Abstract

Conjugate folds and cleavages in the central Sierra Nevada, California, are developed in isolated domains along, or near, ductile shear zones. The conjugate folds form by rotation of older (Jurassic-Cretaceous) fabrics and removal of material along microlithon boundaries and apparently postdate all significant metamorphic events; the conjugate structures in this region are, thus, Cretaceous or Tertiary. Maximum shortening directions associated with these folds are in most cases northwest-southeast, parallel to the present strike of the Sierra Nevada.

Conjugate folds near Briceburg, California, are unusual in that they are found over a relatively large region, are associated with a well-developed crenulation cleavage, and were formed due to west-southwest-east-northeast shortening. Shortening in microlithons of as much as 34% is associated with the crenulation cleavage and formed by buckling, but with a small component of flattening, of an older fabric. Removal of material and shear along microlithon boundaries caused an unknown amount of additional shortening.

Models based on gravity relaxation, elastic recovery, and folding of fabrics in shear zones do not fully explain the orientation and style of conjugate structures in the study area. Instead, the conjugate structures reflect Cretaceous or Tertiary deformation possibly associated with movement along brittle faults. In addition, shortening directions calculated from incremental strains associated with the conjugate folds near Briceburg are in direct conflict with shortening directions based on geometries of experimentally produced conjugate structures, indicating that different mechanisms may operate during formation of experimentally and naturally formed conjugate folds.

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