Reply on RC2

Abstract. Volgo–Uralia is a Neoarchaean easternmost part of the East European craton. Recent seismic studies of the Volgo–Uralian region provided new insights into the crustal structure of this area. In this study, we combine satellite gravity and seismic data in a common workflow to perform a complex study of Volgo–Uralian crustal structure, which is useful for further basin analysis of the area. In this light, a new crustal model of the Volgo–Uralian subcraton is presented from a step-wise approach: (1) inverse gravity modelling followed by (2) 3D forward gravity modelling. First, inversion of the satellite gravity gradient data was applied to determine the Moho depth for the area. Density contrasts between crust and mantle were varied laterally according to the tectonic units present in the region, and the model is constrained by the available active seismic data. The Moho discontinuity obtained from the gravity inversion was consequently modified and complemented in order to define a complete 3D crustal model by adding information on the sedimentary cover, upper crust, lower crust, and lithospheric mantle layers in the process of forward gravity modelling, where both seismic and gravity constraints were respected. The obtained model shows crustal thickness variations from 32 to more than 55 km in certain areas. The thinnest crust with a thickness below 40 km is found beneath the Precaspian basin, which is covered by a thick sedimentary layer. The thickest crust is located underneath the Ural Mountains as well as in the centre of the Volgo–Uralian subcraton. In both areas the crustal thickness exceeds 50 km. At the same time, initial forward gravity modelling has shown a gravity misfit of ca. 95 mGal between the measured Bouguer gravity anomaly and the forward calculated gravity field in the central area of the Volgo–Uralian subcraton. This misfit was interpreted and modelled as a high-density lower crust, which possibly represents underplated material. Our preferred crustal model of the Volgo–Uralian subcraton respects the gravity and seismic constraints and reflects the main geological features of the region with Moho thickening in the cratons and under the Ural Mountains and thinning along the Palaeoproterozoic rifts, Precaspian sedimentary basin, and Pre-Urals foredeep.

Geophysical Inverse Theory and Regularization Problems

We present a digital crustal model in North China Craton (NCC). The construction of crustal model is based on digitization of original seismic sounding profiles, and new results of three-dimensional structure images of receiver functions. The crustal model includes seismic velocity and thickness of crustal layers. The depths to Moho indicate a thinning crust ~30 km in the east areas and a general westward deepening to more than 40 km in the west. The P wave velocity varies from 2.0 to 5.6 km/s in the sedimentary cover, from 5.8 to 6.4 km/s in the upper crust, and from 6.5 to 7.0 km/s in the lower crust. By analyzing regional trends in crustal structure and links to tectonic evolution illustrated by typical profiles, we conclude that: (1) The delimited area by the shallowing Moho in the eastern NCC represents the spatial range of the craton destruction. The present structure of the eastern NCC crust retains the tectonic information about craton destruction by extension and magmatism; (2) The tectonic activities of the craton destruction have modified the crustal structure of the convergence boundaries at the northern and southern margin of the NCC; (3) The Ordos terrene may represent a relatively stable tectonic feature in the NCC, but with the tectonic remnant of the continental collision during the assembly of the NCC in the north-east area and the response to the lateral expansion of the Tibetan Plateau during the Cenozoic in the south-west.

Aegean crustal thickness inferred from gravity inversion. Geodynamical implications

Seismic structures of Zhenbei and Huangyan seamounts and their postspreading volcanic evolution

Crustal velocity structure in the southern edge of the Central Alborz (Iran)

Seismic velocity structure across the Fennoscandia–Sarmatia suture of the East European Craton beneath the EUROBRIDGE profile through Lithuania and Belarus

