摘要:

Depth/diameters (d/D) for fresh craters on the intermediate‐sized icy satellites of Uranus and Saturn have been determined using photoclinometry and shadow lengths, and are compared with similar measurements on the terrestrial planets. Simple bowl‐shaped craters on icy satellites, including those on Miranda for which the highest resolution data are available, are systematically 20–40% shallower than on the terrestrial planets. This pronounced difference between crater depths on icy and rocky surfaces indicates that differences in impact velocity or surface gravity are not as important as the differences in mechanical properties between ice and “rock” in controlling simple crater morphology. Experimental impact studies indicate that differences in material properties such as angle of internal friction can control crater depth. Alternatively, breccia lenses may be thicker in icy satellite simple craters. Complex craters on the icy satellites become significantly deeper with increasing crater diameters, unlike complex craters on terrestrial planets, which are nearly constant in depth. Central peaks, and hence floor rebound, are also volumetrically and morphologically more prominent than wall slumping (rim collapse) on the icy satellites. The magnitude of viscous relaxation of very large craters (e.g., Herschel and Odysseus) on the icy satellites can be estimated from extrapolation of thed/Dfits at smaller crater diameters. The transition diameter from simple to complex morphology is inversely correlated with gravity, as it is on the terrestrial planets, but at significantly smaller diameters than would be expected from a simple extrapolation of the terrestrial trend. Thus crater modification, especially floor rebound, is easier to initiate on icy satellites. Estimated effective cohesions for shocked icy material are lower by a factor of 10–20 relative to rock, although estimated effective viscosities are similar. The distinct, systematic differences between crater morphology on icy and rocky worlds indicate that gross material property differences between rock and ices play a key role in crater formation and modification, but gravity is still the primary dr

ISSN:0148-0227    

DOI:10.1029/JB094iB04p03813

年代:1989

数据来源: WILEY

摘要:

Deformation of Long Valley caldera in eastern California has been closely monitored since June 1983 with a two‐color geodimeter, which has a precision between 0.12 and 0.20 ppm of the baseline length. Initial trilateration measurements were concentrated in the south moat of the caldera, the source of the January 1983 earthquake swarm. Since then, the network has been expanded from 13 to 42 baselines. The additional baselines span the resurgent dome and the western half of the caldera. With a combination of measurements made several times per week on 7–11 baselines and monthly or yearly measurements on the remaining baselines, we have measured both the temporal and spatial deformation of the caldera. Although theM6.4 Chalfant Valley earthquake was located 35 km from the nearest baseline in the network, we observed coseismic strain changes of up to 1.5 ppm on several baselines. Using fault planes selected to coincide with the hypo‐centers of the main shock and theM5.9 foreshock, we have inferred a combined moment of 3.0×1025dyn cm from the two‐color data. This cumulative moment of both earthquakes is equivalent to a magnitude 6.3 shock. Over the period from mid‐1983 to mid‐1988, the long‐term trends determined from the length changes of the baselines within the caldera show a general decrease in the rate of extension. During 1983, measurements show a maximum extension rate of 5.0 ppm/yr. After mid‐ to late‐1984, the extension rates decreased by a factor of 2–4 relative to those in the first year. With the exception of the coseismic step, the extension rates have been fairly constant during the interval between late 1984 and mid‐1988. The long‐term changes in rate can be modeled by using two Mogi point sources representing inflation beneath the resurgent dome of the caldera and a dislocation surface representing slip in the south moat. Modeling indicates that both the rates of slip and inflation have decreased since mid‐1983, but these two processes are still occurring at a mea

ISSN:0148-0227    

DOI:10.1029/JB094iB04p03833

年代:1989

数据来源: WILEY

摘要:

