摘要:

Recent progress in the application of satellite altimetry from Seasat and GEOS 3 to the observation of the oceanic mesoscale variability and general circulation is reviewed. The lack of accurate geoid models has been the major obstacle in the study of the general ocean circulation from altimetry. The use of geoid‐independent methods that utilize the temporal differences in altimetric measurements taken at fixed locations, however, has made significant contributions to our knowledge of the mesoscale variability of the ocean. The mesoscale energies of the sea surface height and geostrophic current have been mapped on a global basis. Their distributions in wave number space have also been analyzed. Because of many of the deficiencies of existing altimeter data (short duration, inadequate orbit, poor accuracy, etc.) most of these results describe only a small portion of the frequency‐wave number spectrum of the variability, but they have nonetheless demonstrated the great value of an optimally designed altimetric mission in advancing our knowledge of the global mesoscale variability. The current technology allows satellite altimetry to detect oceanic variability at periods from a few days to 3–5 years, and wavelengths from 50 to 10,000 km. Determining the time‐averaged general ocean circulation from altimetry is more problematic because an accurate geoid is indispensable. The currently available global geoid models have useful accuracies only at wavelengths greater than about 7000 km. There have been several attempts at mapping the global ocean circulation at those scales using existing altimeter data and geoids. When these results are compared with hydrographic surveys, some qualitative agreement can be observed, but the quantitative differences are mostly inconclusive because of the geoid and orbit errors. It has been suggested, however, that an altimetric mission that is optimally designed with the current technology, when complemented by a state‐of‐the‐art gravimetric mission to map the earth's gravity field, is able to determine the ocean circulation quantitatively at scales from the ocean basin to

ISSN:8755-1209    

DOI:10.1029/RG021i008p01657

年代:1983

数据来源: WILEY

摘要:

The Ries Crater, an impact structure of 26 km diameter in south Germany, is the largest terrestrial crater where substantial amounts of ejecta are preserved, on occasion>100 m deep. Further, the target stratigraphy is well known, and it is possible to relate specific clasts and breccia lithologies to initial target depth. As a consequence the continuous deposits of the Ries, also known as Bunte Breccia, may be studied with exceptional field control. We report field observations and laboratory analyses obtained from 560‐m core materials, taken at nine different locations that range from 16 to 37 km in radial distance from the impact center. The objective is to relate the Ries observations to ejection, and to emplacement processes of large‐scale, planetary crater deposits. The observations regarding the modal‐stratigraphic characteristics of the Bunte Breccia may be summarized as follows: only1‐cm clasts (29.1%) and as highly comminuted, fine‐grained “matrix” (10 GPa constitute<0.1% (weight) of the entire deposit, which indicates that Bunte Breccia was emplaced at essentially ambient temperatures. When possible, the above observations are quantified via linear regressions throughout the text. All of these observations are consistent with, if not predicted by, a ballistic emplacement scenario as postulated by Oberbeck and co‐workers: primary crater ejecta are expelled ballistically and will form secondary craters in the local substrate; a mixture of primary and secondary ejecta results and combines into a highly turbulent, ground‐hugging debris surge as the final phase of ejecta emplacement. Total emplacement time for the Bunte Breccia (⪖200 km³) is estimated to be of the order of 5 min only. These findings are compared with cratering theory relating to a number of ejecta thickness decay models and with the so‐called Z model, addressing material flow during various stages of crater formation. Qualitative to fair agreement of observations and predictions results. An initial crater radius of 6.5 km, an excavation depth of 1650 m, an excavation volume of 136 km³, and an associated transient cavity volume of aproximately 230 km³ appear to be reasonable estimates. Approximately 170 km³ of material was involved in slumping and restoration of the transient cavity for the above radius and Z=2.7. The modal composition of Ries ejecta with regard to preimpact target stratigraphy indicates that materials contained in the continuous deposits of large, complex planetary craters are predominantly derived from depths as small as one‐hundredth the apparent crater diameter. A number of implications are addressed regarding remote sensing of planetary surfaces and investigatio

ISSN:8755-1209    

DOI:10.1029/RG021i008p01667

年代:1983

数据来源: WILEY

摘要:

