Geophysical Investigation of Cenozoic Extension in Higley Basin, Southern Arizona

Higley Basin, located near the western margin of the Basin and Range Province in Central Arizona, is a deep (>3,000 m), complicated half-graben characterized by displacement along both high- and low-angle normal faults; displacement along these faults has created accommodation space for thick volcanic sequences followed by more recent fluvial, alluvial, and colluvial deposits. Higley Basin occupies a unique location near a transition in the vergence of major detachment fault systems largely responsible for the exhumation of metamorphic core complexes in the region. To the west and north, major detachment faults generally dip to the northeast, while to the south and east of the basin, major detachment faults generally dip to the southwest. Recently collected and vintage industry seismic reflection data, combined with gravimetric observations and exploration drilling, provide new insights into the linkage between these two oppositely vergent extensional domains, and the processes by which extension is accommodated in this transitional zone. Two opposing styles of extensional deformation are observed in the subsurface within the basin, and in outcrop in regions surrounding the basin: early low-angle, large-offset normal faults, and later high-angle, smaller-offset normal faults, more typical of faults associated with Basin-and-Range-style deformation. The orientation of pre-existing low-angle faults appears to strongly influence the geometry of subsequent high-angle faulting, as the approximate trend and dip direction are frequently similar for both stages of faulting. Northwest-trending, southwest-dipping high-angle normal faults steadily lose displacement moving toward Higley Basin from the southeast. The northeastern boundary of Higley basin itself does not appear to be defined by high-angle normal faults. High-angle normal faults, though numerous, display minor extension amounts relative to that of the low-angle normal faults. Interpretation of seismic data and regional structural data suggests that Higley Basin represents a depocenter that formed at the transition between two linked low-angle detachment systems of opposing vergence, and therefore would represent a region of near minimal extension within a zone of extreme crustal extension. However, Higley Basin remains one of the deepest basins in southeastern Arizona. Possible explanations for this apparent contradiction include: 1) low-angle fault systems may branch minimally, concentrating displacement on centralized faults up to the edge of each fault system, 2) re-activation of low-angle normal faults during subsequent extensional events, and 3) increased local subsidence, perhaps due to mid-crustal flow away from the basin and towards adjacent uplifted core complexes.