The processes of erosion and deposition must be included in foreland basin models to predict correctly basin geometry and stratigraphy. We present a synthetic stratigraphic model of the development of nonmarine foreland basins that predicts progressive geometry, topography, and facies patterns. In the model, steady crustal shortening occurs according to a wedge‐thickening model, erosion and deposition follow a diffusive process, and the lithosphere is compensated elastically. Erosion and deposition are controlled by the transport coefficients κ of the diffusion equation. For a range of thrust velocities and lithospheric rigidities, transport coefficients are of order 102–103m2/yr in the mountain belt; these values are much higher than those derived from the study of scarp degradation. In the sedimentary basin, transport coefficients of order 104m2/yr are appropriate and are compatible with previous studies of fluvial and deltaic deposition. Rapid thrusting results in a narrow underfilled basin, while slow thrusting results in a wide overfilled basin. In addition, by varying the erosional and depositional transport coefficients while holding other parameters constant, we generate both overfilled and underfilled basins. These results suggest that changes in the rate of thrust loading, the climate, or the source rock lithology can create stratigraphic signatures that have been interpreted to record viscoelastic relaxation of the lithosphere. Clearly, to understand either the long‐term behavior of the lithosphere or to interpret orogenic history from preserved foreland strata, the manner in which a basin was filled must be considered. We apply the model to the evolution of the modern sub‐Andean foreland and find that an erosional transport coefficient of 3000 m2/yr and a depositional transport coefficient of 20,000 m2/yr successfully predict the observed basin

ISSN:0148-0227    

DOI:10.1029/JB094iB04p03851

年代:1989

数据来源: WILEY

摘要:

We carried out gravity surveys along four routes across the Peruvian Andes, one in northern Peru, one in central Peru, and two in southern Peru. Two of the routes extend from the coast to the flat lands of the Amazon Basin across the high Andes, while the other two routes extend only from the coast to the central high plateau or the Altiplano. One route through Nazca contains 250 data points over a length of 800 km and thus offers one of the best gravity profiles across the Andes. This profile shows quite asymmetric gravity anomalies associated with the Western Cordillera and the Eastern Cordillera, in marked contrast with the symmetric pattern of topographic profile. Using these profiles, we constructed crustal models varying layer thicknesses rather than layer densities. The crustal thickness has a maximum beneath the Western Cordillera, although the maximum thickness varies from 45 km in northern Peru to 55 km in central Peru and 65 km in southern Peru in contrast to the assumed thickness of 35 km beneath the stable Brazilian shield. Possible effects of lateral variations of crustal and mantle densities tend to reduce the above estimates of maximum thickness. The very shallow Moho beneath the Peruvian coast steeply deepens eastward in correspondence to the steep western slope of the Andes. The crust is thicker in the Western Cordillera than in the Eastern Cordillera. The Moho beneath the Eastern Cordillera tends to be relatively flat and shallows abruptly eastward across the sub‐Andes. The Wadati‐Benioff zone lies, in general, well below the model South American continental crust, leaving a mantle wedge in between. A notable exception is the Nazca profile, where the base of the model South American continental crust is in direct contact with the top of the Wadati‐Benioff zone. This provides a direct evidence for uplifting of the buoyant slab with an aseismic ridge (the Nazca ridge) at its top, which tends to attach itself to the bottom of the continental crust. We examined whether or not the load due to mountain topography is balanced with the buoyancy load due to excess mass of the crust in the Andes. The Western Cordillera is found to be approximately in isostatic equilibrium, but the Eastern Cordillera is not. This contrast in mechanical state and the difference in recent geology suggest that the Western Cordillera and the Eastern Cordillera have not uplifted through entirely the same tectonic pr

ISSN:0148-0227    

DOI:10.1029/JB094iB04p03867

年代:1989

数据来源: WILEY

摘要:

