taking place. The South Sandwich Trench and the adjacent island volcanoes may 
be a result of thrust faulting and folding by lateral compressive forces, a surface 
expression of deep-seated movement within the earth's mantle. 
The area of confluence of one of the two east-west shear zones and the island- 
trench compressional zone are shown in Figure 27 (Inset Fig. 26). In addition to an 
interpretation of detailed bathymetry, this figure shows the tracks of GLACIER over 
the trench. 
The floor of the South Sandwich Trench is 15 to 30 miles wide and has a moun- 
tainous bottom topography. Many trenches have narrow widths and relatively feature- 
less bottoms composed of great thicknesses of sediments derived from nearby volcanic 
islands. The topographic expression in the South Sandwich Trench suggests the possi- 
bility of block-faulting, resulting from lateral thrust. An alternative explanation 
for the hummocky nature and great width of the floor follows: The floor of the trench 
in recent geologic time was much deeper than at present, perhaps nearly 5,000 
fathoms deep. The floor then had the narrow, V-shaped or rounded profile typical 
of many other deep trenches. The inner wall of the trench (Fig. 27) was weakened 
by movement along the east-west shear zone that extends from this wall back through 
South Georgia Island to Cape Horn. A massive slab of this steep trench wall slid 
down into the bottom, largely filling it. By this explanation, the non-linear hum- 
mocks depicted by the profiles on Figures 28 and 29 would be the result of debris 
from an extensive landslide which originated on the south wall and perhaps was 
triggered by an earthquake. Figure 29 shows profile A-B with no vertical exagger- 
ation; the dotted line is the suggested pre-landslide profile. 
48 
