Miocene‐Pliocene half‐graben evolution, detachment faulting and late‐stage core complex uplift from reflection seismic data in south‐east Arizona

Abstract Reflection seismic data show that the late Cenozoic Safford Basin in the Basin and Range of south‐eastern Arizona, is a 4.5‐km‐deep, NW‐trending, SW‐dipping half graben composed of middle Miocene to upper Pliocene sediments, separated by a late Miocene sequence boundary into lower and upper basin‐fill sequences. Extension during lower basin‐fill deposition was accommodated along an E‐dipping range‐bounding fault comprising a secondary breakaway zone along the north‐east flank of the Pinaleño Mountains core complex. This fault was a listric detachment fault, active throughout the mid‐Tertiary and late Cenozoic, or a younger fault splay that cut or merged with the detachment fault. Most extension in the basin was accommodated by slip on the range‐bounding fault, although episodic movement along antithetic faults temporarily created a symmetric graben. Upper‐plate movement over bends in the range‐bounding fault created rollover structures in the basin fill and affected deposition within the half graben. Rapid periods of subsidence relative to sedimentation during lower basin‐fill deposition created thick, laterally extensive lacustrine or alluvial plain deposits, and restricted proximal alluvian‐fan deposits to the basin margins. A period of rapid extension and subsidence relative to sediment influx, or steepening of the upper segment of the range‐bounding fault at the start of upper basin‐fill deposition resulted in a large downwarp over a major fault bend. Sedimentation was restricted to this downwarp until filled. Episodic subsidence during upper basin‐fill deposition caused widespread interbedding of lacustrine and fluvial deposits. Northeastward tilting along the south‐western flank of the basin and north‐eastward migration of the depocentre during later periods of upper basin‐fill deposition suggest decreased extension rates relative to late‐stage core complex uplift.