摘要:

Measurements of Mercurian topography based on Arecibo radar observations are presented. The data, which were obtained from 1978 to 1984, cover much of the equatorial zone of Mercury between 12°N and 5°S latitude. Over thirty continuous altitude profiles were obtained, each spanning from 20 to 90 degrees of longitude at a resolution of 0.15° (longitude) by 2.5° (latitude). Radar depths for large craters support previous indications from imagery that Mercurian craters are shallower than lunar craters of the same size. One very large (800 km) impact basin shows some distinct topographic structure, although its relative shallowness suggests postimpact modification by isostatic relaxation or volcanic filling. The plains of Tir Planitia appear topographically smooth to the radar. These plains extend well into the hemisphere not imaged by Mariner 10, possibly forming part of a large annulus of smooth plains around Caloris Basin. The circum‐Caloris smooth plains are strongly down‐bowed, indicating subsidence under a load. This and other similarities to lunar maria suggest a volcanic origin for these plains. Additional areas of topographically smooth terrain have been found in both the imaged and unimaged hemispheres. Several ridges, scarps, and fault zones have been identified in the altimetry. Three mapped arcuate scarps show heights of about 700 m and cross‐sectional widths of about 70 km. One of these features is clearly a ridge, while the other two scarps have a more ridge‐like appearance than is suggested by images. One large‐scale topographic drop of 3 km correlates well with a mapped system of faults and intracrater scarps. The equatorial zone of Mercury shows 7 km of maximum relief, although the typical elevation difference between highlands and lowlands is closer to 3 km. Three major highland areas are found, the largest two of which are roughly antipodal and aligned within about 10° of the “hot poles” of Mercury. The unimaged hemisphere, possessing both large craters and topographically smooth areas, does not appear to be markedly different in its topography from the imaged hemisphere. No evidence has been found for another Caloris‐type impact structure in the

ISSN:0148-0227    

DOI:10.1029/JB091iB01p00385

年代:1986

数据来源: WILEY

摘要:

A thin‐shell formulation for computing stress distributions on the surfaces of one‐plate planets is developed. The lithospheric model includes laterally varying density anomalies at two depths, corresponding to undulations on a crust‐mantle boundary and variations in upper mantle density. For a given set of model parameters, values for the vertical displacement, crustal thickness anomaly, and mantle density anomaly required to satisfy the topographic and gravitational boundary conditions are computed, along with the resulting stress field. The computed stresses can then be compared to stress directions inferred from observed tectonic features. This formulation is used to study loads on Venus's lithosphere, and it is found that most highland areas appear to have been formed in a state of isostasy or by uplift, with a significant portion of their support probably derived from dynamic processes in the mantle. A possible exception is Ishtar Terra, whose circumferential mountain belts are most consistent with a radially compressive stress regime generated by flexural loading. Reasonable lithospheric models predict low‐density mantle beneath Atla, Beta, Eisila, Thetis, and Tethus Regiones and thick crust accompanied by higher‐density mantle under Ishtar Terra and Ovda Regio. Thus western Aphrodite Terra appears to be more closely related to Ishtar than to the rest of the equatorial highlands. These results favor a hot spot model of Venus tectonics in which Beta, eastern Aphrodite, Eisila, and Tethus are expected to be relatively young and thermally active, while Ovda and Ishtar should be older and le

ISSN:0148-0227    

DOI:10.1029/JB091iB01p00403

年代:1986

数据来源: WILEY

摘要:

The spatial distributions of agmatic complexes and other features thought to be associated with major crustal structures may be important sources of information about large‐scale structural patterns. However, attempts to incorporate these features into quantitative analyses of linear features have used arbitrary or inaccurate criteria to judge hypothetical geological relationships. In this paper, features of limited spatial extent are considered pointlike, and the concept of a probabilistic lattice point distribution is used to formulate a statistical method that leads to a quantitative and reproducible analysis of directional patterns based solely on the locations of the points. Thus this analysis is independent of linear patterns and provides a measure of the directional information intrinsic to point patterns. The procedure determines the most likely trends of structural anisotropies; Monte Carlo simulations of random point patterns provide a reference distribution from which confidence levels can be determined. Applications to published data for magmatic complexes, magnetic contour closures, and structural change points are used as examples. The results suggest that there has been a tendency to overestimate the amount of information available from point pattern

ISSN:0148-0227    

DOI:10.1029/JB091iB01p00421

年代:1986

数据来源: WILEY

摘要:

Thermally induced lateral heterogeneity in the earth's mantle has been studied in terms of idealized two‐dimensional, constant viscosity, numerical models of mantle convection. Steady model solutions of the temperature and velocity fields were analyzed with respect to both the spatial and spectral signatures of their inherent lateral heterogeneity. By examining the changes in spectral signature produced by varying the Rayleigh number, the degree of internal heating, and the depth in the convection cell, it has been possible to identify those features of the spectra of lateral heterogeneity which are most indicative of these variations. Particular attention has been paid to the spectral characteristics of thermal boundary layers within the convecting layer. The spectral analysis of known temperature fields represents the forward problem corresponding to the inverse problem of inferring the thermal structure of the mantle from the spectral components of its lateral heterogeneity as determined by seismic tomography. The forward problem presented here illustrates a new approach in convection modeling which may ultimately provide important diagnostic criteria for the interpretation of recent tomographic dat

