blanket mortgage

from The Century Dictionary.

• noun A mortgage intended to cover an aggregation of property, or secure or provide for indebtedness previously existing in various forms.

from the GNU version of the Collaborative International Dictionary of English.

• One that covers a group or class of things or properties instead of one or more things mentioned individually, as where a mortgage secures various debts as a group, or subjects a group or class of different pieces of property to one general lien.