Crustal Studies Research Articles

Upper mantle flow and crustal deformation patterns beneath the Dangerous Grounds and Borneo where multiple plates converge in South China Sea revealed by 3-D anisotropic magnetotelluric imaging

SUMMARY A large-scale magnetotelluric (MT) study was carried out in offshore Borneo to understand the lithospheric structure in this geologically complex region where three tectonic plates converge and past crustal studies generated long-standing debates. Marine MT data were acquired at 1416 stations with 3 km spacing along 13 regional lines (covering 4677 line-km) with periodicities of 0.1 to 10 000 s. These were inverted in 3-D incorporating electrical anisotropy with cross-gradients constraints between vertical and horizontal resistivities and the results were validated with resistivity well-logs from several exploration wells. The models reveal widespread presence of electrically resistive upper crustal and uppermost mantle layers, each underlain by a laterally varying conductive and anisotropic layer. The geometries of the anisotropic layers suggest large-scale ductile flow, thrusting and folding forming belts of alternating deep roots and thin lithosphere, consistent with multiple underthrusting/subduction. Our results are in agreement with the seismologically detected lithospheric variations in northern Borneo suggesting onshore–offshore continuity and a common lithospheric–asthenospheric origin for the deformation patterns. Over thinned lithosphere, we found consistent low resistivity and high anisotropy anomalies in the mantle above the spatial locations of fast shear-wave bodies imaged by recent seismological workers which we interpret to indicate post-subduction lithospheric–asthenospheric ductile flow in response to multidirectional regional compression. We demonstrate that these zones are spatially correlated with the distribution of mainly Cretaceous ophiolitic rocks and melanges exposed onshore and suggest that serpentinization of mantle peridotite can explain some of the anisotropic conductivity anomalies. The taper zones of these deep anisotropic conductors are also associated with Neogene sub-basins, high thermal gradients, and intrasedimentary magmatic bodies indicating a link between lithospheric thinning and magmatism. We propose that such anomalies could be important pathfinders for geothermal and natural hydrogen systems in the ongoing global drive for carbon-free energy resources.

Tomofast-x 2.0: an open-source parallel code for inversion of potential field data with topography using wavelet compression

Abstract. We present a major release of the Tomofast-x open-source gravity and magnetic inversion code that incorporates several functionalities enhancing its performance and applicability for both industrial and academic studies. The code has been re-designed with a focus on real-world mineral exploration scenarios, while offering flexibility for applications at regional scale or for crustal studies. This new version includes several major improvements: magnetisation vector inversion, inversion of multi-component magnetic data, wavelet compression, improved handling of topography with support for non-uniform grids, a new and efficient parallelisation scheme, a flexible parameter file, and optimised input–output operations. Extensive testing has been conducted on a large synthetic dataset and field data from a prospective area of the Eastern Goldfields (Western Australia) to explore new functionalities with a focus on inversion for magnetisation vectors and magnetic susceptibility, respectively. Results demonstrate the effectiveness of Tomofast-x 2.0 in real-world studies in terms of both the recovery of subsurface features and performances on shared and distributed memory machines. Overall, with its updated features, improved capabilities, and performances, the new version of Tomofast-x provides a free open-source, validated advanced and versatile tool for constrained gravity and magnetic inversion.

Hamiltonian Monte Carlo based elastic full-waveform inversion of wide-angle seismic data

