SOME IMPORTANT CONSEQUENCES OF REACTIVATION TECTONICS: NEW GEOLOGICAL PRINCIPLES DISCOVERED

In a previous GSA talk (1999 Annual mtg), the writer showed conclusive evidence that most faulting within U.S. sedimentary basins results from reactivation of pre-existing shear zones in the underlying Precambrian basement. This conclusion was amply proven by hundreds of correlations of aeromagnetically-mapped basement faults/shear zones with faults, folds, fracture zones and stratigraphic features in the overlying sedimentary section, documented by detailed petroleum industry subsurface and seismic data in 21 U.S. sedimentary basins. If this correlation is true for basins, then it applies to cover rocks overlying cratons everywhere. The consequences of this are profound and far-reaching, and I will present three examples of the type of geologic advances that can result from an understanding of reactivation tectonics: No. 1) Anticlines are the most common, and most studied, geological structures, after faults, that we know of, and most geologists would say there is little more we can learn about their formation. Yet there is an important characteristic of anticlines not usually considered - longitudinal closure. We acknowledge transverse closure due to transverse shortening, but we neglect shortening that occurs along the long axis of the anticline. I will show, by resolution of stress vectors, that this longitudinal shortening must result from reactivation of a causative underlying basement fault not at right angles to maximum compressive stress. In fact, as the strike of the underlying fault departs more and more from this angle, the anticline becomes less elongated until at approximately 45° it becomes dome-shaped. Enigmatic compressional domes are therefore explained by this mechanism. No. 2) Side-stepping fault systems are infrequently recognized on geology maps but appear to be common in compressive structural regimes. They occur when faults or folds are formed in areas where a set of parallel basement faults is not at right angles to maximum compressive stress. No. 3) Jointing of sedimentary rocks is a phenomenon that has never been completely and satisfactorily understood since joints were first mapped by geologists in the 1800's. However, the occurrence of sets of parallel joints over wide regions is readily explained by meter-scale reactivation of sets of pervasive and parallel basement faults.