At depths of greater than 60 m, ice moves by plastic deformation. What causes this transition from brittle to ductile behavior?

Increase in the intensity of pressure allows very less deformation

Explanation:

For the water the variation of pressure at any depth is directly proportional to the depth of the water above that level.

Thus, at depth greater than 60 m the intensity of pressure on ice is very high and therefore very little deformation is possible.

Hence, the transition from brittle to ductile behavior is observed.