Glacial isostasy and long-term crustal movements in Fennoscandia with respect to lithospheric and asthenospheric processes and properties—reply

DISCUSSION

445

Glacial isostasy and long-term crustal movements in Fennoscandia with respect to lithospheric and asthenospheric processes and properties-reply Nils-Axe1 Mijrner Palmgeophysics

& Geodynnm~cs, S-106 91 Stockholm, Sweden

(Received by publisher December 10. 1990)

Wolf (this issue) has raised some comments on my 1990 paper (Morner,

1990),

where I partly

compare results from observations (Miirner, 1979, 1980, 1981) with the FENNOLORA profile as analyzed by Guggisberg (1986) and partly present novel interpretations for the linear uplift factor. Wolf’s comments concern the comparisons between surface observations of crustal movements and the structures of the deep seismic profile. From the surface paleoshoreline data it was possible to establish

observational the following

(Momer, 1979, 1980): (1) That the total absolute uplift was about 830 m (not 330 m as previously held as the maximum uplift). (2) That the uplift was composed by two different factors, one exponentially decaying factor of typical glacial isostatic characteristics (that died out 4500 yrs ago), and a linear factor that is responsible

for the present uplift (and is associ-

ated with quite different rheological parameters). (3) That the mass in the forebulge and appearance of mass in the uplift cone is a 1: 1 ratio and where the disappearance of mass in the forebulge and appearance of mass in the uplift cone records a horizontal mass flow in a low-viscosity asthenosphere channel. (4) That the asthenospheric viscosity was 1.5 X 10” Poise using a viscosity/strain rate diagram and 1.3 - 1.9 x 10” Poise using a relaxation model. (5) That the present linear uplift is consistent with a viscosity of the order of 1 - 2 X 1O22 Poise (and strain rates two orders of magnitudes smaller

than for the glacial isostatic exponentially

decay

factor). (6) That the flexural rigidity seemed significantly larger than usually assumed. (7) That similarity between the geometry of the exponential uplift factor and the present linear uplift factor was not a solid argument for a similar origin because this geometry is also seen in much older uplift and subsidence events, suggesting that this geometric similarity is an effect of the shape of the lithosphere (or crust). Therefore, it was interesting to note, with respect to the deep seismic profile (Guggisberg, 1986), the following: (1) That there really does exist an asthenospheric channel (cf. point (3) above). (2) That the lithospheric

shape shows a deep

downwarping under the Fennoscandian

Shield (cf.

point (7) above). (3) That the lithospheric thickness, if converted to flexural rigidity just on the basis of thickness, corresponds to 1O25 - 1O26 Nm (cf. point (6) above). (4) That the base of the asthenospheric channel is not downwarped (although the lithosphere is), suggesting that the compensation of the subsidence of the lithosphere (like the glacial isostatic downwarping) took place by horizontal flow in this channel (cf. point (3) above). These agreements are both interesting and significant. They may be taken either as support of my model of glacial isostasy or at the very least as showing the absence of negative data from the deep seismic profile.

has now been more or less completely on Lidtn’s

(1938) shore-level

LidCn’s curve eustasy, (B) 600

is good but must

separated

factors,

ice-covered

into exponential

and period

500 m of uplift Observations

extended

revsed,

displacement

or

curve

be corrected

for

(A) and linear

backwards

into

the

by 3700 yrs and an additional

(see Fig. 1). must

guide the models.

not vise-

versa.

Fig. 1. bfi: Lidbn’s original shore-level displecement curve corrected for eustasy (C). Right: The same curve separated into exponential (A) and linear (B) factors and extended back to the onset of uplift (3700 yrs before the freamelting of the area), indicating that numerical treatments of curve C alone cannot provide realistic figures of rheological parameters.

The uplift process is a “complicated function of several parameters” says Wolf. I fully agree. Still, I think it is the numerical modelists that tend to oversimplify the picture. Another basic factor is to select the best data and to treat them correctly. Many classical calculations are based on Sauramo’s (e.g., 1955) shoreline diagram, which

Guggisberg, B.Ch., 1986. Eine zweidimensionale refraktionsseismische Interpretation der Geschwindigkeits-TiefenStruktur des oberen Erdmantels under dem Fennoskandischen Shield (Project FENNOLORA). Ph-D. Thesis, ETH Ziirich. Lid&n, R., 1938. Den senkvart&ra strandfUrskjut~s firrlopp och kronologi i bgermanland. Geol. F&en. Stockholm F&h., 60: 397-404. Miirner, N.-A., 1979. The Fennoscandian uplift and Late Cenozoic geodynamics: geological evidence. C&Journal, 3(3): 287-318. Mamer, N.-A., 1980. The Fennoscandian uplift: geological data and their geodynamical implication. In: N.-A. Miimer (Editor), Earth Rheology, Isostasy and Eustasy. Wiley, p. 251-284. Mdmer, N.-A., 1981. Crustal movements and geodynamics in Fennoscandia. Tectonophysics, 71: 241-251. Miirner, N.-A., 1990. Glacial isostasy and long-term crustal movements in Fennoscandia with respect to lithospheric and asthenosphetic processes and properties. Tectonophysics, 176: 13-24. Sauramo, M., 1955. Land uplift with hinge-lines in Fennoscandia. Ann. Acad. Sci. Fennicae, AIII, 44: l-25.