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- Islam, Md. Tariqul, 1980, et al.
(författare)
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Continuous subsidence in the Thingvellir rift graben, Iceland: Geodetic observations since 1967 compared to rheological models of plate spreading
- 2016
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Ingår i: Journal of Geophysical Research - Solid Earth. - 0148-0227 .- 2156-2202. ; 121:1, s. 321-338
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Tidskriftsartikel (refereegranskat)abstract
- North America-Eurasia relative plate motion across the Mid-Atlantic Ridge in south Iceland is partitioned between overlapping ridge segments, the Western Volcanic Zone (WVZ) and the Eastern Volcanic Zone. The Thingvellir graben, a 4.7km wide graben, lies along the central axis of the WVZ and has subsided >35m during the Holocene. An ~8km long leveling profile across the graben indicates a subsidence rate of ~1mmyr−1 from 1990 to 2007, relative to the first (westernmost) benchmark. Modeled GPS velocities from 1994 to 2003 estimate a spreading rate of 6.7±0.5mmyr−1 or 35% of the full plate motion rate and up to 6.0mmyr−1 subsidence. The combined geodetic observations show that the deformation zone is 10 times wider than the graben width. We utilize these geodetic observations to test the effects of ridge thermal structure on the kinematics across divergent boundaries. We apply a nonlinear rheology, thermomechanical model implemented in a finite element model. A 700°C isotherm is applied for the brittle to ductile transition in the crust, representing a dry olivine rheology. We adjust the depth of this isotherm to solve for the best fit model. The best fit model indicates that the 700°C isotherm is at 8km depth below the ridge axis, which results in an average thermal gradient of 87.5°Ckm−1 in the upper crust. The thermomechanical model predicts a subsidence rate of 4mmyr−1, comparable to our geodetic observations.
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- Islam, Md. Tariqul, 1980, et al.
(författare)
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Rheological responses to plate boundary deformation at the Eastern Volcanic Zone in Iceland
- 2017
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Ingår i: Tectonophysics. - : Elsevier BV. - 0040-1951. ; 717, s. 16-26
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Tidskriftsartikel (refereegranskat)abstract
- Located on the mid-Atlantic ridge, Iceland allows for direct measurement of crustal deformation. Global Positioning System (GPS) data from the Eastern Volcanic Zone (EVZ), Iceland, and crustal deformation of the rift near its southern end at 64°N show a spreading rate of 13.8 ± 1.8 mm yr− 1. About 90% of the deformation occurs in an 80 to 90-km wide zone. To understand how the rheology of the lithosphere influences rifting, we applied a thermo-mechanical stretching model that includes thermal states in Iceland using temperature- and stress-dependent wet and dry olivine rheology. We attempt to reproduce the thermal structure of a rift by defining 700 °C from 5- to 15-km depth at the rift axis that leads to variation in rheological structure, and to estimate the layer (from surface to a depth of 700 °C) where the elastic deformation of the lithosphere is the greatest. At a fixed spreading rate, the deformation field is controlled by the sub-surface thermal state. The vertical subsidence rate at the ridge axis increases almost linearly as the half-velocity increases. The best fitted model suggests a thermal gradient of ~ 54 °C km− 1 at depth below where 700 °C occurs at the ridge axis. The models have little sensitivity to the wet or dry olivine rheology. Estimated viscosity is ~ 1E19 Pa s at 20-km depth at the ridge axis and ~ 1E18 Pa s up to 100-km depth in the model. The spreading rate influences the tangential (non-linearity) shape of the deformation field, and a change in spreading rate affects the deformation field the most. After spreading velocity, the model's second most sensitive parameter is the location of the 700 °C at the rift axis. The thermomechanical model confirms that the rheological responses at the central part of the rift zone in the EVZ, Iceland caused of plate spreading is nonlinear, comparable with surface deformation observed by GPS measurement.
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- Islam, Md. Tariqul, 1980, et al.
(författare)
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Temperature-Dependent Newtonian Rheology in Advection-Convection Geodynamical Model for Plate Spreading in Eastern Volcanic Zone, Iceland
- 2015
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Ingår i: Journal of Geoscience and Environment Protection. - : Scientific Research Publishing, Inc.. - 2327-4336 .- 2327-4344. ; 3, s. 14-26
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Tidskriftsartikel (refereegranskat)abstract
- Geodynamic process as advection-convection of the Mid-Atlantic Ocean Ridge (MAR), that is exposed on land in Iceland is investigated. Advection is considered for the plate spreading velocity. Geodetic GPS data during 2000-2010 is used to estimate plate spreading velocity along a profile in the Eastern Volcanic Zone (EVZ), Iceland striking N102˚E, approximately parallel to the NUVEL-1A spreading direction between the Eurasian and North American plates. To predict subsurface mass flow patterns, temperature-dependent Newtonian rheology is considered in the finite-element models (FEM). All models are considered 2-D with steady-state, incompressible rheology whose viscosity depends on the subsurface temperature distribution. The thickness of lithosphere along the profile in the EVZ is identified by 700˚C isotherm and 1022 Pa s iso-viscosity, those reach 50 ± 3 km depth at distance of 100 km from rift axis. Geodetic observation and model prediction results show the ~90% of spreading is accommodated within ~45 km of the rift axis in each direction. Model predicts ~8.5 mm∙yr−1 subsidence at the surface of rift center when magmatic plumbing is inactive. The rift center (the highest magmatic influx is ~11 mm∙yr−1) in model shifts ~10 - 20 km west to generate observed style surface deformation. The spreading velocity, isotherm and depth of isotherm are the driving forces resulting in the surface deformation. These three parameters have more or less equal weight. However, as the center of deformation in the EVZ shifts and most of the subsidence takes place in the volcanic system that is currently the active which is the located of plate axis.
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