SUMMARY Full-waveform inversion (FWI) of seismic data provides quantitative constraints on subsurface structures. Despite its widespread success, FWI of data around the critical angle is challenging because of the abrupt change in amplitude and phase at the critical angle and the complex waveforms, especially in the presence of a sharp velocity contrast, such as at the Moho transition zone (MTZ). Furthermore, the interference of refracted lower crustal (Pg) and upper mantle (Pn) arrivals with the critically reflected Moho (PmP) arrivals in crustal and mantle studies makes the application of conventional FWI based on linearized model updates difficult. To address such a complex relationship between the model and data, one should use an inversion method based on a Bayesian formulation. Here, we propose to use a Hamiltonian Monte Carlo (HMC) method for FWI of wide-angle seismic data. HMC is a non-linear inversion technique where model updates follow the Hamiltonian mechanics while using the gradient information present in the probability distribution, making it similar to iterative gradient techniques like FWI. It also involves procedures for generating distant models for sampling the posterior distribution, making it a Bayesian method. We test the performance and applicability of HMC based elastic FWI by inverting the non-linear part of the synthetic seismic data from a three-layer and a complex velocity model, followed by the inversion of wide-angle seismic data recorded by two ocean bottom seismometers over a 70 Ma old oceanic crustal segment in the equatorial Atlantic Ocean. The inversion results from both synthetic and real data suggest that HMC based FWI is an appropriate method for inverting the non-linear part of seismic data for crustal studies.

Efficient magnetotelluric data acquisition using irregular stations and compressive sensing reconstruction

A major challenge in magnetotelluric (MT) surveys is the limited number of stations that can be collected due to the cost of receiver instrumentation and the time required to deploy them in large numbers. Many equal-distance acquisition grid used in MT surveys are either over sampling or faced with severe aliasing. We have applied the compressive sensing theory to MT acquisition and developed an efficient acquisition approach that uses optimized station layout with sparse irregular patterns. This acquisition strategy uses the optimized survey designs to acquire a sparse data set first, and then applies a compressive sensing reconstruction to obtain the equivalent dense data set on a regularly spaced grid. Once reconstructed, the dense data can be used in the traditional manner in the subsequent steps of processing, inversion, and interpretation. Our simulations show that comparable information can be obtained as that from a full regular grid in exploration settings by using as fewer as 25% stations. Furthermore, redeploying the same number of stations in a coarse grid used crustal studies can collect much more information. This new acquisition approach can lead to significant savings in both the acquisition cost and operational time and to maximized information from a given number of stations. The substantial savings in the cost, or the increase in the information content, could make many geoscientific investigations viable that might otherwise be impossible due to limited budget.

Crustal study based on integrated geophysical techniques in the Northwestern Himalayas, Pakistan

The present research aims to explicate the tectonics of the northwest Himalayas, Pakistan, based on integrated gravity, magnetic and earthquake data. The raw gravity and magnetic data were processed, and seven isoanomaly maps were prepared for qualitative interpretation, whereas a gravity model was prepared along the selected profile for quantitative interpretation. The local earthquake data have also been used to identify the various seismogenic layers in the subsurface and to understand how seismicity relates to regional structural features. Three northwest‐trending thick‐skinned blind basement faults are identified from the present study, namely the Indus Kohistan Seismic Zone (IKSZ), Bagh Basement Fault and Hazara Lower Seismic Zone. The IKSZ is the tectonically most active among them and responsible for the 2005 Kashmir earthquake (Mw = 7.6) and 1974 Pattan earthquake (Mw = 6.2) in the region. The present study also delineated the prominent location of the Main Central Thrust in which the high‐grade metamorphic rocks of Higher Himalayan Crystalline thrust over low‐grade metamorphic rocks of Lesser Himalayas along this thick‐skinned fault. The field evidence such as ductile deformation, zone of inverted metamorphism, and brittle deformation also confirm this location. The sharp bending of the crystalline crust, as well as deep basement just south of Kohistan Island Arc (KIA), indicate that the high density and susceptibility rocks of this arc thrust on the leading margin of the Indian Plate along the Main Mantle Thrust. This thrust fault is a major suture zone separating the KIA in the north from the Indian Plate in the south. The other faults in the region exist in the sedimentary/meta‐sedimentary wedge and are thin‐skinned. It is inferred from the current investigation that the study area exhibits a composite style of deformation. Moreover, the gravity model combined with earthquake data suggested the existence of multiple décollement zones below the Hazara‐Kashmir Syntaxis. The model envisaged an increase in the thickness of the Indian Plate continental crust in the northeast direction.

