--> ABSTRACT: Distributed Shortening, A Cure for the Excess Displacements Caused by Bed-Length Balancing: Southern Sequatchle Anticline as an Example, by R. H. Groshong, Jr.; #91021 (2010)

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Distributed 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. 

AAPG Search and Discovery Article #91021©1997 AAPG Annual Convention, Dallas, Texas.