This paper presents a critique of the models proposed to explain the formation of back arc basins: those marginal basins located behind active or inactive trench systems and whose origin is inferred to be subduction related. Several marginal basins are not generically back arc, but rather are related to the initiation of subduction, continental rifting, or plate boundary readjustments. Models of back arc basin formation must be consistent with the observed characteristics of back arc basins, which include the following: (1) the crustal structure and magnetic lineation patterns of back arc and major ocean basins are similar, implying that the processes of crustal accretion in both tectonic environments are also similar; (2) arc‐trench systems without currently spreading back arc basins are in the majority; (3) although petrographically similar to and within the compositional range of midocean ridge basalts (MORB), back arc basin basalts (BABB) show consistent geochemical differences from N‐type MORB and are in many respects transitional toward island arc tholeiites (IAT); (4) distinct BABB and IAT mantle sources may be in extremely close proximity during the initial rifting of an island arc; (5) rifting of the volcanic arc may occur on either side of the line of active volcanoes (±50 km) and may vary from one side to the other along strike; (6) a remnant arc is not always developed; (7) back arc spreading may be initiated above subducted lithosphere, but with time the center of spreading migrates away from the arc and no longer overlies a Benioff zone; and (8) there is no simple correlation between the timing of global plate reorganizations and the formation of back arc basins. The three main classes of models proposed to explain back arc basin formation are mantle diapirism, induced aesthenospheric convection, and global plate kinematics. Because the driving mechanism for the first two classes of models is provided by the local subducting slab, these models fail to explain the temporal and spatial distribution of back arc basins. The third class of models proposes that back arc basins should form whenever global plate interactions require divergence between the overriding plate and the trench line, the latter being observed to move only in a seaward direction with respect to a hot spot reference frame. However, the tectonic setting of several back arc basins suggests that they represent more than just a passive response to kinematic boundary conditions. For example, some dynamic back arc force seems required to move the New Hebrides, Tonga, and South Sandwich forearc plates seaward. Neither the local nor the global models adequately specify the necessary and/or sufficient conditions for back arc basin formation. A better understanding of the processes associated with the initiation of rifting and of subduction is required. Arc rifting and back arc spreading are both secondary phenomena to subduction, but they are also primary extensional phenomena, fundamentally analogous to continental rifting and the initial spreading of major ocean ba

ISSN:8755-1209    

DOI:10.1029/RG021i008p01727

年代:1983

数据来源: WILEY

摘要:

The climate system is driven, primarily, by energy absorbed at the surface. Surface albedo sensitivity is incorporated into all types of climate models, and changes can lead to large feedback effects. For example, alterations in the extent and/or state of the cryosphere and large‐scale modification of vegetation cause significant perturbations in climate model results. The specification of surface albedo in general circulation climate models (GCM's) differs. An improved and agreed surface albedo data set is urgently required for climate modeling. It is likely that the most appropriate means of achieving consistent and credible surface albedos is by using well‐designed satellite surveillance to augment global inventories of soils and vegetation. However, retrieval of surface albedo values for all sky and surface conditions from satellite observations is difficult. Atmospheric distortion is especially hard to remove. Some of the sensitivity of GCM's to surface albedo values may be the result of inadequate parameterization of other climatic components. The accuracy of information demanded by climate modelers could be reduced and made more consistent. Recommendations are made for the implementation of a new global scale observational program with the aim of providing surface albedo data at an accuracy of ±0.05 within 5–10 years. Immediate initiation is

ISSN:8755-1209    

DOI:10.1029/RG021i008p01743

年代:1983

数据来源: WILEY

摘要:

Geologists have known for years that many allochthonous bodies in the form of accreted blocks and microcontinents are present in the Alpine‐Himilaya Mountain belt. In the past a few allochthonous bodies have also been identified in the Circum Pacific Mountain belts. More recently, however, the extent of these bodies and their large numbers around the Pacific (Figure 1) have been appreciated. Moreover, it is only in the last few years that the significance of these accreted bodies has emerge

ISSN:8755-1209    

DOI:10.1029/RG021i008p01779

年代:1983

数据来源: WILEY

ISSN:8755-1209    

DOI:10.1029/RG021i008p01787

年代:1983

数据来源: WILEY

ISSN:8755-1209    

DOI:10.1029/RG021i008p01789

年代:1983

数据来源: WILEY