摘要:

Large igneous provinces (LIPs) are a continuum of voluminous iron and magnesium rich rock emplacements which include continental flood basalts and associated intrusive rocks, volcanic passive margins, oceanic plateaus, submarine ridges, seamount groups, and ocean basin flood basalts. Such provinces do not originate at “normal” seafloor spreading centers. We compile all known in situ LIPs younger than 250 Ma and analyze dimensions, crustal structures, ages, and emplacement rates of representatives of the three major LIP categories: Ontong Java and Kerguelen‐Broken Ridge oceanic plateaus, North Atlantic volcanic passive margins, and Deccan and Columbia River continental flood basalts. Crustal thicknesses range from 20 to 40 km, and the lower crust is characterized by high (7.0–7.6 km s−1) compressional wave velocities. Volumes and emplacement rates derived for the two giant oceanic plateaus, Ontong Java and Kerguelen, reveal short‐lived pulses of increased global production; Ontong Java's rate of emplacement may have exceeded the contemporaneous global production rate of the entire mid‐ocean ridge system. The major part of the North Atlantic volcanic province lies offshore and demonstrates that volcanic passive margins belong in the global LIP inventory. Deep crustal intrusive companions to continental flood volcanism represent volumetrically significant contributions to the crust. We envision a complex mantle circulation which must account for a variety of LIP sizes, the largest originating in the lower mantle and smaller ones developing in the upper mantle. This circulation coexists with convection associated with plate tectonics, a complicated thermal structure, and at least four distinct geochemical/isotopic reservoirs. LIPs episodically alter ocean basin, continental margin, and continental geometries and affect the chemistry and physics of the oceans and atmosphere with enormous potential environmental impact. Despite the importance of LIPs in studies of mantle dynamics and global environment, scarce age and deep crustal data necessitate intensified efforts in seismic imaging and scientific drilling in a range of

ISSN:8755-1209    

DOI:10.1029/93RG02508

年代:1994

数据来源: WILEY

摘要:

The continental lithosphere responds to stress by deforming as a generally layered medium. Deep seismic reflection data, coupled with a variety of ancillary geological and geophysical data, are interpreted to provide images of fault zones that tend to form moderately dipping ramp structures in mechanically rigid layers, and flat detachments in mechanically weak layers. This geometry is similar to ramp and flat structures observed at smaller scales in thrust and fold belts and leads to the interpretation that most orogens are underlain by orogen‐scale décollements in a manner that is analogous to so‐called “thin skin” deformation in sedimentary rocks. Décollements may occur within the crust (for example, near the base of a sedimentary section or in the middle crust), near the Moho, in the subcrustal lithosphere, or in the asthenosphere. Even intracratonic basement‐cored (“thick skin”) uplifts that occasionally occur in foreland regions such as the Wyoming province are probably large (crustal) scale versions of ramp/flat features observed in supracrustal rocks and are thus likely caused by the same fundamental tec

ISSN:8755-1209    

DOI:10.1029/93RG02515

年代:1994

数据来源: WILEY

摘要:

Geomagnetic polarity reversal remains one of Nature's most enigmatic phenomena. Dynamo theory admits solutions in pairs with reversed magnetic fieldsBand −B, but detailed calculations are required to understand how the field can change sign. Theory also admits separate solutions with different symmetry across the equatorial plane, the symmetric (ES) “quadrupole” and the antisymmetric (EA) “dipole” solutions, which may be important in the reversal process and which offer a simple framework for interpreting small paleomagnetic data sets. Ordinary secular variation leads to very large changes in the magnetic field over several centuries and could easily develop into full reversal; the theory of secular variation, which is relatively well developed, may therefore help in understanding reversals. Other clues to geomagnetic reversals come from the Sun, whose magnetic field reverses every 11 years. Paleomagnetic data show the Earth’s magnetic field reverses every million years or so, with each transition taking about a thousand years, during which the intensity may fall by as much as 1 order of magnitude. Reversal frequency undergoes a modulation on the long timescale (107years) of mantle convection, and there have been two long intervals in the past with no reversals. Such behavior is typical of a highly nonlinear dynamical system, but the very long timescale of changes in reversal frequency, and its close proximity to the overturn time of mantle convection, suggests some control of the dynamo by the mantle. Short‐term phenomena, such as change in the length of day and secular variation, have been studied extensively for evidence of core‐mantle interactions, and we may draw on this body of evidence in order to understand long‐term effects. Three physical mechanisms have been proposed: topographic, electromagnetic, and thermal, with the last two being most significant for long‐term effects. Symmetries allow the dynamo to generate anEAfield, with the major component a dipole, but lateral variations on the core‐mantle boundary may lead to magnetic fields with no symmetry, reflecting the structure of the boundary anomalies. Changes in reversal frequency on the mantle convection timescale could arise either from changes in total heat flux from the core to the mantle or from instabilities associated with lateral variations at the core‐mantle boundary. Neither mechanism is well understood, but the former involves significant changes to the Earth's overall heat budget, whereas the latter must always arise as a natural consequence of deep‐mantle convection. Recent measurements of transition fields show pole paths that lie close to two preferred longitudes near 90°W and 90°E; if substantiated, the result would provide the first definitive evidence of long‐term mantle control of the geomagnetic field. Further evidence suggests that the geomagnetic pole during stable polarity also lies along these two longitudes and that magnetic flux at the core surface tends to concentrate along the same longitudes, as does the present field. A simple theory is proposed relating changes in the core field to apparent transition paths measured at the Earth's surface. The model shows that longitude confinement of the transition paths can occur for quite complicated core fields and that surface intensities can drop by 1 order of magnitude, on average, simply because of the reduction in length scale of the transitional field. Simple transition paths may be an indication of some organization of flux at the core surface but not of large‐scale

ISSN:8755-1209    

DOI:10.1029/93RG02602

年代:1994

数据来源: WILEY

摘要:

Transport of participate material across continental shelves is well demonstrated by the distributions on the seabed and in the water column of geological, chemical, or biological components, whose sources are found farther landward or farther seaward. This paper addresses passive (incapable of swimming) particles and their transport across (not necessarily off) continental shelves during high stands of sea level. Among the general factors that influence across‐shelf transport are shelf geometry, latitudinal constraints, and the timescale of interest. Research studies have investigated the physical mechanisms of transport and have made quantitative estimates of mass flux across continental shelves. Important mechanisms include wind‐driven flows, internal waves, wave‐orbital flows, infragravity phenomena, buoyant plumes, and surf zone processes. Most particulate transport occurs in the portion of the water column closest to the seabed. Therefore physical processes are effective where and when they influence the bottom boundary layer, causing shear stresses sufficient to erode and transport particulate material. Biological and geological processes at the seabed play important roles within the boundary layer. The coupling of hydrodynamic forces from currents and surface gravity waves has a particularly strong influence on across‐shelf transport; during storm events, the combined effect can transport particles tens of kilometers seaward. Several important mechanisms can cause bidirectional (seaward and landward) transport, and estimates of the net flux are difficult to obtain. Also, measurements of across‐shelf transport are made difficult by the dominance of along‐shelf transport. Geological parameters are often the best indicators of net across‐shelf transport integrated over time scales longer than a month. For example, fluvially discharged particles with distinct composition commonly accumulate in the midshelf region. Across‐shelf transport of particulate material has important implications for basic and applied oceanographic research (e.g., dispersal of planktonic larvae and particle‐reactive pollutants). Continued research is needed to understand the salient mechanisms and to monitor them over a ra

ISSN:8755-1209    

DOI:10.1029/93RG02603

年代:1994

数据来源: WILEY