Mechanics of viscous wedges: Modeling by analytical
and numerical approaches.
Sergei Medvedev
J. Geophys. Res., 107(B6),
10.1029/2001JB000145, pp. ETG 9 (1-15), 2002
Abstract
Although complex rheological models have been used
to study the evolution of orogenic wedges, many features of simple models
remain to be fully explained. Here, we analyze the plane strain evolution
of model orogenic wedges under simple boundary and rheological conditions.
The uniform linear viscosity wedge is driven by motion of a basal boundary
at a constant velocity. Three main analysis techniques are used: analytical
(algebraic analysis of scales involved); semi-analytical (thin sheet approximation),
and; a complete numerical approach. Application of this variety of approaches
provides a better understanding of the underlying physics and outlines
the advantages and disadvantages of the different techniques. The evolution
of wedges can be divided into three phases. Initially, wedge growth is
mainly vertical and symmetrical, and depends little on the viscosity. The
second phase exhibits almost self-similar growth with the appearance of
surface extension, within an otherwise compressional system, and development
of asymmetry. The last phase involves widening of wedge and further development
of asymmetry and surface extension, the average slope of wedge decreases
during this phase. The Ramberg number, the ratio of characteristic gravitational
to shear stress, defines the duration of each phase. Several parameters
introduced here (mean surface slope, asymmetry of the wedge, surface extension
and near-surface strain history) allow observations from natural wedges
to be linked to the bulk viscosity of the moodel wedges. Analysis shows
that the thin sheet approximation does not correctly describe the initial
stages of wedge evolution.