Revised U-Pb Age and Regional Correlation of the Peavine Sequence near Reno, Nevada: A Record of Convergent Margin Arc Activity or Bimodal Volcanism within a Transtensional Basin Late in the Jurassic?

A bimodal suite of volcanic rock and coarse-grained, locally derived, clastic
rock (Peavine sequence) is preserved in a series of roof pendants discontinuously exposed along the eastern margin of the Sierra Nevada between Gardnerville, NV and
Frenchman Lake, CA. While none of the pendant exposures represent a complete
stratigraphic section, each contains an upright depositional succession that is grossly
similar. Coarse-grained clastic rocks (including heterolithic boulder breccia) locally
interbedded with or cut by plagioclase-phyric basalt dominate the lower part of the
sequence while lithic- or fiamme-rich, light-colored, locally welded pyroclastic rock,
crystal tuff and rhyolite dominate the upper part of the sequence. Minor andesitic
volcanic rock is locally present in both parts of the sequence.
Strata of the Peavine sequence unconformably overlies mafic, Middle Jurassic
arc rock of the Northern Sierra terrane. Samples of Peavine sequence volcanic rock
collected from pendant exposures near Frenchman Lake, California and Peavine
Peak, Nevada each yielded Oxfordian U-Pb ages indicating the sequence represents
the youngest Jurassic volcanic rock preserved in western Nevada.
The Peavine sequence appears to have accumulated within a relatively short
timeframe (i.e., 15 m.y.), during which, up to 5 km of locally derived sediment and
bimodal volcanic strata were laid down in environments which transitioned from subaqueous to subaerial. Such thickness is best explained if deposition took place within
extensional or transtensional environments rather than a traditional arc axis. The
local presence of thick breccia at the base of the sequence (containing plutonic clasts)
suggests fault-generated relief during the onset of Peavine sequence deposition. The
presence of thick pyroclastics (locally welded) and rhyolite (flows or domes) within
the upper part of the sequence is permissive of either caldera development, or a faultbound basin within which pyroclastic flows could have ponded.
Extensional conditions terminated sometime after the Kimmeridgian, and the
Peavine sequence depositional sites were subject to basin inversion and differential
uplift and erosion. The sequence appears to be age correlative to other accumulations of Upper Jurassic clastic and volcanic strata present in southeastern California,
westernmost Arizona and northwestern Mexico that are considered to have been deposited under extensional or transtensional conditions following the termination of
Middle Jurassic, convergent margin, arc volcanism. The absence of Upper Jurassic
strata equivalent to the Peavine sequence within the Pine Nut terrane of western Nevada suggests it was not affected by similar transtensional tectonics. As a result, the
Pine Nut and Northern Sierra terranes may have been separated from one another
during the Jurassic. Their current positions may be the result of strike-slip faulting
during the Early Cretaceous, perhaps related to the Mojave-Snow Lake fault and/or
western Nevada shear zone.
Key Words: Oxfordian, volcanics, Peavine, Reno, Sierra, Pine Nut, terrane