Mulugeta, G. 2002. Scale Effects and Rheologic Constraints in Ramp-flat Thrust Models. In: Schellart, W. P. and Passchier, C. 2002. Analogue modelling of large-scale tectonic processes. Journal of the Virtual Explorer, 7, 51-60. | ||||||||||||

Scale
Effects and Rheologic Constraints in Ramp-flat Thrust Models |
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The models represent layered materials arranged in different stratigraphic successions, with nearly initial constant thickness/width ratio (≈0.2) of the hangingwall blocks (Figs. 4 to 8). In a first arrangement the scale effect of ramp-flat accommodation was investigated by deforming models under normal gravity, and alternatively in a centrifuge (Fig. 4a). In a second set-up (Fig.5a), a competent layer with an induced ramp and overlain by ductile strata of various competence was detached above a rigid base. In a third arrangement (Fig.6a) the competent block was detached above ductile layers of various competence. This set-up was intended to study the geometry of ramp-flat accommodation emanating from ductile decollements. In a fourth set-up the plastilina hangingwall-footwall pairs were embedded in ductile strata of various competence (Fig. 7a). All models were deformed in plane strain in the transport direction. In the tests a rigid base lubricated with liquid soap provided a through-going decollement.
^{4}-10^{5} Pa. Creep viscosities after yielding
varied in the range 10^{7}-10^{8} Pa s, depending on strain rate. This material
has the appropriate stress and strain-dependent rheological properties
to simulate the deformation of sedimentary rocks at deeper levels in the
crust (e.g. Hoshino et al, 1972).
Two bouncing
putties were used to represent the matrix materials. In one, DC-3179 bouncing
putty (Hailemariam & Mulugeta, 1998) was mixed with sand to represent
a relatively stiff matrix (hardened putty. 1 in Figs. 3 b and c). In the
other, the same material was softened by admixing with aolic acid (softened
putty, 2 in Figs. 3 b and c). These simulate ductile strata of various
competence in nature, such as evaporites and shales. At room temperature
and strain rate in the range 8x10
For a general 2-dimensional system, and neglecting inertia, the local stress equilibrium equation may be written as equation 1a.
The various quantities in eq. 1a may be written in a non-dimensional form (where primes denote non-dimensionalisation) as scale model ratios (eq. 1b).
where
x The first
term on the left-hand side of eq. 1b expresses the balance between strength
and gravity induced stresses. Dynamic similitude is satisfied in case
this ratio remains invariant in model and prototype. Moreover, the value
which this dimensionless stress ratio acquires determines the style of
ramp-flat accommodation. For example, when this ratio is much bigger than
one; or s If the
value of the dimensionless strength to gravity induced stress ratio is
the same in nature as in experiments;. x
In the
centrifuge tests (e.g. Fig. 4c) scaling based on eq. 1c amounted to 1cm:0.5km,
i.e using N= 800 and a yield strength of 10 M Pa for the prototype rocks
e.g. sandstone, limestone in nature (e.g. Hoshino et al., 1972) and an
average prototype density for sediments (e.g. sandstones, limestones)
of 2.4 gm/cm |
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