TSK 11 Göttingen 2006 Ustaszewski et al.
 Neotectonics in the Swiss
 Alps — A late Alpine to post-
 glacially active fault at the
 Gemmi Pass Vortrag
 Michaela Ustaszewski1 Adrian
 Pfiffner1 Marco Herwegh1
 Introduction
 The area of the central and western
 Swiss Alps reveals not only the highest
 uplift rates of Switzerland (1.5mma−1
 near Brig, Schlatter & Marti 2002),
 but also shows a strong concentration
 of earthquakes (e.g. Deichmann et
 al. 2004). This raised the ques-
 tion, whether the region hosts any lin-
 ear topographic expressions that can
 be attributed to motion along poten-
 tially seismogenic faults. The area was
 therefore chosen for the investigation of
 postglacially active lineaments. Firstly,
 aerial photographs from the entire area
 were searched for linear features, which
 could be of gravitational or tectonic ori-
 gin. Subsequently, selected lineaments
 were visited in the field to study their
 origin. We found scarce but positive ev-
 idence for neotectonic fault movements.
 One particular lineament that exhibited
 the most promising exposures was inves-
 tigated in greater detail. This lineament
 is a prominent NW–SE striking fault lo-
 cated at the Gemmi Pass, runs perpen-
 dicularly to the regional fold axes and
 cuts through the Helvetic nappe stack.
 The position and orientation of the fault
 discounts gravitational reactivation. A
 close examination of the fault rocks re-
 veals a long term evolution of this fault
 starting already at a late stage of Alpine
 nappe emplacement and related defor-
 1 Institute of Geological Sciences, University
 of Bern, Baltzerstrasse 1–3, CH-3012 Bern,
 Switzerland
 mation.
 Late Alpine deformation
 The fault is characterized by a high den-
 sity of fault-parallel joints and veins,
 which become less abundant away from
 the fault zone. Initially the fault ori-
 ginated as a–c joints forming an array
 with variable widths of 10–20m. With
 progressive deformation, the joints con-
 nected in the center of the array gen-
 erating a major 1–3m wide large-scale
 fault zone. Deformation associated di-
 latancy and the presence of a fluid re-
 sulted in filling of the newly formed cav-
 ities by calcite. Cathodo-luminescence
 on the vein filling shows zonation and
 subsequent disruption by brittle defor-
 mation as is indicated by the existence
 of discrete cataclastic areas. Several cy-
 cles of veining and brittle deformation
 can be observed. Changes in cathod-
 ofacies suggest variations in fluid chem-
 istry pointing to episodic fluid pulses.
 The youngest deformation features in
 these fault rocks are micro-scale faults
 impregnated by iron-hydroxide bearing
 minerals. Kakirites are absent, which
 suggests that they have a low preserva-
 tion potential in carbonate rocks. This
 could be due to syndeformational disso-
 lution of the fine grained fault gouge, or
 recent erosion.
 Postglacial deformation
 The fault crosses a small (ca. 60×30m)
 postglacial, sediment-filled depression,
 which was targeted for a large trench
 (15.4m long, 2m wide and up to 2.2m
 deep) in order to verify its postglacial re-
 activation. The trench bottom reached
 limestone bedrock almost all along the
 trench (x in Fig. 1). It delineates a
 basin, which deepens towards the north-
 1
Ustaszewski et al. TSK 11 Göttingen 2006
 east. The base of the sediment-fill of
 this depression is made of an up to 1.5m
 thick dark brown moraine layer. The
 moraine material consists partly of lodg-
 ment till (h in Fig. 1), partly of trans-
 ported till (e, f and g in Fig. 1). Large
 rock boulders (up to 1m in diameter)
 were found in the till material. A very
 constant 20 to 30 cm thick, fine-grained
 (silt to fine sand fraction), yellow layer,
 for which the working term ‘loess’ is
 used (d in Fig. 1), was sedimented on
 top of the moraine. It has a sharp up-
 per contact, whereas the basal contact
 to the moraine material is sometimes
 unclear. This yellow layer delineates the
 basin form as well. An up to 1.5m thick
 grey-brown B horizon (b in Fig. 1) of soil
 is overlaying the yellow loess layer. It
 consists of brown fine-grained silt mate-
 rial intercalated by numerous sand and
 grit lenses, and up to 7m continuous
 clay bands, which are up to 5 cm thick
 (c in Fig. 1). This horizon shows onlap-
 structures onto the loess at both sides
 of the basin. The uppermost 5 to 15 cm
 are made up by the active soil layer, the
 A horizon (a in Fig. 1). A cataclas-
 tic fault zone disrupts the partly kars-
 tified limestone bedrock from meter 6.4
 to 6.8m. This 40 cm wide zone is split
 in the middle by an open joint or fault
 plane. No surface displacement was seen
 on the bedrock surface. Right above
 this fault zone, the about 50 cm thick
 moraine layer does not show any distur-
 bances. However, the yellow loess layer,
 which represents a very continuous hori-
 zon in the trench with a clear upper sur-
 face, is heavily disrupted, incorporating
 moraine material from below and dis-
 playing flame-like structures and up to
 5 cm large vertical displacements at its
 upper boundary (Fig. 2).
 These structures can not be explained
 by any sedimentary or erosional pro-
 cesses. The overlaying B horizon does
 not seem to be displaced at all, thus
 sealing the movement. These obser-
 vations indicate strike-slip kinematics,
 which would also be favored by the
 recent stress-field. Samples for OSL-
 dating of the Loess layer and the B hori-
 zon were taken in order to limit the age
 of the movement.
 Conclusion
 To summarize, this example of an ac-
 tive fault allows studying active and an-
 cient deformation structures/processes
 that occurred at shallow and greater
 depth, respectively. We expect that the
 episodic cycles of brittle deformation
 and fluid pulses forming veins and cat-
 aclasites equivalent to the older struc-
 tures observed at the surface, were on
 going at a few kilometers depth during
 2
TSK 11 Göttingen 2006 Ustaszewski et al.
 Figur
 e
 1:
 T
 ren
 ch-log
 .
 (a
 —
 soi
 l
 h
 or
 iz
 on
 A
 ,
 b
 —
 so
 il
 h
 or
 iz
 on
 B
 ,
 c
 —
 cl
 ay
 ba
 n
 d
 s
 in
 so
 il
 horizo
 n
 B
 ,
 d
 —
 fine
 -graine
 d
 yel
 lo
 w
 ‘l
 oes
 s’
 la
 yer
 ,
 e,
 f
 +
 g
 —
 trans
 p
 orte
 d
 m
 ora
 in
 e
 material
 ,
 h
 —
 lo
 d
 gm
 en
 t
 ti
 ll
 ,
 x
 —
 li
 me
 st
 on
 e
 b
 edr
 oc
 k,
 p
 artl
 y
 karstified
 ,
 y
 —
 ca
 taclas
 it
 ic
 zo
 ne
 )
 3
Ustaszewski et al. TSK 11 Göttingen 2006
 Figure 2: Photo and sketch of a detail of
 Fig. 1 showing the disrupted zone in the
 loess layer.
 the time of post-glacial activity. Given
 the regional seismicity pattern we con-
 clude that such veining and cataclasite
 formation at depth is still recurring and
 in concert with this earthquake activity.
 References
 Schlatter A & Marti U (2002) Neues Lan-
 deshoehennetz der Schweiz LHN95. Ver-
 messung, Photogrammetrie, Kulturtechnik
 1, 13–17
 Deichmann N, et al. (2004) Earthquakes in
 Switzerland and surrounding regions during
 2003. Eclogæ geologicæ Helvetiæ 97-3, 447–
 458
 4