Abstract

Precambrian crystalline rocks and younger sedimentary rocks were involved in Laramide deformation in the western part of the United States. Near Buffalo, Wyoming, two high-angle dextral faults striking N. 10° E. cut both the upturned sedimentary rocks and the crystalline basement. As this trend does not correspond to directions of transcurrent faulting expected with east–northeast-west–southwest compression (indicated by the north–northwest-trending Clear Creek thrust and upturned sedimentary rocks), the writer studied Precambrian rocks to see if basement structure controlled the strike of the N. 10° E. faults.

The Precambrian complex consists of quartz-biotite-plagioclase gneiss and migmatite with minor hornblende gneiss and amphibolite. These rocks are greatly deformed and partly granitized. Mafic dikes and small ultramafic bodies cut the complex. Early foliation and layering are obscured by later shear fracturing. Three prominent directions of high-angle fractures are N. 10° E., N. 45° E., and N. 75°–80° W.

In the sedimentary rocks, N. 45° E. and N. 75° E.–N. 75° W. fracture sets are well developed; the N. 10° E. set is poorly developed.

The pronounced development of the N. 10° E. fractures and shear zones in the Precambrian rocks and absence of these structure trends in the sedimentary rocks suggest that these fractures and shears were formed during a Precambrian deformation and provided zones of weakness which controlled Laramide tear faulting. Three diabase dikes in the Precambrian rocks between the faults also follow this direction.

The other two trends cutting both the gneisses and the sedimentary rocks are probably Laramide in age, although an east-west dike and a northeast-trending dike in the south end of the area suggest that movement was reactivated along older Precambrian fractures.

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