Timing of emplacement of the Haypress Creek and Emigrant Gap plutons: Implications for the timing and controls of Jurassic orogenesis, northern Sierra Nevada, California

Abstract

Pre-Cretaceous rocks in the northern Sierra Nevada are subdivided from west to east into the Smartville, central, Feather River peridotite, and eastern belts. Cretaceous and younger sedimentary rocks form the western boundary of the Smartville belt, but various reverse-fault segments of the Foothills fault system separate the other belts. The Foothills fault system and associated structures involve rocks as young as Kimmeridgian (Late Jurassic) and are truncated by Early Cretaceous plutons. This relationship is often cited as evidence for the Nevadan orogeny which is commonly viewed as a temporally restricted event involving deformation and metamorphism during the Late Jurassic. Recent work, however, suggests that some of the Mesozoic structural fabric in the northern Sierra Nevada may not have been produced during the Late Jurassic, but instead may have formed between Early and Middle Jurassic time. Thus, distinguishing Nevadan-age deformation from older Mesozoic deformation is now one of the more important problems facing geologists working in the northern Sierra Nevada. The Haypress Creek pluton crops out in the eastern belt and historically has been cited as a post-Nevadan pluton. It intrudes the Early to Middle Jurassic Sailor Canyon Formation that, together with the overlying Middle Jurassic Tuttle Lake Formation, contains a domainally developed, locally penetrative, northwest-striking cleavage (S 2 ). S 2 can be traced into the contact metamorphic aureole of the Emigrant Gap composite pluton, where structural and microtextural evidence indicates that it predates pluton intrusion. New U-Pb zircon data for the Haypress Creek pluton suggest an age of 166 ± 3 Ma and previously published U-Pb zircon data for the oldest phase of the Emigrant Gap composite pluton suggest an age of 168 ± 2 Ma. The fossiliferous Sailor Canyon Formation ranges in age from Early Jurassic (Sinemurian) in its lower parts to Middle Jurassic (Bathonian or Bajocian) in its upper parts. The overlying Tuttle Lake Formation contains S 2 , which formed prior to emplacement of the Emigrant Gap and Haypress Creek plutons at ca. 168–166 Ma. This relationship suggests that the Tuttle Lake Formation must have been deposited and deformed entirely within the Middle Jurassic. Thus, S 2 and associated structures within the eastern belt formed prior to Late Jurassic Nevadan deformation associated with the Foothills fault system. There are two end-member models used to explain the plate tectonic evolution of pre-Cretaceous rocks in the northern Sierra Nevada. These are referred to as the arc-continent collision and single, wide-arc models. Data discussed herein do not preclude either of these models for Early to Middle Jurassic time. However, regardless of which of these models is favored, both scenarios place the approximately 168 Ma and younger Jurassic volcanic and plutonic rocks of the Smartville, central, and eastern belts in a distinctly intra-arc setting and further imply that the Foothills fault system and related Late Jurassic structures are also of intra-arc character. We conclude that there is no evidence along 39°30′N latitude for arc-continent collision during the Nevadan orogeny.