摘要:

Cooling and thickening of lithospheric plates with age and subduction result in large‐scale horizontal density contrasts tending to drive plate motions and mantle flow. We quantify the driving forces associated with these density contrasts to determine if they can drive the observed plate motions. First, two‐dimensional models are computed to evaluate the effects of assumed rheologies and boundary conditions. We are unable to obtain platelike behavior in viscous models with traction‐free boundary conditions. The piecewise uniform velocities distinctive of plate motion can be imposed as boundary conditions and the dynamic consistency of the models evaluated by determining if the net force on each vanishes. If the lithosphere has a Newtonian viscous rheology, the net force on any plate is a strong function of the effective grid spacing used, leading to ambiguities in interpretation. Incorporating a rigid‐plastic lithosphere, which fails at a critical yield stress, into the otherwise viscous model removes these ambiguities. The model is extended to the actual three‐dimensional (spherical) plate geometry. The observed velocities of rigid‐plastic plates are matched to the solution of the viscous Stokes equation at the lithosphere‐asthenosphere boundary. Body forces from the seismically observed slabs, from the thickening of the lithosphere obtained from the actual lithospheric ages, and from the differences in structure between continents and oceans are included. Interior density contrasts such as those resulting from upwellings from a hot bottom boundary layer are assumed to occur on a scale small compared to plate dimensions and are not included. The driving forces from the density contrasts within the plates are calculated and compared to resisting forces resulting from viscous drag computed from the three‐dimensional global return flow and resistance to deformation at converging boundaries; the rms residual torque is ∼30% of the driving torque. The density contrasts within the plates themselves can reasonably account for plate motions. Body forces from convection in the interior may provide only a small net force on the plates. At converging boundaries the lithosphere has a yield stress of ∼100 bars; drag at the base of the plates is ∼5 bars and resists plate motion. The net driving forces from subducting slabs and collisional resistance are localized and approximately balance. Driving forces from lithospheric thickening are distributed over the areas of the plates, as is viscous drag. The approximate balance of these two forces predicts plate velocities uncorrelated with plate area, as observed. The model represents a specific case of boundary layer convection; the dynamical results are consistent with either upper mantle or m

ISSN:0148-0227    

DOI:10.1029/JB086iB06p04843

年代:1981

数据来源: WILEY

摘要:

The density distribution within a cooling plate is calculated, which incorporates temperature and pressure effects. From this density distribution the pressure field within the plate and the gravity field at sea level are computed for various degrees of isostatic compensation. In this model the pressure field within the plate has a horizontal gradient at shallow depths away from zero age and a horizontal gradient toward zero age at greater depths caused by the loading of the ocean. Isostatic equilibrium is approached if one allows the loading due to the water to depress the seafloor and at the same time allows mass conservation by flow at depth toward zero age. A viscosity model based on a Newtonian rheology which included temperature and pressure effects has a high gradient close to the plane separating positive and negative pressure gradients which would facilitate the return flow and decouple the lithosphere from the asthenosphere. Addition of a crust to the homogeneous model does not substantially change these conclusions. Comparison of this model with examples of East Pacific Rise data suggests that some areas may not be in complete isostatic equilibrium, implying the existence of horizontal pressure gradients toward zero age in the asthenosphere. This model can be made to fit the general features of the East Pacific Rise but not the detailed gravity and topography near zero age. If one allows convective cooling of the crust by water, partial melting of the upper mantle, and intrusion of this partial melt into the crust, the water depths increase more rapidly near zero age, and an increased positive gravity anomaly is produced over the rise axis, both of which produce a better fit to East Pacific Rise data at 12°N. This study suggests that crustal magma chambers on the East Pacific Rise may be associated with anomalous positive gravity anomalies caused by a positive density contrast between magma and fractured porous crustal rocks

ISSN:0148-0227    

DOI:10.1029/JB086iB06p04868

年代:1981

数据来源: WILEY

摘要:

Results of a similarity theory for spherical mantle convection are presented. The single‐mode mean field equations are analyzed for convection which is so vigorous that temperature disturbances become localized in thin thermal boundary layers. Our purpose is to study effects of spherical geometry, density interfaces, heat source distribution, and cell size. Steady state solutions are found for isoviscous spherical shells in which the field of motion is spatially periodic in a single spherical harmonic degree. Calculations are carried out over the range 2 ≤l≤ 40 and for various fractions of internal versus base heating. Three configurations are examined: (1) convection in a single layer of cells extending through the whole mantle, (2) convection in two layers, separated by a density interface at 670‐km depth, and (3) convection in a single layer terminating at 670 km. Results of these calculations are used to give estimates of surface horizontal velocities in terms of the heat loss, viscosity stratification, amount of internal heating, and depth of circulation. The surface velocity is most strongly affected by the thickness of the convecting shell. Deep mantle convection can achieve surface velocities which agree with observed plate speeds, while convection restricted to the upper mantle does not, at least on the scale of the major plates. The temperature distribution is strongly affected by the spherical geometry and by the presence of density interfaces. The principal difference between convection in one and two layers is that the latter produces a ‘hot’ lower mantle, while the former produces a

