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1.
We use Monte Carlo Markov chains to solve the Bayesian MT inverse problem in layered situations. The domain under study is divided into homogeneous layers, and the model parameters are the conductivity of each layer. We use an a priori distribution of the parameters which favours smooth models. For each layer, the a priori and a posteriori distributions are digitized over a limited set of conductivity values.
  The Markov chain relies on updating the model parameters during successive scanning of the domain under study. For each step of the scanning, the conductivity is updated in one layer given the actual value of the conductivity in the other layers. Thus we designed an ergodic Markov chain, the invariant distribution of which is the a posteriori distribution of the parameters, provided the forward problem is completely solved at each step.
  We have estimated the a posteriori marginal probability distributions from the simulated successive values of the Markov chain. In addition, we give examples of complex magnetotelluric impedance inversion in tabular situations, for both synthetic models and field situations, and discuss the influence of the smoothing parameter.  相似文献   

2.
According to recent estimates, the continental mid-crust contains 35–40 per cent amphibolites. Heating of the crust by an underlying mantle plume, for example beneath continental rifts, high plateaus, and areas of intraplate volcanic activity, releases water. Dehydration of amphibole-bearing rocks at depths of 20–40  km occurs mainly in the temperature range 650–700 °C, and this releases about 0.4  wt per cent of water.
  Seismic tomography studies of the crust in the Kirgyz Tien Shan Range, where the age of the tectonic activity is less than 30  Ma, revealed a low-velocity zone in the mid-crust. The velocity of P waves was 0.4  km  s−1 lower than in normal crust. MT sounding data in the region show the existence of a low-resistivity layer with an average resistivity of about 25  Ω  m at the depth of the low-velocity layer. The spatial correlation of the observed anomalous layers and calculated effect of fluid phase on seismic and electric parameters of rocks suggests the presence of aqueous fluids released by the heating of the mid-crust.  相似文献   

3.
As a part of the Norwegian Antarctic Research Expedition 1984/85, geological mapping was performed in Gjelsvikfjella and western Mühlig-Hofmannfjella, Dronning Maud Land. The northern part of Gjelsvikfjella is dominated by the Jutulsessen metasupracrustals which have been intruded by a major gabbroic body and several generations of dykes. To the south the metasupracrustals gradually transform into the Risemedet migmatites. In western Mühlig-Hofmannfjella the bedrock is dominated by the large Svarthamaren Charnockite batholith. The batholith is bordered by the Snøtoa metamorphic complex outcropping to the south and west in Mühlig-Hofmannfjella and it is characterized by a high content of partly assimilated country rock inclusions. Mineral paragenesis and geothermometry/geobarometry suggest a two-stage tectonothermal-igneous history with an initial intermediate pressure, upper amphibolite to granulite facies metamorphism followed by high temperature transformations related to the charnockite intrusion. The age of the initial tectonothermal event is probably about 1,100 Ma. Geochronological work in the present study (Rb/Sr whole rock) gave an age of 500 ± 24 Ma for the Svarthamaren Charnockite, interpreted to record the age of crystallization. Late brittle faulting and undeformed dolerite dykes outcropping in Jutulsessen are believed to be related to Mesozoic crustal stretching in the Jutulstraumen-Pencksøkket Rift Zone to the west.  相似文献   

4.
M. Ruff  K. Czurda   《Geomorphology》2008,94(3-4):314
The aim of the study is landslide hazard assessment carried out on a working scale of 1:25 000. The study area within the Northern Calcareous Alps was geologically and geotechnically mapped in order to identify causes and mechanisms of active mass movements. The field surveys were digitised by a Geographical Information System and divided into data layers. The geological units were classified according to their geotechnical properties. All layers were converted into grids and spatially analysed together with a Digital Elevation Model. Comparing the layers with the inventory of active landslides, the prevailing factors leading to sliding movements were identified. Because of the complex tectonic setting and the small number of active landslides, a statistical method of hazard assessment was not applicable. Using the heuristic approach of an index method, the data layers of geotechnical class, bedding conditions, tectonic layouts, slope angles, slope orientations, vegetation and erosion were analysed. The susceptibility of each layer has been evaluated with help of bivariate statistics. The layers have been weighted with indices due to their importance iteratively and were combined into a landslide susceptibility map.  相似文献   

