Abstract

Deformation zones along basement-cover contacts, which are commonly loci of significant displacement, may also be zones of limited movement when cover and basement are folded together. Proterozoic basement gneisses and siliciclastic cover rocks of the Uncompahgre Group in the Needle Mountains, Colorado, are in contact along steeply dipping deformation zones. Analysis of structures in the basement and cover, and along their contacts, provides evidence for (1) deformation of the basement prior to deposition of the Uncompahgre Group, (2) north-directed, thin-skinned thrusting, and (3) cuspate infolding of cover into basement accompanied by upward and outward movement of the cover. Opposed senses of movement along basement-cover contacts on opposite limbs of the synclinorium generated during cuspate infolding imply that the Uncompahgre Group is parautochthonous on basement. Conjugate strike-slip shear zones in both units accommodated north-northwest shortening. Deformation of basement and cover into cuspate structures may explain the presence of some cover sequences as isolated remnants within gneissic basement.

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