Jan Vozar

Integrated geophysical-petrological modeling of lithosphere-asthenosphere boundary in central Tibet using electromagnetic and seismic data

We undertake a petrologically driven approach to jointly model magnetotelluric (MT) and seismic surface wave dispersion (SW) data from central Tibet, constrained by topographic height. The approach derives realistic temperature and pressure distributions within the upper mantle and characterizes mineral assemblages of given bulk chemical compositions as well as water content. This allows us to define a bulk geophysical model of the upper mantle based on laboratory and xenolith data for the most relevant mantle mineral assemblages and to derive corresponding predicted geophysical observables. One-dimensional deep resistivity models were derived for two groups of MT stations. One group, located in the Lhasa Terrane, shows the existence of an electrically conductive upper mantle layer and shallower conductive upper mantle layer for the other group, located in the Qiangtang Terrane. The subsequent one-dimensional integrated petrological-geophysical modeling suggests a lithosphere-asthenosphere boundary (LAB) at a depth of 80–120 km with a dry lithosphere for the Qiangtang Terrane. In contrast, for the Lhasa Terrane the LAB is located at about 180 km but the presence of a small amount of water in the lithospheric mantle (<0.02 wt%) is required to fit the longest period MT responses. Our results suggest two different lithospheric configurations beneath the southern and central Tibetan Plateau. The model for the Lhasa Terrane implies underthrusting of a moderately wet Indian plate. The model for the Qiangtang Terrane shows relatively thick and conductive crust and implies thin and dry Tibetan lithosphere.

Constraints on the evolution of crustal flow beneath northern Tibet

Crustal flow is an important tectonic process active in continent-continent collisions and which may be significant in the development of convergent plate boundaries. In this study, the results from multidimensional electrical conductivity modeling have been combined with laboratory studies of the rheology of partially molten rocks to characterize the rheological behavior of the middle-to-lower crust of both the Songpan-Ganzi and Kunlun terranes in the northern Tibetan Plateau. Two different methods are adopted to develop constraints on melt fraction, temperature, and crustal flow velocity in the study area. The estimates of these parameters are then used to evaluate whether crustal flow can occur on the northern margin of the Tibetan plateau. In the Songpan-Ganzi crust, all conditions are satisfied for topography-driven channel flow to be dominant, with partial melt not being required for flow at temperature above 1000°C. Further north, the Kunlun fault defines the southern boundary of a transition zone between the Tibetan plateau and the Qaidam basin. Constrained by the estimated melt fractions, it is shown that channel injection across the fault requires temperatures close to 900°C. The composition of igneous rocks found at the surface confirm those conditions are met for the southern Kunlun ranges. To the north, the Qaidam basin is characterized by colder crust that may reflect an earlier stage in the channel injection process. In the study area, at least 10% of the eastward directed Tibetan crustal flow could be deflected northward across the Kunlun Fault and injected into the transition zone defining the northern margin of the Tibetan plateau.

Updating the map of Earth's Surface conductance

Studying the Earths deep conductivity structures, important for developing our understanding of the dynamics of the Earth, is complicated due to effects of the shallow conductive structures on the electromagnetic (EM) responses for periods larger than hours. The results of the deep EM soundings can be heavily distorted by the surface shell conductance, which varies from fractions of Siemens (S) inland to up to tens thousand of Siemens in the oceans. Thus, separating the effects caused by those variations and by deep conductivity structures is an important step during interpretation of the data. This article reports on efforts to overcome these difficulties by providing high-resolution, global maps of the Earths surface shell conductivity structure, from which deep conductivity can be interpolated. Using finescale regional surface schemes of conductance for the shallow structures (S-maps) overlain and compiled into broader spatial maps, scientists will b e able to use data products from these efforts to accomplish research goals of the currently running USArray (http://www.emscope.org) and for the planned Euro-Array (http://www.euroarrayorg), projects that aim in part to regionally map the conductivity structures at upper and middle mantle depths by using magnetotelluric (MT) and magnetovariation (MV) methods.

Geoelectrical and geological structure of the crust in Western Slovakia

Electrical resistivity of the Earth’s crust is sensitive to a wide range of petrological and physical parameters, and it particularly clearly indicates crustal zones that have been tectonically or thermodynamically disturbed. A complex geological structure of the Alpine nappe system, remnants of older Hercynian units and Neogene block tectonics in Western Slovakia has been a target of recent magnetotelluric investigations which made a new and more precise identification of the crustal structural elements of the Western Carpathians possible. A NW-SE magnetotelluric profile, 150 km long, with 30 broad-band and 3 long-period magnetotelluric sites, was deployed, crossing the major regional tectonic elements listed from the north: Brunia (as a part of the European platform), Outer Carpathian Flysch, Klippen Belt, blocks of Penninic or Oravicum crust, Tatricum and Veporicum. Magnetotelluric models were combined with previous seismic and gravimetric results and jointly interpreted in the final integrated geological model. The magnetotelluric models of geoelectrical structures exhibit strong correlation with the geological structures of the crust in this part of the Western Carpathians. The significant resemblance in geoelectrical and crustal geological structures are highlighted in shallow resistive structures of the covering formations represented by mainly Tertiary sediments and volcanics. Also in the deeper parts of the crust highly resistive and conductive structures are shown, which reflect the original building Hercynian crust, with superposition of granitoids or granitised complexes and lower metamorphosed complexes. Another important typical feature in the construction of the Western Carpathians is the existence of young Neogene steep fault zones exhibited by conductive zones within the whole crust. The most significant fault zones separate individual blocks of the Western Carpathians and the Western Carpathians itself from the European Platform.

