Joint inversion of gravity and magnetic anomalies of eastern Canada

1998 ◽  
Vol 35 (7) ◽  
pp. 832-853 ◽  
Author(s):  
Ying Zheng ◽  
Jafar Arkani-Hamed
Keyword(s):  
Surface Topography ◽  
Joint Inversion ◽  
Gravity Anomalies ◽  
Magnetic Anomalies ◽  
Eastern Canada ◽  
Entire Area ◽  
Long Wavelength ◽  
Degree Correlation ◽  
Crustal Model ◽  
Density Perturbations

The power spectra and degree correlation of the surface topography and free-air gravity anomalies of eastern Canada show that the gravity anomalies are subdivided into three parts. The short-wavelength components (30-170 km, shorter than 30 km are not well resolved) largely arise from density perturbations in the crust and to a lesser extent from the surface topography and Moho undulation, whereas the contribution of intracrustal sources to the intermediate-wavelength components (170-385 km) is comparable with that of the topography. The long-wavelength components (385-1536 km) are overcompensated at the Moho. We present a crustal model for the intermediate- and long-wavelength components which takes into account the surface topography, density perturbations in the crust, and Moho undulation with a certain degree of isostatic compensation. The general characteristics of this model resemble the crustal structure revealed from seismic measurements. The reduced-to-pole magnetic anomalies of eastern Canada show no pronounced correlation with the topography and with the vertical gradient of the gravity anomalies, suggesting that the source bodies are within the crust and Poisson's relationship does not hold over the entire area. Assuming that the magnetic anomalies arise from induced magnetization, lateral variations of magnetic susceptibility of the crust are determined while taking into account the effects of the surface topography and the Moho undulation of our crustal model. The intermediate- and long-wavelength components of the susceptibility contrasts delineate major collision zones as low-susceptibility regions. We interpret this in terms of thermal demagnetization of the high-magnetic crustal roots beneath the collision zones.

Tectonic significance of gravity and magnetic variations along the Lithoprobe Southern Canadian Cordillera Transect

1995 ◽  
Vol 32 (10) ◽  
pp. 1584-1610 ◽  
Author(s):  
Frederick A. Cook ◽  
John L. Varsek ◽  
Jeffrey B. Thurston
Keyword(s):  
Gravity Anomalies ◽  
Magnetic Anomalies ◽  
Canadian Cordillera ◽  
Western Coast ◽  
Potential Field Data ◽  
Long Wavelength ◽  
Geological Features ◽  
Gravity And Magnetic ◽  
Thrust Sheets ◽  
Accreted Terranes

Correlation of potential field data to regional geological features within the Lithoprobe southern Canadian Cordillera transect corridor allows characterization of anomaly patterns according to their likely sources. Long-wavelength Bouguer gravity anomalies are attributed to isostatic effects of topography, which in most areas is compensated. Two notable exceptions occur: in the Foreland belt a large positive isostatic anomaly is likely due to mechanical support of topography formed as Cordilleran thrust sheets were emplaced over the thick craton, and on the west coast, isostatic anomalies are related to active subduction. Long-wavelength magnetic anomalies in the Foreland belt are associated with cratonal basement beneath the thrust sheets, and these can be followed westward to near the Omineca belt. A prominent positive magnetic anomaly along the western Coast belt is probably associated with mafic rocks generated during subduction. Elsewhere, relatively short wavelength gravity and magnetic anomalies correlate well with either plutons (both gravity and magnetic), volcanics (primarily magnetics), or faults (magnetics) within the region of accreted terranes.