Compiling ship and airborne measurements for the Antarctic's second-generation magnetic anomaly map

In 2001, the Antarctic Digital Magnetic Anomaly Project produced the ADMAP-1 compilation that included the first magnetic anomaly map of the region south of 60◦S. To help fill ADMAP-1’s regional coverage gaps, the international geomagnetic community from 2001 through 2014 acquired an additional 2.0+ million line-km of airborne and marine magnetic anomaly data. These new data together with surveys that were not previously in the public domain significantly upgraded the ADMAP compilation for Antarctic crustal studies. The merger of the additional data with ADMAP-1’s roughly 1.5 million line-km of survey data produced the second-generation ADMAP-2 compilation. The present study comprehensively reviews the problems and progress in merging the airborne and ship magnetic measurements obtained in the harsh Antarctic environment since the first International Geophysical Year (IGY 1957–58) by international campaigns with disparate survey parameters. For ADMAP-2, the newly acquired data were corrected for the diurnal and International Geomagnetic Reference Field effects, edited for high-frequency errors, and levelled to minimize line-correlated noise. ADMAP-2 provides important new constraints on the enigmatic geology of the Gamburtsev Subglacial Mountains, Prince Charles Mountains, Dronning Maud Land, and other poorly explored Antarctic areas. It links widely separated outcrops to help unify disparate geologic and geophysical studies for new insights on the global tectonic processes and crustal properties of the Antarctic. It also supports studies of the Antarctic ice sheet’s geological controls, the crustal transitions between Antarctica and adjacent oceans, and the geodynamic evolution of the Antarctic crust in the assembly and break-up of the Gondwana and Rodinia supercontinents.

Elemental Composition Analysis of Soil Samples from Bayelsa State in the Niger Delta Region of Nigeria

This study analysis the elemental composition and concentration of elements in the soils of the study area to ascertain degree of elemental enhancement in the soil resulting from anthropogenic activities with possible soil contamination, human health and environmental detriment. The study area was divided into eight grids and two soil samples per grid from over burden to a depth of 900mm was collected randomly in each grid. The samples were prepared using standard methods and analyzed with a linear accelerator. The number of detectable elements and their quantitative information was extracted from the elemental spectral signatures. . The result showed a high concentration value in some elements in the soil samples above values of elemental concentrations in soils from other reported studied region. In addition, Aluminium, Strontium, Barium, Gallium etc also showed an extremely high value in their concentration that exceeds the world wide mean range upper limit values in crustal soil study published. The study indicates some degree of potential contamination and therefore necessitate a regular periodic monitoring study to reduce potential health detriment to humans and the environment to as low as reasonably possible.

Investigation of the consistency of Swarm data and CHAOS-6 towards crustal magnetic field studies in China

Investigation of the consistency of Swarm data and CHAOS-6 towards crustal magnetic field studies in China

The velocity structures derived from anisotropic inversion of teleseismic receiver functions around Flores Island, Indonesia

We determine the shear wave velocity structure beneath a seismic station located in the Flores Island using anisotropic inversion of Neighbourhood algorithm. The study area is located in the boundary transition zone from subduction to collision in eastern part of Indonesia, providing an ideal and unique place for crustal anisotropic studies. The previous crustal study using receiver function analysis conducted around this region found the amplitude and timing variations of arrivals as a function of back azimuth. These variations suggest the presence of anisotropic structures underneath the region. Thus, in this study we reanalyze the velocity structure derived from receiver functions by incorporating anisotropic parameters in the inversion process. In general, the best fit model derived from the anisotropic inversion is comparable with the previous study. The crustal thickness beneath the study area is around 38 km and the model also shows P and S waves anisotropic layer at the upper and lower crust. The difference of fast anisotropy axis observed on these layers may reveal the different mechanism of deformation between the shallow and deeper parts of the crust underneath the study area.

