摘要:

The problem of inverting for the aspherical structure of the earth is complicated by the nonlinear dependence of low‐frequency seismic waveforms on aspherical structure. In an attempt to overcome this obstable, we report on the application of two complementary techniques. The first, a data space technique called singlet stripping, which linearly recombines seismic recordings to estimate singlet resonance functions, has been applied to 190 International Deployment of Accelerometers and Global Digital Seismographic Network recordings from five large events. More than 290 singlets from 34 low harmonic degree multiplets appear to have been resolved. A subset of these measurements has been compared with those produced from the second technique, a nonlinear regression, which iteratively estimates coefficients which are linear functionals of aspherical structure. Both techniques agree that most multiplets are normally split, with singlet frequency distributions insignificantly different from those predicted for a rotating, hydrostatic (RH) earth model. The main result of this paper is that both techniques also agree that approximately a third of the multiplets are anomalously split, some of which span frequency bands up to 2.5 times greater than predicted for an RH model. All of the anomalously split multiplets are SKS, PKP, or PKIKP equivalent. The observation of anomalously widely split multiplets is highly robust and provides compelling evidence for the existence of deep large‐scale, nonhydrostatic aspherical structure. The inverse problem for the axisymmetric part of aspherical structure has been performed in the hope of illuminating anomalous splitting. Unless a large amount of structure in the core is included, we are unable to construct a smooth axisymmetric model which accurately predicts the splitting characteristics for the anomalous multiplets while simultaneously fitting the normally split multiplets. The location and nature of this core heterogeneity are unclear, but we find that a simple outer core structure is sufficient to give a reasonable fit to the data. There are good theoretical reasons for believing that such nonhydrostatic outer core structure is geophysically unreasonable, yet differential travel time data sensitive to core structure apparently require similar large scale heterogeneity in the core. Although this dilemma remains unresolved, spectral fitting techniques like the nonlinear regression can be applied to many more multiplets than considered here and it is not unreasonable to predict that reliable large scale aspherical models of the deep earth soon will become availa

ISSN:0148-0227    

DOI:10.1029/JB091iB10p10203

年代:1986

数据来源: WILEY

摘要:

The descending plate and overriding block in a subduction zone are analogous to the guide surface and slide block in a slipper bearing, and subducted sediment is analogous to the lubricant. Subduction is more complex and varied, however, because the overriding block is not rigid, the sediment is buoyant, underplating can occur, and sediment supply can vary widely. A model based on the bearing analogy but taking these differences into account makes detailed quantitative predictions for actual sites, which are illustrated by calculations for five diverse examples: Mariana, 16°N; Mexico, 17°N; Lesser Antilles, 13°N (Barbados); Alaska, 153°W (Kodiak); and Japan, 40°N. It requires as input the geometry of the overriding block and the top of the descending plate, the distribution of density and permeability of the overriding block, the speed of subduction, the density and rheological properties of the subducted sediment, and the rate of sediment input. Its predictions include the profile of thickness of the layer of subducted sediment (all sites; maximum of 360 m at Mariana, 5300 m at Japan), the velocities of flow in the layer (all sites), the shear stresses exerted on the walls (all sites; low beneath accretionary prisms, up to 6 MPa beneath Japan), the rate of offscraping (none at Mariana and late Tertiary Mexico; 85% of input at Lesser Antilles; includes melange at Japan), the distribution and rates of underplating (none at Mariana, extensive at Japan), the zones of possible subduction erosion (extensive at Mariana; local at the others), the amount of sediment subducted to the volcanic arc (all sites; 2% of input at Lesser Antilles, 100% at Mariana), the qualitative pattern of flow at the inlet (five basic patterns; all sites), the upward flow of melange in many instances (none at Mariana; extensive at Japan), and, under relatively rare conditions, the formation of large‐scale melange diapirs (only at Lesser Antilles beneath Barbados I

ISSN:0148-0227    

DOI:10.1029/JB091iB10p10229

年代:1986

数据来源: WILEY

摘要:

The surface slopes of submarine accretionary wedges generally decrease away from the toe or deformation front toward the arc. This paper presents evidence that this characteristic convex cross‐sectional shape is a direct mechanical response to the lithification of sediments during accretion. The recently accreted sediments near the toe, which are typically unconsolidated and therefore very weak, must deform until a critical taper is attained at which they are stable in the presence of the horizontal tectonic compression. This critical wedge taper, and thus the bathymetric slope of the accretionary prism, is relatively high for the porous, weak sediments near the toe. In contrast, the compacted sediments farther back in the wedge are stronger, so they require a relatively modest critical taper and therefore a lower bathymetric slope to be stable under the same tectonic compression. The result is a convex cross‐sectional wedge shape. We investigate several possible causes of the increase in sediment strength within the wedge and conclude that the primary cause is an increase in cohesive strength with decreasing porosity. A straightforward generalization of the exact critical taper model is combined with empirical cohesion‐porosity‐velocity relations to infer the distribution of porosity andPwave velocity within the Barbados accretionary wedge. We estimate that the porosity decreases from its near‐toe value of 70% to approximately 30% at a depth of 3 km below the seafloor and a distance of 60 km arcward from the front of