The Central Andes is the middle part of the Andean chain between about 13°S and 27°S, characterized by the parallel running high mountain chains (the Western and Eastern Cordilleras) at the edges of high plateaus with a height of about 4000 m and a width of 200 to 450 km (the Altiplano‐Puna). From the examination of geophysical and geological data in this area, including earthquakes, deformation, gravity anomaly, volcanism, uplift history, and plate motion, we conclude that the continued plate subduction with domination of compressive stress over the entire arc system is the main cause of the tectonic style of the Central Andes. We propose that the present cycle of mountain building has continued in the Cenozoic with the most active phase since the Miocene, and that the present subduction angle (30°) is not typical in that period but that subduction with more shallowly dipping oceanic lithosphere has prevailed at least since the Miocene, because of the young and buoyant slab involved. This situation is responsible for the production of a broad zone of partial melt in the mantle above the descending slab. Addition of volcanic materials was not restricted to the western edge (where active volcanoes of the Western Cordillera exist) but extended to the western and central portion of the Altiplano‐Puna. The western half of the Central Andes is essentially isostatic because the heat transferred with the volcanic activities softened the crust there. In the eastern edge, the thermal effect is small, and the crust is strongly pushed by the westward moving South American plate. This caused the shortening of crustal blocks due to reverse faulting and folding in the Eastern Cordillera and Amazonian foreland. The magmatism and crustal accretion are dominant at the western end of the mountain system and decrease eastward, while the compression and consequent crustal shortening are strongest at the eastern end and wane toward west. These two processes are superposed between the two mountain chains and form high plateaus there: the Altiplano of Bolivia and Peru and the Puna of Argentina. This interpretation is supported by the observation that (1) Neogene sedimentary formations have been uplifted to high elevations without heavy distortion in the Altiplano and the Western Cordillera, (2) no significant reverse fault systems are observed on the Altiplano, (3) Neogene volcanic rocks and volcanic centers since the Miocene are not restricted to the Western Cordillera but are widely distributed over most of the Altiplano, (4) most of the Altiplano is in a zone of high heat flow values, (5) thick Paleozoic rocks are strongly folded and faulted in the Eastern Cordillera with little volcanism and no large‐scale plutonism in the Cenozoic age, (6) crustal earthquakes with reverse fault mechanisms are concentrated on the eastern flank of the Eastern Cordillera and Amazonian foreland, and (7) the crustal thickness suddenly decreases at the junction of the Eastern Cordillera and the Amazon Basin, exactly at the place of reverse ear

ISSN:0148-0227    

DOI:10.1029/JB094iB04p03891

年代:1989

数据来源: WILEY

摘要:

An active fold‐and‐thrust belt is analogous to a wedge of soil or snow that forms in front of a moving bulldozer; such wedges exhibit a critical taper and a regional state of stress that is everywhere on the verge of Coulomb failure. The width of such a critically tapered fold‐and‐thrust belt does not depend on its brittle strength or frictional properties but rather on the accretionary influx rate of fresh material at its toe and on the rate of erosion; a steady state fold‐and‐thrust belt is one in which the accretionary influx is balanced by the erosive efflux. Rocks are accreted at the toe and then horizontally shortened as they are transported toward the rear; those that enter lower in the accreted section are more deeply buried before being uplifted by erosion. Mass balance and isotropy constrain the kinematics of this large‐scale deformation, enabling us to infer the trajectories, residence times, and stress‐strain histories of rocks incorporated into eroding fold‐and‐thrust belts. A typical rock resides in the steady state Taiwan wedge for 2–3 m.y. before it is uplifted and eroded; during its motion through the wedge, it experiences strain rates in the range 10−13to 10−14s−1. The mechanical energy budget of brittle frictional mountain building is described by the equation , whereis the rate of work performed on the base and front of the fold‐and‐thrust belt by the subducting plate, is the rate at which energy is dissipated against friction on the decollement fault,is the rate at which energy is dissipated by internal frictional processes within the deforming brittle wedge, and is the rate of work performed against gravitational body forces in a reference frame attached to the overriding plate. The total mechanical power being supplied by the subducting Eurasian plate to the active fold‐and‐thrust belt in Taiwan is slightly over 3 GW. Approximately 60% of this work of steady state mountain building is being dissipated against friction on the decollement fault, and about another 25% is being dissipated against internal friction; this leaves only 15% or roughly half a gigawatt available to do useful work against gravity. In general, fold‐and‐thrust belts with moderate pore fluid pressures are dominated by work done against friction on the decollement fault; however, those with nearly lithostatic pore fluid pressures may be dominated by work done against gravity. Internal frictional dissipation is always less than basal frictional dissipation, as it is in Taiwan. An alternative and equivalent description of the mechanical energy balance of a steady state fold‐and‐thrust belt is provided by the equation. In this version the quantity on the left, , is the rate at which work is performed on the back of the wedge by the overriding plate andis the rate of work performed against gravity in a reference frame attached to the subducting plate. The latter quantity is always positive for any critically tapered fold‐and‐thrust belt whose decollement fault dips toward its rear, in contradiction to the central premise of the gravit

ISSN:0148-0227    

DOI:10.1029/JB094iB04p03906

年代:1989

数据来源: WILEY

摘要:

