fortran-tab-mode-default

fortran-tab-mode-default is a variable defined in `fortran.el'.
Its value is

nil

• This variable is safe as a file local variable if its value
  satisfies the predicate `booleanp'.

Documentation:
Default tabbing/carriage control style for empty files in Fortran mode.
A non-nil value specifies tab-digit style of continuation control.
A value of nil specifies that continuation lines are marked
with a character in column 6.

You can customize this variable.