Modeling of deep magnetovariation soundings on the rotating earth

Induced magnetic fields in the Earth arise due to two phenomena: induction generated by the time-variable exciting field and the motional induction caused by movement of the conductive planet in the outer magnetic fields. The comparison of both approaches on the spherical Earth has been analyzed in the present work for two sources in the ionosphere and magnetosphere. For this aim, both sources with their natural sizes and positions have been modeled analytically to obtain the fields on the layered sphere at the middle latitudes. The conditions when the steady ring current field is not influenced by the Earth’s rotation have been established theoretically. The synthetic diurnal magnetograms were used for the deep sounding by the magnetovariation spatial gradient method and the result was compared with the one obtained on the nonrotating sphere. Sounding results using both approaches were found different above the 2D inhomogeneous mantle. The precessions of the magnetospheric belt current pole for daily sampling frequency were presented using several geomagnetic observatory data in the northern hemisphere.

Anomalies of geomagnetic field due to a vertical prolate rotational ellipsoid

Anomalies of geomagnetic field due to a vertical prolate rotational ellipsoid We present an exact analytical solution of the forward magnetometric problem for the perturbing body having the shape of the rotational prolate ellipsoid with the longer axis oriented vertically to the surface of the earth. The anomaly of ΔZ and ΔT is calculated for the network of points in the plane z = const above the ellipsoid, as well as for the points on the surfaces of the volcanic hill: i) the cut cone, ii) the smooth shape given by the rotation of the Gaussian curve. Theoretical results can be useful for the interpretation of land or aeromagnetic survey in the volcanic areas.

Carbonate complexes underlying Flysch belt and subsurface Neogene volcanic in the NE part of Slovakia – a potential for geothermal energy and raw materials

Abstract A positive gravity anomaly was observed in the Flysch belt realm. Based on this fact and available geological knowledge we suppose that the source of gravity anomaly might be carbonate rocks lying perhaps in the footwall of Flysch sediments. The carbonates belong perhaps to the Penninic crust cover (a part of Klippen belt), or to lower structural stage of the Flysch belt. Besides this it is interpreted more volume of Neogene subvolcanic bodies in the frame of the Flysch belt based on the results of the newest magnetic measurements in the NE part of Slovakia (Kucharič et. al., in press). These are accompanied by increasing heat flow and hydrothermal alteration within neighbouring rocks what may eventuate into creation of raw materials. These two factors - carbonates and subvolcanic bodies - are important items for appraisal of new perspective in this area not only from hydrocarbon occurrences point of view (a primary intend within this area) but also for enhancement of geothermal potential of Slovak Republic and opening possibilities for prognosis of raw material occurrences as well.

The forward problem of magnetometry for the oblate spheroid

The forward problem of magnetometry for the oblate spheroid We present analytical solution of the forward magnetometric problem for the oblate spheroid (rotational ellipsoid) as a causative body. The shorter semiaxis of the ellipsoid is supposed to be vertical to the surface of the earth. There is proved that the uniform inducing magnetic field B0 induces inside the spheroid also uniform magnetic field but its modulus and direction are different as compared to B0. The isolines and profile curves of ΔZ and ΔT are calculated on the plane z = const above the ellipsoid, as well as on the surface of the hill in the shape of cutted cone.

Interpretation of the Tribeč Mts. deep geological structure based on results of geophysical, mainly magnetotelluric modelling

Interpretation of the Tribeč Mts. deep geological structure based on results of geophysical, mainly magnetotelluric modelling Magnetotelluric measurements (MT) in the southeast (SE) part of MT-15 profile in the frame of CELEBRATION 2000 project identified a subhorizontal zone with higher conductivity in the depth of cca 2-6 km below the Tribeč Mts. surface. This zone is interpreted as a complex of metamorphic rocks under the tectonically overlaying granites. The original presuppositions collected through other geophysical methods on the existence of such a complex are thereby confirmed. Apart from this finding, the MT measurements identified other conductive zones in the Tribeč mountain range and its surroundings. Those are close to the surface and belong to sedimentary rocks and volcanites. The steep, narrow and deeply penetrating conductive zone at the north-western edge of Tribeč Mts. can be interpreted as a deep Neogene shear zone. Subhorizontal conductive zone in SE part of the cross-section under the volcanic complexes is interpreted as the older mylonitized zone. This zone may be saturated by fluids, and is possibly of carbon origin and mineralized, connected with volcanic activity.