scholarly journals Magnetotelluric imaging of the Mérida Andes and surrounding areas in Venezuela

2020 ◽  
Vol 222 (3) ◽  
pp. 1570-1589
Author(s):  
J Cruces-Zabala ◽  
O Ritter ◽  
U Weckmann ◽  
K Tietze ◽  
M Schmitz
Keyword(s):  
Broad Band ◽  
Synthetic Data ◽  
Sedimentary Basins ◽  
Geodynamic Setting ◽  
Active Deformation ◽  
Fault Systems ◽  
Maracaibo Basin ◽  
The North ◽  
The Caribbean ◽  
Surrounding Areas

SUMMARY The Caribbean and South American tectonic plates bound the north-eastwards expulsion of the North Andean Block in western Venezuela. This complex geodynamic setting resulted in the formation of major strike-slip fault systems and sizeable mountain chains. The 100-km-wide Mérida Andes extend from the Colombian/Venezuelan border to the Caribbean coast. To the north and south, the Mérida Andes are bound by hydrocarbon-rich sedimentary basins. Knowledge of lithospheric structures, related to the formation of the Mérida Andes, is limited though, due to a lack of deep geophysical data. In this study, we present results of the first broad-band magnetotelluric profile crossing the Mérida Andes and the Maracaibo and Barinas–Apure foreland basins on a length of 240 km. Geoelectrical strike and dimensionality analysis are consistent with 1-D or 2-D subsurface structures for the sedimentary basins but also indicate a strong 3-D setting for the Mérida Andes. Using a combination of 2-D and 3-D modelling we systematically examined the influence of 3-D structures on 2-D inversions. Synthetic data sets derived from 3-D modelling allow identification and quantification of spurious off-profile features as well as smoothing artefact due to limited areal station coverage of data collected along a profile. The 2-D inversion models show electrically conductive basins with depths of 2–5 km for the Barinas-Apure and 2–7 km for the Maracaibo basins. A number of resistive bodies within the Maracaibo basin could be related to active deformation causing juxtaposition of older geological formations and younger basin sediments. The most important fault systems of the area, the Boconó and Valera Faults, cross-cut the Mérida Andes in NE–SW direction along its strike on a length 400 km and N–S direction at its centre on a length 60 km, respectively. Both faults are associated with subvertical zones of high electrical conductivity and sensitivity tests suggest that they reach depths of up to 12 km. A sizeable conductor at 50 km depth, which appears consistently in the 2-D sections, could be identified as an inversion artefact caused by a conductor east of the profile. We speculate the high conductivity associated with the off-profile conductor may be related to the detachment of the Trujillo Block. Our results partially support the ‘floating orogen hypothesis’ developed to explain the geodynamic evolution of western Venezuela and they highlight the relevance of the Trujillo Block in this process.

Three-dimensional imaging of the Mérida Andes, Venezuela

2021 ◽  
Author(s):  
José Cruces ◽  
Oliver Ritter ◽  
Ute Weckmann ◽  
Kristina Tietze ◽  
Naser Meqbel ◽  
...  
Keyword(s):  
Three Dimensional ◽  
Sedimentary Basins ◽  
Tectonic Plates ◽  
Fault Systems ◽  
Maracaibo Basin ◽  
The North ◽  
Oblique Convergence ◽  
The Caribbean ◽  
Major Strike ◽  
Triangular Block

