摘要:

After 1958, when scientific satellites began exploring the Earth magnetic environment, many puzzling phenomena could be directly examined, especially the polar aurora and disturbances of the Earth's magnetic field [seeStern, 1989a]. The notion of the solar wind, also introduced in 1958, helped clarify the role of the Sun in driving such phenomena. The large‐scale structure of the magnetosphere, the space region dominated by the Earth's magnetic field, was gradually revealed within the next decade: its trapped particles, its boundary, and its long magnetic tail on the nightside. Inevitably, however, at a more fundamental level, the new discoveries led to new questions about the transfer of energy, the flow patterns of plasmas and electric currents, the acceleration of the aurora, and transient events such as magnetic substorms and storms, which energized ions and electrons. Though significant progress has occurred in some of these areas, many unresolved issues still remain. This review outlines the history of magnetospheric research, draws some general conclusions, and provides an extensive bibliograph

ISSN:8755-1209    

DOI:10.1029/95RG03508

年代:1996

数据来源: WILEY

摘要:

Landslides are common on inclined areas of the seafloor, particularly in environments where weak geologic materials such as rapidly deposited, fine‐grained sediment or fractured rock are subjected to strong environmental stresses such as earthquakes, large storm waves, and high internal pore pressures. Submarine landslides can involve huge amounts of material and can move great distances: slide volumes as large as 20,000 km³ and runout distances in excess of 140 km have been reported. They occur at locations where the downslope component of stress exceeds the resisting stress, causing movement along one or several concave to planar rupture surfaces. Some recent slides that originated nearshore and retrogressed back across the shoreline were conspicuous by their direct impact on human life and activities. Most known slides, however, occurred far from land in prehistoric time and were discovered by noting distinct to subtle characteristics, such as headwall scarps and displaced sediment or rock masses, on acoustic‐reflection profiles and side‐scan sonar images. Submarine landslides can be analyzed using the same mechanics principles as are used for occurrences on land. However, some loading mechanisms are unique, for example, storm waves, and some, such as earthquakes, can have greater impact. The potential for limited‐deformation landslides to transform into sediment flows that can travel exceedingly long distances is related to the density of the slope‐forming material and the amount of shear strength that is lost when the s

ISSN:8755-1209    

DOI:10.1029/95RG03287

年代:1996

数据来源: WILEY

摘要:

Mathematical modeling of coupled groundwater flow, heat transfer, and chemical mass transport at the sedimentary basin scale has been increasingly used by Earth scientists studying a wide range of geologic processes including the formation of excess pore pressures, infiltration‐driven metamorphism, heat flow anomalies, nuclear waste isolation, hydrothermal ore genesis, sediment diagenesis, basin tectonics, and petroleum generation and migration. These models have provided important insights into the rates and pathways of groundwater migration through basins, the relative importance of different driving mechanisms for fluid flow, and the nature of coupling between the hydraulic, thermal, chemical, and stress regimes. The mathematical descriptions of basin transport processes, the analytical and numerical solution methods employed, and the application of modeling to sedimentary basins around the world are the subject of this review paper. The special considerations made to represent coupled transport processes at the basin scale are emphasized. Future modeling efforts will probably utilize three‐dimensional descriptions of transport processes, incorporate greater information regarding natural geological heterogeneity, further explore coupled processes, and involve greater field applicati

ISSN:8755-1209    

DOI:10.1029/95RG03286

年代:1996

数据来源: WILEY

摘要:

The Mariana convergent plate margin system of the western Pacific provides opportunities for studying the tectonic and geochemical processes of intraoceanic plate subduction without the added complexities of continental geology. The system's relative geologic simplicity and the well‐exposed sections of lithosphere in each of its tectonic provinces permit in situ examination of processes critical to understanding subduction tectonics. Its general history provides analogs to ancient convergent margin terranes exposed on land and helps to explain the chemical mass balance in convergent plate margins. The Mariana convergent margin's long history of sequential formation of volcanic arcs and extensional back arc basins has created a series of volcanic arcs at the eastern edge of the Philippine Sea plate. The trenchward edge of the overriding plate has a relatively sparse sediment cover. Rocks outcropping on the trench's inner slope are typical of the early formed suprasubduction zone's lithosphere and have been subjected to various processes related to its tectonic history. Pervasive forearc faulting has exposed crust and upper mantle lithosphere. Many large serpentinized peridotite seamounts are within 100 km of the trench axis. From these we can learn the history of regional metamorphism and observe and sample active venting of slab fluids. Ocean drilling recovered suprasubduction zone lava sequences erupted since the Eocene that suggest that the forearc region remains volcanologically dynamic. Seismic studies and seafloor mapping show evidence of deformation throughout forearc evolution. Large portions of uplifted southern forearc are exposed at the larger islands. Active volcanoes at the base of the eastern boundary fault of the Mariana Trough vary in size and composition along strike and record regional differences in source composition. Their locations along strike of the arc are controlled in part by cross‐arc structures that also facilitate formation of submarine volcano chains extending from the base of the fault westward into the back arc basin. The western boundary is the West Mariana Ridge, the western portion of the volcanic arc active prior to formation of the Mariana Trough. The trough evolved in a two‐stage extension process of rifting and subsequent seafloor spreading. The back arc basin varies along strike from rifted arc lithosphere with scattered volcanoes but no real spreading center in the north to a complex mid‐ocean‐type spreading center south of 20°N. The change from initial rifting to true seafloor spreading is also evident across the Mariana Trough from rifted topography near the West Mariana Ridge to spreading ridges in the central to eastern basin south of 20°N. This morphologic change indicates an early stage of extension with basin‐and‐range‐type topography predominant and volcanism restricted to fissure eruptions along fault block boundaries. The spreading ridges and abyssal hill morphology evolved later as new lithosphere was generated at elongate volcanic ridges located in the center of rift valleys. The center of extension intersects the active volcanic front differently at either end of the Mariana Trough. In the north, extension is by rifting of arc lithosphere where it intersects the arc. In the south a major strike‐slip fault extends from the trench axis across the forearc, through the volcanic arc, and into the back arc basin. Arc magmas apparently leak along this fault zone into the forearc and the back arc spreading center. The complexity of interrelated tectonism and magmatism in this convergent margin is daunting, but studies of arc systems such as this provide the best hope of interpreting many of the exposed terranes accreted to continents. Comparison of subaerial terranes with recent studies of intraoceanic convergent margins will add to our understanding of plate interactions and of the evolution of the volcanic arcs and extensional back arc basins generated withi

ISSN:8755-1209    

DOI:10.1029/95RG03476

年代:1996

数据来源: WILEY