Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
BEG Seminar, October 28, 2005
The Kinematics of Giant Submarine Landslides:
Interest in giant submarine landslides has increased in recent years because of several multibillion-dollar development projects planned long unstable continental margins. Notable amongst these is the Ormen Lange Field development, offshore Norway, located in the scar of the supergiant Storegga Slide (volume c. 3,500 km3), which failed catastrophically some 8,200 years ago, leaving a record of a tsunami across Norway and northeastern Scotland. A major unresolved question with these large-scale landslides on continental slopes is the trigger mechanism. One approach to tackling this problem is to analyze the kinematics in detail, now possible through 3D seismic methods. This presentation discusses this approach, presents examples from offshore Israel and Norway, and opens a discussion as to whether these giant slides move in one massive event (quickstep), or have a more protracted evolution (waltz).
Critical to this discussion is the relatively recent identification of impressive toe-thrust domains associated with some giant slides. These consist of an arcuate series of thrusts, ramping up from a basal shear surface cutting across slope sections up to 600 m thick. Displacement on individual thrusts is up to 200 m, and they can be 20 to 30 km long along strike. It is argued that these contractional systems form in a downslope propagating sequence owing to buttressing of the slide mass against the undeformed slope. It is concluded that slides with buttressed toes (confined slides) behave differently than unconfined slides, which are free to run out and over the toe region, and disaggregate to form debris flows. The different modes of behavior are attributed to a critical limit for the depth to detachment. Recognition of these two end members is essential for correct volumetric balancing of slides and ultimately for any assessment of tsunamigenic risk.