MAGNETIC-PROPERTIES AND EMPLACEMENT OF THE BISHOP TUFF, CALIFORNIA

Anisotropy of magnetic susceptibility (AMS) and characteristic remanen ce were measured for 45 sites in the 0.76 Ma Bishop tuff, eastern Cali fornia. Thirty-three sites were sampled in three stratigraphic section s, two in Owens gorge south of Long Valley caldera, and the third in t he Adobe lobe north of Long Valley. The remaining 12 sites are widely distributed, but of limited stratigraphic extent. Weakly indurated, hi ghly porous to dense, welded ash-flow tuffs were sampled. Saturation m agnetization vs temperature experiments indicate two principal iron ox ide phases: low Ti magnetites with 525-570 degrees C Curie temperature s, and maghemite with 610 degrees-640 degrees C Curie temperatures. AF demagnetization spectra of isothermal remanent magnetizations are ind icative of magnetite/maghemite predominantly in the multidomain to pse udo-single domain size ranges. Remeasurement of AMS after application of saturating direct fields indicates that randomly oriented single-do main grains are also present. The degree of anisotropy is only a few p ercent, typical of tuffs. The AMS ellipsoids are oblate with K-min axe s normal to subhorizontal foliation and K-max axes regionally aligned with published source vents. For 12 of 16 locality means, K-max axes p lunge sourceward, confirming previous observations regarding flow sens e. Topographic control on flow emplacement is indicated by the distrib ution of tuff deposits and by flow directions inferred from K-max axes . Deposition east of the Benton range occurred by flow around the sout h end of the range and through two gaps (Benton notch and Chidago gap) . Flow down Mammoth pass of the Sierra Nevada is also evident. At leas t some of the Adobe lobe in the northeast flowed around the west end o f Glass mountain. Eastward flow directions in the upper Owens gorge an d southeast directions in the lower Owens gorge are parallel to the pr esent canyon, suggesting that the present drainage has been establishe d along the pre-Bishop paleodrainage. Characteristic remanence directi ons from 45 sites (267 samples) yield an overall mean of D = 348 degre es, I = 53 degrees for the Bishop tuff. A correlation is found in two of the three profiles between density and remanence inclination. A mea n remanence direction based on 13 localities together with data from u ncompacted xenoliths and data from the ash-fall tuff at Lake Tecopa is : D = 353 degrees, I = 54 degrees, k = 172, alpha(95) = 2.9 degrees, N = 15.