5.
The C -response connects the magnetic vertical component and the horizontal gradient of the horizontal components of electromagnetic variations and forms the basis for deriving the conductivitydepth profile of the Earth. Time-series of daily mean values at 42 observatories typically with 50 years of data are used to estimate C -responses for periods between 1 month and 1  yr. The Z : Y method is applied, which means that the vertical component is taken locally whereas the horizontal components are used globally by expansion in a series of spherical harmonics.
In combination with results from previous analyses, the method yields consistent results for European observatories in the entire period range from a few hours to 1  yr, corresponding to penetration depths between 300 and 1800  km.
1-D conductivity models derived from these results show an increase in conductivity with depth z to about 2  S  m-1 at z =800  km, and almost constant conductivity between z =800 and z =2000  km with values of 310  S  m-1, in good agreement with laboratory measurements of mantle material. Below 2000  km the conductivity is poorly resolved. However, the best-fitting models indicate a further increase in conductivity to values between 50 and 150  S  m-1.  相似文献   

6.
The conductivity structure of the Earth's mantle was estimated using the induction method down to 2100  km depth for the Europe–Asia region. For this purpose, the responses obtained at seven geomagnetic observatories (IRT, KIV, MOS, NVS, HLP, WIT and NGK) were analysed, together with reliable published results for 11  yr variations. 1-D spherical modelling has shown that, beneath the mid-mantle conductive layer (600–800  km), the conductivity increases slowly from about 1  S  m−1 at 1000  km depth to 10  S  m−1 at 1900  km, while further down (1900–2100  km) this increase is faster. Published models of the lower mantle conductivity obtained using the secular, 30–60  yr variations were also considered, in order to estimate the conductivity at depths down to the core. The new regional model of the lower mantle conductivity does not contradict most modern geoelectrical sounding results. This model supports the idea that the mantle base, situated below 2100  km depth, has a very high conductivity.  相似文献   

7.
The seafloor topography of a slow-spreading ridge shows a number of well-documented regularities at the ridge segment scale as the result of the complex interplay between ridge-axis magmatic and tectonic processes. This paper describes the results of a detailed analysis of the seafloor topography of the Mid-Atlantic Ridge near the Atlantis transform, where marine gravity data provide independent, although non-unique, constraints on subseafloor density structure. Using a combined topography and gravity data set, we identified the specific contributions of subseafloor density structure to the seafloor topography. We show that the observed along-axis deepening (0.3–0.8 km) from the midpoint of a ridge segment towards the non-transform offsets in the study area can be explained by the vertical deflection of a zero-age plate in response to along-axis crustal thickness variations. However, this effect can only account for 50–60 per cent of the observed 1.5–1.7 km deepening towards the Atlantis transform, suggesting the presence of significant stresses in the lithosphere near a transform. Results of plate flexural calculations also predict a more elevated rift flank at the inside corner of the ridge–transform intersection than at the conjugate outside corner. Such an asymmetry in rift flank topography is calculated to be greatest near a transform fault with a significant volume of deep transform valley and when adjacent plates across the transform fault are mechanically decoupled or only weakly coupled. Together these results illustrate the complex interplay between various tectonic processes at a slow-spreading ridge.  相似文献   

8.
MT and reflection: an essential combination   总被引：3，自引：0，他引：3  
Summary. At many localities in the world there have been coincident comprehensive electromagnetic (EM) studies and seismic reflection profiles conducted. Unfortunately, over many more regions the seismic reflection images are interpreted without the constraints afforded by electrical conductivity information. This paper is an attempt to convince the reader that a collocated magnetotelluric (MT) study should, in almost every case, be made wherever a seismic reflection survey is undertaken. Examples are shown from six studies in which the EM results aided the geological/tectonic interpretations of the seismic sections.
Also, difficulties with the MT technique are discussed, and the interpretations of conducting zones within the lower crust are examined. Finally, a generalised model is proposed for the continental crust that may account for both the reflectivity and conductivity of the zone at the top of the lower crust.  相似文献   