A 3D Full Stress Tensor Model for Oklahoma

AbstractThe stress tensor is an important property for upper crustal studies such as those that involve pore fluids and earthquake hazards. At tectonic plate scale, plate boundary forces and mantle convection are the primary drivers of the stress field. In many local settings (10–100 s of km and <10 km depth) in tectonic plate interiors, we can simplify by assuming a constant background stress field that is perturbed by local heterogeneity in density and elasticity. Local stress orientation and sometimes magnitude can be estimated from earthquake and borehole‐based observations when available. Modeling of the local stress field often involves interpolating sparse observations. We present a new method to estimate the 3D stress field in the upper crust and demonstrate it for Oklahoma. We created a 3D material model by inverting multiple types of geophysical observations simultaneously. Integrating surface‐wave dispersion, local travel times and gravity observations produces a model of P‐wave velocity, S‐wave velocity, and density. The stress field can then be modeled using finite element simulations. The simulations are performed using our simplified view of the local stress field as the sum of a constant background stress field that is perturbed by local density and elasticity heterogeneity and gravitational body forces. An orientation of N82°E, for the maximum compressive tectonic force, best agrees with previously observed stress orientations and faulting types in Oklahoma. The gravitational contribution of the horizontal stress field has a magnitude comparable to the tectonic contribution for the upper 5 km of the subsurface.

Mechanism for epeirogenic uplift of the Archean Dharwar craton, southern India as evidenced by orthogonal seismic reflection profiles

Plateaus, located far away from the plate boundaries, play an important role in understanding the deep-rooted geological processes responsible for the epeirogenic uplift and dynamics of the plate interior. The Karnataka plateau located in the Dharwar craton, southern India, is a classic example for the plateau uplift. It is explored using orthogonal deep crustal seismic reflection studies, and a mechanism for the epeirogenic uplift is suggested. A pseudo three-dimensional crustal structure derived from these studies suggests a regionally extensive 10 km thick magmatic underplating in the region. It is further constrained from active-source refraction and passive-source seismological data. We interpret the Marion and Reunion mantle plume activities during 88 Ma and 65 Ma on the western part of Dharwar craton are responsible for the magmatic underplating, which caused epeirogenic uplift. Flexural isostasy related to the onshore denudational unloading and offshore sediment loading is also responsible for the persisting uplift in the region. Plate boundary forces are found to be contributing to the plateau uplift. The present study provides a relationship between the mantle plumes, rifting, development of continental margins, plateau uplift, and denudational isostasy. Combination of exogenic and endogenic processes are responsible for the plateau uplift in the region.

Modelling diurnal variation magnetic fields due to ionospheric currents

SUMMARY Accurate models of the spatial structure of ionospheric magnetic fields in the diurnal variation (DV) band (periods of a few hours to a day) would enable use of magneto-variational methods for 3-D imaging of upper mantle and transition zone electrical conductivity. Constraints on conductivity at these depths, below what is typically possible with magnetotellurics, would in turn provide valuable constraints on mantle hydration and Earths deep water cycle. As a step towards this objective, we present here a novel approach to empirical modelling of global DV magnetic fields. First, we apply frequency domain (FD) principal components analysis (PCA) to ground-based geomagnetic data, to define the dominant spatial and temporal modes of source variability. Spatial modes are restricted to the available data sites, but corresponding temporal modes are effectively continuous in time. Secondly, we apply FD PCA to gridded surface magnetic fields derived from outputs of the physics-based Thermosphere–Ionosphere–Electrodynamics General Circulation Model (TIEGCM), to determine the dominant modes of spatial variability. The TIEGCM spatial modes are then used as basis functions, to fit (or interpolate) the sparsely sampled data spatial modes. Combining the two steps, we have a FD model of DV band global magnetic fields that is continuous in both space and time. We show that the FD model can easily be transformed back to the time domain (TD) to directly fit time-series data, allowing the use of satellite, as well as ground-based, data in the empirical modelling scheme. As an illustration of the methodology we construct global FD and TD models of DV band source fields for 1997–2018. So far, the model uses only ground-based data, from 127 geomagnetic observatories. We show that the model accurately reproduces surface magnetic fields in both active and quiet times, including those at sites not used for model construction. This empirical model, especially with future enhancements, will have many applications: improved imaging of electrical conductivity, ionospheric studies and improved external field corrections for core and crustal studies.

