ROLE OF STRUCTURAL STYLE ON CRITICAL TAPER WEDGE STRENGTH AND GEOMETRY FROM DISTINCT-ELEMENT MODELING

Critical-taper theory has given diverse insight into kinematics, roles of erosion and sedimentation, and the morphology of compressive mountain belts, much of which has been aided by extensive analog and numerical modeling. However, some workers have found the equations arduous, as many of the parameters are very hard to define or obtain in natural systems, and thus making quantitative studies of natural systems difficult. Progress has been made by recasting the parameter-rich mathematics into a much simpler form that describes a linear, co-varying relationship between surface slope and detachment dip (α, β), and internal- and basal-sliding strengths (W, F).

Using distinct-element models, we tested this simpler theory over a range of wedge strengths and structural styles. We also obtained W & F from observations of surface slope α and detachment dip β in active natural systems, all of which including the numerical models, show wedges are strong but detachments are weak, with F/W=0.1 or less.

Model-derived W & F vary about a mean that matches geometry-derived values. Time- and spatially-averaged dynamical F & W are observed to be equal to wedge-derived results. Critical taper reflects the dynamical strengths during wedge growth and is controlled dynamically as base friction varies between an assigned quasi-static value and lower values during slip events. In the wedge, W varies more than F, which may also be true for natural systems. Detachments have frictional stick/slip behavior on a basal wall, but the wedge has more going on within it. Tandem faulting & folding serve to simultaneously weaken and strengthen the wedge, and may occur anywhere: structural style may be important to wedge strength evolution.

Relationships between α and W & F are complex. All sudden, stepwise changes in α, W & F with time coincide with spikes in seismicity in the models. Large events either trigger or are triggered by large changes in F and W. Here we examine the complex details of the dynamically-driven changes in W & F by close monitoring of models at single time-step resolution. We consider the whole wedge but also indiviudal vertical swaths of the wedge where we can monitor subsets of W, F and α to determine how and why W varies during wedge evolution, specifically focusing on the role of evolving structures and overall structural style on wedge strength.