A. W. Gerhard Kunze

On the flexural rigidity and effective viscosity of the lithosphere in the Hawaiian area

Abstract The depth below sea level of the Moho increases from about 15 km under Hawaii to about 20 km beneath Oahu. Computer simulation of the elastic flexure of the lithosphere due to the load of the Hawaiian archipelago reveals that the maximum downward displacement of the Moho should occur beneath Hawaii, not Oahu. It is concluded that the lithosphere in the Hawaiian area has a Maxwellian rheology, that the subsidence of the youngest island (Hawaii) may be largely due to elastic flexure, but that the additional 5 km subsidence of the 2.5 m.y. older Oahu represents viscous deformation of the lithosphere. The flexural rigidity of the lithosphere required to produce the subsidence of Hawaii by elastic flexure alone is 5 × 10 30 dyne cm. The 5 km viscous settling of Oahu during 2.5 m.y. was then simulated utilizing an axisymmetric viscous finite-element computer program. The results indicate an effective lithospheric viscosity of approximately 3 × 10 23 poise. This value is lower than most other estimates and may reflect the anomalous thermal regime in the Hawaiian region.

Some aspects of viscous stress relaxation and intraplate seismicity

Abstract Considerable evidence suggests that the lithosphere has a Maxwellian rheology with an effective viscosity of 1026 poise or less. In a Maxwellian lithosphere viscous stress dissipation limits the maximum attainable stresses (due to mechanisms providing a constant stress buildup) to a value σmax given by the product of stress buildup rate ( C ) and viscous relaxation time τ : σ max = C · τ where τ is the ratio of viscosity to rigidity. The effectiveness of several intraplate stress building mechanisms in a Maxwellian lithosphere was analysed with the following results: 1. (1) The maximum stress produced by the latitudinal drift of lithospheric plates is 27 bars or less 2. (2) The maximum stress produced by the cooling of uplifted, eroded regions is 90 bars or less 3. (3) Lithospheric flexure due to isostatic rebound causes maximum stresses of 68 bars or less for the Appalachian rebound and approximately 150 bars for the Laurentide rebound. It is concluded that none of the mechanisms considered are likely to cause earthquakes if the mechanical strength of the lithosphere is on the order of 1 kb. If the strength of the lithosphere is on the order of 100 bars, then intraplate earthquakes may result from the Laurentide deglaciation as well as from the cooling of uplifted, eroded regions; but if the lithospheric viscosity is on the order of 1025 poise, then only the Laurentide deglaciation may produce adequate stresses. The fact that a stress buildup rate of 74 bars/ 200 yrs along the San Andreas fault generates earthquakes requires a minimum lithospheric viscosity on the order of 1022 poise.

Lunar mascons: another model and its implications

A mascon model is proposed in which the mass excess of the mare basalts in the circular maria is supported isostatically by mass deficits at depth. The model predicts the observed positive gravity anomalies surrounded by negative ring anomalies and explains the absence of gravity anomalies over the irregular maria. The model implies that mare basalts were derived by partial melting of a source region at depth due to pressure relief resulting from the excavation of the circular mare basins, and that the crystalline residuum in the source region is of lower density than the original source rock. The trace element enrichment and near cotectic character of Apollo 11 and 12 lavas reported by some investigators may be caused by extensive magma fractionation enroute from an origin in the circular maria to the final, distant emplacement sites.

Seismicity of the Great Lakes region

The Symposium on the Seismicity of the Great Lakes Region was held April 21, 1988, at the University of Akron, Ohio, in connection with the 22nd Annual Meeting of the North-Central Section of the Geological Society of America, sponsored by the Department of Geology, University of Akron. The purpose of this symposium was to reassess the seismicity of the region and its relationship to tectonic features in view of the unexpectedly strong earthquake of January 31, 1986, near the Perry nuclear power plant in northeast Ohio. A total of 13 papers were presented to an audience of about 60–70. Abstracts of these papers appear in Abstracts with Programs, Geological Society of America, volume 20, number 5, March 1988.

Local harmonic analysis of planetary Doppler gravity data

Planetary gravity fields represented in terms of spherical harmonics or surface mass distributions\ do not have the necessary resolution to permit gravity analysis of local features. Doppler gravity maps representing residual line-of-sight (LOS) accelerations have much greater resolution but cannot be used for conventional geophysical analysis due to the geometric distortions inherent in LOS gravity patterns and lack of normalization of LOS data. However, LOS gravity data may be converted to vertical gravity anomalies by expressing the anomalous local gravitational potential over small rectangular areas in terms of a modified double Fourier series constrained by local Doppler gravity data. The vertical derivative of the resulting potential yields the vertical gravity components at desired altitudes. The resolution of the resulting normalized free air anomaly maps is limited only by that of the original Doppler gravity data. Extended gravity maps may be constructed this way using a moving window approach. It is anticipated that much of the lunar frontside can be mapped at resolutions ranging from 1 to 4 deg of arc.

Direct conversion of los acceleration data to vertical gravity anomalies: A new approach

AbstractMost of the existing lunar and planetary gravity data are in the form of LOS (line-of-sight) components which cannot be used for conventional geophysical modelling. Current methods to invert LOS data yield non-unique or poorly constrained results or results of low spatial resolution. An alternate method presented here promises to produce unique, detailed and more reliable results. It utilizes the fact that three non-coplanar LOS acceleration vectors determined at different times at some point of observation uniquely define the total acceleration vector at that point. Vector analysis shows that the local cartesian componentsgi of the total gravity anomaly vector may be obtained by inversion of the system

Lunar crustal density profile from an analysis of Doppler gravity data

a_{ij} g_j = A_i^2

Tidal Water Level Fluctuations in Water Wells on San Salvador Island, Bahamas

where theaij andAis are, respectively, the local cartesian components and scalar magnitudes of the three required LOS acceleration vectors.In principle, the method is applicable to lunar as well as planetary LOS data.