摘要:

The boundaries of three major plates (Africa, India, and Antarctica) meet in a triple junction in the Indian Ocean near 25°S, 70°E. Using observed bathymetry and magnetic anomalies, we locate the junction to within 5 km and show that it is a ridge‐ridge‐ridge type. Relative plate motion is N60°E at 50 mm/yr (full rate) across the Central Indian Ridge, N47°E at 60 mm/yr across the Southeast Indian Ridge, and N3°W at 15 mm/yr across the Southwest Indian Ridge; the observed velocity triangle is closed. Poles of instantaneous relative plate motion are determined for all plate pairs. The data in the South Atlantic and Indian oceans are consistent with a rigid African plate without significant internal deformation. Two of the ridges at the triple junction are normal midocean spreading centers with well‐defined median valleys. The Southwest Indian Ridge, however, has a peculiar morphology near the triple junction, that of an elongate triangular deep, with the triple junction at its apex. The floor of the deep represents crust formed at the Southwest Indian Ridge, and the morphology is a consequence of the evolution of the triple junction and is similar to that at the Galapagos Triple Junction. Though one cannot determine with precision the stability conditions at the triple junction, the development of the junction over the last 10 m.y. can be mapped, and the topographic expressions of the triple junction traces may be detected on the th

ISSN:0148-0227    

DOI:10.1029/JB085iB09p04723

年代:1980

数据来源: WILEY

摘要:

The Eocene Sitkalidak Formation comprises the youngest and seaward‐most deep‐sea sequence exposed in the Kodiak Islands, which border the eastern Aleutian Trench. Fold orientation, structural style, and stereographic projections of poles to bedding define two structural units in the Sitkalidak Formation, which are geometrically distinct from overlying non‐marine and marine shelf rocks of Oligocene and Miocene age. Landward vergence, intense deformation with stratal disruption, a relatively seaward structural position and sedimentologic considerations all argue that the more deformed portion of the Sitkalidak Formation constitutes an obductively offscraped, trenchfilling fan sequence. Conversely, regular folding, stratal continuity, and relatively landward structural position suggest that the less deformed portion of the Sitkalidak Formation accumulated and was deformed as a slope basin or slope apron deposit. In the more deformed portion of the Sitkalidak Formation, zones of stratal disruption resemble melange and are demonstrably of tectonic origin as indicated by elongated, slickensided boudins with microscopic shear and solution zones. The Sitkalidak Formation was rapidly deformed, uplifted, and unconformably overlain by the Oligocene non‐marine Sitkinak Formation. Zeolite facies metamorphism of the Sitkalidak Formation followed, as a consequence of burial during Oligocene and Miocene time. Vitrinite reflectance and porosity data respectively define a peak temperature of 100–125°C and a maximum burial of 2.4–3.9 km, suggesting a paleogeothermal gradient of about 30°C/km during

ISSN:0148-0227    

DOI:10.1029/JB085iB09p04741

年代:1980

数据来源: WILEY

摘要:

A new approach to the topographic correction for terrestrial heat flow measurements is presented. The approach features calculation of a Fourier series fit to the surface temperature‐surface elevation data where the surface temperatures are based on a model that includes surface temperature variations due to microclimate variations. The mathematics of the terrain correction problem are similar to the upward (away from source) continuation problem in gravity and magnetics so several solutions, in addition to the Fourier series approach, are available in the literature that allow an accurate calculation of the correction provided the surface boundary condition is properly specified. However, the usual boundary condition applied, a linear relation between ground surface temperature and elevation, is shown to be inadequate for drill holes in the depth range 30–200 m no matter how low the topographic relief. Thus a model of ground surface temperature is developed that includes the effects of elevation, slope orientation, and slope angle. Because of the effects of microclimate, the classical models that have isothermal surfaces that generally parallel the topographic surface are significantly in error in many cases, and the patterns of isotherms near the topographic surface are more complicated than was previously recognized. This complexity causes gradient variations with depth in 30‐ to 200‐m holes that have not been previously recognized as being related to topographic effects. Because the temperature effects of slope orientation and inclination do not scale with respect to the magnitude of the relief, significant terrain corrections may be required even in areas of relatively low relief. The application of the technique is illustrated by application to a line dipole hill and a group of drill holes near Wilbur, Washington. In addition, several examples of two‐dimensional terrain effects and one example of three‐dimensional terrain effects are illustrated for topographic sections in the northwestern United States. In the United States, most ‘anomalous’ gradients in the upper 100–200 m of drill holes in impermeable rocks can be explained by a combination of topographic and microclimatic effects, without resorting to temporal climatic changes or unknown types of water effects. The depth of the holes necessary for reliable heat flow measurements in such settings is a signal to noise problem where the noise is the effect at depth of the microclimatically related surface temperature variations, coupled with the topographic effect, and the signal is a temperature increase at any depth due to the background geothermal gradient. Typically, the noise has decreased to a few degrees centigrade per kilometer within the depth range 100–200 m. Thus the general conclusion has been that these depths of holes are required for reliable heat flow values. In fact, when linear temperature‐depth data are observed in shallower holes or when appropriate corrections are made, reliable measure ments in impermeable rocks may be consistently made in holes