<p>The Mérida Andes are a 100 km wide mountain chain that extends from the Colombian/Venezuelan border to the Caribbean coast. To the north and south, the Mérida Andes are bound by hydrocarbon-rich sedimentary basins. Uplift of the mountains started in the late Miocene due to oblique convergence of the Caribbean and South American tectonic plates and the north-eastwards expulsion of the North Andean Block (NAB). This tectonic interaction fostered major strike-slip fault systems, with associated high seismicity, and the partitioning of the North Andean Block into smaller tectonic units, whose interaction accelerated the uplift of the Mérida Andes since the Plio-Pleistocene.</p><p>We present the three-dimensional inversion results of broadband magnetotelluric (MT) data from 72 sites gathered along a 240 km long profile across the central part of the MA, the Maracaibo (MB), and Barinas-Apure (BAB) foreland basins. Directionality and dimensionality analyses suggested 3D structures for the MA section, with the induction vectors indicating off-profile structures, particularly at long periods. Since the distribution of sites predominantly along a single profile can have adverse effects on the outcome of the 3D inversion, we rigorously tested all model features for robustness and excluded artefacts.</p><p>One of the main findings is a deep connection (> 10km) between the most prominent faults of the MA, the Valera and Boconó fault systems, with a deep off-profile conductor to the east of our profile. We interpret this conductive structure as a detachment surface of the Trujillo Block, which is part of the NAB and whose expulsion to the NE significantly influences the present-day geodynamic evolution of western Venezuela. A conductive zone under the Maracaibo Basin correlates spatially with the location of a Bouguer low. Both geophysical anomalies may be caused by a SE tilt of the Maracaibo Triangular Block under the Mérida Andes, bound by the north-western thrust system which could reach depths of 30 km.</p>

Geodynamics of the France Southeast Basin

2010 ◽  
Vol 181 (6) ◽  
pp. 477-501 ◽  
Author(s):  
Xavier Le Pichon ◽  
Claude Rangin ◽  
Youri Hamon ◽  
Nicolas Loget ◽  
Jin Ying Lin ◽  
...  
Keyword(s):  
Seismic Activity ◽  
Sedimentary Cover ◽  
Central Basin ◽  
The South ◽  
Western Basin ◽  
Active Deformation ◽  
The North ◽  
The Alps ◽  
Sediment Cover ◽  
Surrounding Areas

AbstractWe investigate the geodynamics of the Southeast Basin with the help of maps of the basement and of major sedimentary horizons based on available seismic reflection profiles and drill holes. We also present a study of the seismicity along the Middle Durance fault. The present seismic activity of the SE Basin cannot be attributed to the Africa/Eurasia shortening since spatial geodesy demonstrates that there is no significant motion of Corsica-Sardinia with respect to Eurasia and since gravitational collapse of the Alps has characterized the last few millions years. Our study demonstrates that the basement of this 140 by 200 km Triassic basin has been essentially undeformed since its formation, most probably because of the hardening of the cooling lithosphere after its 50% thinning during the Triassic distension. The regional geodynamics are thus dominated by the interaction of this rigid unit with the surrounding zones of active deformation. The 12 km thick Mesozoic sediment cover includes at its base an up to 4 km thick mostly evaporitic Triassic layer that is hot and consequently highly fluid. The sedimentary cover is thus decoupled from the basement. As a result, the sedimentary cover does not have enough strength to produce reliefs exceeding about 500 to 750 m. That the deformation and seismicity affecting the basin are the results of cover tectonics is confirmed by the fact that seismic activity in the basin only affects the sedimentary cover. Based on our mapping of the structure of the basin, we propose a simple mechanism accounting for the Neogene deformation of the sedimentary cover. The formation of the higher Alps has first resulted to the north in the shortening of the Diois-Baronnies sedimentary cover that elevated the top of Jurassic horizons by about 4 km with respect to surrounding areas to the south and west. There was thus passage from a brittle-ductile basement decollement within the higher Alps to an evaporitic decollement within the Diois-Baronnies. This shortening and consequent elevation finally induced the southward motion of the basin cover south of the Lure mountain during and after the Middle Miocene. This southward motion was absorbed by the formation of the Luberon and Trévaresse mountains to the south. To the east of the Durance fault, there is no large sediment cover. The seismicity there, is related to the absorption of the Alps collapse within the basement itself. To the west of the Salon-Cavaillon fault, on the other hand, gravity induces a NNE motion of the sedimentary cover with extension to the south and shortening to the north near Mont Ventoux. When considering the seismicity of this area, it is thus important to distinguish between the western Basin panel, west of the Salon-Cavaillon fault affected by very slow NNE gliding of the sedimentary cover, with extension to the south and shortening to the north; the central Basin panel west of the Durance fault with S gliding of the sedimentary cover and increasing shortening to the south; and finally the basement panel east of the Durance fault with intrabasement absorption of the Alps collapse through strike-slip and thrust faults.

