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Numerical simulation of groundwater flow in a foreland basin

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Numerical simulation of groundwater flow in a foreland basin under compressive tectonic stress Asim Yousafzai Yoram Eckstein Department of Geology, Kent State ... – PowerPoint PPT presentation

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Title: Numerical simulation of groundwater flow in a foreland basin


1
Numerical simulation of groundwater flow in a
foreland basin under compressive tectonic stress
  • Asim Yousafzai
  • Yoram Eckstein
  • Department of Geology, Kent State University,
    Kent, OH 44242 USA

2
The hypothesis
  • abnormal fluid pressure is generated in basins
    under tectonic compression,
  • e.g., in foreland basins adjacent to fold and
    thrust belts

3
Conceptualized model of a generic foreland basin
and adjacent fold and thrust belt (after Ge and
Garven, 1992)
Foreland basin
Foreland sag
4
Motivating statement (1)
  • Because groundwater flow is the dominant
    mechanism for transporting chemical mass in
    sedimentary basins, knowledge of the
    hydrodynamics and geochemistry of flow and
    transport is a fundamental prerequisite to
    understanding geologic processes (Garven 1995).

5
Motivating statement (2)
  • Deep groundwater flow can be driven by several
    mechanisms in sedimentary basins. In the case of
    evolving foreland basins, large-scale compression
    and thrusting potentially causes abnormally high
    pressures in the foreland sag that may initiate
    transient fluid flow (Ge and Garven, 1992).

6
Motivating statement (3)
  • Groundwater flow systems also evolve continuously
    during the development of a sedimentary basin.
    That is, transient hydraulic and thermal states
    can develop in response to spatio-temporal
    changes in climate, thermal conditions, landscape
    erosion, sediment compaction, continental uplift,
    tectonic stress and ongoing diagenesis.

7
Motivating statement (4)
  • Such tectonically driven fluids play an important
    role in the processes of faulting, magmatic
    activity, migration of hydrocarbons, mineral
    transport, metamorphism and paleomagnetism
    (Oliver 1986).

8
Motivating statement (5)
  • Foreland basins, such as the Himalayan foredeep,
    characterized by stratigraphic continuity,
    typically host the largest groundwater flow
    systems.

9
History of the Decan/Asia collision
10
Fold and thrust belts of the Decan/Asia collision
11
Schematic history of the last 10 My of the
collision
12
Schematic history of the last 10 My of the
collision
13
The resulting compressive stress
  • The earthquake focal mechanism solutions (Lisa et
    al. 1997) and moment-tensor solutions point to a
    dominant compressive stress regime of about 90
    MPa resulting from the Cenozoic India-Eurasia
    collision (Mandal et al. 2000).
  • For comparison, the topography-driven (artesian)
    pressure of the recharge water from the highest
    Himalaya ridges to the north of the study area
    may attain a maximum of only 25 MPa

14
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15
Major faultlines (SRT Salt Range Thrust MBT
Main Boundary Thrust KF Kalabagh Fault MCT
Main Central Thrust MMT Main Mantle Thrust JF
Jhelun Fault MKT Main Karakoram
Thrust) Major Rivers (IR Indus River KR
Kabul River) Yellow dots our sampling points
16
Geologic map of the study area (after Wandrey and
Law, 1999)
17
Peshawar Basin
18
Boundary conditions
19
The basin aquifers
  • The Peshawar intermontane basin is a broad,
    oval shaped depression comprising of a thick
    sequence of lacustrine, deltaic and fluvial
    sediments overlain by loess and alluvial deposits
    dated at 2.8 to 0.6 Ma (Hussain et al. 1998).
    These sediments, consisting mainly of sand and
    gravel, form productive aquifers in the north and
    south of the basin

20
The basin aquifers
  • In the central part, the coarse sediments are
    interbedded with clay, silt and sandy silt,
    attaining its maximum thickness and providing
    semi-confinement for a number of aquifers.
    Khyber, Attock-Cherat and Lower Swat-Bunner
    piedmont aquifers occur on the periphery of the
    basin, whereas flood plain and lacustrine
    aquifers occupy the central part of the basin.

21
The basin aquifers
  • Sedimentary rocks in the basin are layered
    with regionally continuous aquifers interbedded
    with aquitards. Deep basal aquifers are commonly
    composed of uncemented sandstones and/or karstic
    carbonates, whereas aquitards consist typically
    of fine-grained limestone, mudstone, evaporites,
    and crystalline basement.

