RELATIONSHIP BETWEEN FOLD GEOMETRY AND CHANNEL SINUOSITY OF THE MIDDLE FORK OF THE AMERICAN RIVER NEAR HORSESHOE BAR, SIERRA NEVADA, CALIFORNIA

Near its confluence with the Rubicon River, the Middle Fork of the American River exhibits an unusually high degree of sinuosity compared to other nearby stretches of river. Here, the river cuts through the Lang Sequence of the lower Shoo Fly Complex and is just upstream of the suture zone defined by the Feather River Belt (FRB). Structural features observed in the Shoo Fly suggest a possible causal relationship between fold geometry, structural fabrics and the sinuous channel morphology.

Preliminary findings from field work and GIS-based data analysis indicate the presence of several large scale (~1 km wavelength), moderately- to steeply-plunging asymmetric folds that have an average trend and plunge of 50,160. Fold hinge zones are, in some instances, exactly coincident with the axis of meanders in the river channel. Linear segments of the river lie parallel or subparallel to compositional layering and/or penetrative cleavage planes found on the fold limbs. On some stretches of river, shorter wavelength/smaller amplitude meanders occur along a single fold limb. Here, the apex of meanders are coincident with where fluvial erosion has cut across competent layers while erosion of less competent units allows channel development parallel to layering. Parasitic folds along the limbs of larger folds may also influence channel morphology in some areas. River sinuosity decreases eastward from the FRB suggesting that regional folding may be related to deformation associated with the suture zone.

Although detailed structural analysis is hampered by limited exposures away from the steep river canyon of the Middle Fork, several parasitic fold hinge areas have been studied in an effort to relate their geometry to that of larger regional structures. Folds within these areas are generally asymmetric and vary in shape from gentle to tight. Individual small-scale fold hinges vary in plunge from 12 to 85° toward the southeast. In some locations, small-scale folding has been found to exhibit an abrupt reversal in plunge direction within a single outcrop. Variation in both plunge direction and magnitude indicates that folding within the study area is highly noncylindrical and/or represents a fold interference pattern from refolding during a later deformational event, as documented elsewhere in the Shoo Fly Complex.