9.
The Miocene sedimentary succession of the southern Browse Basin records the response of a tropical reef system to long‐term, strong subsidence on a passive continental margin. Geological interpretation of a comprehensive two‐dimensional (2D) seismic reflectivity data set documents for the first time the development of a continuous Miocene barrier reef on the Australian North West Shelf. With a length of over 250 km, this barrier reef is among the Earth's largest in the Neogene record. A sequence stratigraphic analysis tied to well data shows that the main controls for the evolution, growth and demise of the reef system were subsidence, third‐order global‐scale eustatic variations and antecedent topography. The generally very high Miocene subsidence rates estimated for the study area cannot be explained by typical passive‐margin subsidence controlled by lithospheric cooling and sedimentary loading alone. Additional dynamic subsidence induced by mantle convection, though documented as unusually large on the northern margin of Australia during the Neogene, can be also regarded as being of only minor importance. Therefore, accelerated tectonic subsidence related to the collision of the Australian and Eurasian Plates 250–500 km north of the study area seems to exert an important influence on reef development and demise, complicated by local tectonic inversion. The Miocene tectonic reactivation and inversion of an older structural grain is interpreted to have controlled the reef development considerably by providing localized topographic highs along transpressional anticlines above basement‐rooted faults that served as preferential sites for reef growth and retreat during times of rapidly rising sea level. This exemplarily shows that the far‐field effects of collision‐induced tectonic subsidence can significantly influence carbonate systems on passive margins.  相似文献   

10.
3-D images of P velocity and P - to S -velocity ratio have been produced for the upper crust of the Friuli area (northeastern Italy) using local earthquake tomography. The data consist of 2565 P and 930 S arrival times of high quality. The best-fitting V P and V P / V S 1-D models were computed before the 3-D inversion. V P was measured on two rock samples representative of the investigated upper layers of the Friuli crust. The tomographic V P model was used for modelling the gravity anomalies, by converting the velocity values into densities along three vertical cross-sections. The computed gravity anomalies were optimized with respect to the observed gravity anomalies. The crust investigated is characterized by sharp lateral and deep V P and V P / V S anomalies that are associated with the complex geological structure. High V P / V S values are associated with highly fractured zones related to the main faulting pattern. The relocated seismicity is generally associated with sharp variations in the V P / V S anomalies. The V P images show a high-velocity body below 6 km depth in the central part of the Friuli area, marked also by strong V P / V S heterogeneities, and this is interpreted as a tectonic wedge. Comparison with the distribution of earthquakes supports the hypothesis that the tectonic wedge controls most of the seismicity and can be considered to be the main seismogenic zone in the Friuli area.  相似文献   

11.
Magnetic data were collected during the Wilkes (1973) and Seacarib II (1987) cruises to the Cayman trough. A new interpretation of magnetic data is carried out. An isochron pattern is drawn up from our anomaly identifications. An early Eocene age (49 Ma, Ypresian) for Cayman trough opening is proposed instead of the late Oligocene or middle Eocene ages suggested by previous studies. Our plate tectonic reconstruction is simpler and fits the on-land geology (Jamaica and Cuba) and the tectonics. Our reconstruction shows a southward propagation of the spreading centre between magnetic anomalies 8 and 6 (26 and 20 Ma). The trough width increases by 30 km in this period. The southward propagation of the Cayman spreading centre from the Middle Oligocene to the Early Miocene induced the development of the restraining bend of the Swan Islands, the formation of a 1 km high scarp on the eastern trace of the Cayman trough transform fault (Walton fault) and the formation of a pull-apart basin (Hendrix pull-apart). Magnetic anomalies and magnetization maps give information about the deformation and the rocks. The proposed evolutionary model of the Cayman trough from the inception of seafloor spreading to the present configuration is presented in relation to the tectonic escape of the northern boundary of the Caribbean plate from the Maastrichtian to the Present.  相似文献   