ISSN:0148-0227    

DOI:10.1029/JB091iB10p10246

年代:1986

数据来源: WILEY

摘要:

Several lines of evidence indicate that the space problem caused by southeastward motion of the Juan de Fuca plate toward the east‐west Mendocino transform is resolved by distributed deformation of the oceanic crust north of the transform. A kinematic model for this deformation based on the assumption of nondivergent flow (conservation of area) within the area known as the Gorda plate, and on the assumption of no subduction along the Mendocino transform, successfully matches the isochron pattern inferred from magnetic anomalies. The model assumes strain rate decreasing to zero approaching the rigid Juan de Fuca plate, with no sharp boundary or separate rigid area that would imply a discrete Gorda plate. This interpretation allows magnetic anomalies from south of the Blanco Fracture Zone to be used to constrain the kinematics of the Juan de Fuca plate. These anomalies imply a substantial change in the distance to the rotation pole at roughly 3 Ma, in addition to the clockwise shift in motion direction at 5 M

ISSN:0148-0227    

DOI:10.1029/JB091iB10p10259

年代:1986

数据来源: WILEY

摘要:

An olivine and spinel phyric picrite from Okmok has Fo92.9olivine cores, Mg/(Mg+Fe2+) (Mg#) = 0.79 after correction for olivine accumulation, and a calculated liquidus temperature of 1409°C. A basalt which is also olivine and spinel phyric is different from the picrite in most aspects of trace element and isotope geochemistry and has Mg# = 0.72 and a liquidus temperature of 1286°C. Five basalts are fractionated but have trace element and isotope chemistries in between those of the two inferred primary magmas. All the lavas have Pb, Nd, Sr, and O isotopic ratios, and interelement high field strength element (HFSE) ratios similar to enriched oceanic basalts (EMORB) which are transitional between depleted mid‐ocean ridge basalts (NMORB) and ocean island basalts (OIB), although elemental concentrations of HFSE are lower in the primary Okmok magmas than in primary EMORB. Ratios of heavy rare earth element (HREE), and light REE (LREE), large ion lithophile element (LILE) to HFSE are progressively higher than EMORB ratios. Those samples most removed from the EMORB signature have the lowest206Pb/204Pb. These features suggest progressive enrichment of an EMORB‐like mantle with an incompatible element‐rich component. In the final mantle source, 80–90% of the LILE and 30–40% of the LREE have been added. Given this extent of metasomatism and the isotopic ratios of the lavas, we see little evidence for significant sediment involvement and suggest that the metasomatic fluid is derived from a lower portion of the slab which has undergone little isotopic exchange with seawater. The high liquidus temperatures of the primary liquids, HFSE with concentrations similar to NMORB and relative abundances similar to EMORB, low Ti/V, and high TiO2/Ni in the primary magmas all require high temperatures and high degrees of partial melting of the mantle source. The temperatures required are sufficiently high that induced counterflow and upward migration of hot (1500°–1600°C) mantle is required. Any melting mechanism involving static mantle, or vertically moving diapirs, is specifically prohibited under Okmok. We propose that decompression of the upward moving limb of the induced counterflow is the dominant melting mechanism and that the melting zone is localized by impingement on the subforearc mantle wedge, that is, the melting zone (and thus the location of the volcanic front) is localized for physical rather than physiochemical reasons. Variable mixing between the static and counterflowing mantle and a variety of metasomatic fluids yields a range of primary magma compositions under Okmok and presumably under

ISSN:0148-0227    

DOI:10.1029/JB091iB10p10271

年代:1986

数据来源: WILEY

摘要:

