PROGRESSIVELY DEFORMED GEOMORPHIC SURFACES CONSTRAIN THE BALANCE BETWEEN THRUST WEDGE THICKENING AND FORELAND PROPAGATION

Critical wedge theory for fold-thrust belts requires balance between thickening the orogenic wedge and the foreland propagation of the wedge tip. Thus, over long time scales, fault activity should alternate between hanging-wall thrusts that serve to thicken the thrust wedge, and the range-bounding thrust fault that accommodates foreland propagation, or, the thickening and propagation could occur simultaneously, such that the hanging-wall faults also rupture in large earthquakes on the basal detachment as observed in accretionary wedges. Maps of currently active faults and folds or bedrock deformation alone cannot differentiate between these models, because the former only provides an instantaneous view and the latter cumulative deformation. However, the structural and temporal evolution of a thrust fault system will be recorded in the progressive deformation of a sequence of geomorphic surfaces that are preserved across the deforming region.

To constrain the subsurface fault geometry and examine this balance between wedge thickening and propagation in a foreland fold-thrust belt, we examine an ~50 km transect across the central Alaska Range where a late Pliocene/early Pleistocene alluvial surface and flights of Pleistocene glaciofluvial terraces are preserved across an active thrust system. The Quaternary deformation of these surfaces is characterized by long, planar, progressively rotated or uplifted dip panels separated from adjacent panels by relatively narrow hinge zones. We developed a cross-sectional model for this thrust system, that consists of two north-dipping thrust faults that merge with the major south-dipping basal thrust fault at depth. The hanging-wall thrust faults and associated folds thicken this thrust wedge while the basal thrust accommodates propagation of the thrust tip. We find that the old alluvial surface and terrace deformation clearly define different phases in the timing of development and activity of the hanging-wall thrusts relative to the basal thrust. This supports the model of alternating phases of internal deformation and foreland propagation of continental thrust belts.