KINEMATICS AND STRAIN SIGNATURE OF CA. 1.45 GA SHEAR ZONES IN THE SIERRA MADRE, WYOMING

The tectonic setting of the transcontinental belt of 1.4 Ga intrusions has been the subject of considerable recent debate. Many 1.4 Ga plutons in the western U.S. exhibit evidence for N-S or NNW-SSE shortening deformation and some contain evidence for a dextral component of shear. Some workers have argued that deformation observed within and adjacent to the 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 southeastern Wyoming away from 1.4 Ga plutons.

Structural studies in the Sierra Madre, southern Wyoming, indicate that parts of the 1780-1750 Ma Cheyenne belt were reactivated, probably at ca. 1.45 Ga. New shear zones were also formed at this time. Shear zones are generally cataclastic but locally 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 (technique of Marrett and Allmendinger, 1990) 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 tension axes are subhorizontal and about 070° consistent with NNW shortening accommodated by conjugate strike-slip shear zones.

The NNW-vergent cataclastic fault in the Sierra Madre corresponds to the surface expression of a strong south-dipping reflector imaged by seismic studies associated with the CD-ROM experiment. We interpret this as supporting evidence for thrusting in the Sierra Madre ca. 1.45 Ga with NNW displacement of 10 to 50 km. We conclude that there is significant deformation ca. 1.45 Ga that is related to regional stress rather than emplacement stress.