Isostatic response to loading of the crust in Canada

1970 ◽  
Vol 7 (2) ◽  
pp. 716-727 ◽  
Author(s):  
R. I. Walcott
Keyword(s):  
Close Association ◽  
Ice Sheet ◽  
Ground Surface ◽  
Gravity Anomalies ◽  
A Value ◽  
Free Air ◽  
Ice Loads ◽  
Anomaly Map ◽  
Free Air Gravity ◽  
Free Air Anomaly

A smoothed free air anomaly map of Canada indicates that the central part of the region occupied by the Laurentide Ice Sheet is over-compensated. Due to the close association of the free air gravity, the apparent crustal warping, the time of deglaciation, and the congruence of the gravity anomalies and the Wisconsin Glaciation, it is concluded that the over-compensation is due to incomplete recovery of the lithosphere from the displacement caused by the Pleistocene ice loads. The amplitude of the anomalies, about –50 milligals, suggests that a substantial amount of uplift has yet to occur and that the relaxation time of crustal warping is of the order of 10 000 to 20 000 y.The profile of the ground surface at the edge of a continental ice sheet on an elastic lithosphere is assessed using a value of the flexural parameter of the lithosphere calculated from gravity and deformation studies in the Interior Plains. The conclusions are: (a) a purely elastic forebulge is not likely to reach an amplitude of more than a few tens of meters; (b) the crust will be depressed for a considerable distance beyond the edge of the ice sheet; and (c) for large ice sheets crustal failure will probably occur in a preferential zone several hundred kilometers inside the maximum ice limit.

scholarly journals Configuration and Dynamics of the Laurentide Ice Sheet During the Late Wisconsin Maximum

2007 ◽  
Vol 36 (1-2) ◽  
pp. 5-14 ◽  
Author(s):  
Arthur S. Dyke ◽  
Lynda A. Dredge ◽  
Jean-Serge Vincent
Keyword(s):  
Ice Sheet ◽  
Gravity Anomalies ◽  
Late Glacial ◽  
Laurentide Ice Sheet ◽  
Geological Evidence ◽  
Original Proposal ◽  
Field Evidence ◽  
Free Air ◽  
Free Air Gravity ◽  
Basin Region

ABSTRACT Prior to 1943 the Laurentide Ice Sheet was considered to have three major domes centered in Keewatin, Labrador, and Patricia (TYRRELL, 1898 a, b; 1913). FLINT (1943) argued that these centres were of only local and temporary importance and favoured a single-domed ice sheet. Despite the lack of supporting geological evidence, and despite the proposition of a Foxe Dome in the interim (IVES and ANDREWS, 1963), the single-dome concept was not seriously challenged until the late 1970's and, in fact, is still strenuously supported (HUGHES era/., 1977 ; DENTON and HUGHES, 1981). This paper extends and modifies recent conclusions that the Laurentide Ice Sheet had more than one dome at the Late Wisconsin maximum. We propose a model incorporating five domes (M'Clintock, Foxe, Labrador, Hudson, and (?) Caribou) based on the position of ice divides, ice flow patterns, drift composition, late-glacial features, postglacial isostatic recovery and free-air gravity anomalies. Our Labrador and Hudson domes closely correspond to Tyrrell's Labradorean and Patrician ice sheets; our Caribou and M'Clintock domes together with the Franklin Ice Complex over the Queen Elizabeth Islands north of the Laurentide Ice Sheet, correspond to Tyrrell's original Keewatin Ice Sheet. The style of glaciation of the Foxe Basin region was not known to Tyrrell, but our reconstruction of the Foxe Dome is in close agreement with the original proposal of Ives and Andrews. Like Tyrrell, our reconstruction is based on field evidence obtained through extensive mapping; the single dome model continues to be unsupported by geological data.

FREE‐AIR GRAVITY ANOMALY MAP OF THE GULF OF MEXICO AND ITS TECTONIC IMPLICATIONS, 1963 EDITION

Geophysics ◽  
1965 ◽  
Vol 30 (1) ◽  
pp. 102-110 ◽  
Author(s):  
Peter Dehlinger ◽  
B. R. Jones
Keyword(s):  
Gulf Of Mexico ◽  
Cross Sections ◽  
Seismic Refraction ◽  
Seismic Velocities ◽  
Surface Ship ◽  
Free Air ◽  
Anomaly Map ◽  
Free Air Gravity ◽  
Free Air Anomaly ◽  
Mohorovičić Discontinuity

As part of a continuing program, Texas A. and M. University has been making a surface‐ship gravity survey of the Gulf of Mexico. The 1963 free‐air anomaly map of the Gulf is the second in a series of maps resulting from these investigations; it includes not only a larger area than the first map, but also measurements having considerably higher accuracies. The present map indicates, as the first one suggested, that the Gulf of Mexico is essentially in isostatic equilibrium but contains local mass anomalies. Two cross sections of crustal layers were constructed which are consistent with the gravity observations and published seismic‐refraction results. One section extends from Galveston, Texas, to the Yucatan Peninsula, and the other from the Sigsbee Deep to Florida. Both sections consist of four to six layers, in which layer densities were assumed to be uniform laterally and the density below the Mohorovičić discontinuity constant. Densities of the layers were converted from seismic velocities using the Drake and Nafe curves. A free‐air anomaly profile across the Cayman Trench is included.