Joint Inversion of High-Frequency Receiver Functions and Surface-Wave Dispersion: Case Study in the Parnaíba Basin of Northeast Brazil

ABSTRACTWe assess the performance of the joint inversion of receiver functions (RF) and surface-wave dispersion in the characterization of the sedimentary package comprising the Parnaíba basin. This procedure is routinely utilized in passive-source crustal studies to retrieve S-wave velocity variations with depth, and has seldom been used with higher-frequency datasets to investigate fine sedimentary structure. The Parnaíba basin is a Paleozoic cratonic basin composed of five supersequences, accumulating ∼3.5 km of sedimentary rocks interbedded by Late Cretaceous diabase sills. The dataset used for this research was acquired between 2015 and 2017 through deployment of 10 short-period and one broadband seismic stations distributed along an approximately 100-kilometer-long linear array in the center of the basin. The deployment was carried out under the Parnaíba Basin Analysis Project, a multi-institutional and multidisciplinary effort funded by BP Energy do Brasil. High-frequency RFs (f<4.8 Hz) were calculated from deconvolution of teleseismic P waveforms (30°<Δ<90°) after rotation into the great-circle path, whereas high-frequency dispersion curves (0.25–2 Hz) were obtained through multiple filter analysis of empirical Green’s functions developed from cross-correlation (ZZ component) and stacking (six months) of time–frequency-normalized ambient seismic noise recordings. S-wave velocity–depth profiles down to ∼5 km depth were developed through an iterative, linearized joint inversion approach. Comparison to independent active-source seismic profiles overlapping with our passive-source seismic line reveals the inverted velocity models successfully retrieve sedimentary thickness (top of the Cambrian), sedimentary velocity structure, and depth to the Cenozoic sedimentary sequence. In addition, high-velocity zones at depths ranging from 1.5 to 2.5 km are observed in the inverted velocity–depth profiles, which are interpreted as due to the Late Cretaceous sills interbedding the basin’s sedimentary rocks. The relative low cost of our approach makes it ideal for basic characterization of relatively unknown sedimentary basins.

Lower Crustal Composition in the Southwestern United States

AbstractThe composition of the lower continental crust is well studied but poorly understood because of the difficulty of sampling large portions of it. Petrological and geochemical analyses of this deepest portion of the continental crust are limited to the study of high‐grade metamorphic lithologies, such as granulite. In situ lower crustal studies require geophysical experiments to determine regional‐scale phenomena. Since geophysical properties, such as shear wave velocity (Vs), are nonunique among different compositions and temperatures, the most informative lower crustal models combine both geochemical and geophysical knowledge. We explored a combined modeling technique by analyzing the Basin and Range and Colorado Plateau of the United States, a region for which plentiful geochemical and geophysical data are available. By comparing seismic velocity predictions based on composition and thermodynamic principles to ambient noise inversions, we identified three compositional trends in the southwestern United States that reflect three different geologic settings. The Colorado Plateau (thick crust), Northern Basin and Range (medium crust), and Southern Basin and Range (thin crust) have intermediate, intermediate‐mafic, and mafic deep crustal compositions. Identifying the composition of the lower crust depends heavily on its temperature because of the effect it has on rock mineralogy and physical properties. In this region, we see evidence for a lower crust that overall is intermediate‐mafic in composition (53.77.2 wt.% SiO) and notably displays a gradient of decreasing SiOwith depth.

Surface and Crustal Study Based on Digital Elevation Modeling and 2-D Gravity Forward Modeling in Thandiani to Boi Areas of Hazara Region, Pakistan

