About This Item
Share This Item
Temporal and spatial patterns of extension along the southern boundary of the Transition Zone, Superstition volcanic field, Arizona: Abstract
The 20.5 - 18.0 Ma 5,000 km2 Superstition volcanic field straddles the Transition Zone – Basin and Range structural boundary in central Arizona. High-precision, sanidine, single crystal, laser fusion 40Ar/39Ar dating of key volcanic units provide a detailed chronology of the evolution of tilt domains and magnitudes of extension throughout the life of the field. In the Transition Zone, even the oldest volcanic strata are either undeformed or only gently tilted, but locally, fanning dip sequences in narrow grabens are preserved. In the Basin and Range, northeast tilting began at about 20.5 Ma and was fairly evenly distributed along closely spaced faults throughout the field until eruption of Apache Leap Tuff at 18.6 Ma. Tilting ceased in most areas by 18.6 Ma, but continued along discrete zones, locally very rapidly, until about 18.0 Ma.
The E-W elongated 350 km2 Superstition cauldron (source of the Apache Leap Tuff), lies in the northwestern corner of the field and is bisected by the southwestern structural boundary of the Transition Zone. Northwest of the cauldron, the boundary fault zone was active until about 18.0 Ma, and southwest-side-down normal motion was accompanied by about 2 kilometers of dextral offset between 18.6 and 18.0 Ma. Extension to the southwest of this segment of the boundary, in the Goldfield Mountains, was moderate to extreme. South of the cauldron, the boundary zone turns sharply to the east and extends into the Globe-Miami area. Extension to the south of this boundary was weak to moderate but distributed over a larger area than in the Goldfield Mountains.
The sharp turn in the boundary zone at the Superstition cauldron coincides regionally with the hypothetical southwesterly continuation of the Jemez Lineament, a prominent northwest-trending zone of Neogene magmatism that extends across New Mexico and eastern Arizona. The turn also coincides with an abrupt change in the dip of the detachment faults that bound the core-complexes in the Basin and Range province. To the west, detachment faults dip to the northeast, and to the south they dip to the southwest. Directly southwest of the Superstition volcanic field the absence of core-complexes is conspicuous.
The nearest core complex, in the South Mountains directly to the west, had a long-lived history (~25-17 Ma) of exhumation involving Tertiary granitoids significantly older than the volcanics of the Superstition volcanic field. Along strike to the southeast from the South Mountains a low-angle, northeast-dipping normal fault in the Santan Mountains, which lie directly southwest of the Superstition volcanic field, is overlapped by a flat-lying outlier of the Apache Leap Tuff.
Copyright © 2009 by AAPG Pacific Section and Utah Geological Society