This paper describes a simple thermal model of an actively deforming critically tapered fold‐and‐thrust belt. The model determines the steady state temperature distribution and heat flow, as well as the pressure‐temperature‐time histories of rocks that outcrop at the surface. The main parameters controlling the thermal structure are the accretion and erosion rates, the undisturbed geothermal gradient at the toe, and the amount of frictional heating. Both shear heating on the decollement fault and internal strain heating within the deforming brittle wedge are incorporated in a mechanically consistent manner, and they dominate the effect of radiogenic heating, except in fold‐and‐thrust belts with significantly overpressured pore fluids. The mean stresses, temperatures, and surface heat flow all increase with an increase in the basal and internal coefficients of friction, and this dependence is used to constrain the level of friction on the decollement fault beneath the steady state fold‐and‐thrust belt in Taiwan. Rocks outcropping in the core of the Central Mountain Range of Taiwan experience maximum theoretical temperatures in excess of 400° C and maximum mean pressures in excess of 500 MPa if the coefficient of basal friction is μb= 0.5. Qualitatively, these conditions are in good agreement with the observed high greenschist facies metamorphism. The theoretical surface heat flow, which increases from 95 mW/m2at the front of the fold‐and‐thrust belt to 240 mW/m2at the rear, is in excellent agreement with the results of a recent geothermal survey of Taiwan, and theoretical cooling histories are in good agreement with fission track and other geochronologic studies. Taken together, these results provide strong evidence that sliding on the basal decollement fault beneath Taiwan is governed by a coefficient of friction in the range of typical laboratory measurements, μb= 0.5 ± 0.2. Approximately 35% of the total surface heat flux of 3 GW is heat conducted into the base of the wedge from the top of the basal decollement fault, and somewhat more than 30% is heat advected into the toe by accretion. The remaining heat is generated internally, about 25% by internal strain heating and about 10% by radiogenic heating. Either an increase in the coefficient of basal friction μbor a reduction in the pore fluid pressure ratio λ = λbleads to an increase in the surface heat flow, because of the increased frictional heating within the wedge and on the basal decollement fault. The overall balance of energy in a steady state fold‐and‐thrust belt is described by the equationE˙=W˙G+Q, where Ė is the rate at which both mechanical and heat energy are added from external sources,W˙Gis the rate at which work is performed against gravitational body forces in a reference frame attached to the overriding plate, andQis the rate at which waste heat flows out of both the upper and lower boundaries. The total power input into the Taiwan fold‐and‐thrust belt is approximately 4.2 GW. The mechanical work being done on the base and front of the fold‐and‐thrust belt accounts for 3 of these 4.2 GW. In addition, 0.9 GW of heat are being advected from the subducting plate into the toe by accretion; the remaining 0.3 GW are being supplied by in situ radiogenic heating. The outgoing energy is dominated by the 3 GW of heat conducted out the top in the surface heat flow; however, another 0.8 GW are conducted down beneath the rear portion of the basal decollement fault, to heat the underlying subducting slab. Only 0.4 of the incoming 4.2 GW do useful mechanical work against gravity within the wedge; the efficiency of brittle frictional mo

ISSN:0148-0227    

DOI:10.1029/JB094iB04p03923

年代:1989

数据来源: WILEY

摘要:

A 5‐year campaign is in progress in central and southern California to monitor tectonic motions using observations of the Global Positioning System (GPS) satellites. Evidence from geological and space‐geodetic data indicates that significant horizontal deformations of order 10 mm/yr are occurring west of the San Andreas fault along the California margin. We demonstrate, using data from initial epoch measurements in January 1987, that the goal of measuring horizontal deformations with an accuracy of 5 mm/yr over the duration of the campaign should be readily achievable. The GPS network was designed to span a wide range of baseline lengths within California and across North America to aid in phase ambiguity resolution, an important factor in achieving highest‐accuracy baseline determinations. We present an efficient algorithm for multisession network analysis of GPS data with simultaneous orbit determination and ambiguity resolution. Using this algorithm with the California data, we demonstrate the improvement obtained in station position and satellite orbit estimation when the phase ambiguities are resolved. We demonstrate several millimeters +1 part in 108short‐term repeatability over five single‐session (daily) analyses for horizontal baseline components and length and several tens of millimeters for the vertical. Comparison with the independently determined very long baseline interferometry baseline between Mojave and Owens Valley suggests that the accuracy is at the same level as the repeatability. We show that simultaneous multisession analyses with ambiguity resolution improve the baseline and satellite orbit estimates compared to the single‐session analyses. With the single‐session analyses we were able to resolve all phase ambiguities on baselines within California. With the multisession analyses we were able to resolve the phase ambiguities for most of the network, over regional and conti

