摘要:

The thermal regimes of continent‐continent convergence zones are modelled by a finite difference technique, assuming that there is some subduction of continental crust. Gravity and heat flow profiles are generated from the thermal models. Subducted crust and slab remain cool except at the upper surface where frictional heating is important. Crustal rocks may be metamorphosed or melted by friction while the plates are converging or by radioactive self‐heating more than 30 m.y. after the plates have stopped. In the former case, melting requires frictional shear stresses between 500 and 2000 bars at a plate velocity of 5 cm/yr. At lower velocities the upper limit of frictional stress is greater. The total work performed in continental subduction may be minimized if the angle of underthrusting becomes more shallow, changing the location of subduction. A model for the geometry of oceanic and continental slabs in the Zagros Mountains is presented which satisfies gravity, heat flow, seismic, and geologic‐constraints. Continental underthrusting is beginning along a new fracture at the edge of the Persian Gulf, isolating the Arabian continental shelf from the subduction process. Slippage along this fault is Pleistocene and probably does not exceed 30 km. Subduction of continental crust at the crush zone is probably insignif

ISSN:0148-0227    

DOI:10.1029/JB080i032p04405

年代:1975

数据来源: WILEY

摘要:

Numerical experiments on steady state thermal convection in a cylindrical geometry have been carried out to identify the conditions necessary for plumes to occur in the mantle. Internally and base‐heated flows with both constant and variable diffusion creep viscosity have been considered. These experiments show that plumes occur only for base‐heated variable viscosity flows. Plumes are examined as a possible mode of convection in the upper mantle above 700 km, since it is in this region of the mantle that base heating should be most dominant. The consequences of an upper mantle plume under Hawaii are discussed in terms of available geological and geophysical observations, and good agreement is found. Plumes also explain the shape of the geotherm under South Africa derived by pyroxene geothermome

ISSN:0148-0227    

DOI:10.1029/JB080i032p04417

年代:1975

数据来源: WILEY

摘要:

Surficial manifestations of the long‐term evolution of plates are examined and used to derive constraints on the pattern of flow in the mantle. Generation and consumption of plates are not balanced around single plates, so the sizes of plates and the relative positions of ridges and descending slabs change, a complex return flow thus being required. At present a net flow should occur from the sites of much subduction toward the southern hemisphere, where sea floor spreading predominates. To allow the relative motions between descending slabs, masses in the mantle should be displaced at a rate which is several times larger than the rate of generation of lithosphere. The patterns of plate motions and of the return flow change considerably over periods of 100 m.y. The formation of instabilities occurs much faster than the cycling of mantle material. Hence the flow in the mantle cannot consist of regular steady state cells in which material moves in simple circuits and which are linked to plate boundaries. The deep flow is not parallel to the relative motions of overlying plates, so it is not two dimensional. The magnitude and non‐steady state of the sublithospheric flow make it doubtful that instabilities originating within the lithosphere alone drive the flow. This fact, together with the abrupt changes of plate motions, including jumping of ridges and continental breakup, suggests that instabilities at depth, e.g., mantle diapirs, are most important. The plate tectonic regime helps generate such instabilities in the mantle by disturbing the temperature distribution th

ISSN:0148-0227    

DOI:10.1029/JB080i032p04425

年代:1975

数据来源: WILEY

摘要:

Data pertaining to the instantaneous relative motions of the Caribbean plate with respect to the North American and Cocos plates have been inverted by using the iterative fitting algorithm formulated by Minster et al. (1974). The best‐fitting Caribbean‐North American pole is located at 50°N ± 18°, 116°E ± 9°, and the computed angular rate is 0.20° ± 0.07°/m.y., which corresponds to a spreading rate of 2.1 cm/yr across the mid‐Cayman rise. The model predicts that the present‐day motion of South America with respect to the Caribbean is northwestward and suggests that since the late Tertiary this motion has been accommodated along an en echelon series of northwest‐trending right lateral strike‐slip faults and northeast‐trending thrust faults which occupy a broad zone of deformation extending from the Curaçao Ridge and its westward extension into the South Caribbean Basin to the frontal thrusts along the southern boundary of the Venezuelan Coast Ranges. The model predicts that the relative motion between the Caribbean and Nazca plates is parallel to the continental margin south of the Gulf of Panama. The rate of motion of the Caribbean plate with respect to the mesosphere, deduced from the fixed hot spot hypothesis, is very small, and this plate may in fact be sta

ISSN:0148-0227    

DOI:10.1029/JB080i032p04433

年代:1975

数据来源: WILEY

摘要:

Two gravity and magnetic intensity profiles across the Vema fracture zone in the crestal region of the mid‐Atlantic ridge indicate that the southern flank of the fracture valley is underlain by a zone of excess mass. Also, the ‘south wall’ of the fracture zone is partially ‘nonmagnetic’ and may be composed of rocks of induced magnetization, low in remanence compared to adjacent basaltic crust. The regions of excess mass and low magnetization probably reflect upper‐crustal ultramafic and serpentinized ultramafic rocks located along the fracture zone. Similar rocks may underlie the north flank of the fracture val

ISSN:0148-0227    

DOI:10.1029/JB080i032p04441

年代:1975

数据来源: WILEY

摘要:

A simple theoretical model is presented to describe the statistical consequences of failing to detect short polarity intervals. A relationship is analytically derived between the true rate of reversals, the observed rate, and a parameter of the model which can be interpreted physically. This is a first step toward unambiguous determination of the true rate of reversals.

