The Interrelationship of the Crust, the Upper Mantle, and Isostatic Gravity Anomalies in the United States

This paper addresses the development of an inertial navigation system into a land-based geodetic survey system by the United States Army Engineer Topographic Laboratories from the Position and Azimuth Determining System (PADS) to the Rapid Geodetic Survey System (RGSS). It contains a brief description of the inertial platform and its operation to obtain position, elevation, gravity anomalies and deflections of the vertical. A summary of test results are included. Potential utilization and improvements are also addressed.

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The edge of northwestern North America at ∼1.8 Ga

Canadian Journal of Earth Sciences ◽

10.1139/e05-039 ◽

2005 ◽

Vol 42 (6) ◽

pp. 983-997 ◽

Cited By ~ 18

Author(s):

Frederick A Cook ◽

Kevin W Hall ◽

C Elissa Lynn

Keyword(s):

Gravity Anomalies ◽

Western Canada Sedimentary Basin ◽

Bear Lake ◽

Lithospheric Extension ◽

Isostatic Gravity Anomalies ◽

Isostatic Gravity ◽

North American Craton ◽

Seismic Reflection Profiles ◽

Northwestern North America ◽

Canada Sedimentary Basin

The ∼1.80 Ga edge of the northwestern North American craton is buried beneath Phanerozoic and Proterozoic rocks of the Western Canada Sedimentary Basin and the adjacent Cordillera. It is visible in more than eight deep seismic reflection profiles that have images of west-facing crustal-scale monoclines with up to 1520 km of vertical relief, and it produces regional isostatic gravity anomalies that can be followed for more than 1500 km along strike. The deep reflection profiles include two major transects of Lithoprobe (southern Canadian Cordillera transect and Slave Northern Cordillera Lithospheric Evolution (SNORCLE) transect) and industry profiles that are strategically located to provide depth and geometry constraints on the monoclines. The isostatic anomalies mark the density transition from Paleoproterozoic and older crystalline rocks of the Canadian Shield to less dense supracrustal rocks of westward-thickening late Paleo proterozoic and younger strata. These gravity anomaly patterns thus provide areal geometry of crustal structure variations along strike away from the depth control provided by the seismic data. Although many of the monoclines follow the Fort Simpson geophysical trend along the Cordilleran deformation front, isostatic anomalies near Great Bear Lake delineate a northeast-striking region of low values that may coincide with a failed rift arm or the southern margin of a large basin. The monoclines are interpreted as a series of en echelon structures that probably formed as a result of lithospheric extension at about 1.801.70 Ga following terminal accretion of the Paleoproterozoic Wopmay Orogen.

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Using isostatic gravity anomalies from spherical harmonic models and elastic plate compensation to interpret the lithosphere of the Bolivian Andes

Geophysics ◽

10.1190/geo2012-0378.1 ◽

2013 ◽

Vol 78 (3) ◽

pp. G41-G53 ◽

Cited By ~ 3

Author(s):

Christopher Jekeli ◽

Hyo Jin Yang ◽

Kevin Ahlgren

Keyword(s):

Spherical Harmonic ◽

Flexural Rigidity ◽

Gravity Data ◽

Gravity Anomalies ◽

Surface Gravity ◽

Bolivian Andes ◽

Isostatic Gravity Anomalies ◽

Isostatic Gravity ◽

Spherical Harmonic Models ◽

Surface Gravity Data

We have determined for the Bolivian Andes that the new global gravity models derived from the Gravity field and steady-state Ocean Circulation Explorer (GOCE) satellite mission may be used directly to study lithospheric structure. Toward this end, we have formulated Bouguer and isostatic gravity anomalies in spherical approximation, rather than in the usual planar approach, using spherical harmonic series consistent with the satellite-derived gravitational models. From the approximate equivalency of topographic masses and surface density layers using the Helmert condensation method we further derived and used isotropic transfer relations between the spherical spectra of topographic loads and elastic spherical shell deflections, where the Airy isostatic compensation is the special case of no flexural rigidity. A numerical comparison of these spherical harmonic models to conventional three-dimensional modeling based on topographic data and newly acquired surface gravity data in Bolivia confirmed their suitability for lithospheric interpretation. Specifically, the relatively high and uniform resolution of the satellite gravitational model (better than 83 km) produces detailed maps of the isostatic anomaly that clearly delineate the flexure of the Brazilian shield that is thrust under the Sub-Andes. Inferred values of the thickness of Airy-type roots and the flexural rigidity of the elastic lithosphere agree reasonably with published results based on seismic and surface gravity data. In addition, a local minimum in the flexural rigidity is evident at the sharp bend of the eastern margins of the Sub-Andes in Bolivia. This feature is consistent with earlier theories for counter rotations about a vertical axis at this minimum, associated with the confluence of the subducted Nazca plate and the Brazilian craton. The GOCE model thus generates high-resolution isostatic anomaly maps that offer additional structural detail not seen as clearly from previous seismic and gravity investigations in this region.

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P Wave Amplitude Decay Offers a Glimpse of Earth's Structure

Eos ◽

10.1029/2016eo044035 ◽

2016 ◽

Vol 97 ◽

Author(s):

Lily Strelich

Keyword(s):

United States ◽

Upper Mantle ◽

Seismic Waves ◽

Wave Amplitude ◽

The United States ◽

Deep Earthquake

Scientists look at deep earthquake signals to map how seismic waves lose energy in the upper mantle across the United States.

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The Utility of Geopotential Field Data in Seismotectonic Studies in the Eastern United States

Seismological Research Letters ◽

10.1785/gssrl.59.4.289 ◽

1988 ◽

Vol 59 (4) ◽

pp. 289-297 ◽

Cited By ~ 3

Author(s):

William J. Hinze ◽

Thomas G. Hildenbrand

Keyword(s):

United States ◽

Magnetic Anomaly ◽

Spatial Relationship ◽

Gravity Anomalies ◽

Three Dimensions ◽

Hazard Evaluation ◽

Earthquake Activity ◽

Seismic Zone ◽

Eastern United States ◽

Isostatic Gravity

Abstract The deterministic approach to seismic hazard evaluation utilizes all available geologic/geophysical information to map the structure and nature of the crust in three dimensions that may relate to earthquake activity. However, information on the crystalline crust of the eastern United States from direct observations, drilling and sparse crustal seismic studies is limited. In contrast, regional gravity and magnetic anomaly data exist over the entire eastern United States and are available in a digital grid to facilitate processing and analysis. Although these data have serious limitations for detailed interpretation, they can be used to estimate the strength of the crust and the lithosphere and to map and characterize (1) zones of weakness such as paleorifts, sutures, and faults; (2) regions of potential stress amplifications such as plutons and irregularities in fault zones; and (3) basement terranes of generally consistent structural pattern that may delimit coherent regional seismic zones. Free-air, Bouguer, and isostatic gravity anomalies have different applications in the characterization of the crust for seismogenic purposes and complement magnetic anomaly maps which focus on upper crustal features. In concert, these data have provided the insight to interpret the host structures that together with related seismic and geoscience data, suggest causative mechanisms of the New Madrid seismic zone and other seismogenic regions of the eastern United States. As a result, we conclude that interpretations of geopotential anomalies are an essential ingredient in seismotectonic studies in the eastern United States, but they are only one of several tools required in the concerted effort of assessing seismic hazards. The presence of anomalies with a particular set of attributes neither confirms nor denies the possible spatial relationship to seismicity.

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