USING IMAGE ANALYSIS TO DOCUMENT THE EVOLUTION OF A CRUSTAL-SCALE FAULT THAT CLOSED THE MIDCONTINENT RIFT

Although the formation of the Mid-Continent Rift at ca. 1.1 Ga is commonly considered the last major tectonic event to have affected the central part of North America, the subsequent closure of the rift tens of millions of years after it formed was itself a significant but as yet poorly understood event. Rift-bounding normal faults were reactivated as crustal-scale reverse faults, with net reverse displacements of hundreds of meters. One such structure is the Marenisco-Atkins Lake Fault zone in northern Wisconsin. It is a north-dipping listric fault along which lower Proterozoic metasedimentary rocks and middle-Proterozoic rift-related volcanic rocks have been thrust southward over Archean gneisses (the Proterozoic sequences had occupied a structurally lower position as a resulting of graben formation during rifting). The fault zone is especially well exposed along the Marengo River in eastern Bayfield County. Here the fault zone is ca. 100 m wide and preserves microstructures recording deformation over a wide range of temperatures. Ductilely deformed feldspars are overprinted by cataclastic textures and hydrothermal veins. The samples also show progressive reduction in grain size and degradation of grain boundaries with increasing deformation. Trends in crystal size, size distribution, angularity and the like can be documented quantitatively using freeware image analysis programs such as Image J, yielding new indices that can serve as proxies for strain. This study illustrates the power of image analysis techniques for reconstructing the deformational history of a major fault zone.