The roles of volume loss, coaxial versus noncoaxial flow, and blueschist exhumation in subduction-related accretionary wedges are still poorly understood. In our study at Leech Lake Mountain in the Eastern belt of the Franciscan subduction complex, we focus on these subjects. In the specific example of the Franciscan, the tectonic significance of the boundary between the Eastern and Central belts remains controversial. The Leech Lake Mountain area in northern California is situated immediately above this boundary and, therefore, appears to be of crucial importance for understanding aspects of the tectonic evolution of the Franciscan.
The structural development at Leech Lake Mountain is characterized by three deformational events. D2 produced the regional cleavage (s2) during blueschist-facies metamorphism. D2 is the only entirely ductile deformation event and was associated with accretion-related internal stacking within the Yolla Bolly terrane. The s2 cleavage is folded by F3 folds, which are overturned to the west. Spacing of a related s3 cleavage is generally at the decimeter scale, but locally is spaced at the centimeter scale. The s2 and s3 cleavages, and F3 folds, are overprinted by tight to open F4 folds at the decimeter to map scale. Map-scale F4 folds are largely upright, whereas small-scale F4 folds are either upright or overturned to the east.
Our finite-strain analysis relates deformed lengths to original lengths in the rock and thus provides an absolute reference frame, which allows us to detect deformation-related volume changes. Absolute finite-strain data from 20 samples support earlier studies indicating that the development of the moderately dipping s2 cleavage was accompanied by pronounced volume loss, averaging 36%. The directions of the principal finite-strain axes are scattered; therefore, we calculated a tensor average of the data. The principal stretches of the tensor average are SX:SY:SZ = 1.06:0.91:0.66, indicating that the accumulation of ductile strain was characterized by subvertical shortening, which was largely compensated for by volume loss and not by orthogonal extension. Evidence for noncoaxial deformation in our samples is limited, indicating that the accumulation of ductile strain was largely coaxial.
Our preferred tectonic interpretation is that accretion and blueschist-facies metamorphism of the Yolla Bolly terrane at Leech Lake Mountain occurred during D2 under an almost coaxial deformation regime. During and after D2, the rocks were considerably exhumed and were telescoped by D3 top-to-the-west out-of-sequence thrusts at shallow-crustal levels. In the Leech Lake Mountain area, D3 thrusting juxtaposed the Eastern and Central belts along the Red Mountain fault and cut out the lower tectonic units of the Eastern belt. The difference in the degree of peak metamorphism between the Eastern and Central belts is modest, indicating that displacement at the Red Mountain fault was not large. We propose that D3 out-of-sequence thrusts attenuated the metamorphic and stratigraphic section across the entire Franciscan and overlying Great Valley forearc. The final D4 event produced large-scale folds. All three events resulted from horizontal shortening. There is no evidence for a major phase of horizontal extension, which might have aided exhumation of the Eastern belt blueschists. Modest tectonic exhumation was due to vertical ductile shortening during D2.