness distributions. The type of crust changes gradually from continental to oceanic, from north to south. The zone of crustal thinning coincides mostly with zones of low-density, heated anomalous upper mantle beneath the rift floor (7 km). The eastern plateaus (the Red Sea hills) show by far the largest crustal thickness in the region (32 km). The Moho relief, as indicated from magnetic interpretation, shows a poor flattening especially in the eastern region. This is contrary to what is given by other authors. However, the present results are in good agreement with the geothermal gradient values in the Red Sea. The interpretation of magnetic data of the Red Sea Rift shows that the spreading rate of the part south of latitude 26.5° N agrees well with the theoretical model, in the order of 0.7 cm/yr. Less agreement has been obtained in the part north of latitude 27.5° N. The magnetic anomalies along the axial portion of the rift floor, as deduced from the results of the regional and residual separation and the 3-D magnetic modeling, are mainly caused by the oceanic crustal structures beneath the graben.

Crustal balance and crustal flux from shortening estimates in the Central Andes

The Central Andes of South America form the second largest high elevation plateau on earth. Extreme elevations have formed on a noncollisional margin with abundant associated arc magmatism. It has long been thought that the crustal thickness necessary to support Andean topography was not accounted for by known crustal shortening alone. We show that this may in part be due to a two-dimensional treatment of the problem. A three-dimensional analysis of crustal shortening and crustal thickness shows that displacement of material towards the axis of the bend in the Central Andes has added a significant volume of crust not accounted for in previous comparisons. We find that present-day crustal thickness between 12°S and 25°S is accounted for (∼−10% to ∼+3%)with the same shortening estimates, and the same assumed initial crustal thickness as had previously led to the conclusion of a ∼25–35% deficit in shortening relative to volume of crustal material. We suggest that the present-day measured crustal thickness distribution may not match that predicted due to shortening, and substantial redistribution of crust may have occurred by both erosion and deposition at the surface and lower crustal flow in regions of the thermally weakened middle and lower crust.

Three-dimensional inversion of gravity data from the Main Ethiopian Rift

The crustal thickness distribution beneath the Main Ethiopian Rift is determined using a 3-D inverse approach to gravity data interpretation. The depths to the crust-upper mantle interface form the inversion parameters. These initial model parameters are constrained with the results of the seismic refraction experiment of the region. The degree of ambiguity of the final model parameters is then quantified using a Q-mode factor analysis approach. The results of the modelling indicate a strong crustal attenuation zone (≤31 km) closely associated with the rifting of the graben and an abrupt fall of the Moho interface on either side of the rift (51 km) related to the formation of the western and southeastern plateaus. However, no indication of crustal separation is observed. The semi-quantitative ambiguity analysis reveals the association of the model parameters from the southeastern plateau with the strong ambiguity region. These model parameters represent the depths to the Moho interface where the seismic control is relatively less.

Three dimensional inversion of gravity data from the main ethiopian Rift

A three-dimensional interpretation of the newly compiled Bouguer anomaly map of the Main Ethiopian Rift is discussed. Then, the crustal thickness distribution beneath the Main Ethiopian Rift is confirmed using a — dimensional inverse approach to gravity data interpretation. The depths to the crust-upper mantle interface form the inversion parameters. Both approaches are constrained with the results of the seismic refraction experiments of the region. The degree of ambiguity of the final model parameters is then quantified.The Bouguer anomalies along the axial portion of the rift floor, as deduced from the results of the regional and residual separation, are mainly caused by deep-seated structures. The high resolution 3-D forward modeling reveals a possible crustal thickness and density distribution beneath the graben.The results of the inversion confirm a strong crustal attenuation zone (≤ 31 km) closely associated with the rifting of the graben and an abrupt fall of the Moho interface on either side of the rift (up to 51 km) related to the formation of the western and southeastern plateaus. However, no indication of crustal separation is observed.The ambiguity analysis reveals that greater ambiguity of the model parameters exists in the southeastern plateau. There, these model parameters represent the depths to the Moho interface where the seismic control is relatively less.

An efficient, probabilistic neural network approach to solving inverse problems: Inverting surface wave velocities for Eurasian crustal thickness

Nonlinear inverse problems usually have no analytical solution and may be solved by Monte Carlo methods that create a set of samples, representative of the a posteriori distribution. We show how neural networks can be trained on these samples to give a continuous approximation to the inverse relation in a compact and computationally efficient form. We examine the strengths and weaknesses of this approach and use it to determine the full a posteriori distribution of crustal thickness from surface wave velocities. The solution to this inverse problem shows significant asymmetry and large uncertainties due to trade‐off with shear velocity structure around the Moho. We produce maps of maximum likelihood crustal thickness across Eurasia which are in agreement with current knowledge about the crust; thus we provide an independent confirmation of these models. In this application, characterized by repeated inversion of similar data, the neural network algorithm proves to be very efficient.

