Why rocks are not always deformed in the same way? How
mountain chains are formed? Why rivers are distributed in specific patterns in
a mountain region? How the plate tectonics are deformed in a subduction
boundary? Some geological processes are difficult to understand when direct
observations are limited or almost impossible to obtain. Some million years is
quite a long period to waiting for check how a mountain belt is formed. In the
same way, it would be amazing to go deep into the planet to check the
subduction of a tectonic plate, but it is impossible for the moment.
In order to reach a better understanding about the
earth dynamics, geologists have been using analogue models since the end of the
19th century to simulate geological processes in a more affordable temporal and
dimensional scale. In the present work, we have used analogue modelling
techniques in order to study the evolution of diapirs.
Diapirs are a type of geological structures formed due
to the upward movement of mobile and less dense material (salt or shales)
through more brittle rocks. Diapirs can display a large variety of geometries
due to different acting parameters. Can sedimentation be one of the major
mechanisms controlling the final diapir geometry? The objective of this work is
to use analogue models to understand how diapiric structures are influenced by
sedimentation. To do so, and according to their physical properties, we used
silicone to simulate less dense material constituting diapirs and coloured sand
as brittle rocks and sediments.
To analyse the impact of sedimentation on the
evolution of diapirs, we design a set of analogue models with different
sedimentation patterns. From all models we highlight the model with homogeneous
sedimentation (Model 1 in Figure 1) and the model with a homogeneous sedimentation
phase followed by a prograding sedimentation phase, which implies differential
sedimentation along the model device (Model 2 in Figure 1). In areas with high
sedimentation, diapirs are well-developed with vertical walls as the one shown
in Figure 1. Contrarily, the diapiric structures are less developed and
remain in an early diapiric phase in areas with low sedimentation. This would be
linked to the loading associated to sediments that would cause a major silicone
withdrawal from beneath the sedimentary pile towards diapirs in areas with
higher sedimentation. This silicone withdrawal is lower in areas with limited
sedimentation.
Thus, the comparison among all the models shows
that the amount of sedimentation and how and when this sedimentation occurs have
a major impact in the final geometry of diapiric structures. Applying the
knowledge obtained from our models to real diapiric basins, it is possible to
understand better the dynamics of salt-related basins and improve the
interpretation of the geological history of diapiric structures worldwide.
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Figure 1: Top view of models 1
and 2 showing the different sedimentary pattern applied. Sedimentation in model
1 is homogenous along the entire model and model 2 is composed of a first phase
of homogeneous sedimentation as model 1 and a second phase with a prograding
sedimentation that implies high sedimentation in the right-side part of the
model and low sedimentation in the left-side part of the model. Last picture
show an example of the resulting diapiric geometry in the high sedimentation
area of model 2.
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This work is supervised by Jaume
Vergés (ICTJA-CSIC) and Thierry Nalpas (Geosciences Rennes, Université Rennes
1). This study is funded by Statoil Research Centre, Bergen (Norway) and by the
CSIC-FSE 2007-2013 JAE-DOC postdoctoral research contract (E.S.). Additional
funding was provided by the Generalitat de Catalunya (2014SGR251). Special
thanks to J.J. Kermarrec and Pascal Rolland from Experimental Tectonics Laboratory
of Geosciences Rennes (Université de Rennes 1, France). We are grateful to
Statoil for its support and permission to publish this research.
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