MULTIPLE PHASES OF LOW-ANGLE NORMAL FAULTING FROM THE PRECAMBRIAN TO THE NEOGENE IN THE IBEX HILLS AND SARATOGA HILLS, CA

The Ibex Hills lie in a critical position within the Death Valley extensional terrane including a southern, up-dip position from the Amargosa detachment fault system and the northern limit of a major Miocene syn-extensional basin. New detailed mapping within the range reveals a complex fault array including normal, reverse, and strike-slip faults with complex overprinting ranging from Precambrian structure, Mesozoic contraction, and Neogene extension. The primary structure in the central Ibex Hills is a north-south to northeast striking low angle detachment fault. While previous work has suggested the presence of this structure this study has allowed for a more detailed understanding of the fault system. In the southern portion of the mapped area the fault places the Mesoproterozoic Crystal Springs formation over ~1.4 Ga basement rock. Farther to the north the fault cuts up section and places the lower and middle Noonday formation on the aforementioned basement rock and curves to a northeast strike at the northern limit of our mapping. To the south gently northwest-dipping normal faults repeat a section of the middle Crystal Springs although further mapping is required to clarify the geometry which is obscured by the overprinting of younger features. Structurally below the detachment along the eastern flank of the mapped area is a Precambrian normal fault which places middle Crystal Springs against the Horse Thief formation. This fault is overlapped by the unconformities at the base of the Kingston Peak and Noonday formations supporting a Precambrian age for the fault. At the top of the structural stack brecciated Beck Springs and Noonday dolomite lie atop Crystal Springs and Horse Thief Springs along a flat fault. Farther to the south the Saratoga Hills also contain complex structural overprinting and low angle normal faults potentially related to those in the Ibex Hills. Previous workers have suggested that the low-angle normal faults of the Ibex Hills are the southeastern extent of the Black Mountain Detachment Fault. Although this work is consistent with this conclusion fault kinematics are not yet resolved and complex cross-cutting relationships imply a prolonged extensional history. Continued detailed mapping of this region will provide new insight into this key area in the Death Valley extensional terrane.