<p>The Congo basin (<em>CB</em>), also named Cuvette Centrale for its bowl shape, occupies a large part of the Congo Craton, which is composed of several amalgamated Archean cratonic blocks, surrounded by Paleo- and Meso- Proterozoic mobile belts. It started to form from a rift phase, during the late Mesoproterozoic (about 1100 Myr). This age, obtained from the interpretation of the almost 3000 km of seismic reflection profiles, is older than that assumed in previous studies and corresponds to a time prior to that of Rodinia assembly. In this tectonic scenario, the <em>CB</em> formation can be related to one of the final phases of the supercontinent Columbia break-up, resulted in several-failed rift. The extensional phase that produced the formation of a very heterogeneous basement, characterized by several basins and highs, NW-SE aligned, could have been likely the effect of the action of a slow multi-divergent velocity (i.e., multi-directional extension) on a cratonic lithosphere, which have induced the initial subsidence of the <em>CB</em> in a weaker part of the craton. The amalgamation of the cratons, composing the basement of the <em>CB</em>, likely left a weak zone in the suture areas, corresponding to the central part of the CB, which could have been more easily deformed, under the influence of tectonic stresses.</p><p>We implemented 3D geodynamic models, using the thermomechanical <em>I3ELVIS</em> code to test the hypothesis that the complex structures of the <em>CB</em> basement are the product of a slow multi-divergent velocity, acting on a cratonic area. The results of the numerical models are used to implement forward gravity models to estimate the temporal variations of the gravity effect of the tectonic structures formed during the simulations. Finally, we compared the forward gravity models with the present-day gravity field, in order to demonstrate the consistency between the modelled and observed main structures of the <em>CB</em>. The main results, in terms of topography variations, well reproduce the first-order basement depth variations of the <em>CB</em>. In particular, they produce the formation of an almost circular depressed area in the central part of the model, intersected by two strongly subsided elongated structures, orthogonal each other, whose topography tend to increase with time. The comparison between the forward gravity models and the observed gravity anomalies (gravity disturbance variations), shows that two fields are characterized by a similar alternation of weak positive and strong negative gravity anomalies. However, the modelled anomalies show a smoother trend and higher amplitude, being related to the density and topography variations induced by the upwelling of the asthenosphere, while the observed gravity field is strongly influenced by the sedimentation not simulated in our model.</p>

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.

The vertical and horizontal distribution of present-day continental deformation is examined to see how tectonic movements may be related to large wavelength perturbations to the temperature and pressure experienced by rocks in the crust. Earthquakes are generally restricted to the upper part of the continental crust. The lower crust is usually aseismic and assumed to be weaker. The uppermost mantle beneath continental regions has minor seismic activity that does not account for much deformation, but probably indicates an important strength contrast between the lower continental crust and the upper mantle. The maximum focal depth of earthquakes in any region appears to be limited by temperature, with most restricted to material colder than 350± 100 °C in the crust and colder than 700± 100 °C in the mantle. At length scales long compared with the thickness of the brittle upper crust, the deformation in regions of continental extension or shortening appears to be continuous, even though, in reality, discontinuous movement on faults occurs. This probably indicates that the deformation is dominated by distributed flow in the ductile portion of the lithosphere and not by the behaviour of the thin brittle upper crust. The distribution of seismicity, elevation contrasts and vertical movements at the surface suggests that there is little spatial separation between the brittle deformation in the upper crust and the ductile deformation below on length scales larger than the lithosphere thickness. For this reason, and because of the short thermal time constant of the crust, long-wavelength perturbations to the thermal regime are more influenced by the behaviour of the lithosphere as a whole than by the precise geometry of deformation in the crust. Large-scale regional metamorphism in zones of shortening may result from the re-establishment of the initial geotherm in thickened crust when the lower part of the lithosphere detaches and falls into the asthenosphere. In regions of extension, an increased geothermal gradient is unlikely to result in regional metamorphism unless magmatic augmentation to the heat supply is important. However, if the stretched region is covered by thick sediments, the basement may experience a small increase in temperature and remain significantly hotter than it would be if there were no sediment cover. While unlikely to account for significant metamorphism, this effect may strongly influence the rheological behaviour of the lithosphere in extending regions. The rapid vertical movements associated with syn- or post-orogenic normal faulting in regions of large-scale crustal thickening are probably at least as important in exhuming mid-crustal metamorphosed rocks, and in disrupting patterns of isograds, as those associated with erosion.

The role of gas flushing on magma reservoir crystallization and its consequences for the growth of planetary crust

