摘要:

Crater palimpsests on Ganymede are circular features of high albedo apparently formed by impacts very early in the satellite's history. Some palimpsests have distinct outward‐facing slopes at their outer margins, suggesting that they are distinct deposits of significant thickness. This view is supported by observations of superimposed craters that excavate through palimpsest deposits into underlying materials of contrasting albedo. We suggest that crater palimpsests may result from volcanic extrusions triggered by large impact events early in Ganymede's history. Impact excavation that occurred to a sufficient depth would penetrate the satellite's thin, primordial lithosphere, allowing mobile, buoyant material from a vigorously convecting mantle to rise to the surface. Upon reaching the surface, the material would spread radially to produce a circular deposit of relatively high albedo. As the lithosphere cooled and thickened, palimpsest formation would have ceased, consistent with the observed concentration of palimpsest formation in the earliest part of Ganymede's recorded geologic history. Constraints on the nature of the extruded material may be derived from a simple model of emplacement. We use an analytic model for axisymmetric viscous flow to constrain the flow viscosity in terms of the aspect (height/radius) ratio γ of the palimpsest. A measured value of γ = 0.02 would imply a viscosity marginally consistent with glacial ice viscosities. Smaller values of γ, which are likely, would imply still lower viscosities, more appropriate to partially liquid flow. If such flows were described by a Bingham rheology, their yield strengths were of the order of 1 kPa. This is lower than is typical for even very mobile silicate lavas, but may be reasonable for a water/ice slush. Ice extrusion could have been a slow process taking thousands of years, while slush extension would have been essentially contemporaneous with the impact event. For the case of either solid ice and slush extrusion, our conclusions for the thermal structure of Ganymede at the time of palimpsest formation are similar: There was a thin (∼10 km) conducting lithosphere underlain by a vigorously convecting mantle that was very close to the solidus tempe

ISSN:0148-0227    

DOI:10.1029/JB095iB12p19161

年代:1990

数据来源: WILEY

摘要:

We explore the pattern of two‐dimensional convection in an highly anisotropical porous medium. This physical situation is relevant to passive margin sedimentary basins consisting of interbedded coarse‐grained pervious and shale or limestone semi pervious layers, in particular to deltaic‐type basins with sandstone lenses in a shale matrix. We show that permeability anisotropies of the order of 102–104allow for long convective cells, of aspect ratio greater than 10, but that a combination of this parameter with a slight slope of the order of a few percent of the sedimentary layers is required to stabilize these long cells. As an example, we present the Okinawa basin, an active submarine back arc basin, with a sedimentary thickness of about 2 km and a heat flow profile across this basin, varying from 32 to 232 mWm−2over a distance of 30 km. It is shown that this heat flow variation is difficult to explain with conductive mechanisms only but is well reproduced by different convective models relying on permeability anisotropy plus slope. Although the insuficient thermal and structural constraints did not allow us to build a unique model, the whole set of possible fits to the heat flow data may restrict the mean hydraulic parameters of the basin. A vertical permeabilty of a few tens of millidarcy and an anisotropy greater than 100 are required to produce the expected stable and active large‐scale circulation. It is suggested in conclusion that this type of circulation might be active in oil‐or oreforming elem

ISSN:0148-0227    

DOI:10.1029/JB095iB12p19175

年代:1990

数据来源: WILEY

摘要:

We present new Devonian paleomagnetic results from 59 sites in three stratigraphic sections exposed on the northwestern margin of the Tarim Block. At one section, 1998 m of red sandstone is continuously exposed; the other two sections can be correlated with the first on both magnetostratigraphic and lithologic grounds. Progressive thermal demagnetization reveals three characteristic magnetizations. One is a Late Permian overprint which is isolated below 578°C at sites near contacts with Late Permian dikes. The other two components are approximately antipodal, have a much higher unblocking temperature than the overprint, and are directionally distinct from the Late Permian overprint. We believe that these are characteristic Devonian magnetizations. Within the continuous 1998 m section, our results denote a 395 m reversed polarity zone overlying a 1603 m normal polarity zone. One reversed event is recorded at the base of the section. A paleomagnetic pole calculated by averaging results from 47 normal and reversed sites lies at λp= 16.5°N, ϕp= 165.0°E,K= 25, andA95= 4.3°. This pole is statistically distinct from a previously reported Late Devonian pole for Tarim. On the basis of field geologic and rock magnetic studies, we believe that the previously reported pole, in addition to results from one of our three sections, reflects at least partial contamination in the form of a thermal overprint caused by Late Permian igneous activity. An analysis of Devonian to Late Carboniferous polar wander suggests that the Tarim Block was attached to a subducting plate, and moved northward and rotated clockwise during the late Paleozoic. Comparison of the Devonian paleolatitudes of Siberian, Kazakhstan, Tarim and South China Blocks indicates that both the Tarim and South China Blocks were located in equatorial regions and were far south of the northern Angaran plate (Siberia and Kazakh

