Geology Reference
In-Depth Information
The original concept of a balanced cross section (Chamberlin 1910) is that the de-
formed-state and restored cross sections maintain constant area and so would balance
on a beam balance (Fig. 11.2). This concept was generalized by Dahlstrom (1969) to a
constant volume criterion. In many structures there is little or no deformation along
the axis of the structure, and so in practice the third dimension can often be tempo-
rarily ignored and constancy of volume can be applied to a cross section as a constant-
area rule. Units which maintain constant bed length are said to be length balanced and
units that maintain constant area but not constant bed length or bed thickness are said
to be area balanced. A balanced cross section is generally understood to be one which
is restorable to a geologically reasonable pre-deformation geometry, as well as main-
taining constant area.
The techniques for the restoration of a structure are necessarily based on models
for the evolution of the geometry. A kinematic model defines the evolution through
time of the geometry of a structure. Four basic kinematic models are commonly used
for restoration (Fig. 11.3). The most appropriate model for a given structure will be
determined by the mechanical stratigraphy and the boundary conditions that produced
the structure. The simplest model is rigid-body displacement (Fig. 11.3a) which may
include both translation and rotation. Layer-parallel slip (Fig. 11.3b) implies slip be-
tween closely spaced layers that maintain constant thickness unless otherwise speci-
fied. If the slip is between layers that are visible at the scale of observation, the folding
mechanism is known as flexural slip (Donath and Parker 1964) and so this is called the
Fig. 11.2.
The concept of a balanced
cross section (after Woodward
in Woodward et al. 1989). De-
formed-state section on the left,
restored section on the right
Fig. 11.3.
Basic kinematic models.
a
Rigid-body displacement.
b
Flexural slip.
c
Simple shear
oblique to bedding.
d
Pure
shear