Gravity data indicates that there is a regular relation between crustal structure, crustal density (composition), and surface ascension. In order to delineate surface and subsurface geological structure features, and to calculate the thickness variation of the crust and sedimentary/metasedimentary wedges, integrated approach of Geographic Information System (GIS) i.e. digital elevation models (DEMs) and two-dimensional forward modeling of gravity data were utilized, which provide the best results for the primary objectives. Tectonically, the study area lies in the Lesser Himalayas as well as to an extent in the sub-Himalaya, more concretely in the western limb of Hazara Kashmir Syntaxis. Topographic data was accumulated in XYZ coordinates utilizing point heights method, and DEMs generation, manipulation, interpretation, and visualization process were directed to surfer-15 and ArcGIS software. Determinately the visualization of surface geological structure in the form of DEMs were proposed. The gravity stations in single contour mode have been quantified by using Scintrex CG-5 gravity meter. The collected gravity data was processed by standardizing corrections, two-dimensional forward modeling along with gravity profile were utilized and bouguer anomaly map and gravity model was computed utilizing bouguer density of 2.4 g/cm3, where the subsurface structures are demarcated by the bouguer anomaly and gravity model. In summary this research has allowed the validation of surface and subsurface geological structure visualization. Digital elevation models provide a defensive prediction of the geological structure of the regional surface. The gravity model demarcated a series of stratigraphic units with density boundaries within the basement. The gravity model also suggests that the thickness of sedimentary/metasedimentary wedge in Thandiani area is 11.48 km and in Boi area, the thickness elongates to about 14.43 km. The total thickness of crust in Thandiani and Boi area is 49.53 km and 52.43 km respectively.

Crustal study of the Pyrenees by refraction and wide-angle reflection using earthquakes as a natural seismic source

In this study we analyse three wide-angle seismic profiles in the Pyrenees to provide new insights into their crustal structure. Several earthquakes with magnitudes greater than 2.5 that occurred in the vicinity of the analysed profiles have been used as the seismic source. We demonstrate the feasibility of using natural seismicity as the seismic source, which would allow using deployments of seismic stations designed for other objectives. We have analysed a total of three profiles in this study, two of them being located in the eastern Pyrenees (Cataluña) with N-S and E-O directions, and the third in the western Pyrenees (Navarra) with NE-SO orientation. All of them were originally deployed to record teleseisms for receiver function analysis. Profiles in the eastern Pyrenean edge constrain the top of the lower crust at a depth of 20 km. In the easternmost sector of the E-W profile, Moho has been imaged as an eastward rising ramp, with a depth decreasing from 35-40 km in the middle of the profile to a depth of 25 km in the eastern edge. The Navarra profile allowed us to constrain the crustal structure of the western Pyrenean edge in detail. The obtained model allowed the imaging of the mid-lower crustal indentation between the Iberian and European plates. The European Moho has been constrained at a depth of 30 km in a small region of the central part of the model, whereas the Iberian Moho has been imaged in its northernmost edge dipping northwards to a depth of 45-50 km.

Surface wave analysis: improving the accuracy of the shear-wave velocity profile through the efficient joint acquisition and Full Velocity Spectrum (FVS) analysis of Rayleigh and Love waves

ABSTRACTSurface wave propagation can be exploited to investigate subsurface conditions in terms of shear wave velocities in a number of possible applications (geotechnical site characterisation, seismic-risk assessment, crustal studies and non-destructive testing). Nowadays, one of the most common methods adopted to analyse the dispersion of surface waves is based on the determination of the Rayleigh wave frequency-dependent phase velocities obtained from multichannel active data. The obtained values represent the dispersion curve, which is inverted to determine the vertical shear-wave velocity (VS) profile. After briefly recalling some fundamental facts and problems regarding surface wave propagation and analysis, we present the Full Velocity Spectrum (FVS) approach which here is used to jointly invert the velocity spectra of the Rayleigh and Love waves acquired by means of a set of horizontal geophones only (Rayleigh waves are in fact analysed while considering their radial component). It is shown that for non-trivial data sets for which modal dispersion curves cannot be soundly extracted, the joint FVS approach may represent an efficient way to properly analyse the data, thus eventually obtaining a robust VS profile free from significant ambiguities that would otherwise inevitably affect the results obtained by following the ordinary approach based on the modal dispersion curve(s) of just one component.