ISSN:0148-0227    

DOI:10.1029/JB095iB12p19185

年代:1990

数据来源: WILEY

摘要:

The Kapuskasing Structural Zone (KSZ) in the Superior Province of north‐central Ontario is thought to be an oblique cross‐section through Middle Archean crust. Samples from 30 sites in granulite gneisses and anorthosites of the KSZ have been analyzed using alternating field and thermal demagnetization methods. Three remanent magnetization components were found with mean directions of: D = 23°, I = 50° (k =52, α95= 11°, N = 5 sites) for the A component; D = 73°, I = −68° (k = 50, α95= 5°, N = 18) for the B component; and D = 332°, I = −59° (k = 53, α95= 17°, N = 3) for the C component. V. Constanzo‐Alvarez and D. J. Dunlop found similar A and B components in less metamorphosed rocks from the same units to the immediate west. They attributed the A component to uplift at 2.55 Ga with subsequent 15° to 30° WNW tilt and the B component to thermochemical overprinting at 1.1 Ga as the KSZ was reactivated during Keweenawan rifting and volcanism. We also attribute the A component to uplift during the 2.55 Ga Kenoran Orogeny with concomitant 10°±5° WNW downward tilting before intrusion of the 2.45 Ga Matachewan dikes, to give its present paleopole of 33°E, 67° (dp = 6°, dm = 9°, N = 13) using data from both studies. We attribute acquisition of the B component to uplift at about 1.88 Ga, as suggested by recent thermal modeling and tectonic syntheses, to give a paleopole of 124°W, 30°N (dp = 7°, dm = 8°, N = 20). The C component is found in only three sites and may be either an hybrid of a 1.1 Ga Keweenawan CRM with the B component or a fault‐block tilted B component ne

ISSN:0148-0227    

DOI:10.1029/JB095iB12p19199

年代:1990

数据来源: WILEY

摘要:

The Spences Bridge Group of late Albian age (palynology and U/Pb zircon date of 104.5 Ma) is situated in the Intermontane Belt of the southern Canadian Cordillera. Zeolite‐grade andesite lavas and interbedded volcaniclastic rocks have been sampled through a thickness of about 2900 m. Sampling has been confined to localities whose bedding attitudes are well defined and to rocks that have not undergone internal deformation. Of the 27 sites studied, 25 gave single‐component magnetizations with well‐grouped directions, high coercivities and high unblocking temperatures. Applying corrections for geological tilt to all 25 sites yielded minimum dispersion after 65% untilting. Analysis indicates that this is a consequence of the interaction of two modes of behavior, one (exhibited byYmagnetizations at 8 sites) in which minimum dispersion occurs after 35% untilting, and one (Xmagnetizations at 17 sites) in which minimum dispersion occurs after 100% correction. (Actually the dispersion is essentially constant in the range 88 to 100% correction). TheYmagnetizations are uninterpretable in terms of the paleofield because no estimate of paleohorizontal can be made. Because of their positive response to the tilt test, theXmagnetizations were acquired before tilting, and it is argued that geologically determined bedding planes are good estimators of paleohorizontal at the time of remanence aquisition. There may be errors because of paleoslope, but arguments are presented for believing that these are small. On this basis, the best estimate of the mean direction of the paleofield in Spences Bridge time is estimated to beD,I(declination, inclination) = 38.7°, 63.9° (17 sites,k= 44 compared withk= 11 in situ, α95= 5.4°) with a paleopole at 64.4°N, 321.0°E (K= 24 compared withK= 5 in situ,A95= 7.5°). When compared with the cratonic reference field for mid‐Cretaceous time, this pretilting magnetization indicates a clockwise rotation of the study area of 66.3°±11.8 (P= 0.05), and an apparent displacement from the south of 15.5°±7.1 (1725±790 km). When these, and other data previously obtained from Cretaceous rocks of the Pacific Northwest, are compiled, they show an increase in apparent displacement from east to west across the Cordillera. This result is discussed in terms of three hypotheses: (1) no translation but systematic tilts of about 30° to the southwest; (2) translation of a substantial part of the Pacific Northwest (referred to as Baja British Columbia and is defined more exactly in the text) by variable amounts up to more than 2000 km from the south together with clockwise rotations; and (3) lesser translation of Baja British Columbia by about 1500 km combined with variable tilting predominantly to the south. It is concluded that paleomagnetic data, as they are presently known, are consistent with hypotheses 2 and 3, but, unless serious sources of error are present, are not consistent with hypothesis 1. The most likely candidate for such errors is the uncertainty in estimating initial dip of beds (paleoslope) in

