Geoscience Reference
In-Depth Information
300 m, dried up about 10 000 years ago. As a result of the drying of this lake,
the ground is now domed: the centre of the old lake has risen 65 m relative to the
margins. This doming is a result of the isostatic adjustment which took place after
the water load had been removed. Estimates of the viscosity of the asthenosphere
which would permit this amount of uplift to occur in the time available range from
10
20
10
19
Pasfor a 75-km-thick
asthenosphere. Postglacial uplift of a small region of northeastern Greenland and
of some of the Arctic islands indicates that the lower value is a better estimate.
This low-viscosity channel appears to correspond to the low-velocity zone in the
upper mantle (Fig. 8.1), but, since the geographic distribution of the viscosity
determination is very limited, the correlation is speculative.
Wider and much more extensive loads must be used to determine the viscosity
of the upper and lower mantle. The last Fennoscandian (Finland plus Scandinavia)
ice sheet, which melted some 10 000 years ago, was centred on the Gulf of
Bothnia. It covered an area of approximately 4
Pa s for a 250-km-thick asthenosphere to 4
×
10
6
km
2
×
with a maximum
average thickness of
2.5 km. The maximum present-day rate of uplift, which
is more than 0.9 cm yr
−
1
,isoccurring in the Gulf of Bothnia close to the centre
of the ancient ice sheet (Fig. 5.19). Figure 5.20 shows the uplift calculated to
occur in the northern Gulf of Bothnia for a model with an asthenosphere 75 km
thick, of viscosity 4
∼
10
19
Pa s, overlying a mantle of viscosity 10
21
Pa s. This
predicted uplift is in good agreement with observations. It is predicted that 30 m
of uplift remains. When a load is removed suddenly, rupture can occur. As the last
remnants of the Fennoscandian ice sheet were lost, very large earthquakes (
M
w
up to 8.2) occurred, with fault lengths up to 160 km and average displacements
up to 15 m. Breaks of this size probably ruptured the whole crust (Fig. 10.2).
Determination of the viscosity of the lower mantle requires loads of very
large extent. The Wisconsin ice sheet, formed during the most recent Pleistocene
glaciation in North America, was perhaps 3.5 km thick and covered much of
Canada as well as part of the northern U.S.A. Melting of the ice resulted in rebound
of the continent but also loading of the oceans. Thus, melting of an extensive ice
sheet provides both loading and unloading data for study. Figure 5.21 illustrates
the calculated uplift at various times after removal of a model Wisconsin ice sheet
that had attained isostatic equilibrium on an elastic lithosphere plate underlain
by a constant-viscosity mantle. Uplift is greatest at the centre of the load, and
the rate of uplift decreases with time as expected. Regions peripheral to the load
also undergo initial uplift before undergoing subsidence. Such uplift, followed by
subsidence, is documented in ancient sea-level changes along the east coast of the
U.S.A. (The melting of the ice sheet immediately causes sea level to rise, but the
oceanic plate also deforms in response to the increased water load, thus resulting
in a further change in sea level.) The uplift documented in Canada (present-
day rates around Hudson Bay are about 1 cm yr
−
1
) and the sea-level changes
along the east coast of North America are all in reasonable agreement with a
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