PALEOGRAPHIC INTERPRETATION OF THE FISH LAKE VALLEY/HORSE THIEF HILLS AREA USING GEOCHEMISTRY OF VOLCANIC ROCKS

Samples of volcanic rocks were collected from Willow Wash in Fish Lake Valley and the Horse Thief hills, California. Regional geologic studies classified these rocks as Tertiary basalt. Our purpose was to date and geochemically characterize these rocks to determine if these are the same or different basalt flows. The geochemical signatures for these rocks were determined by X-Ray Fluorescence (XRF). In Willow Wash, several 11.5-11.7 Ma basalt flows that underlie a thick sequence of Pliocene-Quaternary sediments were classified as basalt (47-52% SiO₂). The Zr/Ba and Ce/Y ratios for these basalts were similar to each other as well as basalt outcrops as far as 32 km west in the White Mountains. Based on the decreasing thickness, age and geochemical data we infer that the basalt found in Willow Wash originated in the White Mountains and flowed 32 km east prior to Basin and Range extension that formed the intervening 1000 m of relief and Fish Lake Valley, Deep Springs Valley and the Horse Thief hills. The Horse Thief hills samples were collected from a volcanic breccia that is part of a tilted sedimentary-volcanic sequence exposed on the south side of the hills. XRF data show that the volcanic matrix of the breccia is andesite and the volcanic clasts are trachyandesite (56-60% SiO₂). A previous whole-rock, K-Ar date of 6.3±0.2 Ma on volcanic rock showed that this is a younger sequence representing the remnant of a Miocene basin now uplifted and exposed on the south side of the Horse Thief hills. Our data confirms that at 11.5-11.7 Ma a basalt flowed 32 km east from the White Mountains to Willow Wash before Basin and Range extension began. The andesitic breccia interbedded with basin deposits shows that a small basin formed south of the Horse Thief hills at 6.3 Ma.