摘要:

Estimation of the Rayleigh numbers of the inner‐core leads to the conclusion that, even with low internal heating rates, this region of the Earth is convecting. This work presents an axisymmetrical model able to investigate convection in a sphere and its applications to the Earth's inner‐core. Rayleigh numbers ranging from the critical (onset of the convection) to ten thousand times the critical have been explored. The thermal implications of a convecting inner‐core have been pointed out: in all the cases the temperature profiles are very close to the adiabat, leading to the possibility of a partially molten regions in the inner‐core. Indeed, the most important effects appear in the first tens of kilometers, just below the inner‐core surface and could be at the origin of the low quality factors Qαand Qβrevealed by seismologi

ISSN:0094-8276    

DOI:10.1029/92GL02148

年代:1992

数据来源: WILEY

摘要:

The surface of Venus is so hot that the lower crust may be weak enough to allow decoupling of mantle and crust. An analytic model of such decoupling assumes that the shallow mantle forms the top boundary layers of large scale mantle convection cells. Crustal flow is driven by the motion of the mantle and by topographically induced pressure gradients. The model predicts that the lowest lowlands are sites of mantle upwelling and thinner than average crust. Highlands are places where mantle downwells and the crust is thick. Surface heat flow is inversely correlated with elevation, consistent with recent estimates of brittle layer thickness variations on Venus. If the average crustal thickness is about 20 km then the average lower crustal viscosity must be close to 1018Pa s to allow decoupling. The observed amplitude of geoid highs over highlands requires an Earth‐like increase in mantle viscosity with dept

ISSN:0094-8276    

DOI:10.1029/92GL02462

年代:1992

数据来源: WILEY

摘要:

The mass per unit area of the Venusian atmosphere is ∼106kg/m2, equivalent to a 1‐km‐deep water ocean. An important effect of this “ocean” on cratering is pressure enhancement due to multiple reverberations during the compression stage. We consider impactors with diameters,D, less than a scale height (∼15 km). Such impactors drive before them bow shocks with standoff distances of Δ ∼ 0.05Dinto the lowermost atmosphere. Reflection of this shock at normal incidence from the basalt surface raises the pressure in the atmosphere and surface, but initially not as much as a direct impact would. Several shock reverberations should occur, however, as the shock crossing time in the trapped, compressed atmospheric layer is much less than either the shock crossing time in the impactor or the time for the impactor to penetrate the atmospheric layer. With each reverberation we estimate new Hugoniots for shocked atmosphere, impactor, and target. The first is treated as an ideal gas of γ= 1.2–1.3, and the latter are modeled with Murnaghan equations of state and Grüneisen Γs ∝ ρ−1. The asymptotic pressure enhancement for a 20 km/s carbonaceous chondrite impactor (modeled as serpentine) striking basalt at 20 km/s, relative to the atmosphereless 1‐D impedance match pressure (∼550 GPa), is a factor of ∼1.4. This maximum enhancement factor increases with velocity, reaching 1.6 for serpentine moving at typical Venus‐impacting cometary velocities (∼65 km/s), and is greater for more compressible impactors (comets). Each reverberation sends a shock pulse into the surface or impactor that overtakes the previous ones, hence at the end of the compression stage both impactor and target are shocked to a higher pressure than would occur in the absence of the atmosphere. The impactor is effectively denser or staffer than in the atmosphereless case, but as the energy and momentum delivered are essentially the same, entering efficiency should be unaffected to first order. However, fusion and vaporization efficiency should beenhancedfor impactors whose impact velocities are below (whether initially or due to atmospheric drag) the threshold for complete melting or vaporization, respectivel

ISSN:0094-8276    

DOI:10.1029/92GL02308

年代:1992

数据来源: WILEY

摘要:

A topographic dome, radial drainage pattern, and regional negative gravity anomaly, all centered on Lake Superior, are vestiges of a 1100 Ma mantle plume that formed the Midcontinent Rift System of central North America. The topographic and gravimetric relations suggest that the dome is maintained isostatically by fundamental changes imprinted on the lithosphere by the plume, including magmatic underplating of the crust and depletion of the upper mantle. These changes occurred at distances of up to 500 km or more from the center of the plume, well beyond the margins of the rift.

