Relations between denudation,glaciation, and sediment deposition: implications from the Plio‐Pleistocene Central Alps

Despite abundant data on the early evolution of the Central Alps, the latest stage exhumation history, potentially related to relief formation, is still poorly constrained. We aim for a better understanding of the relation between glaciation, erosion and sediment deposition. Addressing both topics, we analysed late Pliocene to recent deposits from the Upper Rhine Graben and two modern river sands by apatite fission‐track and (U‐Th‐Sm)/He thermochronology. From the observed age patterns we extracted the sediment provenance and paleo‐erosion history of the Alpine‐derived detritus. Due to their pollen and fossil record, the Rhine Graben deposits also provide information on climatic evolution, so that the erosion history can be related to glacial evolution during the Plio‐Pleistocene. Our data show that Rhine Graben deposits were derived from Variscan basement, Hegau volcanics, Swiss Molasse Basin, and the Central Alps. The relations between glaciation, Alpine erosion, and thermochronological age signals in sedimentary rocks are more complex than assumed. The first Alpine glaciation during the early Pleistocene did not disturb the long‐term exhumational equilibrium of the Alps. Recent findings indicate that main Alpine glaciation occurred at ca. 1 Ma. If true, then main Alpine glaciation was coeval with an apparent decrease of hinterland erosion rates, contrary to the expected trend. We suggest that glaciers effectively sealed the landscape, thus reducing the surface exposed to erosion and shifting the area of main erosion north toward the Molasse basin, causing sediment recycling. At around 0.4 Ma, erosion rates increased again, which seems to be a delayed response to main glaciation. The present‐day erosion regime seems to be dominated by mass‐wasting processes. Generally, glacial erosion rates did not exceed the pre‐glacial long‐term erosion rates of the Central Alps.     

Three-dimensional topography corrections of magnetotelluric data

Topographic effects due to irregular surface terrain may prevent accurate interpretation of magnetotelluric (MT) data. Three-dimensional (3-D) topographic effects have been investigated for a trapezoidal hill model using an edge finite-element method. The 3-D topography generates significant MT anomalies, and has both galvanic and inductive effects in any polarization. This paper presents two different correction algorithms, which are applied to the impedance tensor and to both electric and magnetic fields, respectively, to reduce topographic effects on MT data. The correction procedures using a homogeneous background resistivity derived from a simple averaging method effectively decrease distortions caused by surface topography, and improve the quality of subsurface interpretation. Nonlinear least-squares inversion of topography-corrected data successfully recovers most of structures including a conductive or resistive dyke.     

3-D magnetotelluric inversion and model validation with gravity data for the investigation of flood basalts and associated volcanic rifted margins

20 magnetotelluric (MT) soundings were collected on the Isle of Skye, Scotland to provide a high-resolution three-dimensional (3-D) electrical resistivity model of a volcanic province within the framework of a project jointly interpreting gravity, seismic, geological and MT data. The full 3-D inversion of the MT data jointly interpreted with gravity data reveals upper crustal structure. The main features of the model are interpreted in conjunction with previous geological mapping and borehole data. Our model extends to 13 km depth, several kilometres below the top of the Lewisian basement. The top of the Lewisian basement is at approximately 7–8 km depth and the topography of its surface was controlled by Precambrian rifting, during which a 4.5 km thick sequence of Torridonian sediments was deposited. The Mesozoic sediments above, which can reach up to 2.2 km thick, have small-scale depocentres and are covered by up to 600 m of Tertiary lava flows. The interpretation of the resistivity model shows that 3-D MT inversion is an appropriate tool to image sedimentary structures beneath extrusive basalt units, where conventional seismic reflection methods may fail.     

The time-domain magnetotelluric response over three-layer earth models

Magnetotelluric data observed in frequency and time domains are expressed as apparent resistivity. the apparent resistivity is a weighted spatial average of the subsurface resistivity distribution. In this paper, we develop analytical expressions to compute the apparent resistivity in the time domain for various three-layer earth models. the present approach to computing the magnetotelluric response in the time domain is found to overcome the problems encountered by the method of images. the magnetotelluric response in the time domain for various three-level models have been computed and shown graphically. the time-domain responses show a characteristic behaviour with a small change in layered parameters (resistivity and thickness of the layers), whereas frequency-domain responses do not show such behaviour. This characteristic behaviour of time-domain magnetotelluric sounding curves will be useful in the qualitative interpretation of field data.     

