THE CHEYENNE BELT IN THE SIERRA MADRE, WYOMING, IS NOT THE CHEYENNE BELT: IT IS A CA. 1.4 GA THRUST-TEAR FAULT SYSTEM

During the past ten years, there has been a re-thinking of the origin and tectonic setting of the 1.4 Ga magmatic belt that extends across North America. Many 1.4 Ga plutons in the western U.S. record NW-SE shortening strains, in apparent conflict with the more conventional model of extensional tectonism at that time. Some workers argue that shortening deformation associated with 1.4 Ga plutons is due to emplacement-related, rather than regional, strains. To avoid possible complications associated with emplacement-related strains we examined evidence for suspected 1.4 Ga deformation in SE Wyoming away from 1.4 Ga plutons. We have identified a major thrust-tear fault system, the Battle Lake fault zone (BLFZ), in the Sierra Madre, Wyoming. This fault system partially reactivated and displaced parts of the 1780-1750 Ma Cheyenne belt 10-20 km to the north-northwest at ca.1.45 Ma. The BLFZ corresponds to the surface expression of a strong south-dipping reflector imaged by the CD-ROM seismic line. Shear zones associated with this fault are generally cataclastic but do contain transitional brittle-plastic features. Retrogression of former amphibolite-facies assemblages to the greenschist facies accompanied deformation. Two preliminary U/Pb dates of 1326+130 Ma and 1500+46 Ma (synkinematic epidote and sphene, respectively) suggest that deformation may have been broadly coeval with ca. 1.4 Ga magmatism. In the western and central Sierra Madre, cataclastic lineations and crystal fibers are oriented down dip, and kinematic analysis of cataclastic shear zones yields subhorizontal shortening axes of 330-350° and steeply plunging extensional axes, consistent with bulk reverse-sense shear. South-side-up kinematics dominate over subordinate north-side–up movement sense. In the eastern Sierra Madre, lineations are indicative of strike-slip motion. Subhorizontal shortening axes are oriented at 340°, whereas extension axes are subhorizontal and about 070° consistent with NNW shortening accommodated by conjugate strike-slip faults. The kinematics documented in this study are similar to results from many ca. 1.4 Ga plutons throughout the SW U.S. The relatively uniform strain signature of 1.4 Ga deformation throughout the southwest is more consistent with regional, rather than local, emplacement related stresses.