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A one-dimensional model of fluid pressure and porosity evolution is used to investigate the physical processes that control the development and maintenance of overpressure in a compacting sedimentary basin. We show that for shale-dominated sequences the variation of the hydraulic diffusivity in both space and time is such that it produces a minimum between 2 and 4 km depth, consistent with observations from the Gulf Coast basin. This minimum inhibits the upward flow of fluid by acting as a “bottleneck” and thus determines the shallowest position of the depth to the top of overpressure. Above this region of bottleneck, overpressure does not develop because the porosity is sufficiently large to maintain high values of hydraulic diffusivity that are conducive to the rapid dissipation of excess fluid pressure. Within the overpressured shales, compaction propagates downward through the section, releasing fluids from the upper part of the section while continuing to restrain the upward flow of fluids from deeper within the section. As such, overpressures are predicted to be maintained within the deeper regions of a basin for tens to hundreds of millions of years. Further, fluid viscosity plays an important role in defining the depth behavior of hydraulic diffusivity as a function of time. Assuming a temperature-dependent fluid viscosity guarantees that the hydraulic diffusivity minimum will always exist during the development of the basin. On the basis of our results, we find that the depth at which the porosity equals 14±4% correlates with the depth to the local hydraulic diffusivity minimum and thus the depth to the top of overpressure. Moreover, we interpret that the 14±4% represents the threshold porosity for which a shale actually begins to act as a seal. Within the Gulf Coast basin, the gross sediment facies consists of lower massive shales across which deltaic systems have prograded allowing the deposition of an alternating series of sandstones and shales that grade vertically into massive sandstones. The massive sandstones are highly permeable and are connected hydrologically to the surface. We conclude that these sandstones play little role in the development of overpressure because of their high permeability except to the extent that the base of the massive sandstones marks the minimum depth possible for the top of overpressure. In contrast, overpressuring is observed to develop within either the shale-dominated sequence or the region of interspersed/interfingering sands and clays. The clay-encompassed sands play only a passive role in the development and maintenance of overpressure because it is the low-permeability clays that control the movement of fluids into and out of the sands.

The frequency-magnitude distribution (b value) in the upper plane of the deep seismic zone beneath Tohoku, Japan, shows significant variation with depth which we attribute to dewatering of the oceanic crust during the subduction process. High b value from the surface to 40 km in the upper seismic plane and overlying mantle wedge is thought to be caused by the advection of free water from the oceanic crust. The fact that the b value drops to 1.0 is attributed to the passing of the hydrous oceanic crust through a dehydration front. The resulting increase in pore pressure lowers the effective stress and increases the b value in the upper plane. Beneath the volcanic line, b drops back to

Magnetic model studies, rock magnetization, and major element chemical analyses show that unusually large amplitude magnetic anomalies found along the easternmost segment of the Southeast Indian Ridge [zone A] are caused by the high intensities of magnetization of basalts intruded at the ridge axis. In turn, these high magnetizations are caused by a threefold increase in weight percent of titanomagnetites from normal midocean ridge basalts intruded in the west [zones B and C] to ferrobasalts intruded in the high magnetic anomaly zone to the east. The Fe-Ti increase in the opaques reflects a major elemental enrichment in Fe and Ti caused by extensive shallow fractionation of plagioclase, clinopyroxene, and olivine. This shallow fractionation appears related to unusually cold magma crystallization temperatures, perhaps caused by proximity to a postulated downwelling zone in the asthenosphere south of Australia. However, examination of these correlations in greater detail points to several ambiguities: although the entire region is deep, and therefore the lithosphere is integrally colder than normal plates, zone A is shallower than zones B and C, but geothermometrically, its axial magmas are colder. We do not know how cold magmas directly relate to ferrobasalt generation. Also, plumelike geochemical signatures are also seen in some of the analyses. It appears that plume mantle sources are mixing with MORB sources and then undergoing extensive shallow fractional crystallization in zone A.