Abstract. We present three-dimensional (3-D) models that describe the present-day thermal and rheological state of the lithosphere of the greater Kenya rift region aiming at a better understanding of the rift evolution, with a particular focus on plume–lithosphere interactions. The key methodology applied is the 3-D integration of diverse geological and geophysical observations using gravity modelling. Accordingly, the resulting lithospheric-scale 3-D density model is consistent with (i) reviewed descriptions of lithological variations in the sedimentary and volcanic cover, (ii) known trends in crust and mantle seismic velocities as revealed by seismic and seismological data and (iii) the observed gravity field. This data-based model is the first to image a 3-D density configuration of the crystalline crust for the entire region of Kenya and northern Tanzania. An upper and a basal crustal layer are differentiated, each composed of several domains of different average densities. We interpret these domains to trace back to the Precambrian terrane amalgamation associated with the East African Orogeny and to magmatic processes during Mesozoic and Cenozoic rifting phases. In combination with seismic velocities, the densities of these crustal domains indicate compositional differences. The derived lithological trends have been used to parameterise steady-state thermal and rheological models. These models indicate that crustal and mantle temperatures decrease from the Kenya rift in the west to eastern Kenya, while the integrated strength of the lithosphere increases. Thereby, the detailed strength configuration appears strongly controlled by the complex inherited crustal structure, which may have been decisive for the onset, localisation and propagation of rifting.

No abstract

The density structure of the oceanic lithosphere north of Iceland is key for understanding the effects of the Iceland plume on the greater Jan Mayen‐East Greenland Region. We obtain the 3‐D density structure of the sediments and the crust from regional reflection and refraction seismic lines. The temperature and related density structures of the mantle between 50 and 250 km are derived from a shear wave velocity (Vs) tomography model. To assess the density between the Moho and 50‐km depth, we combine forward and inverse 3‐D gravity modeling. Beneath the Middle Kolbeinsey Ridge (MKR) region, a deep, broad negative mantle density anomaly occurs under the Kolbeinsey Ridge. It is overlain by a narrower uppermost mantle NE‐SW elongated negative density anomaly, which is increasingly displaced eastward of the spreading axis northward. It crosses the West Jan Mayen Fracture Zone and becomes weaker approaching the Mohn's spreading ridge. The effect of this anomaly is consistent with significantly shallower basement on the eastern side of the MKR. We interpret this as the result of thermal erosion of the lithosphere by hot asthenospheric flow out from the Iceland plume, possibly the main driver for several eastward jumps of the MKR during the last 5.5 Ma. The cause for the deviation of the flow may be that the West Jan Mayen Fracture Zone is easier to cross in a region where the difference in lithospheric thickness is small. That implies that the bottom lithospheric topography exerts a regional but not local influence on upper asthenospheric flow.

Geophysical data acquisition in oceanic domains is challenging, implying measurements with low and/or nonhomogeneous spatial resolution. The evolution of satellite gravimetry and altimetry techniques allows testing 3‐D density models of the lithosphere, taking advantage of the high spatial resolution and homogeneous coverage of satellites. However, it is not trivial to discretise the source of the gravity field at different depths. Here, we propose a new method for inferring tectonic boundaries at the crustal level. As a novelty, instead of modeling the gravity anomalies and assuming a flat Earth approximation, we model the vertical gravity gradients (VGG) in spherical coordinates, which are especially sensitive to density contrasts in the upper layers of the Earth. To validate the methodology, the complex oceanic domain of the Caribbean region is studied, which includes different crustal domains with a tectonic history since Late Jurassic time. After defining a lithospheric starting model constrained by up‐to‐date geophysical data sets, we tested several a‐priory density distributions and selected the model with the minimum misfits with respect to the VGG calculated from the EIGEN‐6C4 data set. Additionally, the density of the crystalline crust was inferred by inverting the VGG field. Our methodology enabled us not only to refine, confirm, and/or propose tectonic boundaries in the study area but also to identify a new anomalous buoyant body, located in the South Lesser Antilles subduction zone, and high‐density bodies along the Greater, Lesser, and Leeward Antilles forearcs.

Previous thermomechanical modeling studies indicated that variations in the temperature and strength of the crystalline crust might be responsible for the juxtaposition of domains with thin‐skinned and thick‐skinned crustal deformation along strike the foreland of the central Andes. However, there is no evidence supporting this hypothesis from data‐integrative models. We aim to derive the density structure of the lithosphere by means of integrated 3‐D density modeling, in order to provide a new basis for discussions of compositional variations within the crust and for future thermal and rheological modeling studies. Therefore, we utilize available geological and geophysical data to obtain a structural and density model of the uppermost 200 km of the Earth. The derived model is consistent with the observed Bouguer gravity field. Our results indicate that the crystalline crust in northern Argentina can be represented by a lighter upper crust (2,800 kg/m3) and a denser lower crust (3,100 kg/m3). We find new evidence for high bulk crustal densities >3,000 kg/m3 in the northern Pampia terrane. These could originate from subducted Puncoviscana wackes or pelites that ponded to the base of the crystalline crust in the late Proterozoic or indicate increasing bulk content of mafic material. The precise composition of the northern foreland crust, whether mafic or felsic, has significant implications for further thermomechanical models and the rheological behavior of the lithosphere. A detailed sensitivity analysis of the input parameters indicates that the model results are robust with respect to the given uncertainties of the input data.

