A KINEMATIC REEVALUATION OF THE CHEYENNE BELT SUTURE ZONE: TESTING THE TRANSPRESSION HYPOTHESIS

The Cheyenne belt in SE Wyoming is one of the oldest known unambiguous fossil plate boundaries. Rocks of the belt record a rift-to-drift sequence followed by reactivation of the passive margin and suturing of disparate crustal blocks—the >2.5 Ga Wyoming province and the <1.8 Ga Colorado province. High-strain zones interpreted as the suture zone are characterized by subvertical foliations and dominantly down-dip lineations or apparent flattening strain (S-tectonites). Numerical simulations suggest that this strain geometry may record transpressional deformation with a large strike-slip component. To test this transpression hypothesis we are conducting a series of detailed studies across the suture zone. These localities are: 1) the northern mylonite zone near Bear Lake, 2) the northern mylonite zone in the North Mullen Creek canyon, and 3) the northern mylonite zone along the North Platte River. In each area detailed geologic mapping will be combined with quantitative kinematic analyses.

In the Bear Lake locality high-strain rocks are characterized by subvertical foliations and a S-to-N transition from dominantly S-tectonites to a consistent down-dip mineral lineation on the N side of the high-strain zone. Quantitative kinematic analyses of subhorizontal and subvertical faces of S-tectonites indicate a large component of sinistral strike-slip motion in the southern part of the high-strain zone.

In the North Mullen Creek locality high-strain rocks are characterized by steeply NW-dipping foliations and down-dip mineral lineations. Quartzite of the rift sequence is complexly folded with crystalline rocks of the Colorado province. Preliminary quartz c-axis-fabric analyses indicate strain-path partitioning with SE-side-up, simple-shear-dominated flow at the zone margins and an increasing pure-shear component accommodating subvertical elongation in the center of the zone. The northern mylonite zone in this area may represent a stretching fault accommodating vertical extrusion of material out of the Cheyenne belt.

In the North Platte River locality high-strain rocks are characterized by subvertical S-tectonite fabrics. Where mineral lineations are present they vary from subhorizontal to steeply plunging. These observations are compatible with strike-slip dominated transpression.