Steep tilting of metavolcanic rocks by multiple mechanisms, central Sierra Nevada, California

Abstract

For ∼200 km along the eastern Sierra Nevada continental magmatic arc, Mesozoic metavolcanogenic rocks dip steeply to the southwest (∼80°), a feature that must reflect fundamental processes in magmatic-arc construction. Although tight folds can account for such steep bedding tilts, folds in the metavolcanogenic sections are sparse and small scale. We propose that the high bedding tilts were produced by a combination of thrusting, downward 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 ∼200 km belt and provides a relatively large and well exposed Mesozoic volcanic section ranging in age from Late Triassic to mid–Cretaceous. Detailed mapping and ages from U-Pb zircon dates and fossils within the volcanic section reveal five structural blocks (I–V) that are separated by bedding-parallel thrusts, some of which are cryptic. To explain the present difference in bedding orientations between blocks III and IV, we suggest that the thrusting may have had a duplex geometry, which produced a maximum bedding dip of ∼45° in some blocks. Downward displacement of wall rock and ductile strain account for the remaining ∼35° of the observed average bedding dip (∼80°SW). The exact time of thrusting and duplex formation of Late Triassic to Early Jurassic rocks in blocks I–IV is uncertain, but these structures developed either (1) between 105 and 164 Ma, well before the other rotational processes were active, or (2) mostly around 105 Ma, and closer to the time when other rotational processes were active. Much of the subsequent (ca. 91–76 Ma) bedding tilting is related to downward displacement of beds associated with the emplacement of voluminous Late Cretaceous plutons, and to regional ductile 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 and ca. 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 pendants, suggest that bedding rotation by thrusting(?), downward displacement and ductile strain of wall rock may explain the steep dips along this entire ∼200 km segment of the continental arc. Similar mechanisms may operate at midcrustal levels in other continental arcs.