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 m thick, occur as lobate beds in transverse section at two stratigraphic horizons. Each slide lobe shows a series of adjacent concave-up basal shear surfaces, 30–150 m wide, which nearly intersect or overlap with their neighbors. The upward curved or stepped margins of basal shear surfaces, here calledshear wings, exhibit variable length (ca. 5–30 m 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 m long) of the shear wings, the multiple, adjacent concave-up 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-up 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-up basal shear surfaces nearly intersect or overlap with their neighbors marks a zone of differential movement between the subblocks, 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.