Print this pageUsing the Excess-Area Method to Predict Deformation in the
Footwall of Thrust Faults and Draw 
Balanced
, Restorable Cross Sections
SPANG, JOHN H., and RICHARD H. GROSHONG Jr.
The excess-area method can be used to calculate both the displacement and the
depth to the detachment in thrust faults. When the actual depth to detachment is
known or well constrained, the method can be shown to calculate the wrong depth
to detachment in certain cases, which may indicate the existence of deformation
in the footwall of the fault. For the case of a fault-propagation fold with an
undeformed footwall, the excess-area method calculates the correct depth to
detachment and displacement. For the case of a fault-propagation fold which
develops a wedge-shaped syncline in the footwall of the propagating fault, the
excess-area method gives the correct depth to detachment and fault displacement
when the excess-area is calculated using only units in the hanging wall of the
fault at stratigraphic levels below the fault tip. Using unfaulted layers above
the tip line, the excess-area method calculates the correct amount of shortening
due to folding, but the wrong depth to detachment. In this case, the
intersection of the area-depth line with the area axis gives the area of the
sheared material that has been added to the footwall. The wedge-shaped syncline
in the footwall of the fault is due to distributed layer-parallel simple shear
below the stratigraphic level of the tip line, which results in a trailing edge
that is not straight. Predicting the presence of deformation in the footwall of
thrusts is critical in order to discriminate between various models of
fault-propagation folding. Knowing the area added to the footwall and the
geometry of the unfaulted fold, it is possible to draw balanced
, restorable
cross sections which will aid in exploration for footwall traps.