摘要:

The reflectance spectra of combinations of olivine, orthopyroxene, and iron meteorite metal show systematic variations in spectral properties which can be used to constrain many of the physical and chemical properties of the assemblages. The presence of metal most noticeably affects band area ratios, peak:peak and peak:minimum reflectance ratios, and band widths. These parameters are also affected to varying degrees by the abundance and grain size of the metal and silicates and the composition of the silicates. Band width and band areas are useful for determining metal abundance in olivine+metal and orthopyroxene+metal assemblages, respectively. Mafic silicate grain size variations are best determined using band depth criteria. Band centers are most useful for determining mafic silicate composition. Analysis of the reflectance spectrum of the S‐class asteroid (8) Flora on the basis of these spectral parameters indicates a surface composed of approximately 50 wt % metal, 40 wt % olivine (Fa = 35±10), 10 wt % orthopyroxene (Fs = 30−25+35), and perhaps a few weight percent of clinopyroxene. A substantial amount of the surface seems to consist of a fine‐grained (<45 μm size) component. Analysis of the reflectance spectrum of the A‐class asteroid (446) Aeternitas suggests a surface assemblage of approximately 35 wt % metal, 55 wt % olivine (Fa = 20±10), 7 wt % clinopyroxene (Fs<17%), and 3 wt % orthopyroxene (Fs<45). A substantial fine‐grained (<45 μm size) surface component is also indicated for

ISSN:0148-0227    

DOI:10.1029/JB095iB06p08323

年代:1990

数据来源: WILEY

摘要:

The NASA Magellan Venus Radar Mapper spacecraft was launched into an interplanetary transfer trajectory to Venus on May 4, 1989, and will be placed into orbit around Venus on August 10, 1990. The orbiter carries a 12‐cm‐wavelength, multimode radar system. In the synthetic aperture mode it is capable of imaging most of the Venus surface at a resolution of better than 300 m, approaching 120 m over more than half the planet. In the altimeter mode it will determine topographic relief to a vertical accuracy of better than 50 m averaged over a surface resolution cell approximately 10 km in diameter where the surface relief is not too extreme. In the radiometer mode the radar receiver can determine the surface radio emission brightness temperature with an absolute accuracy of 20 K, at a resolution of 2 K. Tracking of the orbiter's coherent radio telemetry transmitter will permit observations of small accelerations related to gravitational inhomogeneities in the planet's interior. Objectives of the mission include deducing the geological history of the surface and the geophysical state of the interior. Specific attention focuses on the origin, present distribution, and activity of four geological processes that modify the surface: (1) volcanic and tectonic; (2) impact; (3) erosional, depositional, and chemical; and (4) isostatic and convective. Data products resulting from the mission will be made available to members of the scientific community through NASA's Planetary Data Sys

ISSN:0148-0227    

DOI:10.1029/JB095iB06p08339

年代:1990

数据来源: WILEY

摘要:

The Ishtar Terra highland region of Venus contains mountain belts morphologically similar to terrestrial orogenic belts. Akna Montes and Freyja Montes are long, linear mountain belts with about 4 km of relief above the 3‐ to 4‐km‐high Lakshmi Planum. The Maxwell Montes mountain belt, however, is more rectangular in plan view and has relief of approximately 8 km. We have investigated the nature of this latter range using complementary radar images obtained by the Arecibo Observatory and the Venera 15/16 spacecraft, as well as allimetry data obtained by the Venera 15/16 and Pioneer Venus spacecraft. Geological and structural maps have been compiled on the basis of these data, the individual features and their stratigraphic relationships interpreted, and a model for the origin and evolution of Maxwell Montes is proposed. The dominant features of this mountain range are long, parallel ridges and valleys that are intepreted to be anticlines and synclines and to have resulted from ENE‐WSW oriented compression. Two major shear zones bound Maxwell Montes on the north and south, converging toward the west. Nine narrow linear parallel features several hundreds of kilometers long (identified as cross‐strike discontinuities, CSDs) cut obliquely across the strike of the ridges and valleys in a NW‐SE direction and disrupt the structural fabric of the mountain range, dividing Maxwell Montes into 10 crustal domains. Among several possible origins, we find that the data are most consistent with their interpretation as right‐lateral strike‐slip faults with offsets of up to 125 km. Retrodeformation and reconstruction of Maxwell Montes using the offsets determined for individual domains produces a long linear mountain range similar to Akna Montes. Using geologic unit maps and topographic maps of the present configuration of Maxwell, similar reconstructions were made; these reconstructions restored several linear tectonic elements, topographic trends, and sinuous unit boundaries to more contiguous positions. On the basis of these data and observations we suggest that Akna Montes may represent the initial form of compressional orogenic belts on Venus, while Maxwell Montes attained its present morphology through a second stage of deformation involving large‐scale strike‐slip faulting. We examine several models for this second stage and favor the interpretation that it occurred as the Akna Montes‐like proto‐Maxwell Montes was transported to the west between two converging shear zones, accompanied by rotation of the entire mountain range. Thirty percent shortening of the mountain range is implied by the geometries of the present and reconstructed Maxwell Montes. On the basis of the deforamation in Maxwell Montes and the evidence for large‐scale transport we conclude that large‐scale horizontal a movement of crustal materials is a significant aspec

