Discrete Element Modeling of Extensional Fault-Related Monocline Formation and Fault Zone Evolution
The interplay of faulting and folding has long been recognized in extensional systems, and outcrop, subsurface, and physical and numerical modeling investigations illustrate the influence of mechanical layering on development of normal-fault related monoclines. Recognizing the conditions for development of normal-fault related folding is important for accurate seismic interpretation, geometric modeling, fault seal investigations, and trap analysis. We use discrete element modeling to explore the impact of mechanical layering and fault geometry on normal-fault related folding. To develop and calibrate this approach, we use a well-exposed field example from Iceland where the layered sequence consists of strong basaltic lava flows and intercalated weak volcaniclastic deposits. The exposure provides a 3D view of the breached monocline that has 35-40 m of throw from horizontal layers along the footwall to horizontal layers on the hanging wall of the fault, and a hanging wall monocline with a limb width of 85-115 m and limb dip of 15-23°. The 2D discrete element models honor the mechanically stratified character of the deformed sequence and attempt to replicate the monocline geometry by simulating reactivation of a buried fault beneath a mechanically stratified sequence. A suite of models explored the role of fault dip, thickness and proportion of incompetent layers, and amount of displacement. The best match was obtained using a refracted fault geometry with a steep upper segment that transitioned to a moderate dip at depth, and a cover sequence with 40-m-thick strong layers alternating with 10-m-thick weak layers. Model results show that initial faulting is accommodated by folding (monocline development) of weak intervals and opening-mode (extension) fracturing of strong layers. Maximum monocline width is developed early and the width remains stable or narrows with increasing displacement. Brittle deformation in each strong layer is initially decoupled from the other strong layers. As fault displacement increases, through-going fracture connections develop that link segments and become the active fault with a distinct scarp. Consistent with anecdotal field observations, model results suggest that significant subsurface fracture porosity and permeability is developed in brittle competent layers during early deformation. This modeling approach is well suited for exploring extensional deformation processes involving interlayered competent and incompetent layers.
AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018