TECTONIC HISTORY OF MAJOR FAULTS IN THE YUCCA MOUNTAIN REGION: POTENTIAL PATHWAYS FOR FLUID MIGRATION

Kinematic analysis of regional fault systems and field studies (mainly west of Mercury, NV) provide insight into the tectonic history of Yucca Mountain. Our results support the interpretation of the Yucca Mountain region as a pull-apart basin filled with volcanic units erupted through thinned crust. The boundaries of the southern part of the pull-apart basin coincide with major faults (Kawich Greenwater rift or Gravity fault, Highway 95/Carrara, and Bare Mountain). These major fault structures may influence and possibly control the transmission of water from the proposed repository into Amargosa Valley. Tens of kilometers of right-lateral displacement along the Las Vegas Valley Shear Zone (LVVSZ) were accommodated by formation of a pull-apart basin at a northward releasing step along Forty Mile Wash. East of Forty Mile Wash, the trace of the LVVSZ trends westerly (roughly between Lathrop Wells and Mercury) probably along a pre-existing structure. Upright folds with local vertical limbs in layers of the rocks of Pavits Spring (approximately 17 to 14 Ma) south of Little Skull Mountain record transpression along the constraining bend. Tuff of the Crater Flat Group (approximately 14 Ma) that directly overlies the folded beds of Pavits Spring records eruption during deformation. Gentle folds and irregular bed geometries that have been mapped in units as young as the Timber Mountain Group are evidence that contraction continued with volcanism until 10 Ma. Contraction recorded by folds is not evident in units younger than 10 Ma suggesting cessation of transpression related to strike-slip movement along the LVVSZ. Paleozoic formations comprising the floor of the Tertiary basin were deformed concurrently as shown by steep bedding, brecciation, detachments and thrusts. Subsequent development of north-striking normal faults commonly linked by northwest-striking right-lateral strike-slip faults (e.g., Yucca Wash; Sever Wash) and complementary northeast-striking left-lateral strike-slip faults (e.g., Mine Mountain; Rock Valley) records the transition to pure shear with local north-south contraction. Normal faults at releasing steps along the Rock Valley Fault that break across west-trending transpressional folds in the Specter Range reveal the relative ages of simple and pure shear deformation.