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4 MODEL DESIGN
Sandbox modeling, as a form of physical modeling
has proved to be an effective method for the study of
deformation mechanisms of extensional
basins(McClay and White, 1995)
Figure 4: Sketch of side view experimental model.
It was concluded that X-style faults as conjugate
normal faults, may happen easier in a very
homogeneous deformation under extension. For the
sake of homogeneity and clarity, many sandbox
modelings were conducted to determine the length
between the walls and the thickness of sand layers.
The eventual length was designed 14 cm and the
total thickness of sand layers was placed
20cm(Figure 4).
Loose quartz sands have been proven to be the
ideal material for the physical simulation of brittle
deformation in the shallow crust . The sands used in
the experiments are 0.3-0.4 mm in diameter, with
inner friction angles of 31°. In the models, one layer
of rubber sheet is used for deformation transfer
media at the basal of the sag area and connected to
the ends of the movable walls respectively. The
rubber sheet will be gradually stretched as the
motor-driven walls moves outwards, and normal
faulting will take place in the overlying sand layers.
The displacement velocity is 2.58×10
-3
s/cm and the
total displacement is 3.5 cm. The model has been
tested more than twice and similar results have been
obtained. The experiments were undertaken on the
tectonic deformation physical modeling
experimental apparatus of China University of
Petroleum, Beijing.
Although the Nanpu Sag had been controlled by
the main syndepositional faults during Cenozoic
period, for the sake of the observation of the
framework, all experiments omitted the
syndeposition process but the results could not be
affected (Diraison, et al., 2000; Keep, 2003).
5 MODEL RESULT AND
DISCUSSION
After 1 cm of extension displacement, two crossing
faults with opposite inclination appeared, and
constitute the X configuration, but still remain
obscure, the lower part of the X-style faults showed
larger displacements than the upper part, it meant
the deformation was conducted graually from
basement to cover. After 2 cm of extension
displacement, a new set of faults with opposite
inclination cut the upper part of pre-X-style faults,
all the faults constitute complex X configuration;
After 3.5 cm extension displacement, the newly
formed faults were developed at the sides of pre-
faults and cut the upper part of X-style faults
continuously. It can be imagined that the newly
shallow faults would appear with the further
extension, more and more complex X-style faults
would be shown in the sandbox modeling(Figure 5).
Figure 5: X-style faults sequence of the sandbox modeling.
6 SUMMARY
The Nanpu sag is a classical extensional basin
developed in Cenozoic time, X-style faults as a basic
structural style were developed since the deposition
phase of Nm.
It is known that almost all the X-style faults in
the style were developed in a relative short
geological time, confined to the resolution of strata
classification, the detailed development of X-style
cannot be restored by the application of balanced
geological sections method. The sandbox modeling
reproduced the development of X-style faults, it
A Section Sandbox Modeling on the Evolution of Conjugate Normal Faults
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