Bouguer anomaly

About: Bouguer anomaly is a research topic. Over the lifetime, 2860 publications have been published within this topic receiving 51640 citations.

Approximating edges of source bodies from magnetic or gravity anomalies

Abstract: Cordell and Grauch (1982, 1985) discussed a technique to estimate the location of abrupt lateral changes in magnetization or mass density of upper crustal rocks. The final step of their procedure is to identify maxima on a contoured map of horizontal gradient magnitudes. We attempt to automate their final step. Our method begins with gridded magnetic or gravity anomaly data and produces a plan view of inferred boundaries of magnetic or gravity sources. The method applies to both local surveys and to continent-wide compilations of magnetic and gravity data (e.g., Zietz, 1982; Simpson et al., 1983a; Kane et al., 1982).

The inversion and interpretation of gravity anomalies

Abstract: A rearrangement of the formula used for the rapid calculation of the gravitational anomaly caused by a two‐dimensional uneven layer of material (Parker, 1972) leads to an iterative procedure for calculating the shape of the perturbing body given the anomaly. The method readily handles large numbers of model points, and it is found empirically that convergence of the iteration can be assured by application of a low‐pass filter. The nonuniqueness of the inversion can be characterized by two free parameters: the assumed density contrast between the two media, and the level at which the inverted topography is calculated. Additional geophysical knowledge is required to reduce this ambiguity. The inversion of a gravity profile perpendicular to a continental margin to find the location of the Moho is offered as a practical example of this method.

Gravity anomalies and flexure of the lithosphere at mountain ranges

Abstract: The Bouguer gravity anomaly over the Himalayan, Alpine, and Appalachian mountains is characterized by a generally asymmetric gravity “low”, which spans the mountains and associated foreland basins. The minimum of the gravity “low” is generally systematically displaced from the region of greatest topographic relief and shows no obvious relationship to surface geology. In addition, the Alps and Appalachians are associated with a generally symmetric gravity “high” that is unrelated to the topographic relief. Together the gravity low and high form a characteristic positive-negative anomaly “couple”. The steep gravity gradient between the positive and negative couple correlates with the Insubric line and its equivalents in the Alps and the Brevard Zone in the southern Appalachians. This gravity anomaly couple is interpreted as evidence for flexure of the continental lithosphere by subsurface (buried) and surface (topographic) loading. The magnitude of the subsurface load is estimated from the integrated excess mass represented by the positive anomaly and the magnitude of the surface load from the topography. By combining these loads the flexure of the lithosphere and the associated Bouguer gravity anomaly are calculated for different assumed values of the elastic thickness Te of continental lithosphere. The best fitting value of Te for the Himalayas is 80–100 km, for the Alps 25–50 km, and for the Appalachians 80–130 km. The model calculations suggest that the main contribution to the gravity couple in the Alps and Appalachians arises not from surface loads such as thrust sheets and nappes but from subsurface loads. However, in the case of the Himalayas the topography appears to be a sufficient load to explain the observed asymmetric gravity low. When subsurface loads do exist therefore, we would not expect a close correlation between mountain topographic relief and depth to the M discontinuity as required by classical models of isostasy. Rather, the M discontinuity is expected to relate to the center of mass of all the loads acting on the lithosphere. The surface and subsurface loads, inferred from the gravity anomaly, play a major role in the development of mountains and foreland basins. The emplacement of large subsurface loads represents the primary event in the Alps and Appalachians, while in the Himalayas the emplacement of the surface load is the primary event. The emplacement of the subsurface load probably marks the initiation of foreland basin development. The regional characteristics of the basin are controlled by the primary load, while local variations are controlled by the secondary, thrust sheet loads. Subsequent movement of either load will produce migration of the basin depocenter. The origin of the subsurface load is not clear, but its association with the insubric line in the Alps and the Brevard Zone in the Appalachians suggests that it may be related to the “obduction” of crustal blocks/flakes onto the underlying plate during continental collision or possibly to the preloaded crustal structure of the underlying plate.

Closed Covariance Expressions for Gravity Anomalies, Geoid Undulations, and Deflections of the Vertical Implied by Anomaly Degree Variance Models.

Abstract: : A new anomaly degree variance model is developed by considering potential coefficient information to degree 20, and updated values of the point anomaly variance, the 1 degree block variance, and the 5 degrees block variance. This new model and several other models were used to develop closed expressions for the covariance and cross-covariance functions between gravity anomalies, geoid undulations (or height anomalies), and deflections of the vertical. (Modified author abstract)

Gravity anomalies of the ridge-transform system in the South Atlantic between 31 and 34.5° S: Upwelling centers and variations in crustal thickness

Abstract: To decipher the distribution of mass anomalies near the earth's surface and their relation to the major tectonic elements of a spreading plate boundary, we have analyzed shipboard gravity data in the vicinity of the southern Mid-Atlantic Ridge at 31–34.5° S. The area of study covers six ridge segments, two major transforms, the Cox and Meteor, and three small offsets or discordant zones. One of these small offsets is an elongate, deep basin at 33.5° S that strikes at about 45° to the adjoining ridge axes. By subtracting from the free-air anomaly the three-dimensional (3-D) effects of the seafloor topography and Moho relief, assuming constant densities of the crust and mantle and constant crustal thickness, we generate the mantle Bouguer anomaly. The mantle Bouguer anomaly is caused by variations in crustal thickness and the temperature and density structure of the mantle. By subtracting from the mantle Bouguer anomaly the effects of the density variations due to the 3-D thermal structure predicted by a simple model of passive flow in the mantle, we calculate the residual gravity anomalies. We interpret residual gravity anomalies in terms of anomalous crustal thickness variations and/or mantle thermal structures that are not considered in the forward model. As inferred from the residual map, the deep, major fracture zone valleys and the median, rift valleys are not isostatically compensated by thin crust. Thin crust may be associated with the broad, inactive segment of the Meteor fracture zone but is not clearly detected in the narrow, active transform zone. On the other hand, the presence of high residual anomalies along the relict trace of the oblique offset at 33.5° S suggests that thin crust may have been generated at an oblique spreading center which has experienced a restricted magma supply. The two smaller offsets at 31.3° S and 32.5° S also show residual anomalies suggesting thin crust but the anomalies are less pronounced than that at the 33.5° S oblique offset. There is a distinct, circular-shaped mantle Bouguer low centered on the shallowest portion of the ridge segment at about 33° S, which may represent upwelling in the form of a mantle plume beneath this ridge, or the progressive, along-axis crustal thinning caused by a centered, localized magma supply zone. Both mantle Bouguer and residual anomalies show a distinct, local low to the west of the ridge south of the 33.5° S oblique offset and relatively high values at and to the east of this ridge segment. We interpret this pattern as an indication that the upwelling center in the mantle for this ridge is off-axis to the west of the ridge.