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Numerical modelling of rise and fall of a dense layer in salt diapirs
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences.
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences.
2008 (English)In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 172, no 2, 798-816 p.Article in journal (Refereed) Published
Abstract [en]

Numerical models are used to study the entrainment of a dense anhydrite layer by a diapir. The anhydrite layer is initially horizontally embedded within a viscous salt layer. The diapir is down-built by aggradation of non-Newtonian sediments (n = 4, constant temperature) placed on the top of the salt layer. Several parameters (sedimentation rate, salt viscosity, perturbation width and stratigraphic position of the anhydrite layer) are studied systematically to understand their role in governing the entrainment of the anhydrite layer. High sedimentation rates during the early stages of the diapir evolution bury the initial perturbation and, thus, no diapir forms. The anhydrite layer sinks within the buried salt layer. For the same sedimentation rate, increasing viscosity of the salt layer decreases the rise rate of the diapir and reduces the amount (volume) of the anhydrite layer transported into the diapir. Model results show that viscous salt is capable of carrying separate blocks of the anhydrite layer to relatively higher stratigraphic levels. Varying the width of the initial perturbation (in our calculations 400-800 m), from which a diapir triggers, shows that wider diapirs can more easily entrain an embedded anhydrite layer than the narrower diapirs. The anhydrite layer is entrained as long as rise rate of the diapir exceeds the descent rate of the denser anhydrite layer. We conclude that the four parameters mentioned above govern the ability of a salt diapir to entrain an embedded dense layer. However, the model results show that the entrained blocks inevitably sink back if the rise rate of the diapir is less than the rate of descent of the anhydrite layer or the diapir is permanently covered by a stiff overburden in case of high sedimentation rates.

Place, publisher, year, edition, pages
2008. Vol. 172, no 2, 798-816 p.
Keyword [en]
numerical solutions, geomechanics, sedimentary basin processes, dynamics : gravity and tectonics, diapir and diapirism, mechanics, theory, and modelling
National Category
Earth and Related Environmental Sciences
Identifiers
URN: urn:nbn:se:uu:diva-97535DOI: 10.1111/j.1365-246X.2007.03661.xISI: 000252433400024OAI: oai:DiVA.org:uu-97535DiVA: diva2:172521
Available from: 2008-09-03 Created: 2008-09-03 Last updated: 2013-04-08Bibliographically approved
In thesis
1. Modeling internal deformation of salt structures targeted for radioactive waste disposal
Open this publication in new window or tab >>Modeling internal deformation of salt structures targeted for radioactive waste disposal
2008 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis uses results of systematic numerical models to argue that externally inactive salt structures, which are potential targets for radioactive waste disposal, might be internally active due to the presence of dense layers or blocks within a salt layer.

The three papers that support this thesis use the Gorleben salt diapir (NW Germany), which was targeted as a future final repository for high-grade radioactive waste, as a general guideline.

The first two papers present systematic studies of the parameters that control the development of a salt diapir and how it entrains a dense anhydrite layer. Results from these numerical models show that the entrainment of a dense anhydrite layer within a salt diapir depends on four parameters: sedimentation rate, viscosity of salt, perturbation width and the stratigraphic location of the dense layer. The combined effect of these four parameters, which has a direct impact on the rate of salt supply (volume/area of the salt that is supplied to the diapir with time), shape a diapir and the mode of entrainment. Salt diapirs down-built with sedimentary units of high viscosity can potentially grow with an embedded anhydrite layer and deplete their source layer (salt supply ceases). However, when salt supply decreases dramatically or ceases entirely, the entrained anhydrite layer/segments start to sink within the diapir. In inactive diapirs, sinking of the entrained anhydrite layer is inevitable and strongly depends on the rheology of the salt, which is in direct contact with the anhydrite layer. During the post-depositional stage, if the effective viscosity of salt falls below the threshold value of around 1018-1019 Pa s, the mobility of anhydrite blocks might influence any repository within the diapir. However, the internal deformation of the salt diapir by the descending blocks decreases with increase in effective viscosity of salt.

The results presented in this thesis suggest that it is highly likely that salt structures where dense and viscous layer/blocks are present undergo an internal deformation processes when these dense blocks start sinking within the diapir. Depending on size and orientation of these blocks, deformation pattern is significantly different within the diapir. Furthermore, model results applied to the Gorleben diapir show that the rate of descent of the entrained anhydrite blocks differs on different sides of the diapir. This suggests that if the anhydrite blocks descent within the Gorleben diapir, they initiate an asymmetric internal flow within it.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2008. 48 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 551Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 551
Keyword
salt, diapir, anhydrite, deformation, entrainment, numerical modeling, rheology, sinking, Gorleben
Identifiers
urn:nbn:se:uu:diva-9279 (URN)978-91-554-7281-8 (ISBN)
Public defence
2008-09-30, Axel Hambergsalen, Geocentrum, Villavägen 16, Uppsala, 10:00
Opponent
Supervisors
Available from: 2008-09-03 Created: 2008-09-03Bibliographically approved

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