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Rotational Folding in the Western Transverse Ranges, California

N. W. Onderdonk
California State University, Long Beach, CA, [email protected]

Vertical-axis rotation of the western Transverse Ranges during late Miocene time to the present was accompanied by the development of a fold and thrust belt at the northern edge of the western Transverse Ranges. These faults and folds exhibit increased amounts of shortening along strike as a result of differential rotation. Reconstructed cross-sections across the northern edge of the rotated domain show that approximately 25% of the total rotation was facilitated by folding of Miocene and older sedimentary strata. This observation, and similar relationships observed in fold and thrust belts in other parts of the world, indicate that rotational folding is an important mechanism in vertical-axis crustal rotation.

Rotational folds exhibit distinct geometric characteristics that can be used to identify rotational deformation and estimate the amount of rotation. Some identifying characteristics include a consistent change in apical angles along axis, a consistent change in fold amplitude along axis, and anticline and syncline axes that plunge at different angles (in some cases opposite directions). In an upright rotational fold pair, the difference between anticline and syncline plunge angles will increase with increasing rotation and can be used to calculate the rotation amount across one fold wavelength.

Rotational folding can present complications to the interpretation of paleomagnetic data. Paleomagnetic vectors collected from the limbs of rotational folds may exhibit a component of vertical-axis rotation in the opposite sense of the rotation of the fold axis, and if not restored correctly will result in an underestimation of rotation amount.

AAPG Search and Discovery Article #90088©2009 Pacific Section Meeting, Ventura, California, May 3-5, 2009