A three-dimensional inversion technique is developed to investigate the structure of the oceanic crust, using high quality offshore bathymetry, gravity and seismic data. The gravity signatures associated with variations in the thickness of the oceanic crust are isolated from the observed free-air anomaly by subtracting the gravitational effects of seafloor topography and the upper mantle thermal structure, downward continued to the mean depth of the crust/mantle interface and converted onto the relief on that surface. The thickness of the oceanic crust is then calculated by subtracting sea water depth from the depth of the gravity-inferred crust/mantle interface. Seismic refraction data was introduced directly as a constraint in the construction of the initial model for the configuration of the crust/mantle interface and the iterative process of the 3- D joint inversion to reduce the ambiguity in gravityinterpretation. This technique can be easily applied to the off-shore areas to interpret bathymetry, gravity and seismic data that have been routinely collected for the purpose of geophysical exploration. Compared to the unconstrained gravity inversion, this technique can predict a 3-D crustal model that fits better both gravity and seismic observation data of the study area. Introduction The non-uniqueness is an inherent problem in gravity inversion: the observed gravity field cannot be inverted to obtain a unique subsurface structure, unless some additional restrictions are imposed'. The ambiguity in gravity interpretation may he reduced by placing bounds on the density contrast and depth of the subsurface structure, which can be determined by applying certain physical principles. In the present study we propose a method to use seismic data to constrain the depth and configuration of a subsurface density discontinuity in a 3-D gravity modeling. The results of the seismically constrained gravity interpretation are in good agreement with seismic results, and the misfit of the predicted gravity field to the observed does not increase. Methodology The method is simple in principle: in a study area where both seismic and gravity data are available, we use seismic data to construct a framework for gravity modeling, and use gravity inversion technique to resolve the details of the configuration and depth of the subsurface density discontinuity. The implementation of the method may include the following steps. In the initial model for a subsurface interface, those points (nodes) that follow the projections of the seismic profiles are given the depths determined in seismic refraction (or reflection) surveys. Other points on the interface may be chosen at a constant depth (e.g., the average depth of the interface in the area, or the mean value of !he seismic depths). Doing so may introduce discontinuity, or the artificial impulse-shaped variations, to the configuration of the interface. To solve this problem we further CU1several narrow bands on that interface, These narrow hands are centered by the projections of the seismic profiles.

Oceanic and continental lithosphere distribution within the eastern Mediterranean is not well understood. A gravity inversion, incorporating a lithosphere thermal gravity anomaly correction, has been used to map Moho depth, crustal thickness and lithosphere thinning for the eastern Mediterranean, from which the distribution of oceanic and continental lithosphere, the structure of the ocean–continent transition (OCT) and the location of the continent–ocean boundary (COB) can be determined. The gravity inversion results show thin crust and high continental lithosphere thinning under the Ionian Sea and the Herodotus Basin, consistent with these basins being underlain by oceanic crust. Moho depths from gravity inversion are in agreement with seismic refraction estimates in these basins. Highly thinned continental crust is predicted under the offshore Sirte and Levant basins. The sharp decrease in crustal thickness predicted by gravity inversion off the Libyan and Egyptian coast gives an indication of COB location. Crustal thickness and continental lithosphere thinning determined from gravity inversion have also been used to explore the relationship between the Cretaceous West and Central African Rift System (WCARS: Benue Trough, Chad, Central African Shear Zone (CASZ) and Sudan basins) and the eastern Mediterranean basins; continuity between the Cretaceous WCARS and the eastern Mediterranean basins is not apparent in the gravity inversion results.

The latest seismic data and improved information about the subglacial bedrock relief are used in this study to estimate the sediment and crustal thickness under the Antarctic continent. Since large parts of Antarctica are not yet covered by seismic surveys, the gravity and crustal structure models are used to interpolate the Moho information where seismic data are missing. The gravity information is also extended offshore to detect the Moho under continental margins and neighboring oceanic crust. The processing strategy involves the solution to the Vening Meinesz-Moritz’s inverse problem of isostasy constrained on seismic data. A comparison of our new results with existing studies indicates a substantial improvement in the sediment and crustal models. The seismic data analysis shows significant sediment accumulations in Antarctica, with broad sedimentary basins. According to our result, the maximum sediment thickness in Antarctica is about 15 km under Filchner-Ronne Ice Shelf. The Moho relief closely resembles major geological and tectonic features. A rather thick continental crust of East Antarctic Craton is separated from a complex geological/tectonic structure of West Antarctica by the Transantarctic Mountains. The average Moho depth of 34.1 km under the Antarctic continent slightly differs from previous estimates. A maximum Moho deepening of 58.2 km under the Gamburtsev Subglacial Mountains in East Antarctica confirmed the presence of deep and compact orogenic roots. Another large Moho depth in East Antarctica is detected under Dronning Maud Land with two orogenic roots under Wohlthat Massif (48–50 km) and the Kottas Mountains (48–50 km) that are separated by a relatively thin crust along Jutulstraumen Rift. The Moho depth under central parts of the Transantarctic Mountains reaches 46 km. The maximum Moho deepening (34–38 km) in West Antarctica is under the Antarctic Peninsula. The Moho depth minima in East Antarctica are found under the Lambert Trench (24–28 km), while in West Antarctica the Moho depth minima are along the West Antarctic Rift System under the Bentley depression (20–22 km) and Ross Sea Ice Shelf (16–24 km). The gravimetric result confirmed a maximum extension of the Antarctic continental margins under the Ross Sea Embayment and the Weddell Sea Embayment with an extremely thin continental crust (10–20 km).

Evidence for Proterozoic Collision from Airborne Magnetic and Gravity Studies in Southern Granulite Terrain, India and Signatures of Recent Tectonic Activity in the Palghat Gap