22
The basin aquifers
  • Hydraulic conductivity in the basin ranges
    from 30-60 m/day and average specific yield is
    12.
  • Water table elevation varies considerably in
    the area. It ranges from less than 100 m in the
    southern portion to more than 1500 m in the
    mountainous north (in relation to the mean sea
    level).

23
The basin aquifers
  • The main sources of recharge to the aquifer
    are precipitation, seepage from rivers, surface
    storage reservoirs, and irrigation networks. A
    large number of drilled wells and dug wells are
    present in the area. They are extensively used
    for irrigation, industrial and domestic purposes.
    Drilled wells range in depth from 50 to 150 m
    whereas dug wells are up to 20 m in maximum
    depth.

24
  • Both horizontal and vertical loads provide the
    tectonic force for squeezing pore fluids out of
    the sheet and buried strata. This load has been
    represented by using the compressibility values
    in the model.

25
The protocol
  • identify the type of model that should be used
  • identify the type of aquifer boundaries
  • determine values for aquifer materials
  • determine initial values for boundary conditions,
    and
  • calibrate and verify the model.

26
The protocol
  • We chose finite element (FEMWATER) method for
    this study because of its flexibility in handling
    irregular basin geometry and boundary conditions,
    tensoral properties, and deforming media.
  • Input parameters for the model include recharge,
    discharge, and bulk compressibility of the media
    and water.

27
The protocol
  • The compressibility of water is a function of
    pressure and temperature (Strauss and Schubert
    1977), but it does not vary much for the
    conditions present in this model therefore, a
    constant value of 5x 10-10 Pa was assumed.

28
The protocol
  • Typical values of compressibility for
    sedimentary rocks range from 1x10-8 to 1x10-11 Pa
    (Birch 1966 Palciauskas and Domenico, 1989),
    with shaly rocks tending to be somewhat more
    compressible than sandstones and carbonates.
    However, because deformation within a thrust belt
    occurs over periods of tens of millions of years,
    inelastic deformation undoubtedly takes place.
    Palciauskas and Domenico (1989) theoretically
    treat the irreversible process of pressure
    solution in a sandstone and determined that the
    inelastic value of compressibility is 50 times
    greater than for elastic deformation for a time
    scale on the order of millions of years. It was
    assumed for this study that this conclusion is
    applicable to all rock types and therefore
    different values were used for different
    materials of the model.

29
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30
Recharge/Discharge
31
3-D representation of the model layers. Every
layer is bound by a Triangulated Irregular
Network (TIN). Main Mantle Thrust (MMT) Main
Central Thrust (MCT). Main Central Thrust (MCT).
32
Sensitivity analysis for various input parameters
in FEMWATER modeling of the Peshawar Basin. The
vertical axis represents transient heads (m).
33
Effect of compressibility at selected nodes
34
Total heads (m) as obtained from the transient
simulation at the last stress period. Y-axis
indicates the north. Total heads (m) are highest
in the northern part and show a steady decrease
towards the south. Green bars indicate error
within the range.
35
Results of the transient simulation at the last
stress period, shown as regional pressure heads
(m). Pressure heads attain maximum values where
the two fault zones (MMT, MCT) originate at
depth.
36
Conclusions
  • The topography-driven pressure from the highest
    ridges to the north of the study area may attain
    a maximum of 25 MPa as compared to the tectonic
    stress of 90 MPa. Results of the transient
    simulations indicate that topography alone is not
    sufficient to induce pressure heads observed in
    the field (during July/August 2003).
  • Simulation results show that positive residuals
    are obtained when tectonic compression is
    incorporated as an input parameter. The positive
    residuals of 0.98-2.90 m are reduced to 0.40 m at
    the last stress period of the transient
    simulation, showing that tectonic compression is
    playing an important role in driving deep
    groundwater to the shallow levels observed in the
    water wells in the study area.

37
Conclusions
  • It is proposed here that the higher pressure
    regime is breaking the sealed thrusts in the
    foreland fold-and-thrust belt and adjoining
    areas.
  • The numeric calculations have shown that tectonic
    compressions can create periods of transient
    flows in foreland basins, with excess flow rates
    of the order of 10-4 to 10-3 m/yr for thrust
    sheet loads from 1 to 10 km thick.
  • Most of the excess pressure generated by
    compression appears to dissipate in about 104 to
    105 years before a new steady state can be
    reached in about 104 to 105 years. Ge and Garven
    (1992) arrived at the same conclusion for the
    Arkoma basin in central Arkansas and Oklahoma.

38
Acknowledgements
  • We gratefully acknowledge financial support from
    Geological Society of America, and Sigma Xi in
    conducting fieldwork for this research.
    Department of Geology at Kent State University
    facilitated analytical and computational
    procedures.
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