Quantitative Geometry of Thrust and Fold Belt Structures

Abstract

Principles of step-faulting are simulated in computer-synthesized and balanced structural cross sections of faulted and folded terranes. Isolation of geometry from mechanics and chronology allows a review of kinematics and chronology of deformation in layered rocks. Blind faults and duplex structures are geometric consequences of step-faulting, and are responsible for disparities in crustal shortening between superposed tectonostratigraphic units. These disparities are greatest in fold belts, which overlie belts of blind thrusts. Failure to recognize blind thrusts leads to incorrect estimates of both timing and amount of deformation. Geometric modeling demonstrates the intuitive nature of some widely accepted geologic assumptions. Geometric considerations alone suggest th t faults and folds, which can be represented as products of faulting, form in response to a balance between differential tangential and vertical stresses. Where tangential stress provides the driving force, distribution of overburden load appears to control location of thrust ramps. Forward modeling by computer generates balanced cross sections at all stages of growth of a given structure, and shows that purportedly balanced cross sections may include impossible intermediate stages. Forward modeling also reveals geologic questions that must be answered before more sophisticated computer modeling programs can be written.