The formation and structural evolution of the jungrau syncline is
described, based on excellent outcrops occurring in the lotschental, in
the central alps of switzerland. the quality of the outcrops allows us
to demonstrate that the external massifs of the swiss alps have
developed due to internal folding.
The jungfrau suncline, which separates the autochtonous gastern dome
from the aar massif basement gneiss folds, is composed of slivers of
basement rocks with their mesozoic sedimentary cover. in the inner
faflertal, a side valley of the lotschental, the 200 m thick syncline
cp, roses fpir imots, the gastern massif with a reduced mesozoic
sedimentary cover in a normal stratigraphic succession, two units of
overturned basement rocks with their mesozoic sedimentary cover, and the
overturned lower limn of the tschingelhorn gneiss fold of the aar massif
with lenses of its sedimentary cover. stratigraphy shows that the lower
units, related to the gastern massis, are condensed and that the upper
units, deposited farther away from a gastern paleo-high, form a more
complete sequence, linked to the doldenhorn meso-cneozoic basin fill.
the integration of these local observations with published regional data
leads to the following model. on the northern margin of the doldenhorn
hbasin, at the northern fringe of the alpine tethuys, the pre-triassic
crystalline basement and its mesozoic sedimentary cover were folded by
ductile deformation at temperatures above 300 degrees C and in the
presence of high fluid pressures, as the helveti c and penninic nappes
were overthrusted towards the northwest during the main alpine
deformation phase, the visosity contrast between the basement gneisses
and the sediments caused the formation of large basement anticlines and
tight sedimentary sunclines (mullion-type structures). The edges of
basement blocks bounded buy pre-cursor se-dipping normal faults at the
northwestern border of the doldenhorn basin were deformed bu simple
shear, creating overturned slices of crystalline rocks with their
sedimentary cover in what now forms the hungfrau syncline. the
localisation of ductile deformation in the vicinity of pre-existing
se-dipping faults is thought to have been helped by the circulation of
fluids along the faults; these fluids would have been released from the
mesozoic sediments by metamorphic dehydration reactions accompanied by
creep and dynamic recrystallisation of quartz at temperatures above 300
degrees C. Quantification of the deformation suggests an strain
ellipsoid with a ratio (1 + e(1)/+ e(3)) of approximately 1000.
The jungfrau suncline was deformed bu more brittle nw-directed shear
creating well-developed shear band cleavages at a late stage, after
cooling by uplift and erosion. It is suggested that the external massifs
of the apls are basement gneiss folds created at temperatures of 300
degrees C by detachment through ductile deformation of the upper crust
of the european plate as it was underthrusted below the adriatic plate.