Relationships

The position of thrust sheets around

the margins of landslide toe blocks, and their

morphology and direction of thrusting, suggests that

they were formed as a result of toe block pressing and

movement in the surrounding sand. Toe-thrust sheets

therefore can be considered as the morphological

expression of ongoing instability at the landslide

toe. The upthrust nature of these sheets at West

Runton suggests that rotation of toe blocks,

generating forward movement of the surrounding loose

beach sand, is the principal process of toe-thrust

sheet formation (fig. 6). Passive pressing of toe

blocks into the surrounding sand under gravity is

unlikely to result in either brittle failure of the

sand or differential movement of the sand away from

the toe blocks (i.e., different thrust sheet widths).

The presence of thrust sheets therefore suggests that

landslide blocks are actively excavating into the

softer and unconsolidated beach sand that is displaced

outward as a result of this process.

The size and extent of the thrust sheets can be used

as a proxy for the scale, rate, and timing of block

movement. For example, the bigger landslide blocks are

associated with more extensive thrust sheets, and

sheet width is likely associated with excavation

depth. The presence of multiple and overlapping thrust

sheets that vary in extent along the front of

landslide blocks (figs. 4, 5) also suggests that

different parts of the toe are active at different

times and therefore that sliding rates and volumes

averaged across the entire landslide (Waltham and

Dixon 2000) likely conceal wide spatial and temporal

variations.

[Graphic omitted] The presence of delicate toe-thrust

sheets within the intertidal zone at West Runton is of

interest because these features are easily destroyed

by waves and will be wiped out by every high tide.

Figure 6 is a schematic cartoon illustrating a

possible formation mechanism for these features.

During high tides, the elevated position of the

external water plane (mean high water level) against

the landslide toe means that there is a small

difference in head, and low hydraulic potential

gradient, between the landslide toe and its external

environment (fig. 6a). The depth of marine water also

likely increases interstitial pore water pressure both

within the submerged beach sand and within the

fine-grained landslide sediments and influences

effective pressure (cf. Dixon and Bromhead 2002).

Elevated external water pressure at high tide helps to

hold back toe advance (Hutchinson 1988).

At low tide, when the ground water table is located

within the beach sand and is under lower (atmospheric)

interstitial pore water pressure, a large difference

in head and therefore steepened hydraulic potential

gradient exists between the landslide toe and the

external environment (sea level) (fig. 6b). Under

these conditions, sliding and the formation of

toe-thrust sheets can occur. The depth of thrusting

may be limited