12.
To decipher the thermal structure of the sedimentary veneer in southern Israel, new values of thermal conductivity and porosity as well as of the radiogenic heat budget are provided. Thermal conductivity is measured for lithotypes and scaled up for geological formations. The new data are higher than most of the previously measured values, in particular for sandstones and siltstones, whose mean values are 5.0 and 2.9 W m?1 K?1. Mean values of the most abundant lithotypes, which are dolomites and limestones, are on the order of 4.1 and 2.7 W m?1 K?1, respectively. The total radiogenic heat production of the sedimentary cover varies slightly over southern Israel, due to variable lithology and total sediment thickness, yielding a maximum heat flow on the order of 4 mW m?2 where the sedimentary section is thickest (ca. 7 km). A temperature prognosis was made by calculating temperature profiles to the top of the crystalline basement at five well locations applying the new thermal‐conductivity data set and three scenarios of surface heat flow (50, 55 and 60 mW m?2). The calculated temperatures best match with measured drillstem‐test temperatures by using heat‐flow values close to the upper bound of range. Surface heat flow on the order of 55–60 mW m?2 is supported by a reevaluation of an existing temperature log and the application of thermal conductivity from this study. The temperature prediction for southern Israel shows values of 100–120°C at 3500–4500 m depth, indicating a geothermal potential that can be used for heating as well as electricity production.  相似文献   

13.
Geological studies show evidence for temporal clustering of large earthquakes on individual fault systems. Since post-seismic deformation due to the inelastic rheology of the lithosphere may result in a variable loading rate on a fault throughout the interseismic period, it is reasonable to expect that the rheology of the non-seismogenic lower crust and mantle lithosphere may play a role in controlling earthquake recurrence times. We study this phenomenon using a 2-D, finite element method continuum model of the lithosphere containing a single strike-slip fault. This model builds on a previous study using a 1-D spring-dashpot-slider analogue of a single fault system to study the role of Maxwell viscoelastic relaxation in producing non-periodic earthquakes. In our 2-D model, the seismogenic portion of the fault slips when a predetermined yield stress is exceeded; stress accumulated on the seismogenic fault is shed to the viscoelastic layers below and recycled back to the seismogenic fault through viscoelastic relaxation. We find that random variation of the fault yield stress from one earthquake to the next can cause the earthquake sequence to be clustered; the amount of clustering depends on a non-dimensional number, W , called the Wallace number defined as the standard deviation of the randomly varied fault yield stress divided by the effective viscosity of the system times the tectonic loading rate. A new clustering metric based on the bimodal distribution of interseismic intervals allows us to investigate clustering behaviour of systems over a wide range of model parameters and those with multiple viscoelastic layers. For models with   W ≥ 1  clustering increases with increasing W , while those with   W ≤ 1  are unclustered, or quasi-periodic.  相似文献   

14.
The tectonic subsidence and gravity anomalies in the Malay and Penyu Basins, offshore Peninsular Malaysia, were analysed to determine the isostatic compensation mechanism in order to investigate their origin. These continental extensional basins contain up to 14  km of sediment fill which implies that the crust had been thinned significantly during basin development. Our results suggest, however, that the tectonic subsidence in the basins cannot be explained simply by crustal thinning and Airy isostatic compensation.
The Malay and Penyu Basins are characterized by broad negative free-air gravity anomalies of between −20 and −30  mGal. To determine the cause of the anomaly, we modelled four gravity profiles across the basins using a method that combines two-dimensional flexural backstripping and gravity modelling techniques. We assumed a model of uniform lithospheric stretching and Airy isostasy in the analysis of tectonic subsidence. Our study shows that the basins are probably underlain by relatively thinned crust, indicating that some form of crustal stretching was involved. To explain the observed gravity anomalies, however, the Moho depth that we calculated based on the free-air gravity data is about 25% deeper than the Moho predicted by assuming Airy isostasy (Backstrip Moho). This suggests that the Airy model overestimates the compensation and that the basins are probably undercompensated isostatically. In other words, there is an extra amount of tectonic subsidence that is not compensated by crustal thinning, which has resulted in the discrepancy between the gravity-derived Moho and the Backstrip Moho. We attribute this uncompensated or anomalous tectonic subsidence to thin-skinned crustal extension that did not involve the mantle lithosphere. The Malay and Penyu Basins are interpreted therefore as basins that formed by a combination of whole-lithosphere stretching and thin-skinned crustal extension.  相似文献   

