ABSTRACT: Concordant Geometry of Map-Scale Folds and Plutons: Implications for the Structural Framework of the Sierra Nevada, California
SAWLAN, MICHAEL G., U.G. Geological Survey, Menlo Park, CA
Analysis of the geometry of pendants and plutons from published maps of the Sierra Nevada suggests that a major northwest-trending fold belt formed in the Sierra Nevada in the Late Jurassic. Hingelines of inferred folds trend N20-45 degrees W and are large: crest to crest distances range from 7 to 17 km and some hingelines appear to extend along trend for more than 150 km. The inference of map-scale folds derives primarily from an interpretation that arcuate shapes of pendants represent remnants of fold closures; linear pendants, characteristically elongate to the northwest, represent homoclinal parts of fold limbs. South of 37 degrees N, folds appear to plunge gently (less than or equal to 25 degrees ) to the northwest; north of 37 degrees N, fold axes may be subhorizontal or plunge ently southeast. Facing directions of homoclinal sequences in elongate pendants and in ophiolite sequences along hinge zones of northwest-plunging folds in the western Sierra foothills were used as well-controlled data points to infer the sense of folding. Consistent sense of closure along trends of folds and an alternating sense of closure in a direction normal to fold axes were also used to establish the sense of folding in those hinge lines where no independent determination is currently possible.
The geometry and internal structures of many Cretaceous plutons are concordant with the inferred folds, suggesting that folds served as structural traps for these magmas. Individual plutons commonly exhibit arcuate, sinusoidal, or elongate forms coaxial with the trend of plunge of folds inferred for the country rock. Structural analysis of attitudes of contacts between nested plutons of the Mitchell and Whitney Intrusive Series shows pseudofolds with axes plunging ~15-35 degrees to N20-45 degrees W. Map-scale pseudofolds defined by igneous foliations in undeformed plutons are concordant with inferred folds. Pseudofolds in the Lamarch granodiorite (37.1 degrees N), are subhorizontal; a pseudosynform in the Coyote Pass granodiorite, located further south (36.3 degrees N), plunges 10-15 egrees to the northwest. Development of pseudofolds in undeformed plutons is attributed to rotation and alignment of crystals during emplacement of magma into fold structural traps. The exceptional length of the Whitney Intrusive Series (WIS), more than 80 km in a direction of N35 degrees W, may reflect emplacement of magma into the hinge zone of a single antiform. Individual plutons of the WIS have quasicylindrical surfaces that plunge gently to the northwest. In plan view, contacts between nested plutons of the WIS describe arcs, convex to the northwest, and dike-like limbs of individual plutons dip steeply away from their long axes. Asymmetry in limbs of WIS plutons suggests that folds in the country rock verge steeply to the northeast, provided that pluton geometry reflects shapes of the confirming surfaces. The similar geometry of inferred folds in both metamorphic rocks and in quasicylindrical forms in plutons is attributed to structural control of magma emplacement by fold geometry. A corollary is that the maximum widths of plutons may be partly a function of fold half-wavelengths.