ABSTRACTS / RESUMES - Comitato Glaciologico Italiano

preservation of these remnants is primarily a function of
preglacial relief, and not as much a function of climate and
glacial configuration.
Till sheets and till patches occur in a pronounced pattern
across the mountain range. Generally, till sheets are extensive
east of the mountain range, and their thickness and
extent taper out westwards and approach zero at the
mountain range elevation axis and in the western part of
the mountain range (Norway). This pattern reflects the basal
thermal zonation and is complementary to the pattern
of glacial erosion. Where glacial erosion rates were high (at
low elevations and in the western sections of the mountain
range), till deposition was limited in space and time. Where
glacial erosion rates were low (intermediate elevations in
the eastern part of the mountain range, and east thereof),
till deposition dominated. Where the mountain ice sheets
and Fennoscandian ice sheets were frozen to their substrates
(highest elevations in the mountains), erosion and deposition
were negligible.
The setting of the Transantarctic Mountains is similar to
that of the Scandinavian mountains during the ice ages. In
both regions, ice sheets expanded over an inland depression
(Wilkes and Pensacola basins, Gulf of Bothnia) and
expanded through, or overtopped, mountain ranges along
the coast. In contrast, evidence for glacial erosion and till
deposition is present throughout the 4000 m of relief in
the Transantarctic Mountains. At the highest elevations
(along the mountain range elevation axis), consolidated tills
mapped as the Sirius Group crop out (e.g.) Stroeven,
1997). Some of these deposits are of local alpine glacier
origin. However, most of these deposits are considered to
be of East Antarctic Ice Sheet origin te.g., Webb & alii,
1984).
We identify the following problems in the East Antarctic
Ice Sheet interpretation of the high-elevation Sirius Group.
For example, at Mount Feather, Dry Valleys, Transantarctic
Mountains, the Sirius Group rests on an interfluve separating
a >1000 m ice free relief from the Ferrar Glacier
trough of >1000 m relief. However, an interfluve in a highrelief
landscape is the least likely location for till deposition.
Also, given the present relief, we exclude the possibility
that the East Antarctic Ice Sheet deposited the Mt
Feather Sirius Group (and tills in similar morphological
positions).
We identify another problem, which concerns an assumed
late Pliocene age of deposition (e.g.) Webb & alii, 1984).
We regard it as impossible that the high-elevation Sirius
Group on Mt Feather is of Middle Miocene or younger
age. This is because the >1000 m topography surrounding
the Sirius Group was in existence in the Late Pliocene based
on (i) 4°Ar/39Ar dated in situ ashes in the ice free valleys
of Early Pliocene through Middle Miocene age (e.g.) Marchant
& alii, 1993), and (ii) the Oligocene-to-present age of
the Ferrar Glacier trough (implying that there has probably
been a topographical depression of significance since the
Middle Miocene). Given that >1000 m of relief was present
in the Middle Miocene, ice would have been the thinnest,
coldest, and least erosive on the interfluves, and thickest,
warmest, and most erosive in the valleys. This model can-
not explain glacial erosion of the interfluve and deposition
of the Sirius Group on top of it, and a preservation of fragile
morphology in adjacent Middle Miocene-or-older ice
free valleys. Because the subglacial temperature zonation is
robust with relief, but probably uncoupled to the absolute
elevation of that relief, inferring that the Transantarctic
Mountains were lower in the past offers no support. Instead,
deposition must have occurred on reduced relief.
The implication is that the Sirius Group on Mt Feather,
and probably other units of this group in similar morphological
positions, are older than Middle Miocene in age.
KURT STOWE
Constraints on the geomorphological evolution
of the Eastern Alps
Department of Earth Science, Monash University Clayton vic 3168,
Australia
Questions concerning the heat budget of Alpine metamorphism,
the shape of metamorphic PT paths and a large
range of geodynamic questions depend critically on the
thickness geometry of crust and mantle part of the lithosphere.
One data set that is useful to constrain these parameters
independently is the paleotopography. Two more
parameters are necessary: 1. An appropriate isostatic model
and 2. knowledge of lateral tectonic motions. These
two latter parameters are reasonably well known for the
eastern Alps (e.g, Molnar & Lyon-Caen, 1988 1 Ratschbacher
& alii, 1991), but the paleotopography is still very illconstrained
(although see e.g.: Winkler-Hermaden, 1957;
Sakaguchy, 1973; Stiiwe & Sandiford, 1994)
In this contribution, the first results of a current project
are presented in which we attempt to constrain first order
features of the paleotopography of the Eastern Alps since
the Cretaceous. The project aims at interpreting the following
key observations: (i) the strong correlation of topography
with depth of exhumation; (ii) the strong correlation
of topography with tectonic units; (iii) the strong correlation
of the drainage systems with the first order geological
boundaries; (iv) the interesting «L-shaped» pattern of
drainages including the rivers Rhein, Inn, Salzach and
Enns in the north (Stiiwe & Sandiford, 1994). All these
drainages flow eastward before turning abruptly northward
(for the significance of this pattern see: Braun &
Sambridge, 1996). Following datasets are used for our integrated
interpretative approach: 0) the distribution of
depth of exhumation through time as known from geochronology,
geobarometry and the sedimentological record
of the Molasse basins; (ii) digital elevation models describing
the current topography and (iii) the current denudation
rates as derived from stream sediment data (Stiiwe &
Fabel, 1995).
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