Steep tilting of metavolcanic rocks by multiple mechanisms, central SierraNevada, California

For similar to 200 km along the eastern Sierra Nevada continental magmatic are, Mesozoic metavolcanogenic rocks dip steeply to the southwest (similar to 80 degrees), a feature that must reflect fundamental processes in magmat ic-arc construction, Although tight folds ran account for such steep beddin g tilts, folds in the metavolcanogenic sections are sparse and small scale. We propose that the high bedding tilts were produced by a combination of t hrusting, down ward displacement, and ductile deformation of the beds, The last two processes accompanied emplacement of the Sierra Nevada batholith, The Ritter Range pendant lies within this similar to 200 km belt and provid es a relatively lar ge and well exposed Mesozoic volcanic section ranging i n age from I,ate Triassic to mid-Cretaceous, Detailed mapping and ages from U-Pb zircon dates and fossils within the volcanic section reveal five stru ctural blocks (I-V) that are separated by bedding-parallel thrusts, sonic o f which are cryptic. To explain the present difference in bedding orientati ons between blocks III and IV,vp suggest that the thrusting may have had a duplex geometry, which produced a maximum bedding dip of similar to 45 degr ees in sonic blocks. Downward displacement of wall rock and ductile strain account for the remaining similar to 35 degrees of the observed average bed ding dip (similar to 80 degrees SW). The exact time of thrusting and duplex formation of I,ate Triassic to Early Jurassic rocks in blocks I-IV is uncertain, but these structures developed either (1) between P05 and 164 Ma, well before the other rotational proces ses were active, or (2) mostly around 105 Rla, and closer to the time when other rotational processes were active. Much of:the subsequent (ca, 91-76 M a) bedding tilting is related to downward displacement of beds associated w ith the emplacement of voluminous I,ate Cretaceous plutons, and to regional duc tile deformation of the wall rocks during that period: the majority of the tilting probably took place between ca, 91 and 86 Ma. Bedding tilts of early to mid-Cretaceous rocks in blocks IV and V is bracketed between ca, 98 land ra, 90 Ma, Comparisons with metavolcanic sections to the northwest near Tioga Pass and to;the southeast in the Mount Morrison, Mount Goddard and Oak Creek pendan ts, suggest that bedding rotation by thrusting(?), downward displacement an d ductile strain of wall rock may explain the steep dips along this entire similar to 200 km segment of the continental iarc, Similar mechanisms may o perate at mid-crustal levels in other continental arcs.