Continuous subsidence in the Thingvellir rift graben, Iceland: Geodetic observations since 1967 compared to rheological models of plate spreading

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 4.7 km wide graben, lies along the central axis of the WVZ and has subsided >35 m during the Holocene. An ~8 km long leveling profile across the graben indicates a subsidence rate of ~1 mm yr^-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 ± 0.5 mm yr^-1 or 35% of the full plate motion rate and up to 6.0 mm yr^-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 8 km depth below the ridge axis, which results in an average thermal gradient of 87.5°C km^-1 in the upper crust. The thermomechanical model predicts a subsidence rate of 4 mm yr^-1, comparable to our geodetic observations.