by Willy Fjeldskaar IRIS Modelling technique Glacial isostasy

by Willy Fjeldskaar IRIS

ü Modelling technique Glacial isostasy Iceload data Calibration data ü Development 2006

Glacial isostasy The earth’s crust may…be considered as a slowly flexible sheet of solid rock floating on a viscous substratum Nansen, 1928

Model Lithosphere Asthenosphere Upper mantle 670 km A layered viscous Earth with an elastic, uniformly thin lithosphere (Fjeldskaar & Cathles, 1991) Lower mantle

load subsidence (m) thickness (m) Isostatic response distance (km)

Lithosphere as lowpass filter

Decomposition of ice load

Load removal 20 000 BP Ice load I(t, k) Difference between two timesteps 15 000 BP

Ice extent and thickness during the last 20 000 years The glaciation rate from one time step to the next is assumed constant

Equilibrium displacement Nadai, 1950

Transient displacement Relaxation time The Exponential Decay of Beer Foam

Relaxation time wavelengths Filtered relaxation time Relaxation time is the time required for a function to decrease to 1/e (36. 8%) of the equilibrium value.

Relaxation time (40 x 1023 Nm; 70 km) 4000 km 400 km Order no k = 2 pr/l – 1/2

Uplift history

1) present rate of uplift 2) palaeo shoreline tilt

The Earth's response to the deglaciation in Fennoscandia is modelled using a layered viscous model with elastic lithosphere. “The most likely ice model gives a flexural rigidity of 1023 Nm (te = 20 km) at the Norwegian coast, increasing to more than 1024 Nm (te = 50 km) in central parts of Fennoscandia” (Fjeldskaar, 1997) (Fjeldskaar & Cathles, 1991)

Viscosity vs. thickness 140 120 100 80 60 40 0 1 2 3 19 4 5 Viscosity (10 Pa s) A uniform mantle viscosity of 1021 Pa s. 6 7

Best-fit model Observed uplift

Modelling uplift of Svalbard

Sea level changes Storøya Wilhelmøya Kongsøya Hopen Bjørnøya

Sea level changes Storøya Wilhelmøya Kongsøya Hopen

Svalbard rheology The post-glacial shoreline displacement on Svalbard indicates a high viscosity mantle A flexural rigidity of 2 x 1023 Nm (te = 25 km) and a uniform mantle viscosity of 1021 Pa s

Crustal thickness

Lateral uniform: F(kx, ky, t) = e-t a(kx, ky)/t a(kx, ky)-1 a(kx, ky) = 1 + D (kx, ky) k 4/rg Lateral varying: F(kx, ky, x, y, t) = e-t a(kx, ky, x, y )/t a(kx, ky, x, y)-1 a(kx, ky, x, y) = 1 + D(x, y) k 4/rg

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