Vesicle analysis of Sheep Creek Range flows points to tectonic tilting
Author:
Julia DefigueiredoMentor:
Scott Bogue, Professor of Geology, Occidental CollegeA new analysis of existing paleomagnetic and structural data suggests that the 15.1° declination anomaly in 15.2 ma basaltic lava flows in the Sheep Creek Range in Nevada is caused by the tectonic tilting of the flows. The expected declination of these flows is 355.6°, while the measured declination is 340.5°. Previous work interpreted the difference in the anticipated and actual declination of nearby flows and shallow intrusives of the same age as resulting from approximately 19° of counterclockwise tectonic rotation of a crustal block in central Nevada. I evaluated how vesicularity varied through the thickness of two typical Sheep Creek lava flows by calculating the density of samples from different heights within the flow. I also measured the size of visible vesicles using a caliper and categorized them by size and abundance. I concluded that the flows inflated during emplacement. By comparison, observations of active flows in Hawaii demonstrated that lava flow inflation occurs only on near-horizontal slopes (<2°), implying that the Sheep Creek flows were initially horizontal. The flows were measured to have an average attitude striking 350° and dipping 6°, indicating that these flows must have been tilted after they were deposited and magnetized, accounting for the deviation in declination. Mathematically rotating the lava flows to horizontal yields a tilt-corrected declination of 351.5°±15.1°, indistinguishable from the expected declination. The paleomagnetism of the Sheep Creek flows provides no support for the hypothesis that the crust in North Central Nevada experienced substantial counter clockwise rotation since the Miocene.