Electromagnetic and electrical studies in and around Schirmacher oasis, Antarctica by Indian Antarctic scientific expeditions

Transient electromagnetic (TEM), vertical electrical soundings (VES) and self-potential (SP) measurements were carried out in Schirmacher Oasis and in the adjoining shelf ice regions of Antarctica in the year 1984. TEM measurements with MPPO system of varying square loops were employed to measure thickness of shelf ice in the neighbourhood of “Dakshin Gangotri” station (since abandoned) which was estimated of the order of 100 m. The thickness of the ice cap in the vicinity of oasis, however, was found to be of the order of 400 m. VES and SP measurements were carried out in Schirmacher oasis in the vicinity of the “Maitree” station. VES measurements showed that the thickness of scree varied from 1.0 m to 2.5 m and the resistivity in the range of 50 Ω-m to 125 Ω-m. The thickness of weathered zone varied widely between 4.9 m and 21.5 m with resistivity varying in the range of 215 Ω-m to 450 Ω-m. The basement resistivity varied ranging from 700 Ω-m to 2,25,000 Ω-m. The purpose of SP measurements was to study the nature of SP processes in the oasis which remains in the subzero condition round the year. SP profiles in the oasis are featureless. SP measurements were made also to examine the possibility of reliable electric field measurements for determining the impedance of the ground, as a guideline, for future magnetotelluric (MT) measurements over the oasis for crustal studies. The Ground Penetrating Radar (GPR) survey detected the entrance shaft of size 95 cm × 99 cm at a depth of 2.00 m of the buried Dakshin Gangotri hanger, built in 1984–85. Magnetotelluric investigations subsequently made over the Schirmacher oasis indicate cratonic-type crust.

Western Pyrenees geodetic deformation study using the Guipuzcoa GNSS network

Abstract The Basque Country in the north of Spain is located inside the Basque-Cantabrian basin of the western Pyrenees which remarkable seismic-tectonic implications justify the need of geodetic control in the area. In order to perform a crustal deformation study we have analysed all daily observations from the GNSS permanent network of Guipuzcoa and external IGS stations, from January 2007 to November 2011. We have carried out the data processing applying double differences methodology in the automatic processing module BPE (Bernese Processing Engine) from Bernese GNSS software version 5.0. Solution was aligned to geodetic reference framework ITRF2008, by using the IGS08 solution and updated satellite and terrestrial antennas calibration. This five years network study results: Coordinate time series, velocities and baseline lengths variations show internal stability among inner stations and from them with respect to outer IGS stations, concluding that no deformations have been observed.

Application of gravity and magnetic methods for the crustal study and delineating associated ores in the western limb of Hazara Kashmir Syntaxis, Northwest Himalayas, Pakistan

The present geophysical study deals with the ores and crustal demonstration of southeastern Hazara and its adjoining areas of Azad Jammu and Kashmir, Pakistan, on the basis of terrestrial gravity and magnetic data. Tectonically, the study area lies in the Lesser Himalayas as well as to an extent in the sub-Himalaya, more specifically in the western limb of Hazara Kashmir Syntaxis. In this study, 567 gravity and 508 magnetic stations have been measured with CG-5 gravimeter and proton precession magnetometer, respectively. The collected data have been processed by applying standard corrections and then different types of maps were prepared. The ores in the area have been delineated by the qualitative interpretation of residual Bouguer anomaly and reduction to pole total magnetic intensity maps, whereas regional structures are demarcated by the Bouguer anomaly and regional Bouguer anomaly maps. The positive contour closures on the residual Bouguer anomaly map indicate the iron ore and phosphate, whereas negative contour closures are the effects of low-density material which consists of gypsum and soapstone. The pole-reduced total intensity map also shows the negative and positive contour closures almost in the same localities and confirms the residual Bouguer anomaly map. The geological model computed on the basis of Bouguer anomaly demarcated a series of faults between different rock units in the study area. The Kashmir Boundary Thrust cuts the western limb of Hazara Kashmir Syntaxis near the apex in the north of Muzaffarabad and marks the boundary between Murree Formation and carbonates of Abbottabad Formation. The gravity model also suggests that the thickness of the crust increases towards the northeast.