International Geologiical Congress - Oslo 2008

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GHZ-11 Rock slope movements and early warning of catastrophic failure and related tsunamis

 

The Tjellefonna fault system of western Norway: Linking late-Caledonian extension, post-Caledonian normal faulting, and Tertiary rock column uplift with the landslide-generated tsunami event of 1756

 

T Redfield, Norwegian Geological Survey (Norway)
Per Terje Osmundsen, Norwegian Geological Survey (Norway)
 

 

We argue here that the great Tjelle rockslide/tsunami of 1756 was a direct product of the ongoing development of tectonic topography in Møre og Trøndelag county. The Paleozoic Møre-Trøndelag Fault Complex was reactivated as a normal fault during the Mesozoic and, probably, throughout the Cenozoic until the present day. Its NE-SW strands crop out between the coast and the base of a c. 2.5 km high NW-facing topographic 'Great Escarpment.' Well-preserved kinematic indicators and multiple generations of fault products are exposed along a well-defined structural and topographic lineament parallel to both the Langfjorden and the Escarpment. Moderate to steeply dipping main foliation along the flanks of Devonian folds was reactivated and cut by steeply dipping NE-SW and NW-SE-trending faults and joints, compartmentalizing some 15 million cubic meters of bedrock. The catastrophic release of this compartment as a giant rockslide into the Langfjorden on February 22, 1756 caused a sequence of ∼40 meter high tsunami waves that devastated Tjelle and several other local communities.

Because the region is seismically active in oblique-normal mode, and in accordance with scant historical sources, we speculate that an earthquake may have caused the release of the rockslide at Tjelle. Reactivation of N-S to NW-SE-trending normal faults along much of coastal Mid Norway suggests a structural link exists between the processes that destroy the present-day mountains and those that created them. In the Langfjorden region in particular, structural geometry suggests additional unreleased rock compartments may be isolated and under normal fault control. Although post-glacial rebound might in large part help drive present-day seismicity, the normal-fault-controlled escarpments of western Norway were at least partly erected in pre-glacial times. Because normal faulting is active along the westernmost onshore portion of Scandinavia, the prevailing concept that tectonics relinquish control to thermal subsidence during the post-rift phase of passive margin evolution bears re-examination.

 

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