摘要:

Geologic mapping has provided observations that some features on Mars are morphologically similar to lunar Imbrium Sculpture materials. These grooved features occur in clustered sets of a dozen or more, are spaced 1 to 10 km apart, and are unlike other linear Martian features such as yardangs, faults, ancient valley networks, and mass‐wasting features. We propose that they represent impact materials from an ancient basin. The orientations of these features were plotted as lines on a Schmidt net, and the density of the resulting intersections suggests a likely source area at approximately −26.0°, 125.0°, or roughly the center of Daedalia Planum. Numerous features, geologic materials, and Earth‐based radar data indicate that a basin with a diameter of approximately 4500 km with an early Noachian age is centered at this location. Four interior rings with diameters of 1100, 1500, 2200, and 3200 km and an exterior ring with a diameter of approximately 6400 km were found. The rings are spaced with a ratio of2±0.1, which is similar to the basin ring spacing on the other terrestrial planets. Both concentric and, infrequently, radial basin‐related faults were recognized, including Claritas Fossae, Gordii Dorsum, and the Thaumasia graben. Faults or zones of weakness related to the basin rings may have influenced the position of the Mangala Valles source area and Arsia Mons. Based on the location and morphology of early Hesperian to early Amazonian volcanics composing Daedalia Planum, the Daedalia basin may also have extensive, deep‐seated interior faults capable of extruding mare materials to

ISSN:0148-0227    

DOI:10.1029/JB095iB07p10729

年代:1990

数据来源: WILEY

摘要:

Furrows in ancient dark terrain on Ganymede form three systems that are each hemispheric or greater in scale. The oldest of the systems, designated system III, is dominated by approximately concentric troughs centered on about 60°N, 50°W. System I, in the anti‐Jovian hemisphere, contains concentric and subradial furrows arrayed around a large, degraded palimpsest centered at 15°S, 165°W. Furrows in each system formed on and locally are buried by dark volcanic materials that embay and infill preexisting topographic features; they crosscut extremely few well preserved older craters; and they occur on surfaces having significantly different relative crater ages. System II, also in the anti‐Jovian hemisphere, contains widely spaced, radially arrayed furrows commonly 500–2000 km in length, which are organized around a large area of dark smooth resurfacing material, intense dark terrain fracturing, and some of the globally oldest light material. The total thickness of dark terrain resurfacing, estimated using the stratigraphy of different dark material deposits and the rim heights of the largest crater size class whose density was depleted by each deposit, is probably in the range of 3–8 km. Multiple models of the origin of each furrow system were tested using observed geologic features and patterns. Systems I and III were found to be most consistent with reactivation of impact‐generated, multiringed structures by endogenic global extension, during a period of widespread dark material volcanism that obliterated a dense, ancient crater population. System II was found to be most consistent with fracturing of a single, circular, isostatic uplift covering an entire hemisphere. On the basis of geologic observations and interpretations and theoretical models of convection in spheres, it is hypothesized that the uplift developed by long‐term warming of the upwelling current of a single axisymmetric convection cell in an initially cooler, undifferentiated interior. Such wanning would also have created global expansion and supplied the tensional stress inferred to have formed systems I and III. This hypothesis is supported by the concentration around the center of system II of intense fracturing and relatively young dark volcanic deposits, suggestive of high lithospheric heat flow, lithospheric thinning, and stress concentration. An observed long‐term decrease in the width of extensional tectonic features interpreted to be of endogenic origin is also consistent with lithospheric thinning due to warming o

ISSN:0148-0227    

DOI:10.1029/JB095iB07p10743

年代:1990

数据来源: WILEY

摘要:

Catalogs of old magnetic measurements, contemporary publications, and ship's logs have been researched to compile a data set of geomagnetic observations of declination and inclination in the interval 1601 A.D. to 1694 A.D. This search produced 2766 measurements of declination, with quite good global coverage, and 37 of inclination. All inclinations were in the northern hemisphere and most in Europe. A literature search revealed many archeomagnetic and paleomagnetic measurements of field components for the seventeenth century: 40 of these were selected and used in the modeling procedure (intensity measurements were not found sufficiently reliable to allow any conclusions to be made about the overall strength of the geomagnetic field and declinations were too few to be of use). This data set was analyzed to produce a field model for the seventeenth century; the model was denoted epoch 1647 A.D. although no corrections were made for secular variation. The model exhibits many of the stationary features of the recent geomagnetic field and indicates absence of secular variation from the Pacific and a westward drift rate of about 0.25° per year in areas of the core‐mantle boundary beneath the south Atlantic and Indian oceans between 1647 A.D. and 1715 A.D., the date of Bloxham's (1986) earliest field model. This is likely to remain the earliest field model because only three inclination measurements have been found in the sixteenth century; the data set will be used in the future to form a time‐dependent model for the historical pe