scholarly journals Modeling and detection of regional depth phases at the GERES array

2015 ◽  
Vol 41 ◽  
pp. 5-10
Author(s):  
M.-T. Apoloner ◽  
G. Bokelmann
Keyword(s):  
Array Processing ◽  
Synthetic Data ◽  
Region Of Interest ◽  
Sedimentary Basins ◽  
Depth Estimation ◽  
Vienna Basin ◽  
North West ◽  
The North ◽  
Propagation Behavior ◽  
Moderate Seismicity

Abstract. The Vienna Basin in Eastern Austria is a region of low to moderate seismicity, and hence the seismological network coverage is relatively sparse. Nevertheless, the area is one of the most densely populated and most developed areas in Austria, so accurate earthquake location, including depth estimation and relation to faults is not only important for understanding tectonic processes, but also for estimating seismic hazard. Particularly depth estimation needs a dense seismic network around the anticipated epicenter. If the station coverage is not sufficient, the depth can only be estimated roughly. Regional Depth Phases (RDP) like sPg, sPmP and sPn have been already used successfully for calculating depth even if only observable from one station. However, especially in regions with sedimentary basins these phases prove difficult or impossible to recover from the seismic records. For this study we use seismic array data from GERES. It is 220 km to the North West of the Vienna Basin, which – according to literature – is a suitable distance to recover PmP and sPmP phases. We use array processing on recent earthquake data from the Vienna Basin with local magnitudes from 2.1 to 4.2 to reduce the SNR and to search for RDP. At the same time, we do similar processing on synthetic data specially modeled for this application. We compare real and synthetic results to assess which phases can be identified and to what extent depth estimation can be improved. Additionally, we calculate a map of lateral propagation behavior of RDP for a typical strike-slip earthquake in our region of interest up to 400 km distance. For our study case RDP propagation is strongly azimuthally dependent. Also, distance ranges differ from literature sources. Comparing with synthetic seismograms we identify PmP and PbP phases with array processing as strongest arrivals. Although the associated depth phases cannot be identified at this distance and azimuth, identification of the PbP phases limits possible depth to less than 20 km. Polarization analysis adds information on the first arriving Pn wave for local magnitudes above 2.5.

Hydrothermal activity in the Upper Permian Ravnefjeld Formation of central East Greenland – a study of sulphide morphotypes

1998 ◽  
pp. 81-87
Author(s):  
Jesper Kresten Nielsen ◽  
Mikael Pedersen
Keyword(s):  
Base Metal ◽  
Early Period ◽  
Sedimentary Basins ◽  
Hydrothermal Activity ◽  
Source Rocks ◽  
Upper Permian ◽  
Thermal Activity ◽  
East Greenland ◽  
The North ◽  
Alum Shale