ISSN:0148-0227    

DOI:10.1029/JB095iB06p08357

年代:1990

数据来源: WILEY

摘要:

In studies of the anisotropy of susceptibility or remanence of paleomagnetic samples it is conventional to specify the anisotropy in terms of the parameters of the anisotropy ellipsoids, namely the directions of the principal axes of the ellipsoid and their associated eigenvalues. Confidence intervals for these parameters have in the past often been estimated by using a linearization scheme to propagate the effect of small changes through the eigenvalue decomposition. The validity of these approximations is explored using a Monte‐Carlo simulation from measurements that are presumed normally distributed, showing that there are circumstances in which the linearization scheme gives confidence intervals that are much too small. Q‐Q plots indicate that the common assumption that the noise in the measurements is Gaussian does not always hold. Because of these shortcomings in the conventional technique we propose using a bootstrap resampling scheme to find empirically the distribution of uncertainties in the results. Confidence intervals for the eigenvalues are found directly from their empirical distributions. For the principal axes, approximate elliptical regions of confidence on the unit sphere are parameterized in terms of the Kent orFB5distribution. The number of modes observed in the distribution of eigenvalues obtained by bootstrapping is used to classify the shape of the susceptibility ellipsoid as spherical, oblate, prolate or triaxial. The empirical nature of the bootstrap technique allows the extension of the analysis of uncertainties to parameters derived from the principal susceptibilities, such as percentage anisotropy or shape fac

ISSN:0148-0227    

DOI:10.1029/JB095iB06p08383

年代:1990

数据来源: WILEY

摘要:

Paleolatitudes determined from paleomagnetic study of azimuthally unoriented sediment cores recovered by deep sea drilling on the Pacific plate have been previously used in determining reference poles for the Pacific plate apparent polar wander (APW) path. From discordant paleoinclinations and corresponding paleolatitudes determined from these data, some workers have inferred tectonic tilting or large horizontal motion of some deep‐sea drilling sites relative to other parts of the Pacific plate. Here the reliability of these paleolatitudes is evaluated by comparing them with paleolatitudes determined from a Pacific plate reference APW path determined from seamount poles, skewness and amplitudes of magnetic profiles across magnetically lineated seafloor, and paleocolatitudes from azimuthally unoriented basalt cores and from equatorial sediment facies. The reference APW path includes a new determination of the Early Cretaceous pole from several types of paleomagnetic data, which agree well with one another. Sediment paleolatitudes are found to be systematically less southerly than those predicted from the reference APW path, a discrepancy that corresponds to inclinations shallower than expected. Thus most deep‐sea drilling sediment paleomagnetic paleolatitudes are unreliable and should be omitted from the data used in determining reference APW paths. Inclination and paleolatitude discrepancies noted in prior studies may be due not to tectonic causes but to shallowly biased inclinati

ISSN:0148-0227    

DOI:10.1029/JB095iB06p08397

年代:1990

数据来源: WILEY

摘要:

Hole 504B of the Ocean Drilling Program is dedicated to the study of crustal structure and hydrothermal processes in 5.9‐m.y.‐old oceanic basement. Continuing the work of previous legs, hole 504B was extended 212.3 m to a total depth of 1562.3 m below seafloor (bsf) during leg 111 in 1986. Quartz‐sulfide veins occur at a depth of 1369–1388 m bsf in basalts of the sheeted dike complex. The ore minerals are predominantly pyrite, less chalcopyrite, rare Corich Cu‐Fe‐S phases, and a thiospinel (linnaeite/carrollite). Microprobe analyses yield a high Co content in zoned vein pyrites (>8 wt %) as well as in the Cu‐Fe‐S phases (>5 wt %). Up to 35.8 wt % Co was detected in the thiospinel. A Co/Ni ratio of>100 distinguishes the vein pyrite from pyrite in the basaltic wall rock and from pyrite formed as an alteration product of olivine (Co/Ni<5). The Co/Ni ratios correlate positively with Cu and negatively with As. Co‐rich, nonstoichiometric Cu‐Fe‐S sulfides in chalcopyrite are interpreted as metastable phases which have been quenched at a high temperature and prohibited from exsolution of the stable products chalcopyrite and pyrite. Fluid inclusions in quartz from the quartz‐sulfide veins are two‐phase and vary from liquid‐ to vapor‐dominated. Their salinities range from 4.2 to 7.2 wt % equivalent NaCl and average 5.5 wt %. Pressure (360 bars) corrected average filling temperatures vary from 271° to 408°C with a maximum of 486°C. This is consistent with calculated quartz formation temperatures for a single quartz separate (+4.2‰ δ18O) using oxygen isotope thermometry. The δ18O value of the hydrothermal fluid was determined to be +1.7‰. The temperature data indicate fluid alteration of the sheeted dikes at about 350° to 500°C. The maximum homogenization temperatures intersect the liquid/vapor two‐phase boundary above the critical point of seawater. Thus phase separation could have occurred before or during the formation of the mineralized veins and t