Gravitationally induced stresses at structural boundaries

1980 ◽  
Vol 17 (9) ◽  
pp. 1286-1291 ◽  
Author(s):  
A. K. Goodacre ◽  
H. S. Hasegawa
Keyword(s):  
Upper Mantle ◽  
Gravity Anomalies ◽  
Positive Anomaly ◽  
Crust And Upper Mantle ◽  
Free Air ◽  
Induced Stresses ◽  
Free Air Gravity ◽  
Free Air Anomaly ◽  
Structural Boundary ◽  
High Seismicity

Prominent gravity anomalies, consisting of paired positive-negative belts, occur in Canada at structural boundaries between geological provinces. The associated anomalous masses produce what are termed gravitationally induced stresses. These stresses may contribute to the failure of rocks along preexisting faults, or other zones of weakness. In the case of a typical structural boundary, failure at shallow depths in the crust is likely to occur in the region outlined by the negative gravity anomaly, whereas failure deeper within the crust and upper mantle may occur beneath the positive anomaly. Along the lower St. Lawrence valley, good spatial correlation is found between regions of high seismicity and those negative free-air anomaly areas which are adjacent to prominent free-air gravity highs. It is suggested that in a heavily faulted region, such as the lower St. Lawrence valley, gravitationally induced stresses may be a contributing factor to the production of earthquakes in regions which are otherwise already close to failure.

scholarly journals Evaluation of the BGM-3 sea gravity meter system onboard R/V Conrad

Geophysics ◽  
1986 ◽  
Vol 51 (7) ◽  
pp. 1480-1493 ◽  
Author(s):  
Robin E. Bell ◽  
A. B. Watts
Keyword(s):  
Gravity Anomaly ◽  
Gravity Data ◽  
Gravity Anomalies ◽  
Test Range ◽  
Free Air ◽  
Marine Gravity ◽  
Abrupt Changes ◽  
Spring Type ◽  
Free Air Gravity ◽  
Free Air Anomaly

The first Bell Aerospace BGM-3 Marine Gravity Meter System available for academic use was installed on R/V Robert D. Conrad in February, 1984. The BGM-3 system consists of a forced feedback accelerometer mounted on a gyrostabilized platform. Its sensor (requiring no cross‐coupling correction) is a significant improvement over existing beam and spring‐type sea gravimeters such as the GSS-2. A gravity survey over the Wallops Island test range together with the results of subsequent cruises allow evaluation of the precision, accuracy, and capabilities of the new system. Over the test range, the BGM-3 data were compared directly to data obtained by a GSS-2 meter onboard R/V Conrad. The rms discrepancy between free‐air gravity anomaly values at intersecting ship tracks of R/V Conrad was ±0.38 mGal for BGM-3 compared to ±1.60 mGal for the GSS-2. Moreover, BGM-3’s platform recovered from abrupt changes in ship’s heading more rapidly than did the platform of GSS-2. The principal factor limiting the accuracy of sea gravity data is navigation. Over the test range, where navigation was by Loran C and transit satellite, a two‐step filtering of the ship’s velocity and position was required to obtain an optimal Eötvös correction. A spectral analysis of 1 minute values of the Eötvös correction and the reduced free‐air gravity anomaly determined the filter characteristics. To minimize the coherence between the Eötvös and free‐air anomaly, it was necessary to prefilter the ship’s position and velocity. Using this procedure, reduced free‐air gravity anomalies with wavelengths as small as a few kilometers can be resolved.

Implications of Late Pleistocene Glaciation of the Tibetan Plateau for Present-Day Uplift Rates and Gravity Anomalies

1997 ◽  
Vol 48 (3) ◽  
pp. 267-279 ◽  
Author(s):  
Georg Kaufmann ◽  
Kurt Lambeck
Keyword(s):  
Tibetan Plateau ◽  
Late Pleistocene ◽  
Ice Sheet ◽  
Gravity Anomalies ◽  
The Tibetan Plateau ◽  
Pleistocene Glaciation ◽  
Free Air ◽  
Uplift Rates ◽  
Free Air Gravity ◽  
Central Tibet

Minimal and maximal models of Late Pleistocene Glaciation on the Tibetan Plateau are considered. The large ice sheet models indicate that disintegration of the ice sheet could have contributed up to 7 mm/yr of present vertical uplift and 2 mm/yr of horizontal extension. The former value can account for more than 50% of the observed uplift in central Tibet. The peak free-air gravity anomaly arising from the deglaciation would be around −5.4 mGal. In contrast, the smaller ice sheet models do not contribute significantly to the signals of present uplift and gravity anomalies. Modern geodetic measurements therefore have the potential to constrain the Late Pleistocene glaciation of the Tibetan Plateau. Assuming a large ice sheet over the Tibetan Plateau, the disintegration can contribute up to 6 m of eustatic sea-level rise.

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