Interpretation of Long Wavelength Magnetic Anomalies

1981 ◽  
pp. 231-255 ◽  
Author(s):  
Paolo Gasparini ◽  
Marta S. M. Mantovani ◽  
Wladimir Shukowsky
Keyword(s):  
Magnetic Anomalies ◽  
Long Wavelength

Analytical expressions for gravity anomalies due to homogeneous polyhedral bodies and translations into magnetic anomalies

Geophysics ◽  
1979 ◽  
Vol 44 (4) ◽  
pp. 730-741 ◽  
Author(s):  
M. Okabe
Keyword(s):  
Gravitational Potential ◽  
Computing Time ◽  
Gravity Anomalies ◽  
Magnetic Anomalies ◽  
Two Dimensions ◽  
First Derivative ◽  
Second Derivatives ◽  
Analytical Expressions ◽  
Derivatives Of ◽  
The Magnetic Field

Complete analytical expressions for the first and second derivatives of the gravitational potential in arbitrary directions due to a homogeneous polyhedral body composed of polygonal facets are developed, by applying the divergence theorem definitively. Not only finite but also infinite rectangular prisms then are treated. The gravity anomalies due to a uniform polygon are similarly described in two dimensions. The magnetic potential due to a uniformly magnetized body is directly derived from the first derivative of the gravitational potential in a given direction. The rule for translating the second derivative of the gravitational potential into the magnetic field component is also described. The necessary procedures for practical computer programming are discussed in detail, in order to avoid singularities and to save computing time.

Comoros – New Evidence and Arguments for Continental Crust

2016 ◽  
Author(s):  
John Milsom ◽  
Phil Roach ◽  
Chris Toland ◽  
Don Riaroh ◽  
Chris Budden ◽  
...  
Keyword(s):  
Continental Crust ◽  
Fracture Zone ◽  
Seismic Data ◽  
Gravity Anomalies ◽  
Magnetic Anomalies ◽  
Magnetic Data ◽  
The West ◽  
Sea Floor ◽  
Inappropriate Use ◽  
Sea Floor Spreading

ABSTRACT As part of an ongoing exploration effort, approximately 4000 line-km of seismic data have recently been acquired and interpreted within the Comoros Exclusive Economic Zone (EEZ). Magnetic and gravity values were recorded along the seismic lines and have been integrated with pre-existing regional data. The combined data sets provide new constraints on the nature of the crust beneath the West Somali Basin (WSB), which was created when Africa broke away from Gondwanaland and began to move north. Despite the absence of clear sea-floor spreading magnetic anomalies or gravity anomalies defining a fracture zone pattern, the crust beneath the WSB has been generally assumed to be oceanic, based largely on regional reconstructions. However, inappropriate use of regional magnetic data has led to conclusions being drawn that are not supported by evidence. The identification of the exact location of the continent-ocean boundary (COB) is less simple than would at first sight appear and, in particular, recent studies have cast doubt on a direct correlation between the COB and the Davie Fracture Zone (DFZ). The new high-quality reflection seismic data have imaged fault patterns east of the DFZ more consistent with extended continental crust, and the accompanying gravity and magnetic surveys have shown that the crust in this area is considerably thicker than normal oceanic and that linear magnetic anomalies typical of sea-floor spreading are absent. Rifting in the basin was probably initiated in Karoo times but the generation of new oceanic crust may have been delayed until about 154 Ma, when there was a switch in extension direction from NW-SE to N-S. From then until about 120 Ma relative movement between Africa and Madagascar was accommodated by extension in the West Somali and Mozambique basins and transform motion along the DFZ that linked them. A new understanding of the WSB can be achieved by taking note of newly-emerging concepts and new data from adjacent areas. The better-studied Mozambique Basin, where comprehensive recent surveys have revealed an unexpectedly complex spreading history, may provide important analogues for some stages in WSB evolution. At the same time the importance of wide continent-ocean transition zones marked by the presence of hyper-extended continental crust has become widely recognised. We make use of these new insights in explaining the anomalous results from the southern WSB and in assessing the prospectivity of the Comoros EEZ.

Forward  Modelling of Bouguer Anomalies along a transect of the Southern Apennines and the Tyrrhenian back-arc basin.

