DEVELOPMENT OF GREGG BASIN AND THE SOUTHWESTERN GRAND WASH TROUGH DURING LATE STAGE FAULTING IN EASTERN LAKE MEAD

We have documented the stratigraphy, structure, and ages of Miocene strata in three critical areas on the north and east side of the Lost Basin Range in the eastern Lake Mead domain. These new data from the three areas allow us to make important interpretations about the history of faulting and tilting of hanging wall and footwall blocks along the Wheeler Ridge/Infernal Mountains fault system and the Lost Basin Range block. The geometry of the fault system is that of a relay ramp between two normal faults and therefore the Gregg basin is interpreted as a relay ramp basin. Sedimentation in Gregg basin and along the east side of Lost Basin Range started at 15.3 Ma based on dated tuffs and both sections have evidence for active faulting starting at 15.3 Ma; faulting continues until after 13 Ma along the southeastern Lost Basin Range, while the Gregg basin is poorly constrained. Final exhumation of the Lost Basin Range based on apatite fission track is also 15 Ma. Our favored interpretation is that the Wheeler Ridge and Infernal Mountains fault system was active starting at 15.3 Ma. An alternative interpretation is that faulting on the south Wheeler fault began later, between approximately 15 and 14 Ma. Based on an open syncline in the hanging wall of the fault system (Gregg basin) and correlation of the limestone of Gregg basin to the 11 – 6 Ma Hualapai limestone in Grand Wash, faulting may have slowed at about 11 Ma, but it likely continued to 11 – 6 Ma; the Infernal Mountains fault tilts the limestone, so faulting was active after 6 Ma.