MESOZOIC GEOLOGY OF THE LOBDELL, SWEETWATER, AND PINE GROVE HILLS PENDANTS, EASTERN CALIFORNIA AND WESTERN NEVADA (38.5° N): IMPLICATIONS FOR THE PLACEMENT OF THEMOJAVE-SNOW LAKE FAULT

Three roof pendants situated in the Sweetwater Range, CA and Pine Grove Hills, NV, occur as tectonic packages of metamorphosed marine and volcanic strata that are separated from metavolcanic rocks by faults that dip to the NW and NE at low angles. These faults in turn are cut by high-angle normal(?) faults that parallel the geographic trend of the three pendants, suggesting that the pendants are preserved in an E-NE trending graben of probable Mesozoic age.

The metasedimentary rocks are derived from limestones and calcareous siliciclastic sandstones, indicating a mixed siliciclastic-carbonate depositional system. Turbidites are common. Ammonites and pelecypods indicate a Sinemurian (Early Jurassic) age. The metavolcanic units are comprised of felsic-intermediate pyroclastic rocks and lesser andesite flows. Thick silicic ignimbrites suggest caldera-style eruptions in subaerial environments. Volc-litharenites within the volcanic units appear to represent both fluvial and marine environments; pillow lavas have not been recognized. The stratigraphic relations between the volcanic and marine sedimentary units are uncertain.

The low-angle (thrust?) faults are characterized by fault zones with 10 m-scale slices of foliated carbonate rock, suggesting that fault style was influenced mechanically by competence contrasts between carbonate and metavolcanic rock. Fault slices of carbonate rock bound by meta-volcanic thrust sheets suggest significant tectonic imbrication. A low-angle fault is cut by the Desert Creek Pluton (~155 Ma). The plutonic age and the fossil age of the strata (>195 Ma) constrain the activity on the low-angle faults to a 40 m.y. period from ~195-155 Ma.

The absence of exposed Paleozoic or older passive margin rocks from the pendants suggests that the pendants are not part of the Snow Lake Block of Lahren, et al. (1990). Thus it is unlikely that the Mojave-Snow Lake fault is oroclinally folded near the latitude of 38.5° N to affect western Nevada.