ISSN:0148-0227    

DOI:10.1029/JB095iB07p10769

年代:1990

数据来源: WILEY

摘要:

New global maps of the vector magnetic anomaly field of Earth are derived from Magsat data. Improvement on earlier published vector and scalar maps is achieved with the following procedure: (1) The noise level of vector measurements associated with the discontinuities of the in‐flight sensor attitude determination is reduced by selecting the data. (2) A monthly model of the main field is computed to account for secular variation, using only quiet day data. (3) The time dependence of the ring current effect is taken into account using hourly values of theDstindex based on ground obseivations. (4) Sectorial indices based on ground observations are introduced to select profiles corresponding to low magnetic activity. These indices provide a better discrimination of quiet data than the global Kp index usually used for that purpose. (5) A time‐ and longitude‐dependent dynamic model of the equatorial electrojet is derived and used to correct magnetic measurements at low latitudes. Anomaly maps ofXandZcomponents are obtained using selected measurements within the reduced altitude range: 350–425 km. These maps are compared with earlier Magsat anomaly maps, and their respective resolutions are discussed. These maps evidence large anomalies of lithospheric origin within the oceanic domain. In particular, theZmap displays a characteristic pattern of long‐wavelength anomalies in these areas. It is first shown that some of these anomalies, particularly in the North Atlantic and Indian oceans, may be correlated with the age of the lithosphere. Then, a significant similarity between magnetic and geoid anomalies above ocean basins is evidenced. This suggests that long‐wavelength oceanic magnetic anomalies may also be attributed to variations of the thickness of the magnetized layer associated with ocean floor

ISSN:0148-0227    

DOI:10.1029/JB095iB07p10783

年代:1990

数据来源: WILEY

摘要:

Sonar reflectance is used to quantify the relative ages of young volcanic terrains on a seamount chain and a nearby spreading center in the north equatorial Pacific. The reflectance is a measure both of the burial of lava by pelagic sedimentation and of the microrelief of the lava. The extent of sediment cover and the lava type distribution are calibrated with thousands of bottom photographs. The relative ages indicate that the locus of volcanic eruptions has migrated toward the East Pacific Rise axis. Zero‐age activity lies just to the west of the East Pacific Rise south of the Clipperton Transform and is probably the cause of an anomalously shallow neovolcanic zone. The evidence fits a scenario in which eruptive centers appear, evolve, and become extinct in succession, with little or no overlap in time. The eruptive centers are fed in sequence through conduits from buoyant, tightly focused magma plumes. The near identical heights of many of the centers at the time of their latest eruptions suggest that the seamounts tap the same mantle depth range. Broad summit plateaus with calderas are formed if the plume volume is such that the hydraulic head is reached and magma is trapped within the volcano in ephemeral magma chambers or conduits. If the plume volume is small and the hydraulic head is not reached, the seamount remains small with only a small crater. The plumes ascend asynchronously from a common region of nucleation in the asthenosphere. Plumes that follow in close succession build a composite seamount with a string of summit calderas. Plumes that rise less often result in a line of discrete circular volcanoe

ISSN:0148-0227    

DOI:10.1029/JB095iB07p10801

年代:1990

数据来源: WILEY

摘要:

The 800‐km‐long Jemez lineament is the most active volcanic feature in the southwestern United States. It is the southeastern tectonic boundary of the Colorado Plateau and crosses the Rio Grande rift at the Jemez Mountains. The primary volcanism of the lineament is basaltic and has occurred in the last 4.5 m.y. To infer spatial distributions of partial melt in the upper mantle source zones for the Rio Grande rift and the Jemez lineament, we investigated the lateral variations ofPwave velocity in the upper mantle beneath these features. We used teleseismicPwave delays recorded at a 22‐station network to perform a damped least squares, three‐dimensional inversion for these lateral variations. Our technique employed velocity interpolation within a three‐dimensional grid of points, rather than using blocks of constantPwave velocity. This method allows highly realistic computation of seismic ray paths as well as accurate computation of the matrix elements in our system of equations. Determinations of resolution of results were done in two independent ways, both of which gave consistent estimates of resolution. In our best resolved volume the inversion showed no significant concentration of relative low velocity forPwaves beneath the Rio Grande rift. However, directly beneath the Jemez lineament there is a ∼100‐km‐wide, 1–2% low‐velocity feature in the depth range of 50–160 km. Because of the association of the lowPwave velocity with the Jemez volcanic lineament but not with the Rio Grande rift, because loweredPwave velocity can be associated with increased partial melt, and because the volume of recent volcanism at the lineament greatly exceeds that at the rift, we infer that a large magmatic source zone exists beneath the Jemez lineament but not beneath the Rio Grande rift. This implies that the volcanic potential of the Jemez lineaments continues to greatly exceed that of the Rio Grande rift. The mantle source zones for volcanics of the Jemez lineament are not overridden by, but rather track, the motion of the North American plate; this implies that these sources are within the lithospheric plate, as is clarified in the discussion. The magmatic source zones of the Jemez lineament are modeled as due to clockwise rotation of the Colorado Plateau about a pole in northeastern Colorado. This rotation caused extension of the lithosphere beneath the Jemez lineament, permitting concentration there of partially melted roc