ISSN:0148-0227    

DOI:10.1029/JB086iB06p04881

年代:1981

数据来源: WILEY

摘要:

Delamination of the lithospheric thermal boundary from overlying continental crust propagates laterally from the line of initiation, accelerating as the sinking slab of detached lithosphere grows longer. This propagation has been numerically modeled with steady state equations in a moving reference frame by matching an interior finite element solution to flexible boundary conditions which represent the mechanical and thermal response of the surroundings. The form of the solution depends on the shear coupling of intruding asthenosphere to the top of the sinking slab across a thin layer of crustal material. Without coupling, the tip of the intrusion cools and stiffens to form a wedge dividing the crust (cold mode). With coupling, the intrusion is forced to convect and remains ductile (hot mode). The cold mode can propagate at all velocities; the hot mode has a lower limiting velocity of 1–2 cm/year but offers less resistance at higher speeds. Resistance to delamination includes a constant term from the buoyant crustal downwarp, plus a velocity‐proportional term representing viscous deformation. However, the proportionality constant of the latter term is only weakly dependent on crust and lithosphere viscosities. Matching this resistance to loading lines of 100‐ to 800‐km slabs sinking in a mantle of 1022P, velocities of 0.3–8.0 cm/year are obtained. Changes in viscosity affect this rate, but cold mode delamination is unstoppable except at continental margins or by failure in the sinking slab. The surface expression of delamination is a leading ‘outer rise’ followed by a submarine trough with a large negative free‐air anomaly, which finally evolves into a 1‐km plateau. If crustal viscosity and velocity are both low, however, there is a montonic crustal uplift with no trough. Thus the present lack of linear supracontinental oceans does not preclude delamination at up to 4 cm/year driven by slabs up to

ISSN:0148-0227    

DOI:10.1029/JB086iB06p04891

年代:1981

数据来源: WILEY

摘要:

Standard time series analysis techniques have been applied to the homogeneous polar motion data recently published by the ILS‐IPMS [Yumi and Yokoyama, 1980] in order to study some of the more controversial features apparently possessed by the older ILS data. The magnitude and direction of the secular trend unbiased by the presence of harmonics in the data were determined, yielding a rate of polar wander ∼3.52×10−3arc sec/yr (which extrapolates to ∼0.98°/m.y.) in direction 80.1°W longitude. The long‐period Markowitz wobble, which dominates the retrograde power spectrum of the data, has a signal to noise ratio in that spectrum of 21:1; its period is well‐determined as 31 years. Variations with time of the annual wobble and Chandler wobble were investigated using complex demodulation; the annual wobble was found to undergo relatively insignificant variations in amplitude and phase, in contrast to some analyses of the older ILS data, while the amplitude modulation and 1925–1940 phase change of the Chandler wobble

ISSN:0148-0227    

DOI:10.1029/JB086iB06p04904

年代:1981

数据来源: WILEY

摘要:

Synthesis of geodetic, geologic, and seismic data from the White Wolf fault, California, indicates that the fault separates an area of late Quaternary and continuing rapid uplift in the Tehachapi Mountains and Transverse Ranges from even more rapid subsidence in the southern San Joaquin Valley. On July 21, 1952, rupture of the White Wolf fault produced theML= 7.2 Kern County earthquake. We used the aftershock zone to delimit the size of the faulted slip surface and applied constraints imposed by the known 1952–1953 horizontal shear strains to model the measured coseismic vertical displacements, with an elastic dislocation model. A curved fault trace with decreasing fault depth (27 to 10 km from the surface vertically to the base), slip (3 to 1 m), and dip (75° to 20°) from the 1952 epicenter at the southwest end of the fault toward the northeast provides the fit most consistent with the geodetic record, the measured seismic moment, the fault‐plane solution, and the pattern of surface rupture. Two short releveled lines near the 1952 epicenter tilted 4 and 17 μrad down to the north from 5–10 years before the earthquake; the preseismic tilts differ significantly from ten other surveys of these lines. Left‐lateral fault‐crossing shear strain from 0.2–20 years before the quake was two times greater than both preseismic off‐fault strains and the post‐seismic fault‐crossing strains. During the first seven years after the earthquake, aseismic deformation was negligible. From 1959 to 1972 uplift reached 160 mm over an area larger than the aftershock zone, rising first in the epicentral region and then at the northeast end of the fault. This was unaccompanied by any surface fault slip. Reconstruction of the vertical separation on the White Wolf fault from late Quaternary and late Miocene stratigraphic marker beds shows that the rate of reverse fault slip increased forty‐fold, from 0.1–0.2 mm/yr to 3–9 mm/yr, between the past 10–15 m.y. and the most recent 0.6–1.2 m.y. We estimate a 170‐ to 450‐yr average recurrence interval for earthquakes on the White Wolf fault with slip equivalent to that in 1952. The 1952 earthquake appears to be characteristic of the Quaternary record of fault displacement in the increase in White Wolf slip toward the San Andreas fault, the ratio of reverse to lateral slip (1.3:1), and the ratio of vertical fault slip to emergence of the hanging wall block (3:1). The>8500‐m‐deep sedimentary basin on the down‐thrown block cannot be explained by repeated slip of t

