Stress, Pore Pressure, and Porosity in Fold-and-Thrust Belt System
Baiyuan Gao, and Peter Flemings
Jackson School of Geosciences, The University of Texas at Austin, Austin, TX, US
ABSTRACT
We present a large-strain coupled poro-mechanical model to investigate the evolution of stress
and pore pressure in fold and thrust belt systems. In the model, a 100 km long and 3km thick
sediment layer overlies a frictional boundary (µ = 0.4). We initialize the model with geostatic
stress such that the beginning stress ratio of horizontal stress and vertical stress is 0.8. Then the
left boundary of the material is subject to displacement at a constant rate and the final
displacement is 2.5km. A tapered wedge develops as the model proceeds. Compaction is
simulated by Elfen SR3 soil model to characterize stress, strain and porosity behavior under
various mean and differential stresses. As displacement proceeds, the horizontal effective stress
increases until it is limited by the strength of the material. At final stage, the horizontal effective
stress increases to about 2.3 times vertical effective stress in the accretionary wedge region
(Fig.1). The great increase of horizontal effective stress leads to a strong compaction. The stress
rotation exists beyond the accretionary zone. We categorize the thrust belt into 3 zones
according to their stress conditions: critical state region, transition region, and geostatic
compaction region. Within the accretionary wedge, the sediment is at critical stage condition.
During the thrust belt evolution, close to the backstop, the sediment stress path starts from
geostatic compression condition and moves towards critical state condition (Fig.2b). As a
results, the sediment porosity changes deviate from the geostatic compression trend and reaches
toward critical state compression (Fig.2c). This study provides insights of stress paths and the
relationship of porosity and stress in compressional regimes and will assist field stress and
pressure prediction. It will also help to constrain geomechanics related studies in compressional
regimes such as tectonic reconstruction and fault slip analysis.
Figure 1: Initial stress vs. Final stress.
Figure 2: Stress and compaction evolution
(a) Initial stress and final stress comparisons of (a) The initial and final position of the
tracked sediment.
a pseudo well shown in Fig.b.
(b)Horizontal effective stress distribution. At
initial stage, the ratio of horizontal effective (b) Stress path of the tracked sediment.
stress and vertical effective stress is 0.8
whereas it becomes about 2.3 at final stage (c) Compression history of the tracked
sediment.
within the accretionary wedge.
Figure 1: Initial stress condition vs. Final stress condition
(a) Initial stress and final stress comparisons of a pseudo well shown in Fig.b. The pink, light
green and light blue lines represent the initial horizontal effective stress, initial mean
effective stress, and initial vertical effective stress, respectfully. The red, green and blue
lines with round markers represent the horizontal effective stress, mean effective stress
and vertical effective stress at final stage.
(b) Horizontal effective stress distribution. At initial stage, the ratio of horizontal effective
stress and vertical effective stress is 0.8 and it increases to about 2.3 at final stage within
the accretionary wedge.
Figure 2: Stress and compaction evolution
(a) The initial and final position of the tracked sediment. The initial tracked sediment (blue dot)
is at a distance of 7km to the left side wall and it moves to the upper right at final stage (blue
arrow).
(b) Stress path history of the tracked sediment. The blue line represents the stress path history of
the tracked sediment. At initial stage, the tracked sediment is at Ko stress path (black line)
and it walks towards the critical state line (red line) as the thrust belts develops.
(c) Compression history of the tracked sediment. The blue line represents the porosity of the
tracked sediment. At initial stage, the sediment porosity is at Ko compression line (black
line). As the thrust blest evolves, the compaction deviates from the Ko compression curve
and moves to the critical state compression curve (red line).