Deformation history of the Yolla Bolly terrane at Leech Lake Mountain, Eastern belt, Franciscan subduction complex, California Coast Ranges

The roles of volume loss, coaxial versus noncoaxial Row, and blueschist exh umation in subduction-related accretionary wedges are still poorly understo od, In our study at Leech Lake Mountain in the Eastern belt of the Francisc an subduction complex, we focus on these subjects, In the specific example of the Franciscan, the tectonic significance of the boundary between the Ea stern and Central belts remains controversial, The Leech Lake Mountain area in northern California is situated immediately above this boundary and, th erefore, appears to be of crucial importance for understanding aspects of t he tectonic evolution of the Franciscan. The structural development at Leech Lake Mountain Is characterized by three deformational events. D-2 produced the regional cleavage (s(2)) during blu eschist-facies metamorphism. D-2 is the only entirely ductile deformation e vent and was associated with accretion-related internal stacking within the Yolla Bolly terrane. The s(3) cleavage is folded by F-3 folds, which are o verturned to the west. Spacing of a related s(3) cleavage is generally at t he decimeter scale, but locally is spaced at the centimeter scale. The s(2) and s(3) cleavages, and F-3 folds, are overprinted by tight In open F-4 fo lds at the decimeter to map scale. Map-scale F-4 folds are largely upright, whereas small-scale F-4 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 s(2) cleavage was accompanied by pronounced volume loss, averaging 36%. The directions of the principal finite-strain axes are scat tered; therefore, we calculated a tensor average of the data. The principal stretches of the tensor average are S-x:S-y:S-z 1.06:0.91:0.66, indicating that the accumulation of ductile strain was characterized by subvertical s hortening, which was largely compensated for by volume loss and not by orth ogonal extension, Evidence for noncoaxial deformation in our samples is lim ited, indicating that the accumulation of ductile strain was largely coaxia l. Our preferred tectonic interpretation is that accretion and blueschist-faci es metamorphism of the Yolla Bolly terrane at Leech Lake Mountain occurred during D-2 under an almost coaxial deformation regime. During and after D-2 , the rocks were considerably exhumed and were telescoped by D-3 top-to-the -west out-of-sequence thrusts at shallow-crustal levels. In the Leech Lake Mountain area, D-3 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 Easter n and Central belts is modest, indicating that displacement at the Red Moun tain fault was not large. We propose that D-3 out-of-sequence thrusts atten uated the metamorphic and stratigraphic section across the entire Francisca n and overlying Great Valley forearc. The final D-4 event produced large-sc ale folds. All three events resulted from horizontal shortening. There is n o evidence for a major phase of horizontal extension, which might have aide d exhumation of the Eastern belt blueschists, Modest tectonic exhumation wa s due to vertical ductile shortening during D-2.