ISSN:0148-0227    

DOI:10.1029/JB095iB12p19213

年代:1990

数据来源: WILEY

摘要:

Major and trace element data for a suite of lavas from fifty‐six dredges and ALVEN dives on the ridge axis and adjacent abyssal hills have been used to investigate the geometry and evolution of magmatic systems beneath the Endeavour Segment, Juan de Fuca Ridge. The morphology of the Endeavour Segment between the northward propagating Cobb Offset and the recently formed (<0.2 m.y.) Endeavour Offset is dominated by a shallow, rifted, elongate crestal volcano (Endeavour Ridge) that deepens along‐strike into a broad, deep basin at each offset. A set of ridges, interpreted to be previous crestal volcanoes rifted apart by spreading, flank the Endeavour Ridge and chronicle the “dueling” propagator history of the Cobb Offset The tectonic evidence strongly suggests that a large portion of the Endeavour Segment may be a failing rift segment at this time. Lavas from the current axis of the Endeavour Segment are moderately fractionated (MgO: 6–8.5 wt %) and have generally higher SiO2, Al2O3, Na2O, and K2O, and lower FeO*man lavas from south of the Cobb Offset (SOCO lavas). Incompatible trace element abundances and ratios indicate the Endeavour lavas are primarily enriched E‐MORBs and T‐MORBs (e.g., Zr/Nb: 7–24; Zr/Y: 2.5–5.9; and Ba/TiO2: 6–64), in contrast with the SOCO lavas, which are more depleted in character. Thus, the 30‐km wide Cobb Offset appears to mark a major geochemical boundary beneath the Juan de Fuca Ridge. In contrast with the Endeavour Segment axial lavas, samples from adjacent abyssal hills are more similar to the SOCO lavas in their major and trace element characteristics. These observations suggest that the parental magmas of the Endeavour Segment exhibit temporal variability, with more enriched material arriving only recently beneath the ridge axis. Pronounced compositional variability is observed at small spatial scales within the Endeavour Segment axial lavas, which does not correlate with axial morphology. This variability is interpreted to reflect ubiquitous small scale mantle heterogeneity and poor mixing of multiple parental magmas during migration from the melt region or within axial magma chambers. Highly enriched samples (Zr/Nb<10) are localized near the summit region of Endeavour Ridge, whereas slightly less enriched samples occur along the length of the axis and on the older flanking ridges. Recent enrichment may result from diminishing extents of partial melting of a heterogeneous source in response to tectonic reconfiguration, causing more fusible enriched domains to dominate the chemical signature of melts produced. Small scale heterogeneity along‐strike seems incompatible with models of centralized upwelling of melts beneath the summit region of the ridge axis, with shallow lateral injection of melts to distal ends of the segment, unless these spatial variations actually reflect temporal variations in the source composition and collapse of the shallow magmatic systems toward the summit region as rift

ISSN:0148-0227    

DOI:10.1029/JB095iB12p19235

年代:1990

数据来源: WILEY

摘要:

The basal slip systems of biotite and their mechanical expressions have been investigated by shortening single crystals oriented to maximize and minimize shear stresses on (001). Samples loaded at 45° to (001) exhibit gentle external rotations associated with dislocation glide. High‐angle kink bands in these samples, unlike those developed in micas loaded parallel to (001), are limited to sample corners. Samples shortened perpendicular to (001) show no evidence of nonbasal slip and fail by fracture over all conditions tested. The mechanical response of biotite shortened at 45° to (001) is nearly perfectly elastic‐plastic; stress‐strain curves are characterized by a steep elastic slope, a sharply defined yield point, and continued deformation at low (mostly1%. Stresses measured beyond the yield point are insensitive to confining pressure over the range 200 to 500 MPa and exhibit weak dependencies upon strain rate and temperature. Assuming an exponential relationship between differential stress σdand strain rateε˙of the formε˙=Cexp(ασd)exp(−Q/RT), the data collected over strain rates and temperatures of 10−7 to 10−4 s−1 and 20° to 400°C, respectively, are best fit by an exponential constant α of 0.41±0.08 MPa‐1 and an activation energyQof 82±13 kJ/mol. A power law fits the data equally well withn= 18±4 andQ= 51±9 kJ/mol. Samples oriented favorably for slip in directions [100] and [110]are measurably weaker than those shortened at 45° to [010] and [310], consistent with the reported Burgers vectors 〈100〉, 1/2 〈110〉, and 1/2 〈110〉. The anisotropy of biotite is further revealed by contrasting these plastic strengths with results of samples deformed parallel and perpendicular to (001). Previous studies have shown that biotite loaded in the (001) plane is strong prior to the nucleation of kink bands. The strength of biotite shortened perpendicular to (001) exceeds that measured parallel to (001) and is pressure dependent. Application of the results to deformation within the continental crust suggests that biotite oriented favorably for slip is much weaker than most other silicates over a wide range of geologic conditions. Its presence within foliated rocks and shear zones may limi

