Gravity modelling of the lherzolite body at Lers (French Pyrenees); some regional implications

AbstractLherzolites outcrop throughout the North Pyrenean Zone of the Pyrenees and are everywhere associated with metamorphosed carbonates. It has been suggested that heat from the cooling of the lherzolites was responsible for the high temperature metamorphism of the carbonates. A gravity survey reported here shows that the volume of the lherzolite body at Lers is approximately 0.8 km3. The maximum volume of carbonates that such a body could metamorphose is 3.2 km3. This latter value is so much less than the volume of carbonates inferred from field mapping that the lherzolite body cannot have been the sole source of heat for metamorphism of the carbonates.It has been suggested from seismic data that there is a step in the Moho beneath the North Pyrenean Fault in the central Pyrenees. Gravity anomalies reported here show that either the step is less than 10 km high or that the density contrast is very low at the base of the crust in the Pyrenees.

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A gravity survey of the central Labrador Trough, northern Quebec

Canadian Journal of Earth Sciences ◽

10.1139/e77-005 ◽

1977 ◽

Vol 14 (1) ◽

pp. 45-55 ◽

Cited By ~ 1

Author(s):

P. Kearey

Keyword(s):

Gravity Anomalies ◽

Igneous Rocks ◽

Gravity Survey ◽

Iron Formation ◽

The North ◽

Density Control ◽

Residual Gravity Anomaly ◽

Fold Belts ◽

Persistent Depression ◽

North And South

The Labrador Trough is the best preserved and exposed of several Aphebian (lower Proterozoic) fold belts which surround the Archaean Ungava Craton of northern Quebec and is characterised by three longitudinal facies zones: predominantly meta-sedimentary rocks in the west and east and predominantly basic meta-igneous rocks in the centre. The results of a detailed gravity survey of the central part of the Labrador Trough between latitudes 55° 45′ and 57° 30′ and longitudes 66° 30′ and 70° are presented. Over 1500 rock samples provide density control for the interpretation of four residual gravity anomaly profiles in terms of the surface geology.In the eastern part of the Labrador Trough positive gravity anomalies correlate with outcrops of basic meta-igneous rocks. Their causative bodies extend subsurface to the east and reach depths of up to 9 km in the central part of the area, but are considerably thinner to the north and south. This interpreted depth is considerably less than the 15–20 km that has been inferred by other workers from surface geological investigations. Small positive gravity anomalies are associated with iron formation. A persistent depression in the observed gravity field over the centre of the trough in the south coincides with deposits of the basal sedimentary unit. Uncertainty in the location of the regional level prohibits accurate thickness determinations of the causative bodies of negative anomalies in this area, but the approximate values of 2–3 km obtained for the basal unit are of the same order as estimates based on geological investigations. In the northern part of the area the causative bodies of the negative anomalies are probably elevated areas of granitic basement. These elevated basement features may be related to a ridge that controlled sedimentation during much of the trough's history.

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Constraining the geometry at depth of La Maladeta and Andorra-Mont Louis granites (Central Pyrenees) through gravity modelling

10.5194/egusphere-egu2020-5688 ◽

2020 ◽

Author(s):

Conxi Ayala ◽

Pilar Clariana ◽

Ruth Soto ◽

Joan Martí ◽

Aina Margalef ◽

...

Keyword(s):

Gravity Anomaly ◽

Cross Sections ◽

Gravity Data ◽

Gravity Modelling ◽

The North ◽

Geological Surveys ◽

Residual Gravity Anomaly ◽

Paleozoic Rocks ◽

Subsurface Geometry ◽

Central Pyrenees

In the Central Pyrenees, where density contrast between the Paleozoic rocks and the intruded granitic bodies is measurable, geological cross-sections constrained with gravity data help to unravel the subsurface geometry of the granites.With this goal in mind, during 2018 and 2019 several gravimetric surveys were carried out in the Central Pyrenees to improve the existent spatial resolution of the gravity data from the databases of the Spanish and Catalan Geological Surveys, especially in La Maladeta and Andorra Mont-Louis granites&#8217; area. After the gravity reductions, we obtained the Bouguer gravity anomaly from which we calculated the residual gravity anomaly by subtracting a third degree polynomial which represents the regional anomaly in agreement with the geometry of the crust in this region.The gravimetric response over La Maladeta and Andorra Mont-Louis granites is markedly dissimilar pointing out differences in the composition and geometry at depth of the two granites. La Maladeta granite shows a gravimetric zonation with small variations in its amplitude from one zone to the next, consistent with small lateral changes in its composition, predominantly granodioritic. By contrast, the Andorra Mont-Louis pluton is characterized by a relative minimum suggesting a more granitic composition.With respect to the inferred geometry at depth, the results obtained from gravity modelling show that the La Maladeta granite displays a laccolithic shape with its basal contact deeping to the North whereas the Andorra Mont-Louis granite has a more batholitic shape. Although the emplacement age of both granites is similar (Late Carboniferous &#8211; Early Permian), their different geometry at depth suggests that either (1) their emplacement mechanisms were different or (2) the subsequent Alpine orogeny affected both granites in different ways better preserving the original geometry of the Andorra Mont-Louis granite.

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A gravity model of the North Eurasia crust and upper mantle: 1. Mantle and isostatic residual gravity anomalies

Russian Journal of Earth Sciences ◽

10.2205/2001es000062 ◽

2001 ◽

Vol 3 (2) ◽

pp. 125-144 ◽

Cited By ~ 24

Author(s):

M. K. Kaban

Keyword(s):

Gravity Model ◽

Upper Mantle ◽

Gravity Anomalies ◽

Crust And Upper Mantle ◽

Residual Gravity ◽

The North ◽

North Eurasia

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Chemical Th–U–total Pb dating by electron microprobe analysis of monazite, xenotime and zircon from the Archean Napier Complex, East Antarctica: evidence for ultra-high-temperature metamorphism at 2400 Ma

Precambrian Research ◽

10.1016/s0301-9268(01)00228-5 ◽

2002 ◽

Vol 114 (3-4) ◽

pp. 249-275 ◽

Cited By ~ 55

Author(s):

Masao Asami ◽

Kazuhiro Suzuki ◽

Edward S Grew

Keyword(s):

High Temperature ◽

Electron Microprobe ◽

Microprobe Analysis ◽

Electron Microprobe Analysis ◽

East Antarctica ◽

High Temperature Metamorphism ◽

Napier Complex ◽

Ultra High Temperature

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Seismic reflection test on the granite of the Skye Tertiary igneous centre

Geological Magazine ◽

10.1017/s0016756800021804 ◽

1992 ◽

Vol 129 (5) ◽

pp. 633-636 ◽

Cited By ~ 7

Author(s):

N. R. Goulty ◽

M. Leggett ◽

T. Douglas ◽

C. H. Emeleus

Keyword(s):

Seismic Data ◽

Seismic Reflection ◽

Intrusive Complex ◽

Gravity Modelling ◽

Test Location ◽

Basic Rocks

AbstractWe have conducted a seismic reflection test over a short profile on the granite of the Skye Tertiary central intrusive complex. From previous gravity modelling work it had been inferred that the granite is approximately 1.5 km thick and overlies basic rocks. The seismic data indicate that the granite is at least 2 km thick at the test location. Reflection events of alternating polarity between depths of 2.1 and 2.4 km suggest that basic and acidic sheets are interlayered at the base of the granitic mass.

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

Geophysics ◽

10.1190/1.2235858 ◽

2006 ◽

Vol 71 (5) ◽

pp. C81-C92 ◽

Cited By ~ 17

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.

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