ISSN:0148-0227    

DOI:10.1029/JB080i032p04446

年代:1975

数据来源: WILEY

摘要:

An analytic model (Bote model) is presented as a means to describe the pressures at and arrival time of the spherically diverging stress wave resulting from an underground explosion. The model is based on a linear combination of the forms used to describe the two extremes in the stress wave lifetime: a strong shock at early times and a simple elastic wave at late times. The Bote model traces the propagation of the spherically diverging stress wave through the hydrodynamic, plastic, crush‐up, and elastic pressure regimes and predicts the pressures at and arrival times of the shock front. While able to account for the effects of porosity and water content of the surrounding medium, the Bote model requires knowledge of only two points on the pressure‐volume (PV) curve of the medium: the pressurePEbelow which the medium is assumed to behave elastically and the pressurePCat which all air‐filled voids in the medium have been crushed out. Simple expressions are given for determiningPCandPEas a function of water content and gas fill porosity. Other necessary Bote model inputs include explosive yield (Y) and medium type as well as geologic characteristics, including grain density (ρg), in situ density (ρ0), water content (W), and sound speed (C). Compared with both calculated and measured ground motion data, Bote results show good agreement for times ranging from tens of microseconds to tens of milliseconds, for distances out to a scaled range of 107 m/kt1/3, and for pressures to of the order o

ISSN:0148-0227    

DOI:10.1029/JB080i032p04449

年代:1975

数据来源: WILEY

摘要:

The bottom of the magnetized crust determined from the spectral analysis of residual magnetic anomalies is generally interpreted as the level of the Curie point isotherm. This paper studies the spatial variation of the Curie point isotherm level in Yellowstone National Park with the help of aeromagnetic data. A very shallow isothermal surface at a depth of only 5–6 km below sea level is associated with the central part of the Yellowstone caldera. It seems to extend along a narrow corridor toward the southwestern and eastern edges of the map. Except in a few localized spots, the isotherm deepens considerably in the areas outside the caldera. Because the caldera encloses most of the areas of hydrothermal alteration, fumaroles, and boiling springs in the park, this study indicates a strong correlation between the spatial variation of the Curie isotherm level and the concentration of subsurface geothermal energ

ISSN:0148-0227    

DOI:10.1029/JB080i032p04461

年代:1975

数据来源: WILEY

摘要:

The vacuum thermal conductivity of lunar fines sample 14163 was measured for the approximate temperature range of 100°–400°K. Several sample densities from 1100 to 1800 kg/m3were used. The temperature dependence of the conductivity was found to be well represented by the relationk=A+BT3, which is predicted by elementary theory. The coefficientsAandBwere obtained by least squares analysis of the data. The thermal diffusivity was calculated for the various densities by using specific heat data from the literature along with the measured conductivities. The results are compared with those obtained for Apollo 11, Apollo 12, and terrestrial basalt samp

ISSN:0148-0227    

DOI:10.1029/JB080i032p04466

年代:1975

数据来源: WILEY

摘要:

Using crystals grown into vugs from oriented lunar rocks as directional detectors of cosmic dust particles and solar flare nuclei, I have measured the angular distribution of the flux of micrometeoroids of mass ∼3×10−17to ∼8×10−14g. Hypervelocity impact craters of diameter 500 Å to 10μm and tracks from solar flare nuclei of energy ∼100 keV/amu to ∼20 MeV/amu were observed in crystals from rock 71055 facing lunar south and in crystals from rock 74255 facing lunar east. The vug geometry restricts the region of space viewed by the crystal detectors such that only particles streaming nearly normal to the ecliptic could enter the vug from 71055, while only particles confined to the ecliptic could enter the vug from 74255. By comparing the size‐frequency distributions and circularity distributions of craters observed on these orthogonal crystals, I have found that dust grains both in ecliptic orbits and in orbits inclined to the ecliptic have virtually identical mass‐frequency distributions and similar shapes. I have evaluated the micrometeoroid fluxes after determining the exposure ages of the individual vug crystals by measuring the solar flare track density gradients in each crystal. The flux of particles of mass ≳4 × 10−15g confined to the ecliptic is ∼1.1×10−2/m2s 2π sr. This value is in good agreement with satellite measurements, suggesting that the micrometeoroid flux has been relatively constant over the past 35,000 yr. The flux of particles producing microcraters on a lunar surface facing south is lower than the ecliptic flux by a factor of ∼7. This experiment was unable to distinguish between interstellar dust or interplanetary particles in highly inclined orbits as the source of craters observed on the south‐facing surface. The solar flare energy spectra observed in vug crystals from both 71055 and 74255 deviate from the long‐term spectrum determined by Hutcheon et al. (1974) at energies below ∼10 MeV/amu. I conclude that the shallow energy spectrum cannot be a result of erosion and suggest that the experimental data from which the long term low‐energy Fe spectrum has b

ISSN:0148-0227    

DOI:10.1029/JB080i032p04471

年代:1975

数据来源: WILEY