Laterally contiguous, concave-up basal shear surfaces of submarine land-slide deposits (Miocene), southern Cyprus: differential movement of sub-blocks within a single submarine landslide lobe

Abstract

Detailed analysis of submarine landslide deposits from extensive outcrops of a Miocene slope succession (southern Cyprus) reveals significant information on basal shear surfaces of the slides. The deposits, 3-25 in thick, occur as lobate beds in transverse section at two stratigraphic horizons. Each slide lobe shows a series of adjacent concave-tip basal shear surfaces, 30-150 in wide, which nearly intersect or overlap with their neighbors. The upward curved or stepped margins of basal shear surfaces, here called shear wings, exhibit variable length (ca. 5-30 in long) and inclination. The basal shear surfaces were probably initiated along a bedding-parallel weak horizon, but propagated upward at some point where downslope driving stresses decreased or resisting forces increased. Considering the dimension (ca. 5-30 in long) of the shear wings, the multiple, adjacent concave-tip basal shear surfaces can be seen as a single basal shear plane in most high-resolution subbottom and seismic images because of the limit of acoustic resolution. The multiple contiguous, concave-tip basal shear surfaces in each lobe suggest that a submarine landslide lobe probably moved downslope as several sub-blocks in transverse section, rather than as a single unified one. The boundary of the sub-blocks where concave-tip basal shear surfaces nearly intersect or overlap with their neighbors marks a zone of differential movement between the sub-blocks, each probably showing very subtle differences in magnitude or speed of downslope movement. This subtly differential movement would create intense sediment deformation at the boundary between the sub-blocks, and may lead to longitudinal shear ridges on the upper surface.