ISSN:0148-0227    

DOI:10.1029/JB086iB06p04913

年代:1981

数据来源: WILEY

摘要:

Two new geodetic measurements of strain accumulation in the state of Washington for the interval 1972–1979 are reported. Near Seattle the average principal strain rates are 0.07 ± 0.03 μstrain/yr N 19°W and −0.13 ± 0.02 μstrain/yr N71°E, and near Richland (south central Washington) the average principal strain rates are −0.02 ± 0.01 μstrain/yr N36°W and −0.04 ± 0.01 μstrain/yr N54°E. Extension is taken as positive, and the uncertainties quoted are standard deviations. A measurement of shear strain accumulation (dilatation not determined) in the epoch 1914–1966 along the north coast of Vancouver Island by the Geodetic Survey of Canada indicates a marginally significant accumulation of right‐lateral shear (0.06 ± 0.03 μrad/yr) across the plate boundary (N40°W strike). Although there are significant differences in detail, these strain measurements are roughly consistent with a crude dislocation model that represents subduction of the Juan de Fuca plate. The observed accumulation of strain implies that large, shallow, thrust earthquakes should be expected off the coast of Washington and British Columbia. However, this conclusion is not easily reconciled with either observations of elevation change along the Washington coast or the focal mechanism solutions for shallow

ISSN:0148-0227    

DOI:10.1029/JB086iB06p04929

年代:1981

数据来源: WILEY

摘要:

Data from a precision multi‐wavelength distance measuring (MWDM) instrument, located near Hollister, California, have been analyzed in terms of strike‐slip faulting in the region covered by the network. Four episodes of deformation that are readily identifiable in the MWDM data for the year following September 1975 can be modeled as slip on the Calaveras fault. Creepmeter data tend to support this interpretation, since the creep events and the observed episodes of deformation appear to be correlated in a temporal sense. Lower bounds on the moment for each episode of slip can be determined from the MWDM data by the technique of linear programing. The results indicate that aseismic slip is the dominant mechanism for strain release, since the combined moment of 1.16 × 1024dyn cm for the four episodes far exceeds the moment of all of the earthquakes that occurred during the year. Creepmeter data and geologic evidence that are taken in conjunction with the lower bound of the moment indicate that the depth of aseismic slip for some of the episodes of fault slip could extend to below the greatest depth of earthquake hypocenters for this region of central Califo

ISSN:0148-0227    

DOI:10.1029/JB086iB06p04941

年代:1981

数据来源: WILEY

摘要:

Revised estimates of seismic slip rates along the Middle America Trench are lower on the average than plate convergence rates but match them locally (for example, Oaxaca). Along the Cocos‐North American plate boundary this can be explained by nonuniformities in slip at points of aseismic ridge or fracture zone subduction. For at least 81 yr (and possibly several hundred years), no major (Ms≥ 7.5) shallow earthquake is known to have occurred near the Orozco Fracture Zone and Tehuantepec Ridge areas. Compared with the average recurrence periods for large earthquakes (33 ± 8 yr since 1898 and 35 ± 24 yr between 1542 and 1979), this suggests that either a large (M≥ 8.4) event may be anticipated at such locations, or that these are points of aseismic subduction. Large coastal terraces and evidence suggesting tectonic uplift are found onshore near the Orozco Fracture zone. The larger discrepancy between plate convergence and seismic slip rates along the Cocos‐Carribbean plate boundary is more likely due to decoupling and downbending of the subducted plate. We used the limited statistical evidence available to characterize both spatial and temporal deficiencies in recent seismic slip. The observations appear consistent with a possible forthcoming episode of more intense seismic activity. Based on a series of comparisons with carefully delineated aftershock zones, we conclude that the zones of anomalous seismic activity can be identified by a systematic, automated analysis of the worldwide earthquake catalo

ISSN:0148-0227    

DOI:10.1029/JB086iB06p04949

年代:1981

数据来源: WILEY

摘要:

Shallow (upper 1.2 km) compressional wave velocity models from line 2A of the Socorro, New Mexico, Consortium for Continental Reflection Profiling (COCORP) survey of the Rio Grande rift supplement a previous refraction study of Socorro COCORP lines 1 and 1A. First‐arrival refraction analysis and slant stacks of the refraction profiles (derived from the reflection data) are used to determine velocity models of Socorro COCORP line 2A. Exposure of Paleozoic and Mesozoic sedimentary formations in the vicinity of Socorro line 2A allow direct correlation of a unit's velocity to its geological era. Weathered Mesozoic and Paleozoic formations within the rift are inferred to have significantly lower velocities than previously believed. Reinterpretation of the Rio Grande rift structure in the survey area results in a model that is consistent with available gravity data and provenance studies. The 13 km of Cenozoic spreading determined from the model is at least 5 km higher than that found 80 km to the north. There is evidence of minor Paleozoic deformation or nondepositio

ISSN:0148-0227    

DOI:10.1029/JB086iB06p04960

年代:1981

数据来源: WILEY