ISSN:0148-0227    

DOI:10.1029/JB085iB09p04757

年代:1980

数据来源: WILEY

摘要:

Seismic reflection surveys carried out by the Consortium for Continental Reflection Profiling (COCORP) in the Rio Grande Rift near Socorro, New Mexico, have successfully mapped large‐scale structural variations down to the base of the crust. A total of 155 km of 24‐fold common reflection point coverage was obtained parallel and transverse to major rift structure, including a continuous east‐west profile which spans the central rift from the Sierra Lucero on the west (at the edge of the Colorado Plateau) to the Manzano Mountains on the east. Stacked seismic reflection sections derived from these surveys indicate coherent reflected energy from depths as great as 35 km, with substantial evidence of structural complexity extending throughout the crust. Among the major features apparent on these sections are (1) large‐scale relief on the floor of the Albuquerque Basin, including a major intragraben horst standing over 3 km above the surrounding deep basin floors, and an extensive, shallow structural bench beneath the southeast portion of the basin, (2) well‐developed, antithetic Tertiary normal faulting on the east side of the rift, (3) marked lateral and vertical variation in intrabasement reflection character, including relatively long correlatable events, distinct bands of short, discontinuous reflections, and seismically transparent zones characterized by few, if any, reflections, (4) a complex band of reflections from depths appropriate for the Moho, and (5) unusually strong, coherent reflections from a depth of approximately 20 km, corresponding closely with the top of an active magma body previously postulated to exist beneath the rift in this area on the basis of microearthquake studies. The magma body reflections are among the most prominent deep events yet recorded by COCORP; their geometry and relationship to surrounding structural elements provide important constraints on the depth of brittle faulting and new insight on magma migration and accumulation. Extensional faulting appears to be primarily near vertical near the surface with no clear evidence of large scale listric faulting at depth. The eastern rift boundary at Abo Pass is marked by a high‐angle planar zone devoid of coherent reflections, possibly representing intense structural disruption extending through the crust. In contrast, the seismic section across the western rift boundary is defined by a gently dipping, relatively simple reflection of uncertain origin, with little evidence of a throughgoing fault in the lower crust. The seismic characteristics of many of the intrabasement features suggest particular igneous and/or metamorphic origins and support models which emphasize lateral and vertical heterogenei

ISSN:0148-0227    

DOI:10.1029/JB085iB09p04773

年代:1980

数据来源: WILEY

摘要:

Some inverse problems are characterized by a model consisting of a piecewise continuous function and a set of discrete parameters. For linear problems of this general type, which we call mixed, we show that when the number of datadis greater than the number of parametersp, it is always possible to construct a set of a leastd‐pequations that are independent of the values of the discrete part of the model. These equations, which we call the annulled data set, can be used to estimate the continuous part of the model. The discrete part of the model can be estimated from a second set ofpequations that relate the discrete and continuous parts of the model. The linearization of the nonlinear travel time functional that enter in the hypocenter location problem leads to a mixed inverse problem. The splitting procedure is natural to this problem if the hypocenters are estimated initially by conventional nonlinear least squares by using travel times calculated from some initial estimate of the velocity model. The annulled data are a set of linear combinations of the residuals that are unbiased by that initial location, and as a result, they can be used directly to estimate a perturbation to the velocity model by a Backus‐Gilbert procedure. This makes an iterative algorithm possible that consists of a conventional hypocenter location followed by estimating a perturbation of the velocity model from the annulled data set. The uniqueness of the final velocity model is assessed via the linear resolution analysis of Backus and Gilbert (1968, 1970). We also construct a set of Frechet derivatives that relate perturbations of each hypocenter component to perturbations of the velocity model. These kernels are used to assess the possible error of the hypocenters due to inadequate knowledge of the velocity structure by an application of the generalized prediction approach of Backus (1970a). Good results are obtained when the procedure is applied to a simple synthetic data

ISSN:0148-0227    

DOI:10.1029/JB085iB09p04801

年代:1980

数据来源: WILEY

摘要:

Large‐aperture seismic array (LASA) subarray amplitude anomalies are investigated using 395 medium sized events, distributed in ten azimuthally divided sectors. Although LASA magnitudes, when averaged over all azimuths, are only slightly biased in relation to National Earthquake Information Services (NEIS) magnitudes, the amount of bias varies with azimuth and subarray, suggesting that a simple station correction for mbbias is not adequate. In addition, fluctuations among LASA subarrays are about 0.15 in standard deviation even when these magnitudes are calibrated in sectors. Details of such fluctuations are explained in part by local crustal and upper mantle heterogeneities under LASA. The amplitude anomalies are linearly related to the amount of travel time anomalies in each sector, implying that both effects are due to crustal focusing. Using a fixed effects model, the authors attempt to separate the cause of mbbias into sector (azimuth) effect, subarray effect, and subarray‐sector interaction. However, even this detailed modeling could not, with confidence, explain mbbias. Using the reciprocity principle, even with a well‐calibrated station, a factor of 2 in uncertainty results for predictions of a stations's amplitude for an event only 50 km away from a calibration event in a region as complex as LASA. The event magnitude uncertainty would probably be reduced by network averaging, or by a nonstatistical detailed crust and mantle structure which could be analyzed by ray tracing to remove receiver ef

ISSN:0148-0227    

DOI:10.1029/JB085iB09p04811

年代:1980

数据来源: WILEY

摘要:

The Gissar‐Kokshal fault zone south of the Tien Shan Mountains in central Asia is one of the most active areas of intraplate seismicity in the world. Since 1949 four earthquakes greater than magnitude 6.5 have occurred along the section of this fault north of the Pamir Mountains, almost completely filling a seismic gap which had existed for at least 80 years. The two most recent of these earthquakes, the 1974 Markansu (Ms= 7.4) and 1978 Zaalai (Ms= 6.7) events, were each preceded by a short period of increased activity in the future epicentral area some years prior to the main shock. The largest earthquake within this active period occurred at the same location as the future large shock. A segment adjacent to the 1978 earthquake, occupying the only unbroken portion of the former gap, has recently shown a similar burst of activity, suggesting that it may be the site of the next large earthquake of this sequence. Large premonitory earthquakes are also observed near the sites of seven other earthquakes with magnitudes 7.0 to 8.3 in the Tien Shan since 1880. In all of the cases studied, the premonitory time interval and magnitude of the premonitory earthquake increase with the size of the main shoc

ISSN:0148-0227    

DOI:10.1029/JB085iB09p04829

年代:1980

数据来源: WILEY

摘要:

The uppermost identifiable head waves from the vicinity of the seafloor andgroup velocities developed from multiples of critical reflections are utilized to contour sound velocities from near the floor of the Barents Sea, an area with divergent velocity structures. Where the low‐velocity sediments are 30 to 80 m thick and cap relatively high‐velocity materials, the seafloor sound velocities determined from critical reflections are strongly dependent on frequency. Multiple reflections from thick‐layered sediments, without major velocity discontinuities, are not affected by passbands that vary from 15–40 up to 100–200 Hz. Certain sonobuoy records are compared with wave equation solutions at fluid/solid and solid/solid interfaces modeled from the Barents Sea data. Results indicate that the presence of quite thin sediments (∼30 m), not otherwise detectable, may be required to account for the behavior ofpwaves from apparently high‐velocity seafloors. Under the fairly restrictive conditions of a fluid/solid interface, shear wave velocities can be estimated from the pattern of reflecte

ISSN:0148-0227    

DOI:10.1029/JB085iB09p04838

年代:1980

数据来源: WILEY

摘要:

Marine seismic refraction data taken in and near Prudhoe Bay, Alaska, are presented. The higher compressional sound wave velocity in ice‐bonded materials as compared to unbonded materials was used to locate areas of subsea permafrost and depths to the top of the permafrost. These data are compared favorably with similar information from available drilling data. A map is presented which shows the extent of permafrost within 40 m of the ocean surface along approximately 40 km of the Beaufort Sea coast. The permafrost surface is shown to vary irregularly in depth without regard to the distance from shore. Large areas without shallow permafrost are attributed to possible paleo‐river‐valleys and to the lack of clays which may have been removed in these va

ISSN:0148-0227    

DOI:10.1029/JB085iB09p04845

年代:1980

数据来源: WILEY

摘要:

Microseisms (seismic surface waves) in the frequency range of 0.1–1 Hz recorded at Palisades, New York, are compared with microbaroms (infrasound) in the same frequency range recorded simultaneously. Both are thought to be generated by interfering ocean waves, and the question addressed is how often does their comparison imply that they have the same source. Compared were their approximate regions of formation, the formation conditions (both meteorological and oceanographic), amplitude variations, and frequency spectra. It is concluded that for the signals recorded at this location the two phenomena appear to have the same ocean wave sources 2/3 of the times and at least some of the remaining times one of the signals is prevented from arriving owing to propagation difficultie

ISSN:0148-0227    

DOI:10.1029/JB085iB09p04854

年代:1980

数据来源: WILEY