Numerical modeling of frontal and basal accretion at collisional margins

Abstract

We investigate the deformation of orogenic wedges that form in the early stages of continent‐continent collisions using a two‐dimensional numerical model limited to the upper lithosphere. Our models show that deformation at the plate margins is influenced by rheology, surface processes, and the balance between inward mass flux and outward subduction flux, as controlled by the subduction load (which represents the effects of slab pull and resistive forces) and flexural downbending. We find three characteristic deformation modes: (1) near‐pure subduction with little or no accretion; (2) frontal accretion with development of an accretionary wedge built up by offscraping of the sediment layer at shallow depth; and (3) independent frontal and basal accretion where a retrothrust allows stacking of basement nappes at crustal to mantle depths. Near‐pure subduction is enabled for “ordinary‐rheology” materials, characterized by brittle and viscous material behavior (approximating a “Christmas tree‐type” depth profile), and almost zero friction along the subduction shear zone. Frontal accretion occurs when slightly increased friction along the subduction shear zone allows offscraping of the sediment layer from the subducting plate. Independent frontal and basal accretion develops in strong‐rheology models with an almost fully brittle material behavior. Major surface erosion or a reduction of the subduction load promote the development of large basement nappes. Frontal accretion is favored by major sedimentation during convergence, a large backstop, and in the case of a lateral transition from a “strong‐rheology” to an “ordinary‐rheology” subducting plate. Our numerical models develop first‐order characteristics as observed in natural orogenic wedges, for example upper crustal nappe stacks, frontal and basal accretion, or extension in the core of an orogen. Frontal and basal accretion are interdependent, and tend to stabilize the subduction system.

Thick-Skinned and Thin-Skinned Tectonics: A Global Perspective

“…[144] explain this change by the effect of the crustal root beneath the Alps, which induces a component of gravity spreading in the uppermost part of the orogen. In fact, numerical modeling also show that such an extension parallel to the overall contraction develops in the core of model orogens as a consequence of gravitational spreading [39,55].…”

“…[39] discuss the importance of erosion and could show that basal accretion is directly linked to erosion.…”

“…Simplified drawing of numerical experiment SL2E by [39]. Numbers show sequence of formation of thrust sheets.…”

“…One may also consider that any rock uplift by motion and thickening of thrust sheets will be enhanced because erosion removes some of the load, which has to be dragged up passively. Numerous numerical and analog models have shown the importance of surface processes (erosion and sedimentation) on the evolution of the internal structure of (model) orogens ( [39,[53][54][55][56][57] just to name a few). At orogen scale, a mountain range with a crustal root formed in a thick-skinned tectonic style that is submitted to horizontal shortening and to erosional surface processes may decay by "erosional collapse" or "subsurface collapse", depending on the intensity of erosion [58].…”

Thick-Skinned and Thin-Skinned Tectonics: A Global Perspective

“…Nappe stacking increases the volume of the orogen with rock material being accreted at its front and base. [44] studied the factors influencing frontal and basal accretion at collisional margins by numerical modeling and found that the most influential factors are the rheology of the material, surface processes and the balance between inward and outward material flux. These models suggest that a mechanically strong crust with almost fully brittle material behavior develops frontal and basal accretion, which occur independently and spatially separated.…”