15.
The Nova Basin contains an upper Miocene to Pliocene supradetachment sedimentary succession that records the unroofing of the Panamint metamorphic core complex, west of Death Valley, California. Basin stratigraphy reflects the evolution of sedimentation processes from landslide emplacement during basin initiation to the development of alluvial fans composed of reworked, uplifted sections of the basin fill. 40Ar/39Ar geochronology of volcanic units in middle and lower parts of the sequence provide age control on the tectonic and depositional evolution of the basin and, more generally, insights regarding the rate of change of depositional environments in supradetachment basins. Our work, along with earlier research, indicate basin deposition from 11.38 Ma to 3.35 Ma. The data imply sedimentation rates, uncorrected for compaction, of ~100 m Myr−1 in the lower, high-energy part to ~1000 m Myr−1 in the middle part characterized by debris-flow fan deposition. The observed variation in sediment flux rate during basin evolution suggests that supradetachment basins have complex depositional histories involving rapid transitions in both the style and rate of sedimentation.  相似文献   

16.
Summary. Geomagnetic variation data from the north-eastern United States, analysed using both the transfer function and the hypothetical event techniques, reveal the presence of a linear crustal conductivity anomaly. To the west of the New York–Vermont border, the anomaly runs parallel to a regional aeromagnetic lineament and it may be associated with a basement fracture zone. To the east, there is no obvious correlation with Appalachian tectonic structure but a kink in the axis of the anomaly correlates well with high heat flow in the White Mountains.  相似文献   

17.
Summary . In this paper the accuracy of velocity-depth profiles derived by matching WKBJ seismograms to observations is quantitatively evaluated. Seismograms computed with the WKBJ method are generally quite reliable but possess predictable, systematic inaccuracies in the presence of strong velocity gradients. The effects of these inaccuracies on models derived through WKBJ waveform inversion are studied, using reflectivity seismograms as 'data'. The velocity structure used is an oceanic lithosphere model that contains several transition regions separated by relatively homogeneous layers, producing partially-reflected reverberations in the reflectivity synthetics that are absent from the WKBJ seismograms. The inversion incorporates the 'jumping' strategy to solve for the smoothest models consistent with the data. We find these solutions to be independent of the starting model and to have a stable basic structure that agrees well with the correct model. The differences, everywhere less than a seismic wavelength, depend on the frequency content of the seismograms. Reverberations in the reflectivity seismograms that are well separated from WKBJ arrivals are treated as 'noise' in the inversion.  相似文献   

18.
This paper focuses on Messinian Salinity Crisis (MSC) evaporites in the Cyprus Arc (eastern Mediterranean) using high‐resolution reflection seismic and multi‐beam data. The results shed new light on the Miocene to Present tectonic evolution of this area and contribute to our general knowledge of the MSC in a deep basin setting. The evaporites and overlying formations show a complex deformation pattern due to a combination of thick‐skinned plate‐tectonic convergence and thin‐skinned disharmonic deformation related to the mobile evaporite‐bearing unit. Several MSC markers are identified and precisely mapped: the base of the MSC unit is a ‘decollement’ level, whereas the top is clearly identified as a toplap surface. Intra‐MSC markers and two MSC subunits are identified and mapped over the entire study area. The geometry of MSC markers shows that the lower MSC subunit was deposited in a relatively quiet tectonic setting. The nature of the anisopachous upper unit indicates a syn‐depositional phase of large‐scale plate‐tectonic activity. A thin‐skinned phase of compressional deformation during the Late Miocene affected the entire MSC unit, overlain by undeformed Pliocene–Quaternary layers. A second thin‐skinned phase, well expressed in the bathymetry, occurred from the Pliocene to Recent, resulting in extensional gravity‐gliding within the evaporites and the Pliocene–Quaternary sequence. We show that the MSC had a dramatic impact on the regional structure. For instance, the erosive nature of the top of the MSC unit is linked to the desiccation episode rather than to the cessation of tectonic activity. This particularly strong and short‐lived erosion may have been enhanced by the regional effects of the MSC, owing to differential uplift/subsidence caused by the drawdown. The evaporites are essential markers for constraining the tectonic framework, provided that active deformation can be distinguished from passive gliding associated with extensional/contractional deformation.  相似文献   