A model for the structure, composition and evolution of the Kenya rift

The seismic refraction-wide-angle reflection experiments carried out in 1985 and 1990 in the Kenya rift (KRISP '85 and KRISP '90) show major crustal thickness variations both along and across the rift. Along the rift axis crustal thickness varies from 35 km in the south beneath the Kenya dome to 20 km in the north beneath the Turkana region. Due to the distribution of crustal thickness beneath the rift flanks, it can be stated that the major amount of variation in crustal thickness along the rift axis is due to the Tertiary rifting episode. The northwards decrease in crustal thickness can be correlated with changes in surface topography (northwards decrease), rift width (northwards increase), surface estimates of extension (5–10 km in the south and 35–40 km in the north) and Bouguer gravity, the regional northwards increase of which can be explained entirely by the change in crustal thickness. Below the 750 km long axial rift profile, uppermost mantle P n velocities are low, being 7.5–7.7 km/s. However, under the northern part of the rift two layers with velocities of 8.1 km/s and 8.3 km/s are embedded in the low-velocity mantle material at 40–45 km and 60–65 km depth, respectively. In contrast, the wide-angle data show that beneath the Kenya dome, in the southern part of the rift, low mantle velocities occur down to at least 65 km depth. This mantle velocity structure is indicative of the depth to the onset of melting being at least 65 km beneath the northern part of the rift and thus not being shallower than the depth (45–50 km) to the onset of melting under the Kenya dome to the south. A profile across the rift north of the Kenya dome at the latitude of Lake Baringo shows that the low uppermost mantle P n velocity of 7.5–7.7 km/s and crustal thinning of 5–10 km is confined to below the surface expression of the rift. An abrupt change in Moho depths and P n velocities occurs as the rift boundaries are crossed. Beneath the rift flanks, normal P n velocities of 8.0–8.2 km/s occur. The presence of hot mantle material beneath the Kenya dome since the onset of volcanism here at 15–20 Ma is still compatible with the abrupt change in mantle P-wave velocities as the rift boundaries are crossed. Petrological interpretation of the seismic velocities indicates a few (up to 5) percent basaltic melt in the mantle below the rift except in the two layers with velocities greater than 8.0 km/s under the northern part of the rift where some crystal orientation (anisotropy) is necessary. Below about 45–50 km depth beneath the southern part of the rift the magma could exist as in situ partial melt. The above results, taken together with results from teleseismic studies, petrology and surface geology, indicate anomalously hot mantle material appearing below the present site of the Kenya rift at about 20–30 Ma. The active uprising of this anomalously hot mantle material since this time has given rise to widespread volcanism along the whole length of the rift and has modified the crust beneath the rift by mafic igneous underplating and intrusion, especially into the basal crustal layer. Accompanying the uprise of the anomalously hot mantle material minor crustal extension (5–10 km) has occurred beneath the Kenya dome in the southern part of the rift where crustal thickness is large (35 km). Under the Turkana region in the northern part of the rift, a greater amount of extension (35–40 km) has taken place and the crustal thickness is small (20 km), although the depth to the onset of melting under the northern part of the rift is, if anything, greater than under the southern part of the rift.

Role of pre-rift rheology in kinematics of extensional basin formation: constraints from thermomechanical models of Mediterranean and intracratonic basins

The role of pre-rift rheology on the kinematics of extensional basin formation is examined. Constraints obtained on the effective elastic thickness and level of necking, inferred from forward modelling of a number of Alpine/Mediterranean basins (including the Gulf of Lion margin, the Valencia Trough, the Tyrrhenian Sea and the Pannonian Basin), are interpreted in terms of the pre-rift rheology of the lithosphere underlying these basins. The Gulf of Lion/Tyrrhenian Sea basins and the Pannonian Basin appear to be end-members in terms of the inferred depth levels of necking. The models support the existence of spatial variations in crustal and lithospheric strength, as inferred from previous rheological modelling for other segments of the European lithosphere, and provide constraints on the ratio of crustal and subcrustal strength during extension. The results of these studies are compared with predictions on the kinematics of extension for a number of intracratonic basins, including the Black Sea basins, the Transantarctic Mountains/Ross Sea and Saudi Arabian Red Sea margins, and the Baikal and East African rifts. The kinematics of extension appears to be largely controlled by the (transient) thermal regime of the pre-rift lithosphere and the crustal thickness distribution. These usually result from orogenic processes operating on the lithosphere before extensional basin formation. Predictions are made for the level of external forces required to initiate rifting in intracratonic and Alpine/Mediterranean settings. The models also shed light on the relative parts played by far-field versus near-field stresses and inferred variations in strain rate during the evolution of these basins.

Crustal extension in the Oslo Graben, SE Norway: a method incorporating magmatism and erosion

Three methods for determining crustal extension were examined for use in the Late Paleozoic Oslo Graben of southeast Norway: 1. (1) analysis of faults and dikes, 2. (2) volume budget/isostasy, 3. (3) analysis of crustal thickness distribution assuming constant crustal volume during extension. These methods were found not to be suitable mainly due to rift age and magmatism. Instead, a new integrated model which combines the effect of Moho contours, volumes of mantle injecta and erosion, was used. Volumes of intracrustal mantle injecta are found from a model combining data on upper crustal felsic magmatic rocks with petrological and gravity inversion models. For the Vestfold Graben Segment in the south this implies twice the stretching estimate as compared to stretching derived from Moho contours alone, and two to three times the estimate from an (inadequate) set of fault and dike information. The batholiths mark zones of high crustal thinning, and magmatism was most likely caused by a combination of a thermal anomaly and lithospheric stretching.