Balanced Cross Sections of Small Fold-thrust Structures

Abstract

Balanced, restorable cross sections for the Precambrian-cored Sage Creek anticline in the Wind River Basin are combined with constraints on the mechanical behavior of various types of basement rocks and sedimentary rocks to give a new model of the apparent folding of the upper basement surface and the early stages of a fold-thrust structure. Vertical relief on the upper basement surface develops as a result of motion along a zone of discrete, parallel faults, giving an upper basement surface which appears to be folded on some scale of observation. The sedimentary rocks above the uplift are folded into a tight anticline-syncline pair. Progressive restoration of the cross sections using the proposed geometry of the early stages of uplift shows that folding and overturning above the fault zone is the major component of shortening in the cover rocks. The late "mountain flank" thrusts, which cut the fold increase the vertical relief of the upper basement surface and extend the cover rocks which increases the fold amplitude in the cover rocks. Cross sections depicting the present geometry must balance throughout the development of the structure and satisfy the predicted mechanical behavior of the rocks involved at the conditions under which the structure formed.