ROTATION OF LATE CENOZOIC EXTENSIONAL STRESSES, YUCCA FLAT REGION, NEVADA TEST SITE, NEVADA
ANDER, HOLLY DOCKERY
Doctor of Philosophy
The Nevada Test Site (NTS) is located in the southern Basin and Range where the geology is typified by complexly deformed Paleozoic sedimentary rocks underlying Tertiary and Quaternary volcanics and alluvium all displaced by Cenozoic normal faults. The purpose of this study is to interpret the history of change in Cenozoic extensional stress orientations using ash-flow tuff distributions, surface fault configurations, and slickenside analyses. Extensive drill hole data collected from Yucca Flat within NTS were used to construct isopach and structure contour maps of Cenozoic units occupying the northerly-trending basin. The configuration of these units indicates that the north-south-trending faults controlling present day basin morphology were inactive during deposition of the volcanic rocks from approximately 25 to 11 myBP. However, after 11 myBP, the overlying sedimentary sequence was strongly influenced by these faults and consequent basin development. In particular, an inordinately thick section of late Tertiary and Quaternary alluvium occurs at the southwestern end of Yucca Flat. Southwest-striking faults at the southwestern of Yucca flat are postulated to be deflected at their northeast ends, becoming continuous with the north-south basin forming fault sets. The northeast faults exhibit predominantly left-lateral displacement occurring post-11 myBP. This sense of motion is incompatibile with the N50(DEGREES)W extension determined today via in situ measurements. The north-south faults are primarily dip-slip with a small component of right-lateral motion. The thick sedimentary section found in southern Yucca Flat at the intersection of the southwest-striking and north-south-striking faults formed as a pull-apart basin which developed syndepositionally with the alluvium. Observed offsets of volcanic units by the southwesterly striking faults as well as slickenside analyses of data from the major fault zones throughout the area indicates a N78(DEGREES)W extension operating since 11 myBP. After formation of much of the Yucca flat basin, the least principal stress rotated to N50(DEGREES)W. This rotation apparently occurred very recently and the new stress orientation has had little effect on the fault patterns or fault displacements of the area. Synthesis of this work with other studies throughout the southern Basin and Range show a consistent clockwise rotation of least principal stress through an angle of 90(DEGREES) in the past 17 my.