Magnetometer array study in a tectonically active region of Quebec, Canada

Summary. An array of 26 magnetometers deployed in a tectonically active area of Quebec has mapped a boundary in terrestrial electrical conductivity for 200 km along strike. The contrast in conductivity across the boundary, from previous magnetotelluric soundings, is about one order of magnitude. Anomalous variation fields associated with electric currents flowing along the boundary are readily detected at pulsation periods only when the horizontal field is polarized transverse to the structure (the E -polarization case). The anomaly is hardly visible in transfer functions from substorms, for a number of reasons: a predominant H -polarization orientation of the substorm fields, the single order-of-magnitude contrast in conductivity, and the probably small depth extent of the structure. Attempts were made to estimate the response of a one-dimensional earth via the inductive scale length with gradients evaluated from polynomial surfaces fitted to the smoothly varying substorm fields. The results were inconsistent, owing to vertical fields with strong external components and to horizontal fields with scale lengths too small relative to their penetration distances.     

Two-dimensional magnetotelluric inversion

Summary. When complex structure is encountered in magnetotelluric surveys, interpretation by locally fitted layered models is of questionable validity. However, when the processed data show two-dimensional structure, numerical inversion schemes for two-dimensional models may be constructed as an aid to regional data interpretation.
The two-dimensional magnetotellurics inversion problem is here formulated in a way that may be applied to many problems. A resulting computer program is analysed carefully in terms of its cost relative to that of simpler layered modelling.
As an example, the method is applied to some field data where the interpretive advantages of the program become evident.     

Electrical conductivity models for the continental crust based on laboratory measurements on high-grade metamorphic rocks

Summary. Deep electrical conductivity soundings are increasingly being used as an additional source of information regarding the nature of the lower continental crust. However, a lack of relevant laboratory-based conductivity measurements makes interpretation of such soundings difficult. Laboratory measurements have been made on saturated and unsaturated samples of possible lower crustal rock types subjected to confining pressures up to 0.4 GPa, temperatures up to 300°C, and with variable pore fluid pressure up to the confining pressure. Extrapolation of these results suggests that the surprisingly high conductivities deduced for depths of approximately 20 km in certain stable continental areas may result from a combination of basic rock type and high pore fluid pressures, for whereas the conductivities measured in acid rock types can be explained in terms of conduction through the pore fluid alone, the conductivities measured in basic rock types imply enhanced conduction presumably through the matrix or along grain boundaries. The lower conductivities deduced from field experiments for the upper crust may be due to more acid rock types and/or lower pore fluid pressures, perhaps due to hydration reactions. In areas where the high conductivity layer is coincident with a low velocity layer an explanation in terms of changing pore fluid pressure, i.e. low to high, with increasing depth seems more likely.     

Magnetotelluric Soundings In the Canadian Rocky Mountains

Summary. Magnetotelluric soundings have been made at 25 stations in the Rocky Mountain Trench (RMT) and Main Ranges near 53° N, close to the centre of a major conductivity anomaly which had been mapped in a magnetovariation array study. Most stations covered the frequency range 0.01–500 Hz and three stations 0.0002–500 Hz. the resistivity tensor shows low to moderate anisotropy in the RMT, but is strongly 2-D or 3-D in the Rocky Mountains. Apparent resistivities as a function of frequency are displayed in pseudosections along the Trench and along a transverse profile across the RMT and into the Main Ranges. In preparation for 2-D modelling, 1-D inversions have been used to construct resistivity-depth sections satisfying both magnitudes and phases of the MT responses. These show very low resistivities, in the range 1—10Ωm, in the upper crust under the RMT and even lower values under the Main Ranges. the latter values give strong confirmation of the Northern Rockies conductor reported by Bingham, Cough & Ingham and are in agreement with models of the conductors fitted to long-period magnetovariation fields by Ingham, Gough & Parkinson. the MT results here reported add some essential depth and resistivity information. It is suggested that the conductors beneath the Rocky Mountains Main Ranges and Trench constitute a thickening at the edge of the Canadian Cordilleran Regional (CCR) conductor. Gough has argued that a wide variety of geophysical and geological parameters indicate high temperatures and partial melting in the mantle under the CCR conductor. At the upper crustal depths penetrated in this magnetotelluric study, it is considered more probable that the high conductivity is caused by hot, saline water of mantle origin rather than silicate melt. the CCR in general may have two layers of fluid producing its high conductivity, silicate melt below and saline hot water above.     

The results of deep geomagnetic soundings in the West Carpathians

Summary. The results of geomagnetic soundings in the West Carpathians are presented. The number of observation sites in this region was approximately 90. The data treatment included the calculation of single-station transfer functions and the separation of the fields into external and internal parts on some profiles. The interpretation was based on the construction of two-dimensional models explaining the observed features. The observed distribution of geomagnetic induction vectors is very regular and the axis of the anomaly runs in the contact region between the outer and inner Carpathians. The thickness of the well-conducting rock complex exceeds 10 km. The well-conducting rocks seem to be the sediments which accumulated in the contact region of the platform. The asymmetry in the distribution of induction vector amplitudes on some profiles can be accounted for by asymmetrical geometry, which is characteristic of the underthrusting of rocks.     

