Catastrophic rockslides in the Alps

In carbonate-lithic rockslides, the hitherto undocumented phenomenon of diagenetic cementation of the sturzstrom deposits will be used to determine proxy ages of rockslide events by 234 U/230 Th disequilibrium dating of cements. Large-scale rockslides exceeding 106 m 3 in volume are not only a major process of mountain erosion and orogenic mass balance but, in densely populated regions such as the Alps, also represent a major threat to humans and facilities. Establishing the distribution of rockslides in time is a prerequisite of hazard assessment for future events and for a better understanding of potential triggers, such as climatic change or phases of enhanced earthquake frequency. The ages of many rockslides of the Alps, however, are still poorly defined. Our preliminary investigations of major carbonate-lithic rockslides of the Alps revealed that indeed nearly all of them contain pockets, thicker crusts and patches wherein the rockslide material underwent cementation into a breccia. As already proven for the Fern Pass rockslide (Austria), breccia cement can provide a proxy age of the sturzstrom event by dating the cement with the 234 U/230 Th disequilibrium method. A cross-check of the U/Th age with 36Cl surface exposition ages underscored the validity of the U/Th age, and showed that the U/Th age constrains the comparatively wide error range of exposition dating (Ostermann et al., 2007). The common phenomenon of cementation in rockslide deposits to date is practically unknown to the geoscientific community. Our preliminary data strongly suggest that these cements may be routinely used for U/Th proxy-dating the events. In addition, different `generations` of cement as distinguished by petrographic analysis may provide time constraints on the further post-depositional development of a rockslide mass. The U/Th disequilibrium method has recently been successfully applied to age-date cements of Quaternary deposits of the Alps, such as talus breccias, fluvial conglomerates and spring tufas (Ostermann, 2006). In the frame of the project proposed herein, age determination of selected rockslides shall be done by both U/Th dating of cements and by surface exposition dating with cosmogenic radionuclides. Exposition dating has the advantage of providing the `direct` age of the rockslide event, yet the resulting ages are fraught with wide 2s standard errors. Combining surface exposition dating with U/Th dating of cements thus has the potential to arrive at more precise proxy ages of rockslides than can be achieved by a single method alone.

In carbonate-lithic rockslides, the hitherto undocumented phenomenon of diagenetic cementation of the sturzstrom deposits will be used to determine proxy ages of rockslide events by 234 U/230 Th disequilibrium dating of cements. Large-scale rockslides exceeding 106 m in volume are not only a major process of mountain erosion and orogenic mass balance but, in densely populated regions such as the Alps, also represent a major threat to humans and facilities. Establishing the distribution of rockslides in time is a prerequisite of hazard assessment for future events and for a better understanding of potential triggers, such as climatic change or phases of enhanced earthquake frequency. The ages of many rockslides of the Alps, however, are still poorly defined. Our preliminary investigations of major carbonate-lithic rockslides of the Alps revealed that indeed nearly all of them contain pockets, thicker crusts and patches wherein the rockslide material underwent cementation into a breccia. As already proven for the Fern Pass rockslide (Austria), breccia cement can provide a proxy age of the sturzstrom event by dating the cement with the 234 U/230 Th disequilibrium method. A cross-check of the U/Th age with 36Cl surface exposition ages underscored the validity of the U/Th age, and showed that the U/Th age constrains the comparatively wide error range of exposition dating (Ostermann et al., 2007). The common phenomenon of cementation in rockslide deposits to date is practically unknown to the geoscientific community. Our preliminary data strongly suggest that these cements may be routinely used for U/Th proxy-dating the events. In addition, different `generations` of cement as distinguished by petrographic analysis may provide time constraints on the further post-depositional development of a rockslide mass. The U/Th disequilibrium method has recently been successfully applied to age-date cements of Quaternary deposits of the Alps, such as talus breccias, fluvial conglomerates and spring tufas (Ostermann, 2006). In the frame of the project proposed herein, age determination of selected rockslides shall be done by both U/Th dating of cements and by surface exposition dating with cosmogenic radionuclides. Exposition dating has the advantage of providing the `direct` age of the rockslide event, yet the resulting ages are fraught with wide 2s standard errors. Combining surface exposition dating with U/Th dating of cements thus has the potential to arrive at more precise proxy ages of rockslides than can be achieved by a single method alone.