Abstract

The Pliocene-Pleistocene orogeny that formed Santa Lucia Range folded its granitic and metamorphic basement as well as the overlying sedimentary rocks, which were 1−2½ miles thick in the area studied. The folds tend to flatten up-section from the basement and to become disharmonic high in the sedimentary sequence. Most are cut by reverse faults that also tend to die out up-section. Both folds and faults trend roughly parallel to basement foliations, which are older than latest Cretaceous. The folds are broad and open over granitic plutons and more closely spaced and tight (locally isoclinal) over metamorphic rocks. Within belts of metamorphic rocks lie the major reverse faults, which dip in the same direction as the metamorphic foliations. The deformation was thus directed appreciably by older rock units and structures.

Basement folding involved some flexure of foliated rocks, but the principal mechanism was a gross flowage due to piecemeal slip on pre-existing fractures. In the more deformed sectors, minor faults lie about 1 foot apart and have an average displacement of 6 inches. Groove directions on 1500 minor faults measured in 43 subareas indicate systematic movement patterns. Analyzed on equal-area nets, the patterns are so like that predicted for ideal shear within a fractured-rock body that their σ1 orientations can be approximated. These orientations are roughly accordant to petrofabric compression axes determined from calcite {011̄2} lamellae in 19 marbles and 26 sandstones. Most of the 81σ1 orientations trend about normal to nearby fold axes and traces of reverse faults, while σ2 orientations are parallel to these features and roughly horizontal. The vertical angles between σ1 orientations and the reverse faults measure about 30 degrees.

The average of all the σ1 orientations in the basement is a line plunging 30° N. 30° E. The range trends N. 40° W. The basement rocks of the range were shortened about 12 per cent in a direction roughly normal to the range. Despite the pronounced ordering of folds along older structures, the range itself extends in nearly a straight line across major divisions of the California basement. These relationships indicate that the range was produced by deep-seated deformation, possibly a northeast-to-southwest flowage under the crust.

The σ1 orientations in the upper sandstones typically plunge parallel to bedding. This and the occurrence of disharmonic folds high in the section indicate that basement shortening caused the upper part of the sedimentary sequence to buckle more or less independently of the basement.

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