NOTE: This article was published in a former series of GEUS Bulletin. Please use the original series name when citing this article, for example: Kresten Nielsen, J., & Pedersen, M. (1998). Hydrothermal activity in the Upper Permian Ravnefjeld Formation of central East Greenland – a study of sulphide morphotypes. Geology of Greenland Survey Bulletin, 180, 81-87. https://doi.org/10.34194/ggub.v180.5090 _______________ Bituminous shales of the Ravnefjeld Formation were deposited in the subsiding East Greenland basin during the Upper Permian. The shales are exposed from Jameson Land in the south (71°N; Fig. 1) to Clavering Ø in the north (74°20′N) and have attracted considerable attention due to their high potential as hydrocarbon source rocks (Piasecki & Stemmerik 1991; Scholle et al. 1991; Christiansen et al. 1992, 1993a, b). Furthermore, enrichment of lead, zinc and copper has been known in the Ravnefjeld Formation on Wegener Halvø since 1968 (Lehnert-Thiel 1968; Fig. 1). This mineralisation was assumed to be of primary or early diagenetic origin due to similarities with the central European Kupferschiefer (Harpøth et al. 1986). Later studies, however, suggested base metal mineralisation in the immediately underlying carbonate reefs to be Tertiary in age (Stemmerik 1991). Due to geographical coincidence between the two types of mineralisation, a common history is a likely assumption, but a timing paradox exists. A part of the TUPOLAR project on the ‘Resources of the sedimentary basins of North and East Greenland’ has been dedicated to re-investigation of the mineralisation in the Ravnefjeld Formation in order to determine the genesis of the mineralisation and whether or not primary or early diagenetic base metal enrichment has taken place on Wegener Halvø, possibly in relation to an early period of hydrothermal activity. One approach to this is to study the various sulphides in the Ravnefjeld Formation; this is carried out in close co-operation with a current Ph.D. project at the University of Copenhagen, Denmark. Diagenetically formed pyrite is a common constituent of marine shales and the study of pyrite morphotypes has previously been successful from thermalli immature parts of elucidating depositional environment and thermal effects in the Alum Shale Formation of Scandinavia (Nielsen 1996; Nielsen et al. 1998). The present paper describes the preliminary results of a similar study on pyrite from thermally immature parts of the Ravnefjeld Formation which, combined with the study of textures of base metal sulphides in the Wegener Halvø area (Fig. 1), may provide an important step in the evaluation of the presence or absence of early thermal activity on (or below) the Upper Permian sea floor.

scholarly journals Anomalous azimuthal variations with 360° periodicity of Rayleigh phase velocities observed in Scandinavia

2020 ◽  
Vol 224 (3) ◽  
pp. 1684-1704
Author(s):  
Alexandra Mauerberger ◽  
Valérie Maupin ◽  
Ólafur Gudmundsson ◽  
Frederik Tilmann
Keyword(s):  
Seismic Anisotropy ◽  
Broad Band ◽  
Shear Velocity ◽  
Velocity Variation ◽  
The South ◽  
Study Region ◽  
Lithospheric Thickness ◽  
Phase Velocities ◽  
The North ◽  
Southern Norway

SUMMARY We use the recently deployed ScanArray network of broad-band stations covering most of Norway and Sweden as well as parts of Finland to analyse the propagation of Rayleigh waves in Scandinavia. Applying an array beamforming technique to teleseismic records from ScanArray and permanent stations in the study region, in total 159 stations with a typical station distance of about 70 km, we obtain phase velocities for three subregions, which collectively cover most of Scandinavia (excluding southern Norway). The average phase dispersion curves are similar for all three subregions. They resemble the dispersion previously observed for the South Baltic craton and are about 1 per cent slower than the North Baltic shield phase velocities for periods between 40 and 80 s. However, a remarkable sin(1θ) phase velocity variation with azimuth is observed for periods >35 s with a 5 per cent deviation between the maximum and minimum velocities, more than the overall lateral variation in average velocity. Such a variation, which is incompatible with seismic anisotropy, occurs in northern Scandinavia and southern Norway/Sweden but not in the central study area. The maximum and minimum velocities were measured for backazimuths of 120° and 300°, respectively. These directions are perpendicular to a step in the lithosphere–asthenosphere boundary (LAB) inferred by previous studies in southern Norway/Sweden, suggesting a relation to large lithospheric heterogeneity. In order to test this hypothesis, we carried out 2-D full-waveform modeling of Rayleigh wave propagation in synthetic models which incorporate a steep gradient in the LAB in combination with a pronounced reduction in the shear velocity below the LAB. This setup reproduces the observations qualitatively, and results in higher phase velocities for propagation in the direction of shallowing LAB, and lower ones for propagation in the direction of deepening LAB, probably due to the interference of forward scattered and reflected surface wave energy with the fundamental mode. Therefore, the reduction in lithospheric thickness towards southern Norway in the south, and towards the Atlantic ocean in the north provide a plausible explanation for the observed azimuthal variations.

