THE FOWLER PASS SHEAR ZONE: A PERSISTENTLY REACTIVATED PROTEROZOIC STRUCTURE IN THE SOUTHERN ROCKY MOUNTAINS

The Fowler Pass Shear zone is a NW striking ductile shear zone exposed in the Cimarron River Canyon of the Sangre de Cristo Mountains in New Mexico. The shear zone is composed of NW striking fabrics that are the result of multiple episodes of deformation that began in the early Proterozoic and occurred as recently as the early Cenozoic Laramide orogeny.

The earliest deformation phase recognized within the Fowler Pass shear zone is associated with the emplacement of >1678 Ma Proterozoic calc-alkaline plutons. These plutons contain NW striking, subvertical magmatic foliations which are concordant with foliations in the low grade schists the plutons intrude. Fabrics within the country rocks are defined principally by chlorite and fine grained aggregates of albite and quartz indicating greenschist facies metamorphism. This combined with narrow to nearly absent contact aureoles suggests the plutons were emplaced at shallow crustal levels. Isoclinal folds near the pluton margins suggest NE-SW directed shortening.

Overprinting these fabrics are mylonites which increase in intensity towards the SW towards upper amphibolite to granulite facies gneisses that make up the most of the Cimarron range. Mylonitic fabrics strike NW and are steeply NE dipping. Mineral lineations plunge to the NW and record sinistral-normal displacements. Metamorphic mineral assemblages within the mylonites suggest upper greenschist facies conditions and Ar dates suggest deformation at 1.4 Ga.

Separating the mylonites from high grade rocks of the Cimarron range is a NW striking SW dipping brittle reverse fault. The last motion on this fault has to be pre-45 Ma and post Triassic, suggesting a Laramide age. However, combined gravity data and remote sensing imagery suggests that this brittle fault may have also been active during the Ancestral Rockies orogeny.

These observations demonstrate that the Fowler Pass shear zone represents a zone of persistent weakness that has controlled the location of major structures within this portion of the southern Rocky Mountains. It is likely to be typical of such crustal flaws providing a window into the processes that control crustal reactivation.