page_title: Sedimentary isostasy
section_title: Cyclicity mechanism underpinning
hierarchical_section_title: Sedimentary isostasy / Cyclicity mechanism underpinning
caption_attribution_description: English: Water reaches supercritical phase at temperature and pressure in the crust reached at approximately 10 km. This water phase is an extremely effective solvent.
context_page_description: Isostasy is a model that describes how upper lithospheric buoyancy adjusts to any significant change in overburden pressure; it responds to significant additional masses of ice or sediment by subsiding. This equilibrium buoyancy model is supported by a hypothetical mobile magma that exists at depths not yet penetrated by the drilling rig.
  Sedimentary isostasy is a related model that addresses how space is created below base level to accommodate additional influxes of sediment. This model assumes that relative sea level remained stable for extensive periods of geological time, despite the continental crust's level changing intermittently throughout the Phanerozoic. The significance of the model is that it can help explain complex geological events observed in sedimentary basins across the globe. In basin analysis it provides a viable alternative to the Sequence stratigraphy model, which implies that sea levels frequently rose above present sea-level and that the crust independently oscillated up and down throughout geological time. It also provides firm theoretical support for the emerging precepts of tectonostratigraphy.
context_section_description: The structure of the Viking Graben and adjacent areas was established from seismic and well data during the late 20th century. 
   Its relationship with the Scandian high level peneplain was established only in 2018 when Japsen et al. described evidence on the Hardangervidda Mountains in southern Norway that helped validate Herschel's model.
  Herschel's letter apparently influenced Charles Lyell's Principles of Geology Book frontispiece (5th edition 1837). This shows rocktype sequences in a stable sedimentary basin. Below about 3,000 metres of porous water- and hydrocarbon-saturated sedimentary rocks are a further 2,000 metres of sediment with porosity mostly filled by diagenetic cement. Beyond that level, sediments are metamorphosed by temperature and pressure to form quartzites, schists and marbles with ground water absorbed as crystal components. Deeper still, rocks reach a stage of ultrametamorphism into gneiss and migmatite, a contorted metamorphic rock containing a granitic partings.  Finally,
   the combination of temperature and pressure exceeds the phase transition threshold of supercritical water,gas   which assists in the production of an extremely mobile granitic melt(granulite) at a depth of 10 to 15 kilometres.  The current chronosome model assumes that the thickness of this fluid zone is limited by a very precise combination of physical and chemical factors to form a thin layer, implied by the amount of intermittent subsidence to be less than 100m. thick. This zone likely coincides with the Conrad discontinuity. Above that level, any reduction in temperature and pressure causes water to drop below its supercritical phase threshold and minerals in the granulite crystallize in sequence. This occurs not far above the Conrad level because granulite rarely reaches the surface as rhyolite lava; if it were to reach the surface, it could cause a catastrophic eruption.
  At the Conrad Discontinuity migmatite encounters a process of anatexis and differentiation, the melting and gravity separation of the basic heavy minerals, which sink to form a restite of immiscible basic magma. The separate granulite residue has its mobility further enhanced as a consequence, and overburden pressure causes it to seek a lateral passage away from the centre of the basin. As the granulite's initial volume is expelled laterally, the entire basin sinks into the space vacated by magma, thus placing the next level of transitional migmatite directly on top of the restite, where it begins to melt and perpetuate cyclical processes throughout the basin.
  There is usually an ancient craton on at least one side of an extensive sedimentary basin. When the lateral impetus of granite magma reaches a craton boundary it splits the craton horizontally at Conrad level and raises or tilts a fault block upwards to form a new horst feature. Successive magma injections beneath it raise the horst block further by stages and related uplift and erosion of the block contribute a further influx of sediment to form a new chronosome in the basin.
caption: Phase diagram of water related to temperature and pressure in the crust
exif: {}