Two major types of passive margins are recognized, i.e. volcanic and non-volcanic, without proposing distinctive mechanisms for their formation. Volcanic passive margins are associated with the extrusion and intrusion of large volumes of magma, predominantly mafic, and represent distinctive features of Larges Igneous Provinces, in which regional fissural volcanism predates localized syn-magmatic break-up of the lithosphere. In contrast with non-volcanic margins, continentward-dipping detachment faults accommodate crustal necking at both conjugate volcanic margins. These faults root on a two-layer deformed ductile crust that appears to be partly of igneous nature. This lower crust is exhumed up to the bottom of the syn-extension extrusives at the outer parts of the margin. Our numerical modelling suggests that strengthening of deep continental crust during early magmatic stages provokes a divergent flow of the ductile lithosphere away from a central continental block, which becomes thinner with time due to the flow-induced mechanical erosion acting at its base. Crustal-scale faults dipping continentward are rooted over this flowing material, thus isolating micro-continents within the future oceanic domain. Pure-shear type deformation affects the bulk lithosphere at VPMs until continental breakup, and the geometry of the margin is closely related to the dynamics of an active and melting mantle.
This paper deals with problems relating to ground‐water movement in aquifers under water‐table conditions, and more particularly with the unsteady flow caused by variations in some boundary condition or in the replenishment from the ground surface. The cases to be treated are diagrammatically shown; the first refers to the flow between two reservoirs, and the second to the flow between a water divide and a reservoir.
The treatment will be based on the so‐called Laplace transformation method, which is useful for solving some typical cases, often met in hydrology and various fields of engineering practice. Definite formulas are established, which include the effect of a moderately sloping bottom, and the inclination of the ground‐water level is also taken into consideration. Separate formulas are given for the more simple case that the bottom of the aquifer is horizontal.
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