A wealth of chemical information may provide a false sense of security to the petrologist interested in extracting petrogenetic information from a set of major and/or trace element abundances. Such data are said to be closed, as each analysis sums to a constant (100%). Failure to sum to a constant results from a combination of errors of commission and omission. A correlation coefficient between a pair of closed components has a contribution from the linear association between the components and a contribution from the effects of closure itself. Standard statistical procedures cannot separate these two sources; therefore, the investigator cannot tell if a strong correlation (as revealed by a nearly linear trend on a binary scatter diagram, for example, is due to a strong linear association between the components or due to closure. Techniques developed by Aitchison (1984a, b) appear to be capable of providing a framework within which the user can begin to assess the relationship among closed components. These techniques are applied to a set of 35 TiO2, Zr, Y, and Sr analyses of basalts. A statistical analysis permits rejection of Aitchison's (1984a, b) hypothesis of complete subcompositional independence, indicating that there is a degree of dependency within the data set. The first two principal components extracted from the covariance matrix of the log‐centered form of these data account for more than 90% of the total variation, and three major tectonic‐related fields can be clearly recognized in the space defined by the first two principal components: ocean floor basalts, within‐plate basalts and arc‐related basalts. Analyses from six additional data sets taken from the literature were plotted on this diagram, and all reclaim the tectonic settings stated in the literature. Simple modeling reveals that the addition or subtraction of Sr from an analysis results in a linear locus of points which is parallel to the boundary between the ocean floor and the within‐plate basalts, suggesting that such a diagram may prove useful in the analysis of basalts that have been subjected to low‐ to moderate‐grade metamorphism

ISSN:0148-0227    

DOI:10.1029/JB091iB10p10289

年代:1986

数据来源: WILEY

摘要:

Gaussian band‐fitting analysis has been applied to the reflectance spectra of mafic silicates, which are of geologic importance throughout much of the solar system. Reflectance spectra obtained over a sample temperature range of ≈80 to 448 K of olivine, clinopyroxene, and orthopyroxene were used in order to characterize the spectral changes in the 1‐μm wavelength region of these minerals as a function of temperature. Four Gaussians are required to characterize the olivine, while two Gaussians are necessary to characterize both the orthopyroxene and clinopyroxene. The trends of the individual Gaussian parameters (integrated area, width, and center position) are presented as a function of temperature for each mineral. The characterization by Gaussian analysis of the 1‐μm absorption feature of all minerals yields consistent trends in center position, integrated area, and width that may prove useful in quantitative determination of these minerals in mineral mixtures. The Gaussian analysis of olivine, for the three primary band components, is qualitatively consistent with previous crystal field transition assignments of Fe2+in the M(1) and M(2) crystallographic sites. The trends of the determined olivine Gaussian center positions imply that site symmetry is more important in determining transition energies than metal‐oxygen distances. The result of the Gaussian analysis for orthopyroxene is consistent with the need for two Gaussians to describe an asymmetric band. The Gaussian analysis of the clinopyroxene is consistent with previous assignments of crystal field transitions due to Fe2+in the M(1) and M(2) crystallogr

ISSN:0148-0227    

DOI:10.1029/JB091iB10p10301

年代:1986

数据来源: WILEY

摘要:

Deep Sea Drilling Project hole 504B penetrates 1076 m into oceanic layer 2 and is the first hole to pass through the transition from pillow basalts altered at low temperatures into hydrothermally metamorphosed sheeted dikes. Alteration of the crust at site 504 occurred in four stages, related to the movement of the crust away from the spreading axis: (1) Dikes reacted with seawater (200–>300°C) in the upwelling zone of an axial convection cell at the spreading axis, resulting in the formation of greenschist facies parageneses in veins and host rocks. Mixing of the upwelling hydrothermal fluids with seawater circulating in the overlying more permeable pillow section occurred in the upper part of the lithologic transition zone, causing a steep temperature gradient at the base of the pillow section. Secondary minerals formed in the lower pillow section from the resultant reducing “mixed” fluids at temperatures of around 100°C. At the same time, the initial effects of “seafloor weathering” began in the upper 320 m of the pillow section at low temperatures (<50°C) and under conditions of open circulation of oxidizing seawater. (2) Following refracturing of the dikes, a second stage of axial upwelling occurred; hydrothermal fluids (200°–380°C) were probably similar to those presently sampled from spreading ridges on the seafloor. Mixing of these fluids with seawater in the lithologic transition zone caused deposition of quartz, epidote, and sulfides in veins and formation of a sulfide‐rich stockworklike zone within this transition zone. Alteration of the pillow section proceeded under the prior conditions, with the effects of seafloor weathering extending progressively downward into the crust. (3) Seawater recharge penetrated to depths of at least 1075.5 m subbasement and deposited anhydrite locally in veins. (4) Off‐axis alteration of the dikes was characterized by formation of zeolites, calcite, and prehnite in veins and host rocks from more highly evolved and lower temperature (100°–250°C) fluids. Alteration in the upper pillow section evolved from “seafloor weathering” conditions to more reducing and rock‐dominated, as cracks in the basalt were sealed with secondary minerals and the basement was covered with a layer of sediment. Zeolites and calcite were the last phases to form throughout the pillow section. Sealing of the crust to convective cooling also allowed conductive reheating of the crust from below. Calculations of seawater‐crustal chemical fluxes from whole rock data are complicated by the low recovery and heterogeneity of alteration effects but indicate that basalt‐seawater interactions are a sink for seawater M