ISSN:0148-0227    

DOI:10.1029/JB095iB07p10829

年代:1990

数据来源: WILEY

摘要:

Between 1091 and 1098 Ma, most of a 15‐ to 20‐km thickness of dominantly tholeiitic basalt erupted in the Midcontinent Rift System of the Lake Superior region, North America. The Portage Lake Volcanics in Michigan, which are the youngest MRS flood basalts, fall into distinctly high‐ and low‐TiO2types having different liquid lines of descent. Incompatible trace elements in both types of tholeiites are enriched compared to depleted or primitive mantle (La/Yb = 4.3–5.3; Th/Ta = 2.12–2.16; Zr/Y = 4.3–4.4), and both basalt types are isotopically indistinguishable. Sr, Nd, and Pb isotopic compositions of the Portage Lake tholeiites have87Sr/86Sri≈ 0.7038, εNd (1095 Ma)≈ 0±2, and μ1≈ 8.2. Model ages with respect to a depleted mantle source (TDM) average about 1950–2100 Ma. Portage Lake rhyolites fall into two groups. Type I rhyolites have Nd and Pb isotopic characteristics (εNd (1095 Ma)≈ 0 to −4.7; μ1≈ 8.2–7.8) consistent with contamination of tholeiitic rocks by 5–10% Archean crust. The one type II rhyolite analyzed has Nd and Pb isotopic compositions (εNd (1095 Ma)≈ −13 to −16; μ1≈ 7.6–7.7) which are consistent with partial melting of Archean crust. Early Proterozoic crust was not a major contaminant of MRS rocks in the Lake Superior region. Most reported Nd and Pb isotopic compositions of MRS tholeiites from the main stage of volcanism in the Lake Superior region and of the Duluth Complex are comparable to the Nd and Pb isotopic data for Portage Lake tholeiites. The isotopic enrichment of the MRS source compared to depleted mantle is striking and must have occurred at least 700 m.y. before 1100 Ma. There are two likely sources for such enriched MRS tholeiitic magmatism: subcontinental lithospheric mantle enriched during the early Proterozoic or enriched mantle derived from an upwelling plume. Subcontinental lithospheric mantle alone as a source cannot be ruled out strictly on isotopic grounds, but melting of enriched lithosphere potentially ranging in age from 3.6 to 1.8 Ga would have been less likely to generate either the homogeneity or the volume of MRS tholeiites. Decompression melting of an upwelling enriched mantle plume in a region of lithosphere thinned by extension could have successfully generated the enormous volume (850×103km3) of relativel

ISSN:0148-0227    

DOI:10.1029/JB095iB07p10851

年代:1990

数据来源: WILEY

摘要:

The Proterozoic Midcontinent Rift System of North America is remarkably similar to Phanerozoic rifted continental margins and flood basalt provinces. Like the younger analogues, the volcanism within this older rift can be explained by decompression melting and rapid extrusion of igneous material during lithospheric extension above a broad, asthenospheric, thermal anomaly which we call the Keweenaw hot spot. Great Lakes International Multidisciplinary Program on Crustal Evolution seismic reflection profiles constrain end‐member models of melt thickness and stretching factors, which yield an inferred mantle potential temperature of 1500°–1570°C during rifting. Combined gravity modeling and subsidence calculations are consistent with stretching factors that reached 3 or 4 before rifting ceased, and much of the lower crust beneath the rift consists of relatively high density intruded or underplated synrift igneous material. The isotopic signature of Keweenawan volcanic rocks, presented in a companion paper by Nicholson and Shirey (this issue), is consistent with our model of passive rifting above an asthenospheric mantle