2021 ◽  
Author(s):  
Assel Akimbekova ◽  
Paolo Mancinelli ◽  
Massimiliano Rinaldo Barchi ◽  
Cristina Pauselli ◽  
Giorgio Minelli
Keyword(s):  
Gravity Anomalies ◽  
Moho Depth ◽  
Positive Trend ◽  
Tyrrhenian Sea ◽  
Southern Apennines ◽  
Bouguer Anomalies ◽  
Crustal Model ◽  
Density Values ◽  
Best Fitting ◽  
Fitting Model

<p>Abstract</p><p> </p><p>In the present study, starting from original measurement stations, we created the Bouguer anomaly map of Southern Italy with a reduction density of 2670 kg m<sup>-3</sup>. We perform a regional gravity modelling at crustal scale along the trace of the CROP-04 (on-shore) and MB6 (off-shore) deep seismic reflection profiles crossing the Southern Apennines and the Southern Tyrrhenian Sea. Along the 320 km-long modelled profile, we investigate crustal-scale sources for the observed gravity anomalies. </p><p>After a compelling review of the published Moho geometries in the area, that were retrieved from either active or passive seismic methods, we test them in the observed gravity field through forward modeling of the Bouguer gravity anomalies. The comparison between the different Moho interpretations shows that the steepness of the subducting slab, the position of the step between the western (Tyrrhenian) and the eastern (Adriatic) Moho and Moho depth represent the main features influencing the observed Bouguer anomalies at crustal scale.</p><p>Finally, we provide a best-fitting model across both onshore and offshore areas. In the proposed best-fitting model, the wide wavelength and strong regional Bouguer anomalies correlate with the geometry of the Moho discontinuity and deep tectonic structures. On the other hand, the small-amplitude oscillations of the gravity anomalies were attributed to the low-density values of the Pliocene-Quaternary deposits both on- (e.g. the Bradanic trough) and off-shore (e.g. recent deposits in the Tyrrhenian sea bottom). Gravity minima correspond to the crustal doubling underneath the Southern Apennines where the Tyrrhenian Moho (~27 km depth) overlies the deeper Adriatic Moho (~50 km depth). The positive trend of the observed anomaly toward NE is related to the shallowing of the Adriatic Moho to depths of ~28 km in the Adriatic. Similarly, towards SW, the observed anomaly follows a positive trend towards the maxima located in the Central Tyrrhenian Sea. We model this trend as representative of crustal thinning and shallowing to values of ~12 km depth of the Tyrrhenian Moho. We also model a crustal transition from geometries and density values typical of a continental crust in the Adriatic domain towards a more oceanic structure and composition in the Tyrrhenian domain. This crustal model locates the westward flexure of the Adriatic Moho, mimicking the subduction of the Adriatic lithosphere beneath the Peri-Tyrrhenian block and locates step between the western (Tyrrhenian) and the eastern (Adriatic) Moho beneath the Apennines range.</p><p>The resulted gravity forward model provide contributions to the tectonic settings understanding of the area by providing a robust crustal model ranging from the Tyrrhenian Sea to the Apulian foreland.</p><p> Finally, we believe that the proposed model can serve as a starting point for future studies investigating the upper crustal geometries in the area and addressing open questions about its relations with seismicity distribution.</p><p> </p>

Reduction to the pole of the North American magnetic anomalies

Geophysics ◽  
1990 ◽  
Vol 55 (2) ◽  
pp. 218-225 ◽  
Author(s):  
J. Arkani‐Hamed ◽  
W. E. S. Urquhart
Keyword(s):  
North America ◽  
Gravity Anomalies ◽  
Magnetic Anomalies ◽  
Gravity Gradient ◽  
Plate Boundaries ◽  
Gravity Gradients ◽  
The North ◽  
Vertical Gravity Gradient ◽  
Regional Tectonic ◽  
Vertical Gravity

Magnetic anomalies of North America are reduced to the pole using a generalized technique which takes into account the variations in the directions of the core field and the magnetization of the crust over North America. The reduced‐to‐the‐pole magnetic anomalies show good correlations with a number of regional tectonic features, such as the Mid‐Continental rift and the collision zones along plate boundaries, which are also apparent in the vertical gravity gradient map of North America. The magnetic anomalies do not, however, show consistent correlation with the vertical gravity gradients, suggesting that magnetic and gravity anomalies do not necessarily arise from common sources.

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