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New (U–Th)/He in apatite ages suggest rapid slip rates on the Tertiary Buckskin–Rawhide detachment fault, which crops out in the Buckskin and Rawhide Mountains of western Arizona. Single-crystal (U–Th)/He in apatite ages were determined for a suite of crystalline rock samples from the footwall to this detachment. The samples were collected from a transect parallel to the fault slip direction, and proximal to the fault zone; they show a linear decrease in age with distance, from ∼14 to ∼11 Ma over ∼15 km. The age versus distance relationship is interpreted to show a slip rate of 4.2 (+1.2/−0.8) mm/yr, continuing for a period of >3 Myr. This rate is more-or-less in agreement with, but more precise than, previously published slip rates from this and other large-offset extensional detachments. Such slip rates are alarmingly high, being roughly 5–10 times faster than rates commonly observed on active structures at the Earth’s surface. These rapid slip rates suggest that significant seismic hazard might be posed by some extensional structures, and they raise questions regarding the relationship between such rapidly slipping, large-offset structures and the slower, smaller offset faults that are more commonly observed at the Earth’s surface. [Copyright &y& Elsevier]

Studies of the evolution of drainage systems in extensional settings like the Basin and Range Province of western North America benefit from well drilling as a means of acquiring valuable insight. Cuttings from two wells drilled into sediments of the Phoenix metropolitan region, Arizona, USA, offer new insights into the drainage history of the Salt and Verde river drainage basins. Analyses of detrital zircons with U Pb dating reveal a different signature for Ancestral Salt River Deposits (ASRD) as compared to the underlying basin fill. Trace element, 87Sr/86Sr ratios, and electron microprobe analyses of basalt fragments in the basal deposits of the ASRD in two different wells from Mesa, Arizona, show matches for outcrop sources near Bartlett Dam in the Verde drainage and near Stewart Mountain Dam in the Salt drainage. This indicates that the Salt and Verde rivers were transporting these basalts when the Salt River first flowed into the metropolitan Phoenix area. We employed tephrochronology to determine that the 3.3-Ma Nomlaki tuff accumulated in closed-basin playa deposits located near the present-day junction of the Salt and Verde rivers, providing a maximum-limiting age for the integration of both rivers. Because the age of mountains crossed by the Salt and Verde rivers pre-date <3.3 Ma by tens of millions of years, we rule out antecedence and superimposition as possible mechanisms to explain these transverse drainages. Multiple lines of evidence presented here are inconsistent with drainage piracy from headward erosion for both drainages: (i) headward erosion would have eroded analyzed basalt clasts from outcrop positions prior to drainage integration, and yet these clasts only occur in the lowest deposits of the ASRD; (ii) headward erosion would not be expected to create transverse streams in two distinct river drainages at the same time, and yet basalt clasts eroded from outcrops in the Salt and Verde rivers arrived together in the basal layer of the ASRD deposits, within the temporal resolution of 3 m sampling interval for well cuttings; (iii) headward erosion of the Verde River from the Nomlaki Tuff (providing maximum age of river integration at 3.3 Ma) to the breached Verde Formation-depositing lake at 2.5 Ma would require an extraordinarily fast rate of >12 cm/yr across multiple bedrock uplands; and (iv) Mescal Limestone clasts on the highest Salt River strath terrace cannot be explained by headward erosion. However, all analyzed evidence are consistent with the process of lake overflow. Based on our findings and the use of geomorphic criteria (Douglass et al., 2009), we conclude that lake overflow is the most likely cause of drainage integration of both the Salt River and the Verde River. Unlabelled Image • Verde and Salt rivers integrated drainages after Nomlaki 3.3 Ma eruption • Detrital zircon analyses reveal substantially different source areas of sediment before and after river integration • Mineralogic and geochemical analyses indicate penecontemporaneous arrival of Salt and Verde rivers into the Phoenix area • Antecedence, superimposition, and headward erosion via piracy processes in conflict with new evidence • Lake overflow best explains Verde and Salt rivers integration across the Basin and Range Province of Arizona, USA [ABSTRACT FROM AUTHOR]