The stress force that pulls on the crust where two plates are moving apart is called what

Tension force is a pulling force that acts on the body when it is hanged to objects like cable, chain, or string. It is also the stress force that pulls on the crust where 2 plates move apart. Tension force is usually denoted by T (sometimes also denoted as F sub t). 

This is the formula for tension force: 
T= w± ma 
Where, w is weight of the body, m for mass and a for acceleration of the moving body.