Deep crustal geoelectric structure beneath the Northumberland Basin

Summary. In terms of lateral variations in conductivity structure, the southern Southern Uplands and Northumberland Basin are characterized by a region of attenuated vertical magnetic fields with small spatial gradients reflecting the presence of a substantial conducting zone. Five magnetotelluric data sets from the region have been analysed to provide accurate and unbiased estimates of the impedance tensor. The response data are used to investigate the deep geoelectric crustal structure of the region. Three appropriate sets of response data have been subjected to two construction algorithms for 1-D inversion. The geoelectric profiles recovered identify a deep crustal conducting zone underlying the Northumberland Basin. The zone, modelled as a layered structure, dips steeply from mid-crustal depths underneath the Northumberland Basin to lower crustal depths to the NW. The structure thus correlates, in location and geometry, with a deep crustal reflecting wedge detected offshore by a deep seismic reflection profile.     

欧洲边境区合作模式探析——以上莱茵边境区为例

边境区研究是政治地理学的重要组成部分。经济全球化和区域集团化趋势为边境区合作创造了良好的机遇,边境区合作研究也日益成为区域经济学的一门新分支。欧洲国家通过其悠久的合作传统在边境区合作研究方面积累了丰富的理论和实践经验。在欧洲所有的边境区合作中,上莱茵地区一直被认为是最成功的案例。本文从上莱茵边境区合作的发展历史入手,着重分析其组织机构设置和运行经验,试图为我国边境区合作提供有益的启示。     

The instantaneous apparent resistivity tensor: a visualization scheme for LOTEM electric field measurements

Long-offset transient electromagnetic (LOTEM) data have traditionally been represented as early- and late-time apparent resistivities. Time-varying electric field data recorded in a LOTEM survey made with multiple sources can be represented by an 'instantaneous apparent resistivity tensor'. Three independent, coordinate-invariant, time-varying apparent resistivities can be derived from this tensor. For dipolar sources, the invariants are also independent of source orientation. In a uniform-resistivity half-space, the invariant given by the square root of the tensor determinant remains almost constant with time, deviating from the half-space resistivity by a maximum of 6 per cent. For a layered half-space, a distance–time pseudo-section of the determinant apparent resistivity produces an image of the layering beneath the measurement profile. As time increases, the instantaneous apparent resistivity tensor approaches the direct current apparent resistivity tensor. An approximate time-to-depth conversion can be achieved by integrating the diffusion depth formula with time, using the determinant apparent resistivity at each instant to represent the resistivity of the conductive medium. Localized near-surface inhomogeneities produce shifts in the time-domain apparent resistivity sounding curves that preserve the gradient, analogous to static shifts seen in magnetotelluric soundings. Instantaneous apparent resistivity tensors calculated for 3-D resistivity models suggest that profiles of LOTEM measurements across a simple 3-D structure can be used to create an image that reproduces the main features of the subsurface resistivity. Where measurements are distributed over an area, maps of the tensor invariants can be made into a sequence of images, which provides a way of 'time slicing' down through the target structure.     

An algebraic formulation of geophysical inverse problems

Many geophysical inverse problems derive from governing partial differential equations with unknown coefficients. Alternatively, inverse problems often arise from integral equations associated with a Green's function solution to a governing differential equation. In their discrete form such equations reduce to systems of polynomial equations, known as algebraic equations. Using techniques from computational algebra one can address questions of the existence of solutions to such equations as well as the uniqueness of the solutions. The techniques are enumerative and exhaustive, requiring a finite number of computer operations. For example, calculating a bound to the total number of solutions reduces to computing the dimension of a linear vector space. The solution set itself may be constructed through the solution of an eigenvalue problem. The techniques are applied to a set of synthetic magnetotelluric values generated by conductivity variations within a layer. We find that the estimation of the conductivity and the electric field in the subsurface, based upon single-frequency magnetotelluric field values, is equivalent to a linear inverse problem. The techniques are also illustrated by an application to a magnetotelluric data set gathered at Battle Mountain, Nevada. Surface observations of the electric ( E _y ) and magnetic ( H _x ) fields are used to construct a model of subsurface electrical structure. Using techniques for algebraic equations it is shown that solutions exist, and that the set of solutions is finite. The total number of solutions is bounded above at 134 217 728. A numerical solution of the algebraic equations generates a conductivity structure in accordance with the current geological model for the area.     