ISSN:0148-0227    

DOI:10.1029/JB095iB12p19257

年代:1990

数据来源: WILEY

摘要:

When describing the deformation of poro‐elastic materials subject to pore pressure (PP) and confining pressure (PC), the concept of effective pressure is commonly used. In such a description the deformation is described in ternis of a single stress parameter, the effective stress (PE). Experimental studies which attempt to describe the effective pressure law are troubled by the fact that deformation of geologic materials invariably exhibits loading path dependence (hysteresis). Properties which exhibit hysteresis are not easily described by a single stress state variable and hence the concept of effective pressure becomes clouded. Therefore, the effective pressure law here is formulated to describe the relative effects of pore and confining pressure on a given property, with specific attention to the loading path. Here we develop an experimental technique for measuring the effective pressure law which is useful for many properties of interest, including those that are highly nonlinear and exhibit common types of hysteresis. Applying this method in an experimental study of the effective pressure law for joint closure, we experimentally derive an effective pressure law which describes the values of pore and confining pressure consistent with a given joint closure for a loading path of constant closure. The study can be viewed as an attempt to include both pore and confining pressure in a single constitutive law for joint closure. The constant closure loading path is such that the measurement is not affected by hysteresis caused by joint closure. The results provide insight into the microgeometrical and micromechanical properties of joints. The data are not consistent with a simple extension of commonly used linear elastic constitutive models for joint deformation which have compared favorably with experiments in the absence of pore pressure. For smooth lapped glass joints, the effective pressure relation is found to be dependent on the local joint stiffness, with the relationship between the effective pressure law and the local joint stiffness being insensitive to the measured surface topography. Similar measurements on lapped and fractured rock provide some constraints on the effective pressure behavior of jointed roc

ISSN:0148-0227    

DOI:10.1029/JB095iB12p19279

年代:1990

数据来源: WILEY

摘要:

The melting temperatures of FeS‐troilite and of a 10 wt % sulfur iron alloy have been measured to pressures of 120 and 90 GPa, respectively. Our results document that FeS melts at a temperature of 4100 (± 300) K at the pressure of the core‐mantle boundary. Eutectic‐like behavior persists in the iron‐sulfur system to the highest pressures of our measurements, in marked contrast to the solid‐solution‐like behavior observed at high pressures in the iron‐iron oxide system. Iron with 10 wt % sulfur melts at a similar temperature as FeS at core‐mantle boundary conditions. If the sole alloying elements of iron within the core are sulfur and oxygen and the outer core is entirely liquid, the minimum temperature at the top of the outer core is 4900 (± 400) K. Calculations of mantle geotheims dictate that there must be a temperature increase of between 1000 and 2000 K across thermal boundary layers within the mantle. If D″ is compositionally stratified, it could accommodate the bulk of th

ISSN:0148-0227    

DOI:10.1029/JB095iB12p19299

年代:1990

数据来源: WILEY

摘要:

We describe fundamental thermodynamic relations (Helmholtz free energy as a function of volume and temperature) for solids and liquids, simple physically based expressions which contain all thermodynamic information about a system. The solid fundamental relation consists of Debye and Birch‐Murnaghan finite‐strain theory combined in the Mie‐Grüneisen framework. The liquid fundamental relation is derived by taking the high‐temperature limit of the solid expression. We derive the liquid equation of state, which contains only four parameters, from the liquid fundamental relation and show that it successfully describes measurements of liquid alkali metals, water, and liquid diopside over a wide range of pressure and temperature. We find optimal fundamental relation parameters for diopside, enstatite, ilmenite, and perovskite and find the solid relation to be in excellent agreement with data, including heat capacities, thermal expansion, and MgSiO3phase equilibria. We then combine the liquid and solid fundamental relations to calculate the melting curves of diopside, enstatite, and perovskite, which are found to be in excellent agreement with experiment. All predicted melting curves havedT/dPslopes which decrease steadily with pressure, eventually becoming negative because of liquid‐crystal density inversion. Our predicted melting temperature of perovskite in the D″ region (3750 K) at the base of the mantle is thousands of degrees lower than previous estimates, yet it is consistent with experimental data. The predicted melting curve, although consistent with the lack of widespread melting in the lower mantle, is much lower than recently proposed geotherms in the D″ layer at the base of the mantle. By combining our results with seismic observations of the deep mantle, we propose that the D″ layer consists of magnesiowüstite and silica in the form of stishovite or its recently discovered high‐pr

ISSN:0148-0227    

DOI:10.1029/JB095iB12p19311

年代:1990

数据来源: WILEY