NUMERICAL MODELING OF DESICCATION AND TECTONIC PROCESSES AS DRIVING MECHANISMS FOR MACRO-POLYGON FORMATION

Geologic hazards known as “Earth fissures” are large (hundreds of meters to kilometers in length) surface cracks that occur in semi-arid and arid portions of the U.S. and Mexico. Despite an appearance akin to fault rupture zones, Earth fissures are tension cracks that form as a result of land subsidence and differential compaction in the subsurface driven by groundwater pumping and aquifer depletion. Large cracks of similar appearance and length to Earth fissures also occur in such dry environments, but with a distinctly different morphology of multiple bifurcations that leads to patterned ground of giant, macro-polygon structure. The driving mechanism(s) for macro-polygon crack development is not entirely clear but is conventionally attributed to volumetric contraction by desiccation of clay-rich sediments. When cracks of either type first emerge at the land surface it can be difficult to differentiate their classification and, more importantly, their cause. This study uses numerical modeling to explore different hypotheses for the genesis of two cracks that have recently formed at Fort Irwin, California, where their presence on an active airstrip represents a geologic hazard to ongoing flight operations. We modeled the formation of cracks using a discrete element method and computational resources at the Purdue Rosen Center for Advanced Computing in West Lafayette, Indiana, to solve for the motion integration scheme involving thousands of particle elements. After calibrating the model to laboratory experiments of soil desiccation, we performed numerical experiments to evaluate crack development in large domains via deep desiccation and basin tectonic processes. Results indicate that it may be difficult to generate macro-polygon cracking morphology solely by the desiccation process. However, the 2-D basin simulations suggest that, when subjected to tensile stresses, tectonic cracks can be propagated to the surface by desiccation. These results challenge the accepted notion that macro-polygons are caused by desiccation alone and are proving helpful for developing the hazard mitigation strategy at Fort Irwin.