Print this pageDistributed Shortening, A Cure for the Excess Displacements
Caused by Bed-Length 
Balancing
: Southern Sequatchle Anticline as an Example
GROSHONG, RICHARD H., JR.
Length-balanced cross sections have major displacement problems in triangle
zones and wrench-fault zones that may be reduced up to an order of magnitude by
small amounts of distributed shortening. The Sequatchie anticline, for 240 miles
the frontal structure of the southern Appalachians, requires as much as 4 miles
or as little as 0.5 miles displacement, depending on the balancing
model. Along
nearly all its length it is 5 miles wide, has a relief of approx. 3000 to 4000
feet, and a basal detachment just above the crystalline basement. The standard
length-balanced fault-bend fold interpretation requires approx. 4 miles of
displacement on the lower detachment and nearly the same amount on an upper
detachment. A duplex interpretation can reduce these amounts to approx. 1.5
miles. In the southern portion of the anticline all the faults are blind. The
hypothetical upper-detachment displacement, if transferred into the foreland,
produces no accommodating structures along the southern 80% of the anticline
and, if transferred into the hinterland along a passive-roof backthrust, must
extend across a substantial part of the thrust belt without breaking the
surface. A detachment-fold model that includes heterogeneous layer-parallel
shortening of 5-10%, distributed throughout the structure, eliminates the need
for any upper-detachment displacement and requires only about 0.5 mile
displacement on the lower detachment to form the anticline. The shortening
mechanisms range from grain-scale strains of 2-4.5% to second-order folds and
thrusts. Second-order structures are abundant in compressional folds and the
small amount of internal deformation they represent allows the structure to be
locally balanced without upper-detachment displacement and greatly reduces the
total displacement required on the lower detachment.