Tectonic Evolution of the Condrey Mountain Schist: an Intact Record of
Late Jurassic to Early Cretaceous Franciscan Subduction and Underplating
Abstract
The Klamath Mountains in northern California and southern Oregon are
thought to record 200+ m.y. of subduction and terrane accretion, whereas
the outboard Franciscan Complex records classic ocean-continent
subduction along the North American margin. Unraveling the Klamaths’
late history could help constrain this transition in subduction style.
Key is the Mesozoic Condrey Mountain Schist (CMS), comprising, in part,
a subduction complex that occupies a structural window through older,
overlying central Klamath thrust sheets but with otherwise uncertain
relationships to other, more outboard Klamath or Franciscan terranes.
The CMS consists of two units (upper and lower), which could be
correlated with 1) other Klamath terranes, 2) the Franciscan, or 3)
neither based on regional structures and limited extant age data. Upper
CMS protolith and metamorphic dates overlap with other Klamath terranes,
but the lower CMS remains enigmatic. We used multiple geochronometers to
constrain the timing of lower CMS deposition and metamorphism. Maximum
depositional ages (MDAs) derived from detrital zircon geochronology of
metasedimentary rocks are 153-135 Ma. Metamorphic ages from white mica
K-Ar and Rb-Sr multi-mineral isochrons from intercalated and coherently
deformed metamafic lenses are 133-116 Ma. Lower CMS MDAs (<153
Ma) predominantly postdate the age of other Klamath terranes, but
subduction metamorphism appears to predate the earliest coherent
Franciscan underplating (ca. 123 Ma). The lower CMS thus occupies a
spatial and temporal position between the Klamaths and Franciscan and
preserves a non-retrogressed record of the Franciscan Complex’s early
history (>123 Ma), otherwise only partially preserved in
retrogressed Franciscan high grade blocks.