Strain partitioning and the formation of forearc slivers at oblique convergent margins: Insight from numerical modeling.

摘要

Oblique relative plate motion is ubiquitous at convergent margins, often resulting in a significant component of motion parallel to the margin. Partitioning of relative plate motion can result in deformation that is accommodated as spatially distinct margin-parallel shear and margin-normal thrusting, and lead to the development and migration of crustal slivers. These slivers, bounded by thrust faults at the trench and arc-ward by a well-developed margin-parallel strike-slip fault, are observed at about half of all modern convergent boundaries. Some modestly oblique settings have developed fore-arc slivers while other margins, with higher obliquities, have failed to effectively partition plate motion into distinct zones suggesting mechanisms other than obliquity are important in partitioning. Analog modeling has shown that pure frictional wedges always partition deformation but produce sliver like motion and structures at only very high obliquities. The presence of ductile layers at depth in some analog models, however, can localize shear at much lower obliquities. In light of this, we have performed, for a wide range of obliquities, finite-element numerical simulations of convergent wedges with similar geometries and distributions in strength as layered analog models, with a basal ductile layer. For these models, we solve force-balance equations for Stokes flow using COMSOL Multiphysics in order to quantify the magnitude and style of stress. Our numerical models display a similar distribution of cross-sectional topography and surface velocity fields compared to their counter part oblique analog experiments. The numerical models also demonstrate a progressive localization of margin-parallel shear with the growth of wedge topography. All wedges with a non-zero obliquity eventually show the onset and localization of shear indicative of strike-slip deformation, which we quantify by calculating the principal horizontal stress field, as well as, the margin-normal and margin-parallel components We show that the distribution of the angle 'alpha' across the wedge, where alpha is defined as the angle between the greatest stress and the margin normal, is diagnostic of partitioning. The distribution of alpha allows us to constrain the transition between distributed shear and localized partitioning in the evolving wedges. These results suggest, in conjunction with analog models, that viscous behavior at depth, responding to increasing topography during convergence, work to localize margin-parallel shear in obliquely convergent wedges and gives a mechanism for the development of fore-arc slivers in nature at small obliquities.