ISSN:0094-8276    

DOI:10.1029/92GL02285

年代:1992

数据来源: WILEY

摘要:

Measurements of the lake level of Lake Langisjór at the SW edge of the Vatnajökull ice cap indicate a tilt of 0.26 ± 0.06 μrad/year away from the ice cap in the years 1959–1991. The tilt is too large to be explained as an elastic Earth response to ice retreat this century, or to be caused by change in the gravitational pull of the ice cap, but it can be explained by sub‐lithospheric viscous adjustment. Regional subsidence in historical times in SE Iceland can similarly be attributed to viscous adjustment resulting from the increased load of Vatnajökull during the Little Ice Age. The inferred sub‐lithospheric viscosity is 1 × 1018−

ISSN:0094-8276    

DOI:10.1029/92GL02209

年代:1992

数据来源: WILEY

摘要:

Previously unreported subcrustal earthquakes have been located up to 90 km depth beneath the Northern Apennines (Italy). These earthquakes occur beneath a zone of abundant upper crustal seismicity, mostly confined to the upper 20 km of the crust. Although there are relatively few well located subcrustal earthquakes, there appears to be a general southwestward deepening of the hypocenters, from the Adriatic to the Tyrrhenian Sea, consistent with other geophysical data which suggest that the Adriatic lithosphere is presently subducting beneath the Northern Apennines.

ISSN:0094-8276    

DOI:10.1029/92GL02503

年代:1992

数据来源: WILEY

摘要:

Records from a seismic network deployed as part of an active tomography experiment at 9°30′N on the East Pacific Rise (EPR) provide an opportunity to characterize local microearthquake activity over an 8‐day period. With the exception of the region around the 9°03′ overlapping spreading center (OSC), no events were located on the rise axis. One microearthquake, located from P and S‐wave arrival times, lies adjacent to the network, 18 km to the west of the rise axis. Five more events, located from P‐wave and T‐phase data, are to the south of the network. Three cluster around the western arm of the 9°03′ OSC, while two are located 25–30 km to the east of the OSC. The seismic moments of the microearthquakes range from 5 × 1019to 4 × 1020dyn cm, several orders of magnitude larger than those reported in other studies of shallow events along the axis of the EPR. These results suggest that overlapping spreading centers may be the loci of substantial microearthquake activity and that at this location the normal faults that form on the young flanks of the EPR are active off‐axis to dista

ISSN:0094-8276    

DOI:10.1029/92GL02208

年代:1992

数据来源: WILEY

摘要:

Ambient pressure Mössbauer spectra of Mg0.957Fe0.1SiO3perovskite synthesized at pressure‐temperature conditions of ∼50 GPa and 1700Kshow that the iron is entirely high‐spin Fe2+and appears to be primarily located in the octahedral site within the crystal structure. We observe broad Mössbauer lines, suggesting a distribution of electric‐field gradients caused by disorder associated with the Fe ions. Also, the perovskite exhibits magnetic ordering at temperatures<5 K, implying that there is a magnetic contribution to the absolute (“third‐law”) entrop

ISSN:0094-8276    

DOI:10.1029/92GL02461

年代:1992

数据来源: WILEY

摘要:

High resolution magnetic susceptibility imaging is a new technique for studying the magnetic properties of geological thin sections. The two‐dimensional distribution of both remanent and induced magnetization can be determined with a spatial resolution (<1.0 mm) that is similar to the size of phenocrysts in the sample. Amongst many problems in rock magnetism to which it could be applied, the technique holds great potential for understanding the origin of the anisotropy of magnetic susceptibility in pyroclastic flows. Preliminary tests on a single sample of ignimbrite indicate that secondary iron‐titanium oxide particles deposited within vesicle walls prior to their collapse are responsible for the bulk susceptibil

ISSN:0094-8276    

DOI:10.1029/92GL02322

年代:1992

数据来源: WILEY

摘要:

A magnetotelluric traverse of the Peninsular Ranges in southern California has revealed a pervasive zone of lower resistivity beginning at a uniform depth of 10 km and extending to depths of 60–90 km. Resistivities above 10 km depth are similar to those found in batholiths; very high values correspond to outcrops of crystalline basement. Because seismicity below 11–12 km is sparse, others have concluded that the brittle‐ductile transition is shallow beneath the range. The zone of low resistivity in the lower crust corresponds well to the ductile region, and we conclude that the lower values are caused by fluids trapped below the transition. Because the range has experienced vertical uplifts during the Pliocene era and the top of the low resistivity zone is flat, the present brittle‐ductile transition must have been formed in the last 5 M.y. A possible source for the fluids is the rift to the east in the Salton

ISSN:0094-8276    

DOI:10.1029/92GL02439

年代:1992

数据来源: WILEY