Seismic Energy Release from Intra-Basin Sources along the Dead Sea Transform and Its Influence on Regional Ground Motions

2020 ◽  
Author(s):  
Roey Shimony ◽  
Zohar Gvirtzman ◽  
Michael Tsesarsky
Keyword(s):  
Ground Motions ◽  
Seismic Energy ◽  
Sedimentary Basins ◽  
Seismic Wave Propagation ◽  
Dead Sea ◽  
Strike Slip ◽  
Dead Sea Transform ◽  
Bay Area ◽  
The Dead ◽  
Surrounding Areas

ABSTRACT The Dead Sea Transform (DST) dominates the seismicity of Israel and neighboring countries. Whereas the instrumental catalog of Israel (1986–2017) contains mainly M<5 events, the preinstrumental catalog lists 14 M 7 or stronger events on the DST, during the past two millennia. Global Positioning System measurements show that the slip deficit in northern Israel today is equivalent to M>7 earthquake. This situation highlights the possibility that a strong earthquake may strike north Israel in the near future, raising the importance of ground-motion prediction. Deep and narrow strike-slip basins accompany the DST. Here, we study ground motions produced by intrabasin seismic sources, to understand the basin effect on regional ground motions. We model seismic-wave propagation in 3D, focusing on scenarios of Mw 6 earthquakes, rupturing different active branches of the DST. The geological model includes the major structures in northern Israel: the strike-slip basins along the DST, the sedimentary basins accompanying the Carmel fault zone, and the densely populated and industrialized Zevulun Valley (Haifa Bay area). We show that regional ground motions are determined by source–path coupling effects in the strike-slip basins, before waves propagate into the surrounding areas. In particular, ground motions are determined by the location of the rupture nucleation within the basin, the near-rupture lithology, and the basin’s local structure. When the rupture is located in the crystalline basement or along material bridges connecting opposite sides of the fault, ground motions behave predictably, decaying due to geometrical spreading and locally amplified atop sedimentary basins. By contrast, if rupture nucleates or propagates into shallow sedimentary units of the DST strike-slip basins, ground motions are amplified within, before propagating outside. Repeated reflections from the basin walls result in a “resonant chamber” effect, leading to stronger regional ground motions with prolonged durations.

Stochastic inversion of pressure and saturation changes from time-lapse AVO data

Geophysics ◽  
2006 ◽  
Vol 71 (5) ◽  
pp. C81-C92 ◽  
Author(s):  
Helene Hafslund Veire ◽  
Hilde Grude Borgos ◽  
Martin Landrø
Keyword(s):  
Stochastic Model ◽  
Seismic Data ◽  
Synthetic Data ◽  
Time Lapse ◽  
Bayesian Framework ◽  
Reservoir Modeling ◽  
Data Set ◽  
The North ◽  
Fluid Saturation ◽  
Likelihood Model