19.
Foreland basins are important recorders of tectonic and climatic processes in evolving mountain ranges. The Río Iruya canyon of NW Argentina (23° S) exposes ca. 7500 m of Orán Group foreland basin sediments, spanning over 8 Myr of near continuous deposition in the Central Andes. This study presents a record of sedimentary provenance for the Iruya Section in the context of a revised stratigraphic chronology. We use U‐Pb zircon ages from six interbedded ash layers and new magnetostratigraphy to constrain depositional ages in the section between 1.94 and 6.49 Ma, giving an average sedimentation rate of 0.93 ± 0.02 (2σ) km Myr?1. We then pair U‐Pb detrital zircon dating with quartz trace‐element analysis to track changes in sedimentary provenance from ca. 7.6 to 1.8 Ma. Results suggest that from ca. 7.6 to ca. 6.3 Ma, the Iruya watershed did not tap the Salta Group or Neogene volcanics that are currently exposed in the eastern Cordillera and Puna margin. One explanation is that a long‐lived topographic barrier separated the eastern Puna from the foreland for much of the mid‐late Miocene, and that the arrival of Jurassic‐Neogene zircons records regional tectonic reactivation at ca. 6.3 Ma. A second major provenance shift at ca. 4 Ma is marked by changes in the zircon and quartz populations, which appear to be derived from a restricted source region in Proterozoic‐Ordovician meta‐sediments. Considered in conjunction with the onset of coarse conglomerate deposition, we attribute this shift to accelerated uplift of the Santa Victoria range, which currently defines the catchment's western limit. A third shift at ca. 2.3 Ma records an apparent disconnection of the Iruya with the eastern Puna, perhaps due to defeat of the proto Rio‐Iruya by the rising Santa Victoria range. This study is one of the first applications of quartz trace‐element provenance analysis, which we show to be an effective complement to U‐Pb detrital zircon dating when appropriate statistical methods are applied.  相似文献   

20.
Magnetic field fluctuations have been recorded by an array of portable three-component magnetometers at 60 sites across the Eyre Peninsula in South Australia between December 1993 and March 1995. An additional 54 magnetometer data records, collected prior to 1989 and described by Milligan (1989) and Milligan, White & Chamalaun (1989), were included in the analysis. A major conductive feature in the crust, first noted by White & Milligan (1984) as the Eyre Peninsula Anomaly (EPA), is re-examined to assess its continuity to the north of the original arrays and to investigate its relationship with major tectonic features.
Magnetic-field time-series were converted to induction arrows in the frequency domain. These induction arrows were initially inverted using the minimum-structure 2-D Occam approach to estimate the electrical conductance of the crust. Following this, thin-sheet forward modelling was used to examine the relationship between the conductance and the dominant tectonic features. The principal results of the modelling are that a narrow conductive feature extends inland from the coast about 160 km before terminating, and the conductance is in the range 3000 to 10 000 S, which decreases inland.
A strong correlation exists between the electrical conductance of the Eyre Peninsula and Bouguer gravity anomalies, and in particular the EPA is coincident with a significant Bouguer gravity gradient. There is also good agreement between the locations of the foci of earthquakes of magnitude greater than 4.0 and the EPA. We believe that the anomaly is associated with a geological fracture in the Precambrian upper crust as a result of crustal extension prior to the rifting of Australia from Antarctica in the Jurassic (160 Ma).  相似文献   

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