Vincent S. Cronin
A balanced cross section is often assumed to provide a "correct," "geometrically admissible," potentially accurate image of the subsurface distribution of rock units. Conversely, unbalanced cross sections are tacitly assumed to be inaccurate. A small duplex structure involving glacial lakebeds in Skardu intermontane basin, Karakoram-Himalaya, Pakistan, displays a cross-sectional morphology in which essentially all of the basic assumptions used in the construction of balanced cross sections are violated. In the Skardu duplex, a conventionally balanced cross section would not be accurate.
The roof thrust lies about 1.5 m above the subhorizontal floor thrust, supported by imbricate thrust slices within the duplex that are each about 40-50 cm thick and about 2-4 m long. Deformation within each thrust slice indicates that the length, thickness, and volume of a given bed are not the same now as before deformation. Bed cutoff angles generally differ across a given fault, because the two sides were exposed to different local stress histories during deformation. The orthorhombic symmetry of fractures or faults within each thrust slice indicates non-plane strain, although the glacial motion responsible for the duplex was essentially unidirectional.
The Skardu duplex, an analog for economically important structures up to about four orders of magnitude larger within major orogenic belts, demonstrates that rock deformation is a dynamic process controlled by rock strength characteristics under applicable thermal, mechanical, and geochemical conditions. Rock deformation is not a kinematic process controlled by rules of geometric admissibility, or by conservation of geometric quantities such as volume, area, bed length, and bed cutoff angle.
AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.