Effects of pressure and fluid saturation can have the same degree of impact on seismic amplitudes and differential traveltimes in the reservoir interval; thus, they are often inseparable by analysis of a single stacked seismic data set. In such cases, time-lapse AVO analysis offers an opportunity to discriminate between the two effects. We quantify the uncertainty in estimations to utilize information about pressure- and saturation-related changes in reservoir modeling and simulation. One way of analyzing uncertainties is to formulate the problem in a Bayesian framework. Here, the solution of the problem will be represented by a probability density function (PDF), providing estimations of uncertainties as well as direct estimations of the properties. A stochastic model for estimation of pressure and saturation changes from time-lapse seismic AVO data is investigated within a Bayesian framework. Well-known rock physical relationships are used to set up a prior stochastic model. PP reflection coefficient differences are used to establish a likelihood model for linking reservoir variables and time-lapse seismic data. The methodology incorporates correlation between different variables of the model as well as spatial dependencies for each of the variables. In addition, information about possible bottlenecks causing large uncertainties in the estimations can be identified through sensitivity analysis of the system. The method has been tested on 1D synthetic data and on field time-lapse seismic AVO data from the Gullfaks Field in the North Sea.

scholarly journals Nonlinear analysis of the occurrence of hurricanes in the Gulf of Mexico and the Caribbean Sea

2018 ◽  
Vol 25 (2) ◽  
pp. 291-300 ◽  
Author(s):  
Berenice Rojo-Garibaldi ◽  
David Alberto Salas-de-León ◽  
María Adela Monreal-Gómez ◽  
Norma Leticia Sánchez-Santillán ◽  
David Salas-Monreal
Keyword(s):  
Time Series ◽  
Gulf Of Mexico ◽  
Nonlinear Analysis ◽  
Chaotic Behavior ◽  
Caribbean Sea ◽  
Historical Record ◽  
Nonlinear Analyses ◽  
The North ◽  
The Caribbean ◽  
The Individual

Abstract. Hurricanes are complex systems that carry large amounts of energy. Their impact often produces natural disasters involving the loss of human lives and materials, such as infrastructure, valued at billions of US dollars. However, not everything about hurricanes is negative, as hurricanes are the main source of rainwater for the regions where they develop. This study shows a nonlinear analysis of the time series of the occurrence of hurricanes in the Gulf of Mexico and the Caribbean Sea obtained from 1749 to 2012. The construction of the hurricane time series was carried out based on the hurricane database of the North Atlantic basin hurricane database (HURDAT) and the published historical information. The hurricane time series provides a unique historical record on information about ocean–atmosphere interactions. The Lyapunov exponent indicated that the system presented chaotic dynamics, and the spectral analysis and nonlinear analyses of the time series of the hurricanes showed chaotic edge behavior. One possible explanation for this chaotic edge is the individual chaotic behavior of hurricanes, either by category or individually regardless of their category and their behavior on a regular basis.

Temporal distribution and population structure of two congeneric species ofCucullanus(Nematoda: Cucullanidae) inOrthopristis ruberin Venezuela

2008 ◽  
Vol 82 (1) ◽  
pp. 69-76 ◽  
Author(s):  
A.K. Bashirullah ◽  
M.T. Diaz
Keyword(s):  
Temporal Distribution ◽  
Caribbean Sea ◽  
Congeneric Species ◽  
Mean Intensity ◽  
Qualitative And Quantitative ◽  
The North ◽  
Significant Difference ◽  
The Caribbean ◽  
Over Exploitation ◽  
Fish Hosts

AbstractThe qualitative and quantitative parameters of temporal distribution ofCucullanus tripapillatusandCucullanus chrysophrydesin the intestine ofOrthopristis ruberin the Caribbean Sea on the north of Margarita Island, Venezuela were analysed. A total of 540 fish were collected at random from the catch of commercial trawlers during 1982–83 and 1992–93. Both species ofCucullanuswere found throughout the year; prevalence and mean intensity ofC. tripapillatuswere higher than that ofC. chrysophrydes. A significant difference was found in infection between the two years of sampling. Female worms were more abundant than male in both years. Both species exhibited pronounced prevalence and maturity in September and March of each year, indicating seasonality. The patterns of occurrence of the two species ofCucullanusinO. ruberdid not change in the 10-year interval but the host size and number of parasites declined, which may be due to over-exploitation of definitive fish hosts.

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