ISSN:0148-0227    

DOI:10.1029/JB091iB01p00435

年代:1986

数据来源: WILEY

摘要:

Joint determination of the geoid and permanent dynamic sea topography is demonstrated by simulation from direct use of satellite altimetry. The separation is feasible because of the disparities between the error spectra of the geoid and its unique perturbative effects as seen on the orbit. The solution for the components of the permanent topography from 14 cm Seasat altimetry promises to yield accurate results down to half wavelengths of about 1000 km. Present discrimination of this surface from disjoint satellite altimeter results appears to be at a scale near 4000 km. The radial effects of uncertainties in the M2 ocean tide on the Seasat orbit and altimeter measurements will probably be only a minor contaminant of the solution.

ISSN:0148-0227    

DOI:10.1029/JB091iB01p00453

年代:1986

数据来源: WILEY

摘要:

In prior studies paleomagnetic data have been used to determine ancient minimum velocities of the continents. The greatest minimum velocities, ∼50 km/m.y., exceed the present‐day absolute velocities of the major continents. This result is important because the observation that present‐day oceanic plates move faster than present‐day continental plates can be treated as a major constraint on plate dynamics. However, these prior studies are limited because they ignore known errors in the paleomagnetic data. In this paper we present a technique for incorporating these errors and using them to estimate 95% confidence limits on the minimum velocities. This new technique was applied to Pennsylvanian to Cretaceous paleomagnetic data from North America and to the ancient geometry of Laurasia. When errors are considered, minimum velocities are 10 to 50 km/m.y. lower than when errors are ignored. Additional uncertainties owing to a hypothesized clockwise rotation of the Colorado Plateau, a source of many of the data, are considered and found to alter the minimum velocities only negligibly. Allowing for uncertainties in the pole positions, uncertainties in ages, and the uncertainty due to possible rotation of the Colorado plateau, we estimate that Laurasia moved at least 48 km/m.y. from Early Jurassic to Early Cretaceous time, and at least 43 km/m.y. from Pennsylvanian through Late Triassic time. There is only a 5% risk that the true minimum velocities are lower than these lower bounds. These velocities are less than the peak velocities over short intervals found in prior analyses, but exceed the average velocities previously found for Triassic and Jurassic time. These newly estimated velocities are still much higher than the present velocities of the continents and, as in the prior studies, imply that the slow motion characteristic of major continents at present is not a fundamental attribute of plate

ISSN:0148-0227    

DOI:10.1029/JB091iB01p00462

年代:1986

数据来源: WILEY

摘要:

Marine geophysical data from off‐shore of southern Chile are used to define the interaction between the Chile Ridge and Chile Trench during late Cenozoic time. We identify three distinct ridge‐trench collision events. Between 14 and 10 Ma a 700‐km‐long, nearly continuous section of the Chile Ridge was sub‐ducted between 55°S and 48°S. Shorter sections of the ridge, offset by large transform faults, were subducted at 6 and 3 Ma between 48°S and 47°S. At the present‐day triple junction, the subduction of the ridge has a strong influence on the Chile Trench. In this region the landward trench slope has undergone a recent episode of subduction‐driven tectonic erosion: the trench slope is narrower and steeper than along other sectors of the margin and the trench axis has migrated shoreward. Evidence exists for late Neogene and Quaternary uplift and plutonism on the adjacent continental margin. South of 48°S, where collision took place between 10 and 14 Ma, the effects of collision are much less pronounced. In particular, the landward trench slope does not appear to have been subjected to extensive tectonic erosion. We conclude that the configuration of the spreading centers and transform faults on the Chile Rise is the chief factor controlling ridge‐trench tectonic interaction. Tectonic erosion of the landward trench slope and tectonic activity in the adjacent continental margin are much greater when short sections of ridge, offset by large transform faults, are subducted. A major sub‐marine channel leads along the trench axis south‐ward from the triple junction. This channel cuts across the outer trench rise and carries sediment westward to the Mornington Abyssal Plain. The Paleogene tectonic history of the southern Chile Trench includes a southward migrating, ridge‐trench collision involving the Farallon‐Aluk spr

ISSN:0148-0227    

DOI:10.1029/JB091iB01p00471

年代:1986

数据来源: WILEY

摘要:

The paper reports the results of an aeromagnetic study of the Mid‐Atlantic Ridge in the northern South Atlantic. The study consists of 38,000 km of low altitude magnetics tracks centered on the ridge covering nearly 700,000 km2south of Ascension Island. Most tracks extend out to crust of anomaly 5B age. Large areal coverage coupled with high resolution allows comparison of crustal generation processes over space (along ridge strike) and time (across strike). Temporal heterogeneities revealed in the data include a pattern of change in spreading rate and direction of plate motion. The total spreading rate increased sharply at 10 Ma and then decreased between 6 Ma and the present. The time variation of spreading rate is similar to that seen in other areas such as the North Atlantic and the southern and eastern Pacific. The plate boundary has evolved over time in response to the shift in plate motion direction through fragmentation and rotation of ridge segments between fracture zones. In several cases the location of the spreading axis has shifted through jumps or propagation across fracture zones. The periods of plate boundary modification correlate with the increase and decrease in spreading rate. Spatial heterogeneities are caused by the influence of the Ascension hotspot on the Mid‐Atlantic Ridge in the northern portion of the study area, and include two apparent seamount chains formed by the hotspot. Ascension Island and several of the seamounts appear to have been produced some distance away from the hotspot location (presently about 225 km southeast of Ascension Island) by channeled subaxial magma flow beneath the spreading center. The seamount chains, therefore, do not mark hotspot tracks. Large local variations in spreading rate and degree of asymmetry along strike are also fo

ISSN:0148-0227    

DOI:10.1029/JB091iB01p00497

年代:1986

数据来源: WILEY

摘要:

We investigate the problem of retrieving anisotropy as a function of depth in the mantle, from the observed azimuthal variations of Love and Rayleigh wave velocities. Following the approach of Smith and Dahlen, this azimuthal dependence is expressed in terms of a Fourier series of the azimuth θ. For the most general case of anisotropy (provided it is small enough), some simple linear combinations of the elastic tensor coefficients are shown to describe the total effect of anisotropy (both polarization anisotropy and azimuthal anisotropy) on the propagation of surface Waves. For the terms that do not depend on the azimuth the combinations are related to the elastic coefficients of a transversely isotropic mantle. For the azimuthal terms the relevant combinations are explicited. It is found that the partial derivatives of the azimuthal terms with respect to these combinations are easy to compute for they are proportional to the partial derivatives of a transversely isotropic model in the case of a plane‐layered model. In a first approximation the same property holds true for a spherical earth and we calculate from PREM all the partial derivatives needed for performing the inversion of the azimuthal anisotropy of surface waves in the period range 50–300 s. It is observed that very shallow anisotropy can be responsible for substantial azimuthal variations up to the longest periods. With this approach it is also easy to compute the azimuthal variations of surface wave velocities produced by any anisotropic model. When a Ci j elastic tensor is chosen for the upper mantle, azimuthal variations up to 2% are obtained for Rayleigh waves. The azimuthal variations of Love wave velocities are very small. The 2 θ term of the azimuthal variations of Rayleigh wave velocities is the dominant term. Its fast axis corresponds to the fast axis of P

ISSN:0148-0227    

DOI:10.1029/JB091iB01p00511

年代:1986

数据来源: WILEY

摘要:

Sequences of dynamic instabilities are analyzed for a single degree of freedom elastic system which slides along a surface having frictional resistance depending on slip rate and slip rate history, in the manner of Dieterich, Ruina and others. The system is represented as a rigid block in contact with a fixed surface and having a spring attached to it whose opposite end is forced to move at a uniform slow speed. The resulting “stick‐slip” motions are well understood in the classical case for which there is an abrupt drop from “static” to “sliding” frictional resistance. We analyze them here on the basis of more accurate frictional constitutive models. The problem has two time scales, an inertial scale set by the natural oscillation periodTof the analogous frictionless system asT/2π and a state relaxation scaleL/Voccurring in evolution, over a characteristic slip distanceL, of frictional stress τ towards a “steady state” value τss(V) associated with slip speedV. We show that τ ≃ τss(V) during motions for which acceleration a satisfiesaL/V2≪ 1, and that this condition is met during an inertia controlled instability in typical circumstances for which the unstable slip is much greater thanL. SinceV/ais of orderT/2π during inertia controlled motion, one hasL/V≪T/2π, whereasL/V≫T/2π during much of the essentially quasi‐static “stick” part of the cycle when there is a sufficiently small imposed velocity at the load point. Thus the physically irrelevant time scale (L/Vduring inertial controlled motion,T/2π during quasi‐static motion) is much shorter than the relevant scale, which is troublesome from a numerical point of view as it is the shorter time scale which constrains allowable step size. We propose efficient numerical procedures to deal with such response, in which the full equations with inertia and state relaxation are solved only in a transition regime when both time scales are significant. We show results for several friction laws, all having history dependence based on a single evolving state variable and all having properties that ∂τ/∂V>0 for instantaneous changes inV, that τ evolves towards τss(V) as exp (−δ/L) with ongoing slip δ whenV= const, and that dτss(V)/dV<0 except possibly at highV. During the dynamic instabilities we find that motion continues at a nearly steady state condition, τ≃τss(V), until dynamic overshoot becomes so significant that “arrest” begins. In the arrest stage,Vdrops rapidly to very much lower values (never zero in our models) under nearly fixed state conditions, and then

ISSN:0148-0227    

DOI:10.1029/JB091iB01p00521

年代:1986

数据来源: WILEY