EVOLUTION OF SMALL- AND MESOSCALE STRUCTURE AND FABRIC IN AN ACCRETIONARY WEDGE

Synthesis of outcrop observations of structure and fabric to give a picture of the geometry, constitution, and history of an orogenic belt is carried out from smaller to larger scale.  From the synthesis, a model for the kinematics may be formulated – e.g., the sequential emplacement of thrust sheets and their deformation.  How do we simulate outcrop observations from a large-scale dynamical model?  We formulate a simple dynamic model for a large portion of an orogenic belt, and use it to simulate the distribution of structures and fabrics that might be seen at the outcrop.  To make it permissive to set aside complexity associated with metamorphic and igneous processes, we model the portion of a steady-state accretionary wedge near its “backstop.”  A rectangular region of thickness H << L and width L/2, corresponds to ~1/2 the total breadth of the wedge; taper is accounted for by allowing topographic relief, A, over the width.  Plane flow is described by a stream function

Y = UH{a + (z/H) – 1/2 [(1 + a)/(1 + D)](z/H)²(3 – z/H)]}sin(lx)

specified by only three dimensionless groups: the aspect ratio, k = 2p(H/L); the ratio of underplating (W) to subduction drag velocity (U), a = (W/U)(1/k); and the ratio of back-flow accommodated by erosion and transport (~EA) to that by internal deformation, D = (3hE/rgH)(1/k²), where h is wedge viscosity.  Topographic relief is A = (W + kU)/(E + k²rgH/3h).   Deformation of underplated elements is computed along trajectories to give its distribution within the wedge.  A “nested” computation is carried out for small-scale structures, such as asymmetric folds, and fabric within an element, using its large-scale kinematic history. We seek to determine the discriminatory power of the distribution of structures in an inversion for the global parameters k, a, and D.

Research supported by NSF OPP-9815160.