ISSN:0148-0227    

DOI:10.1029/JB095iB07p10869

年代:1990

数据来源: WILEY

摘要:

Abundant gabbroic xenoliths in porphyritic pillow basalt were dredged form the northern Gorda Ridge. The host lava is a moderately fractionated, normal mid‐ocean ridge basalt with a heterogeneous glass rind (Mg numbers 56–60). Other lavas in the vicinity range from near primary (Mg number 69) to fractionated (Mg number 56). On the basis of textures and mineral compositions, the xenoliths are divided into five types: Type A xenoliths consist of large clinopyroxene oikocrysts, enclosing euhedral to subhedral plagioclase. Olivine and glass are present only in trace amounts. Type B xenoliths consist of olivine and plagioclase (±spinel) with abundant intersertal glass. Partially resorbed olivine with deeply embayed margins in places subophitically encloses small plagioclase crystals. Olivine and large plagioclase crystals often contain glass inclusions and/or spinel. Type C xenoliths are a composite of types A and B, with areas of large clinopyroxene oikocrysts adjoining areas of loosely joined clusters of olivine and plagioclase in glass. Type D is represented by a single large xenolith that, except for containing a larger proportion of crystals, appears identical to the host lava. Type E is represented by a single small xenolith that is mineralogically similar to type B but with a distinct, fine‐grained, tightly interlocking texture, formed by small olivine oikocrysts enclosing euhedral plagioclase, in a small amount of intersertal glass. Many single crystals and glomerocrysts in the host lava appear to be xenocrysts. Some forsteritic olivine and anorthitic plagioclase contain glass inclusions more primitive than the host lava, while rare glass inclusions in skeletal olivine suggest entrapment of melt more evolved than the host lava. The overabundance of plagioclase crystals and their textural features suggest accumulation. The xenoliths are not cognate to the host lava, but they are genetically related. Chemistry of mineral phases in conjunction with textural features suggests that the xenoliths formed in different parts of a convecting magma chamber that underwent a period of closed system fractionation. The chamber was filled with a large proportion of crystalline mush when new, more primitive, and less dense magma was injected and mixed incompletely with the contents in the chamber, forming the hybrid host lava. Plagioclase‐rich leucogabbro layers in ophiolites attest to similar processes in magma chambers of past spreading centers. However, xenoliths in ferrobasalt from the southern Juan de Fuca Ridge differ significantly from the Gorda Ridge xenoliths and suggest that the magma chamber at the southern Juan de Fuca Ridge is located at shallower depth and in cooler crust than the one at the northern Gor

ISSN:0148-0227    

DOI:10.1029/JB095iB07p10885

年代:1990

数据来源: WILEY

摘要:

Triaxial experiments were performed at room temperature and confining pressures up to 450 MPa on four pure, dense calcite rocks whose average grain sizes range over four orders of magnitude. Volumetric strain was measured during some of the experiments and microstructural studies were conducted to identify the active deformation mechanisms. The brittle fracture strength and macroscopic initial “plastic” yield stress in the semibrittle field follow empirical Hall‐Petch relations. The confining pressure at the brittle‐ductile transition depends inversely on grain size, but the stress ratio σ3/σ1at the transition is nearly the same for the different rocks. We assume that the initial flaw size scales with grain size and compare our experimental data to the fracture mechanics models of Ashby and Hallam (1986) for brittle fracture and Horii and Nemat‐Nasser (1986) for the brittle‐plastic transition in compression. The first model predicts that small confining pressures are sufficient to inhibit work softening behavior; however, our data indicate that localization occurs for significantly higher values of confining pressure than predicted. Furthermore, we find that localization is inhibited with increased confining pressure because of the increased activity of plastic flow mechanisms, rather than because of the increased difficulty of crack propagation alone. With certain assumptions, the model predicts the experimentally determined slope of the Hall‐Petch relation in the brittle field, although it underestimates the compressive strength of the rocks. The second model predicts that the stress ratio σ3/σ1at the brittle‐plastic transition scales with the, square root of the grain size; however, the experimental data do not corroborate the model unless the square of the ratio of the mode I fracture toughness to the plastic yield stress in shear scales with the grain size. The stress ratio at the brittle‐ductile transition is apparently a constant for many different rock types; we suggest that the physical basis for this relationship is that the ductility of most mineral aggregates falls

ISSN:0148-0227    

DOI:10.1029/JB095iB07p10907

年代:1990

数据来源: WILEY