<p>We introduce a new approach for 3D joint inversion of potential fields and its derivatives under the condition of constraining data and information. The interactive 3D gravity and magnetic application IGMAS (Interactive Gravity and Magnetic Application System) has been around for more than 30 years, initially developed on a mainframe and then transferred to the first DOS PCs, before it was adapted to Linux in the ’90s and finally implemented as a cross-platform Java application with GUI called IGMAS+. The software has proven to be very fast, accurate and easy to use once a model has been established. Since 2019 IGMAS+ has been maintained and developed in the Helmholtz Centre Potsdam – GFZ German Research Centre by the staff of Section 4.5 – Basin Modelling and ID2 – eScience Centre.</p><p>The analytical solution of the volume integral for the gravity and magnetic effect of a homogeneous body is based on the reduction of the three-folded integral to an integral over the bounding polyhedrons (in IGMAS polyhedrons are built by triangles). Later the algorithm has been extended to cover all elements of the gravity tensor as well. Optimized storage enables very fast inversion of densities and changes to the model geometry and this flexibility makes geometry changes easy. The geometry is updated and the gravity is recalculated immediately after each change. Because of the triangular model structure, IGMAS can handle complex structures (multi Z surfaces) like the overhangs of salt domes very well. Geophysical investigations may cover huge areas of several thousand square kilometers but also models of Applied Geophysics at a meter scale. Due to the curvature of the Earth, the use of spherical geometries and calculations is necessary.</p><p>The model technique is user-friendly because it is highly interactive, operates ideally in real-time whilst conserving topology and can be used for both flat (regional) and spherical models (global) in 3D. Modeling is constrained by seismic and structural input from independent data sources and is essential toward true integration of 3D thermal modeling or even Full Waveform Inversion. We are close to the demand for treating all geophysical methods in a single model of the subsurface and aim of fulfilling most of the constraints: measurements and geological plausibility.</p><p>We demonstrate the flexibility of the software by modeling: (1) the southern segment of the Central Andes which is designed to assess the relationship between the characteristics of the overriding plate and the deformation and dynamics of the subduction system; (2) the South Caribbean margin which defines the two flat-slab subductions of the Nazca Plate and the Caribbean Plate, with variable mantle density distribution implemented by voxels; (3) the North Patagonian Massif Plateau in Argentina which provides insight into the main height differences between the plateau and the surroundings; and (4) an Alpine model which interrogates the strength of the lithosphere at different locations through the Alps and their forelands.</p>

<p>In the context of research software sustainability, in this work we present the case of IGMAS+ (Interactive Gravity and Magnetic Application System) – a software tool for interactive 3-D numerical modelling, inversion, visualization and interpretation of potential fields together with some applications.</p><p>Modern workflows for geophysical interpretation and construction of 3-D data-constrained subsurface geophysical models in complex geological environments require software tools capable of handling multiple interdisciplinary and inhomogeneous input data, both seismic and non-seismic, like gravity and magnetics with their gradients, or magnetotelluric. These aspects imply big challenges not only in implementation and development of the modelling software, but also in organizing communication within the user community. A user of a research software often plays a role of a tester. Our joint goal as a research software community is to improve communication between developers and users, foster related technologies and overall culture of testing while using the research software.</p><p>Through the example of IGMAS+ we illustrate how a research software based on clear concepts with a well-established core algorithm can survive in the course of 40 years of development and still be useful, popular and demanded, at the same time being free for research and education purposes with a long-term plan to stay so. The software is largely used in creation of 3-D data-constrained subsurface structural density and susceptibility models at different spatial scales. Both large-scale models (thousands of square km) and regional (hundreds of square km), which we illustrate on several lithospheric-scale case studies, are important for understanding the drivers of geohazards. These models are necessary for efficient and sustainable extraction of resources, such as groundwater, deep geothermal energy or hydrocarbons, from sedimentary basins. Medium-scale models support studies on the usage of subsurface as thermal, electrical or material storage in the context of energy transformation. On the other hand, small-scale (tens of square km) models are largely used in applied geophysics, typically in sub-salt and sub-basalt settings. On the microscale (1 - 5 meters), the software presented here has also been used very successfully in the context of archaeological research and natural cavity localizations. Creation of all these models benefit a lot from the interactive modelling and inversion capabilities.</p><p>IGMAS+ is maintained and developed at the Helmholtz Centre Potsdam – GFZ German Research Centre by the effort of a research and development group limited by staff and time capacities. In these circumstances we find important to share our experience in organizing the development of the software and its documentation, the support of users, as well as our vision on the exchange of experience among the users.</p>

Abstract. We present 3D models that describe the present-day thermal and rheological state of the lithosphere of the greater Kenya Rift region aiming at a better understanding of the rift evolution, with a particular focus on plume-lithosphere interactions. The key methodology applied is the 3D integration of diverse geological and geophysical observations using gravity modelling. Accordingly, the resulting lithospheric-scale 3D density model is consistent with (i) reviewed descriptions of lithological variations in the sedimentary and volcanic cover, (ii) known trends in crust and mantle seismic velocities as revealed by seismic and seismological data, and (iii) the observed gravity field. This data-based model is the first to image a 3D density configuration of the crystalline crust for the entire region of Kenya and northern Tanzania. An Upper and a Basal Crustal Layer are differentiated, each composed of several domains of different average densities. We interpret these domains to trace back to the Precambrian terrane amalgamation associated with the East African Orogen and to magmatic processes during Mesozoic and Cenozoic rifting phases. In combination with seismic velocities, the densities of these crustal domains are indicative of compositional differences. The derived lithological trends have been used to parameterize steady-state thermal and rheological models. These models indicate that crustal and mantle temperatures decrease from the Kenya Rift in the west to eastern Kenya, while the integrated strength of the lithosphere increases. Thereby, the detailed strength configuration appears strongly controlled by the complex inherited crustal structure, which may have been decisive for the onset, localisation, and propagation of rifting.