Skardu Duplex Structure: Implications for Accuracy in
Balanced Cross
Sections
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.