STRUCTURAL ANALYSIS OF THE IBEX HILLS, CA: EVIDENCE FOR MULTIPLE PHASES OF MESOZOIC SHORTENING AND TRANSTENSIONAL RELATED FOLDING, FAULTING, AND EXTENSION

Wright and Troxel worked extensively in southern Death Valley but aside from Wright’s early work in the 50’s, mapping of the Ibex Hills was mostly at reconnaissance level. Our recent mapping in the Ibex Hills demonstrates Mesozoic contractional structures overprinted by Neogene extension and strike-slip deformation. The earliest deformation is interpreted as two phases of Mesozoic shortening, recorded primarily by overprinted fold systems. The oldest folds (F1) are tight to sub-isoclinal, ~E trending folds with an axial planar to fanning pressure solution cleavage. F1 is overprinted by ~N-S trending, more open folds that produce strong curvature in F1 axes and type 1 interference patterns. These folds are cut by a series of Neogene faults. The oldest identified faults consist of currently low to moderate angle, E-NE dipping normal faults which are folded about a SW-NE trending axis. That this folding postdates at least some of the extension suggests a component of syn-extensional shortening that is probably strike-slip related. Cross-cutting these east dipping faults are both NW and NE striking, high angle normal faults. In general, it appears that the NE striking high-angle faults are younger and displace the other normal faults, consistent with NW directed extension superimposed on older SW directed extension. Additionally, a major N-S striking, oblique-slip fault bounds the eastern flank of the range with slickenlines measured at rakes of <30⁰ which together with the map pattern suggests dextral-oblique movement along the east front of the range. In the central Ibex Hills, the oblique fault clearly cross-cuts low angle normal faults, however, to the north it appears to be truncated by normal faults, potentially signifying a zone of accommodation related to the adjacent Sheephead Fault (SF), overprinting by NW directed extension, or both. Transcurrent systems have been proposed to explain extension parallel folding and the geometry of folded low angle faults followed by NW striking faulting in the Ibex Hills aligns with these models. Collectively the field data suggest an early period of SW directed extension is recorded in the Ibex Hills, which was suggested by Troxel. The younger NE striking faults presumably record the younger NW directed extension during or after SF movement.