Structural analysis of a ductile-brittle Precambrian shear zone in the Sierra Madre, Wyoming: western extension of the Cheyenne belt?

Abstract

Abstract Existing plate tectonic models for the Cheyenne belt, a Proterozoic suture at the southern margin of the Archean Wyoming craton, are based largely on data from the Medicine Bow Mountains, southern Wyoming. This paper tests the hypothesis that a major structural and geochronological discontinuity in the Sierra Madre of southern Wyoming is the western extension of the Cheyenne belt. The Sierra Madre shear zone consists of a cataclastic western segment and a mylonitic eastern segment that comprises several mylonite zones. The eastern segment records early ductile thrusting (ca. 1750 Ma) and later dextral strike-slip faulting as revealed by macrostructural, microstructural, and quartz fabric analysis. Structures associated with these events are kinematically compatible and coeval with structures within the suture zone in the Medicine Bow Mountains. The correlation of shear zones in the two ranges is supported further by lithologic similarity of distinctive rocks north of and within the shear zones and the presence of unusual quartz fabrics documenting 〈 c 〉 slip in synkinematic granites in both ranges. Isolated exposures of mylonite along the dominantly cataclastic western segment suggest that the mylonitic shear zone was formerly more extensive. The western segment of the Sierra Madre shear zone is a single zone of intense cataclasis that truncated and dismembered the earlier ductile shear zone. Kinematic analysis within the cataclastic zone suggests that it formed during north-directed thrusting. East-striking, south-dipping thrust faults and associated overturned-to-the-north recumbent folds are developed in supracrustal rocks directly north of the cataclastic fault zone. This spatial association of north-vergent structures and their apparent kinematic compatibility suggests that they are genetically related. Two major northwest-striking, cataclastic, dextral strike-slip in the eastern Sierra Madre appear to merge with the cataclastic thrust fault. These cataclastic faults are interpreted as parts of a north-vergent, thrust-tear system that cut across the original suture zone as rocks in the upper plate of the thrust were transported to the north. It is not known whether this deformational event occurred in response to continued, post-suture convergence to the south or if it represents a separate and distinct Precambrian event. The prominent bend in the Sierra Madre shear zone may be an oroclinal flexure produced as a consequence of northward translation along the cataclastic thrust system.