ISSN:0148-0227    

DOI:10.1029/JB091iB10p10309

年代:1986

数据来源: WILEY

摘要:

Deep Sea Drilling Project hole 504B is located in 5.9 m.y. crust in the eastern Pacific Ocean about 200 km south of the Costa Rica Rift. At 1076 m subbasement, it is the deepest penetration of marine crust yet achieved. We present here magnetic data from this hole, especially from the recently cored leg 83 section (which constitutes the lowermost 500 m). These data, when combined with those of other studies, yield not only the deepest but also the most detailed and comprehensive picture of marine magnetic structure at a single site currently available. The basement rocks from hole 504B can be divided into three major magnetic units. The upper units (the top 500 m or so of basement) are essentially similar to other shallow marine basement sections, consisting of a mixture of various types of extrusive basalt. Low‐temperature altered titanomagnetite is the dominant magnetic carrier, and the magnetic properties of these rocks are comparable to other shallow marine basalts. Below the upper units is a 200‐m transition zone, consisting of a mixture of extrusives and dikes. High‐temperature hydrothermal alteration (much of it greenschist facies grade) has caused oxidation‐exsolution of primary titanomagnetite as well as its partial or total replacement by silicates. The dominant magnetic carrier is an Fe‐rich titanomagnetite, magnetically indistinguishable from pure magnetite. The combination of oxidation‐exsolution and silicate replacement results in a very low natural remanent magnetization (NRM). The remaining 300 m are in the upper portion of the sheeted dike complex. Primary titanomagnetite is exsolved, but alteration is less intense than in the transition zone. NRM values are substantially higher than those of the transition zone, due to less silicate replacement of primary titanomagnetite coupled with an uncertain contribution from secondary magnetite (which occurs as a silicate alteration product). The NRM magnitude is sufficiently high that the sheeted dike complex, in this location at least, may make a significant if not substantial contribution to the magnetic anomaly. Overall, the magnetic properties of this hole, especially the lower 500 m, are strongly influenced by postemplacement alteration and may bear little or no resemblance to their values upon initial cooling. As hydrothermal temperatures do not appear to have exceeded 400°C, the NRM in the lower two sections is apparently a chemical rather than a thermal remanence. As such, models of marine magnetic structure that combine cooling models of the crust with the assumption that magnetic remanence is purely of thermal origin do not appear sufficient to adequately predict the properties of the deeper crust. A better understanding of the nature of postemplacement alteration in the marine crust is required, especially its characteristics at high temperatures, and its effects on magneti

ISSN:0148-0227    

DOI:10.1029/JB091iB10p10337

年代:1986

数据来源: WILEY

摘要:

The Toba depression in north central Sumatra is a complex of several overlapping calderas resulting from three major ignimbrite‐forming eruptions. Within the depression, the upland masses of Samosir and northern Uluan consist of welded ignimbrite capped by coarse breccia and lacustrine sediment, hitherto interpreted to be two parts of a single resurgent dome. This study has demonstrated that the welded tuffs of Samosir and Uluan have different magnetic polarities and therefore at least two different ignimbrites are present; the Samosir/Uluan massif may consist of parts of two resurgent domes. The first ignimbrite eruption occurred at 0.84 Ma and produced a very thick (>400 m), densely welded unit having a reversed polarity. Anisotropy of magnetic susceptibility (AMS) flow direction and lithic size data indicate that the source lies in the southern part of the Toba depression, and the thick deposit of Uluan is thought to have ponded in a 40‐km‐wide caldera. The second ignimbrite is normally magnetized. AMS flow direction data indicate two separate source vents, one to the north in the Haranggaol caldera, and another to the south. The thick deposit at Samosir is thought to have ponded in the southern caldera. Coarse sediments then accumulated over Samosir and northern Uluan and were capped by lacustrine deposits. A renewed episode of resurgence then uplifted Samosir Island and possibly the northern part of Uluan. At approximately 0.075 Ma the last and apparently largest ignimbrite eruption occurred from calderas in the north and south parts of the Toba depression. This ignimbrite is mostly nonwelded and normally magnetized. Part of the Uluan dome was destroyed by collapse of the Sibandung caldera and Latung graben and concomitant with renewed subsidence of the Haranggaol and Porsea cal

ISSN:0148-0227    

DOI:10.1029/JB091iB10p10355

年代:1986

数据来源: WILEY