Deep Schlumberger sounding and the crustal resistivity structure of central Australia

Summary. Three 200 km Schlumberger resistivity soundings have been conducted over the central Australian shield, using telephone lines to obtain the large electrode spacings. These represent the first crustal scale controlled source electrical study to be carried out in this continent. A computer controlled data acquisition system was used which allowed precise measurements to be made with only modest emission currents (0.1–0.5 A).
The three soundings, centred on the towns of Renner Springs, Wauchope and Aileron, showed the southern part of the study area (the Arunta Block) to be an order of magnitude more resistive than the more northerly section (the Tennant Creek Block). This difference correlates with the higher heat flow of the Tennant Creek Block. A lowering of apparent resistivity at large electrode spacings for one sounding (Wauchope) is taken to indicate the presence of a low resistivity layer in the middle crust, at a depth less than 20 km. However, the effect of the highly conductive overburden characteristic of inland Australia, combined with the large transverse resistance of the crust, prevented the other two soundings from detecting such a layer. Without support from these two soundings, it is impossible to be sure that the lowered resistivity at Wauchope is not caused merely by lateral variations in near-surface resistivity.
The data also show that crustal resistivities are much lower than the expected values for dry rock, whether or not a low resistivity layer is included in the model. This implies a widespread occurrence of free water in the crust, with greater amounts occurring at depth if the low resistivity zone exists.     

On the Fréchet differentiability of the one-dimensional magnetotellurics problem

Summary. This paper proves that, subject to certain assumptions, the one-dimensional magnetotelluric response function and the associated electric fields are Fréchet differentiable with respect to the conductivity function. The flaw in the proof offered by an earlier author is explained.     

Automatic model for finding the one-dimensional magnetotelluric problem

Summary. A method is described for finding a resistivity model that fits given magnetotelluric data in the one-dimensional case. The procedure is automatic and objective in that no a priori model structure is imposed. Starting with a uniform half space derived directly from the data, the procedure gradually transforms the half space to one with a continuous and smooth resistivity distribution whose response fits the measured data. The method is illustrated by application to two magnetotelluric data sets.     

Tectonic evolution of the North Sea basin: crustal stretching and subsidence

Summary. The lithospheric stretching model for the formation of sedimentary basins was tested in the central North Sea by a combined study of crustal thinning and basement subsidence patterns. A profile of crustal structure was obtained by shooting a long-range seismic experiment across the Central Graben, the main axis of subsidence. A seabed array of 12 seismometers in the graben was used to record shots fired in a line 530 km long across the basin. The data collected during the experiment were interpreted by modelling synthetic seismograms from a laterally varying structure, and the final model showed substantial crustal thinning beneath the graben. Subsidence data from 19 exploration wells were analysed to obtain subsidence patterns in the central North Sea since Jurassic times. Changes in water depth were quantified using foraminiferal assemblages where possible, and observed basement subsidence paths were corrected for sediment loading, compaction and changes in water depth through time. The seismic model is shown to be compatible with the observed gravity field, and the small size of observed gravity anomalies is used to argue that the basin is in local isostatic equilibrium. Both crustal thinning and basement subsidence studies indicate about 70 km of stretching across the Central Graben during the mid-Jurassic to early Cretaceous extensional event. This extension appears to have occurred over crust already slightly thinned beneath the graben, and the seismic data suggest that total extension since the early Permian may have been more than 100km. The data presented here may all be explained using a simple model of uniform extension of the lithosphere.     

Resistivity studies over the Flinders conductivity anomaly, South Australia

Summary. Seven Schlumberger resistivity soundings with maximum current electrode spacings of 20 km have been conducted south of Lake Frome in South Australia. These experiments were done partly to test new electrical sounding equipment and partly to investigate a large conductivity anomaly previously delineated by other workers using magnetometer array and MT methods (the 'Flinders'anomaly). These previous studies left some doubt as to the depth to the conductive region responsible for the anomaly.
The electrical soundings did not detect a buried conductive zone, which constrains it to lie deeper than 5–7 km. However, the study did show the surface sediments of the region to be very conductive; resistivities of 2–9 μm were measured over thicknesses of 50–400 m, with sediment thickness inferred to be up to 2 km to the north of the studied area. This raises the question of whether current channelling in the surface sediments could have been responsible for the earlier results. Simple modelling and application of the criteria given by Jones suggest this may be so.
The equipment used for this study is a low power (200 W), computer controlled system which employs synchronous stacking and other signal processing to achieve signal to noise improvement ratios of up to 1000.