ISSN:0148-0227    

DOI:10.1029/JB094iB04p03949

年代:1989

数据来源: WILEY

摘要:

Previous techniques for modeling anelastic deformation of the lithosphere have included plane strain models, restricted to two‐dimensional problems, and quasi‐three‐dimensional “plane stress” or “thin plate” models, that did not accurately include the effects of the shallow frictional layer, or of kinematic detachment of crust from mantle. This paper presents techniques to remedy these deficiencies of thin plate models. An iteration strategy in which the rheology is linearized using artificial prestress and a particular effective viscosity tensor causes the calculation of horizontal velocities to converge monotonically, even with a frictional layer at the top of the lithosphere. A technique using two planar grids allows deformation and displacement to be different at the crust and mantle levels, at far less cost than that of a three‐dimensional grid. A finite element technique is developed for computing the changes in thickness of these layers caused by pure shear, simple shear, and pressure gradients. A technique based on relaxation of perturbation eigenfunctions solves the heat equation in the lithosphere during deformation. Accuracy of component numerical methods is good for simple test problems, but in realistic nonlinear problems utilizing all components, only the precision can be determined because of the lack of analytic solutions. Precision of the combined program is tested with a realistic sample problem and presented as a function of the number of iterations in each velocity solution (convergence factor 0.73 to 0.88), size of time step in the predictor/corrector time integration (convergence as Δt0.8), and number of degrees of freedom in the finite element grid (convergence as N−0.5 to −0.8for most variables). Overall cost of a simulation varies with the fractional precision Π as Π−3.3. A new consequence of kinematic detachment, a moving wave of crustal thickness, is found; unfortunately, the form of the wave depends on the finite element size. This means that element size must be chosen to approximate the smoothing by flexural rigidity effects that were n

ISSN:0148-0227    

DOI:10.1029/JB094iB04p03967

年代:1989

数据来源: WILEY

摘要:

Existing geological and geophysical data support the view that the opening of the Tyrrhenian Sea was by rifting of a formerly continuous continental lithosphere, which began in upper Miocene on the upper Sardinian margin and migrated southeastward to the Calabria margin. Heat flow across the Tyrrhenian Sea shows a pronounced asymmetry, from about 50–70 mW m−2over the Sardinian margin, to more than 150 mW m−2over its southeastern margin off southern Italy and Calabria. In this study we use the newly acquired data from Ocean Drilling Program Leg 107, heat flow, seismic velocity, and bathymetric data to constrain a group of models that supposes the Tyrrhenian Sea opened by detachment of a formerly continuous lithosphere along a southeastern facing, low‐angle normal fault. Specific models tested include (1) the Wernicke model, in which thinning and extension of the continental lithosphere are supposed to be accomplished by “simple shear” along a large‐scale, gently dipping detachment fault zone which cut through the entire lithosphere, and (2) the delamination model, in which the low‐angle detachment fault is supposed to cut only the upper and middle crust but to merge subhorizontally into the lower crust, below which concurrent pure shear may occur. A versatile, two‐dimensional finite element procedure is used to evaluate the tectonic and thermal evolution of these models, and the results are compared with observations to test the validity of the models. Delamination models with large pure shear components in the lower lithosphere and the Wernicke model predict heat flow within ±1σ of the measurements. The Wernicke model, however, predicts a basin configuration much too deep in comparison with bathymetry and a subbasinal lithospheric thickness substantially different from Rayleigh wave dispersion results. The delamination model with a large component of pure shear in the subbasinal lower lithosphere, on the other hand, predicts satisfactorily both the bathymetry and the lithospheric thicknes

ISSN:0148-0227    

DOI:10.1029/JB094iB04p03991

年代:1989

数据来源: WILEY