8 Deformation Aside of subvertical ruptures the HB shows other types of deformation structures that, however, are difficult to interpret. Much ambiguity results from the fact that the breccia was overridden and partially eroded by a thick, warm-based glacier during the Last Glacial Maximum, such that many brittle deformation features may have resulted from glacial loading. The southern fringe of the HB near Hungerburg is affected by gravitational instability (”Bergzerreißung”), and the key outcrop at Lepsius adit is located in a block that slid downwards (Ampferer 1936). A syncline was described in the Red Breccia that results from an array of subvertical fractures that increasingly open down-section; this structure may reflect glacial loading or gravitational sag due to erosion of sediment below (Ampferer 1936). From an abandoned gravel pit west of the Mayr quarry Ampferer (1936) described two faults in an unlithified, proglacial succession, probably of early Upper Würmian age. The faults stroke SE– NW and showed a very steep dip towards NE. In addition, one of these faults cross-cut a boulder of White Breccia ~ 5 m in size embedded in the gravels. No information of the magnitude of the fault throw was given. Ampferer (1936) interpreted these faults as ”Absenkungsklüfte”, without specifying a process that caused the fracturation. Unfortunately, this gravel outcrop does not exist anymore. To fracture a boulder of White Breccia embedded in unlithified gravels requires substantial triaxial loading combined with a high differential strain rate. Fault surfaces with kinematic indicators are rare in the HB. On the right flank of the Mühlau gorge, several faults with low-dipping striae were found. A dextral offset is
indicated along one of these faults. Another fault found in the basal HB along Rossfall-Lahner shows low-dipping linears indicating a sinistral offset. These faults fit the seismic evidence of active deformation mentioned above. 9 Excursion route (Fig. 5) Stop 1: Weiherburg: Overview of Quaternary deposits near Innsbruck This first stop provides an introduction to the topic of this half-day field trip and an overview of the topographic and large-scale geological setting of the central Inn Valley, followed by a concise summary of the Quaternary dynamics (ice advances, glacial overdeepening, terrace formation) giving rise to the landscape around the city of Innsbruck. Stop 2: Engländergrab: Lateglacial(?) coarse-grained delta The trail leading from the Alpenzoo towards Hungerburg (Wilhelm Greil Weg) passes outcrops of conglomerate soon after leaving the parking lot. This conglomerate is known in the literature as “Weiherburgdelta” or “Deltaschotter beim Engländergrab”, named after the grave of a young Englishman which is located to the West of the trail on top of this small hill. The conglomerate consists of well-rounded and commonly also well sorted gravel, dominated by metamorphic rocks (Fig. 6A). A few blocks of Red HB up to ca. 1 m in
Fig. 5: Excursion route. Abb. 5: Exkursionsroute.
Excursion F ¬ Innsbruck
89
F