Collaborative Research: Numerical Modeling of Fluid Flow and Transport during out of Sequence Thrusting and Underplating: Application to the NanTroSEIZE Investigation Area

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Understanding the interplay between out-of-sequence thrusts, fluid expulsion, underplating, and the behavior of the plate boundary fault is of fundamental importance to the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE), but these processes have not previously been examined through fluid flow and deformation modeling. Accordingly, this modeling study will address the impact of these major processes on pore pressures and fluid flow within the prism, mega-splay fault, and plate boundary fault. The modeling will be implemented using an existing software package ABAQUS. This package offers the ability to simulate the major features important to this modeling, will allow modeling results to be reproducible by other researchers, and is flexible enough to accommodate needs of future subduction zone modeling. The model will be applied to the Kii transect of the Nankai subduction zone and used to test the following interrelated hypotheses: Hypothesis 1: The mega-splay fault system significantly alters the patterns of excess pore pressures and porosity loss within the accretionary complex. Hypothesis 2: The hydrologic role of the Nankai mega-splay system can be illuminated through observation of pore pressures, porosities, and fluid chemistry. Hypothesis 3: Underplating rejuvenates fluid expulsion at the base of the accretionary prism by transferring sediments to a different stress state and by moving the decollement zone to a less dewatered area. Although this application of the modeling is to the Kii transect of the Nankai subduction complex, an assessment of the importance of underplating and out-of-sequence thrusting for subduction zone dewatering and deformation will be useful for other subduction zones, as well as for workers in fold and thrust belts. Splay faulting and underplating are commonly inferred features that could greatly alter pore pressure and porosity distributions (and thus seismogenesis) in many subduction zones. Broader Impact Understanding seismogenic zone processes has great societal impact related to earthquake and tsunami hazards. This study will constitute an initial step in the long-term investigation of the seismogenic zone. Modeling conducted early in the investigative process can help to shape scientific strategies during drilling and monitoring. The use of the ABAQUS software package will expand the ability of the subduction zone modeling community. Model input data sets will be made available to other researchers. As an active proponent in NanTroSEIZE, Liz Screaton will directly communicate modeling results to fellow proponents and at future workshops and NanTroSEIZE-related meetings. In addition, general interest conclusions will be published in scientific journals and presented at national scientific meetings. This project will result in the training of graduate students. At University of Florida, it will be adapted to either one PhD or two M.S. students. At University of Colorado, the project work will involve a student (either MS or PhD) in the modeling. The graduate students will present at scientific meetings and the project will be a significant component of their dissertation or thesis.