The effects of Pleistocene glacial loading on rock failure, permeability increases, pore pressure evolution, and brine migration within two linked sedimentary basins were evaluated using a multiphysics control volume finite element model. We applied this model to an idealized cross section that extends across the continent of North America from the Hudson Bay to the Gulf of Mexico. Our analysis considered lithosphere geomechanical stress changes (σyy > 35 MPa) in response to 10 cycles of ice sheet loading. Hydrologic boundary conditions, lithosphere rheological properties, and aquifer/confining unit configuration were varied in a sensitivity study. We used a Coulomb Failure Stress change metric (ΔCFSp > 0.1 MPa) to increase permeability by a factor of 100 in some simulations. Results suggest that a buildup of anomalous pore pressures up to about 3 MPa occurred in confining units during periods of glaciations, but this had only a second-order effect on triggering rock failure. In regions prone to failure, permeability increases during glaciations help to explain observations of brine flushing in sedimentary basin aquifers. During the Holocene to present day, deglaciation resulted in underpressure formation in confining units primarily along the northern margin of the northern basin. Holocene-modern geomechanical stress fields were relatively small (<0.6 MPa). However, pore pressure increases associated with postglacial rebound, especially when a basal sedimentary basin aquifer is present, induced rock failure and seismicity up to 150 km beyond the terminus of the ice sheet. Sedimentary basin salinity patterns did not equilibrate after 10 simulated glacial cycles.
Bibliographical noteFunding Information:
This study was supported by NSF grant (Mark Person EAR-1344553 and NSF EPSCoR under grant IIA-130134 and Jennifer McIntosh EAR-0635685). The authors are grateful to Editor Jean Bahr, Associate Editor Larry Murdoch, Dr. Chris Neuzil, and two anonymous reviewers who helped us greatly improve the manuscript. Thanks to Dr. Alex Rinehart for helpful discussions about adopting the Mohr-Coulomb failure criteria. Our model package (CVFEM_Rift2D) has been uploaded onto the CSDMS (Community Surface Dynamics Modeling System) website hosted by the University of Colorado (https://csdms.colorado.edu/wiki/Model_download_portal). The users can download the package by selecting Hydrological Models in the Model Repository Menu and searching for CVFEM_Rift2D. The uploaded files include (1) MATLAB-based control volume finite element two-dimensional geomechancial model cvfem_ice.m, (2) Fortran-based two-dimensional flow and transport model Rift2D, and (3) MATLAB-based one-dimensional permafrost model permafrost_ice.m. The grid generation and model input parameters for the geomechanical model are contained within the MATLAB-based codes. Information on how to modify the input parameters, grid sizes, and boundary conditions can be found in the file readme_cvfem_ice.txt. Information on running Rift2D and creating data sets can be found in the uploaded Rift2D model documentation and graphical preprocessor. Rift2D can be compiled using gfortran. Rift2D and cvfem_ice.m create Tecplot formatted output files that can be graphically rendered using either Tecplot or Pareview. The program permafrost_ice.m generates the subsurface temperature profiles in a txt file every 1,000 years for the entire 100,000-year cycle for each individual column.
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- ice sheet
- mathematical modeling
- solute transport