APPLICATIONS OF AN INTERACTIVE COMPUTER PROGRAM TO MEASURING HETEROGENEOUS STRAIN IN A THRUST SHEET

To study the effects of crustal heterogeneity on deformation, geologists need to estimate strain gradients at a wide range of scales, and better tools to make such time intensive measurements practical. We developed an interactive computer program designed to streamline strain measurements of elliptical objects by the Rf-Phi method. Elliptical objects are loaded from a text file or quickly traced from an imported photograph. The user ‘un-deforms' them by simple shear, pure shear, or rotation. As the ellipses' shapes (Rf) and long-axis orientations (Phi) change in the display window, the Rf-Phi plot is continuously updated. These graphical experiments to un-strain the population using a range of inverse strain ellipses can be done quickly and easily, making it possible to inspect the shapes and orientations of the ‘un-deformed' objects. Further, all the information about size and shape of objects can be exported to formatted files for statistical analysis.

We applied the program to strain measurements in the Day Mountain thrust sheet in western Massachusetts, where Mesoproterozoic gneisses and unconformably overlying Neoproterozoic Dalton Formation and Cambrian Cheshire Quartzite were thrust westward over Cambrian to Ordovician marbles of the Stockbridge Formation. We combined 2-D strain estimates from three surfaces into strain ellipsoids. Conglomerate beds in the Dalton Formation serve as an excellent strain gauge. Strain ellipsoids vary widely over scales of less than 0.5 km; they are prolate (4:1:1), oblate (4:4:1), and most commonly triaxial. The long axis of the strain ellipsoid is commonly parallel to the westward direction of thrusting, but locally it is perpendicular. Thus thrusting was not accomplished by plane-strain, but rather by adjacent regions of extending and constricting flow. Strong thrust-related deformation fabrics are restricted to the Dalton Formation; they did not develop in the underlying gneisses or in the overlying Cheshire Quartzite. Thus, strain partitioning concentrated deformation in the intermediate Dalton Formation between the two other more competent units. The heterogeneous strain recorded in the conglomerate may reflect variations in stratigraphic thickness of the Dalton Formation and/or first-order structures within the thrust sheet that impeded ductile flow.