ISSN:0148-0227    

DOI:10.1029/JB095iB06p08405

年代:1990

数据来源: WILEY

摘要:

The Mendana Fracture Zone and the Trujillo Trough are two major intra‐oceanic features on the Nazca Plate off Peru. During the SEAPERC cruise of the R/VJean Charcotin 1986, Sea Beam bathymetric data, single‐channel seismic records, geomagnetic and gravity measurements, and heat flow data have been obtained over these two areas. In this paper, we confirm that the Mendana Fracture Zone is actively opening perpendicular to its trend, resulting in the formation of new oceanic crust since about 3.5 Ma. We calculate that this new rift is propagating westward along the fracture zone at a velocity of about 10 cm yr−1(or 2 cm yr−1with respect to the trench). This extensional feature appears to be genetically linked with the Trujillo Trough, located 200 km to the north. This trough is bounded to the east by a left: lateral transpressive fault which became active probably at the time when the Mendana Rift started. We then discuss a possible mechanism to explain the origin of these two areas of intraoceanic defo

ISSN:0148-0227    

DOI:10.1029/JB095iB06p08419

年代:1990

数据来源: WILEY

摘要:

The northern Funeral Mountains, southeastern California, comprise a metamorphic core complex that lies within a region of extreme Neogene extension but did not experience a significant late Cenozoic thermal event. This circumstance, unusual among core complexes of the North American Cordillera, permits direct examination of the preextensional thermal evolution of a portion of the Cordilleran hinterland. For pelitic schists from the highest‐grade portions of the Funeral metamorphic core, thermobarometry and thermodynamic modeling of garnet zonation define a P‐T trajectory showing: (1) attainment of “peak” metamorphic conditions at 800–850 K and 800–1000 MPa (30–37 km depths); followed by (2) 400 to 600 MPa of decompression (15–22 km of exhumation) with no substantial change in temperature. Available geochronologic data indicate that peak metamorphism occurred in Early Cretaceous time and that the decompression path developed over the Early to Late Cretaceous interval. The maximum pressures indicated by the petrologic data require substantial Late Jurassic(?)‐Early Cretaceous tectonic burial even though most recognized thrust faults at this latitude that have large stratigraphie throws are assumed or demonstrated to have early Mesozoic ages. We postulate that post‐Early Cretaceous extensional faults may have excised the necessary middle Mesozoic thrust structures. While the majority of extensional structures responsible for this excisement is likely to be of Neogene age, associated with Basin and Range extension, the Cretaceous decompression path described in this paper is similar to the theoretical P‐T paths derived from numerical modeling of extensional unroofing. Given recent evidence for the development of extensional structures in compressional regimes like the Himalayan and Alpine orogens, it seems prudent to search for evidence of Mesozoic extensional structures in future studies of the hinterland of the Nort

ISSN:0148-0227    

DOI:10.1029/JB095iB06p08437

年代:1990

数据来源: WILEY

摘要:

Rates of change of magnetic susceptibility during leaching can characterize the presence of certain common types of magnetic minerals in metamorphic rocks. In this study the results of leaching are confirmed by mineral separation, optical microscopy, scanning electron microscopy (SEM) and microchemical analysis under SEM. Leaching curves provide a simple, rapid way of determining the relative roles of oxides, sulfides, and silicates in carrying the susceptibility in metamorphic rocks.

ISSN:0148-0227    

DOI:10.1029/JB095iB06p08447

年代:1990

数据来源: WILEY

摘要:

Single‐crystal laser fusion40Ar/39Ar analyses and several conventional bulk fusion40K‐40Ar dates have been used to determine the age of volcaniclastic strata within the Olorgesailie Formation and of associated volcanic and sedimentary units of the southern Kenya rift. In the principal exposures along the southern edge of the Legemunge Plain, the formation spans the interval from approximately 500 to 1000 ka. Deposition continued to the east along the Ol Keju Nyiro river where a tuff near the top of the formation has been dated at 215 ka. In these exposures, the formation is unconformably overlain by sediments dated at 49 ka. A possible source for the Olorgesailie tephra, the Ol Doinyo Nyokie volcanic complex, contains an ash flow dated at ∼1 Ma, extending the known age range of this complex to encompass that of virtually the entire Olorgesailie Formation in the Legemunge Plain. These geologic examples illustrate the importance of the single‐crystal40Ar/39Ar dating technique whereby contaminant, altered, or otherwise aberrant grains can be identified and eliminated from the determination of eruptive ages for reworked or altered pyroclastic deposits. We have presented a computer‐modeling procedure based on an inverse‐isochron analysis that promotes a more objective approach to trimming40Ar/39Ar isotope data sets o

ISSN:0148-0227    

DOI:10.1029/JB095iB06p08453

年代:1990

数据来源: WILEY