ABSTRACT The San Francisco volcanic field stretches from Williams, Arizona, in the west, to northeast of Flagstaff, Arizona, on the east. Within the ~5000 km 2 area, more than 600 volcanoes are primarily monogenetic and basaltic, but silicic stratovolcanoes and domes are present as well. This field guide focuses on five broadly basaltic cones (Government Prairie vent, Red Mountain, SP Crater, Colton Crater, and Sunset Crater) and two silicic volcanoes (Kendrick Peak and San Francisco Mountain) in the field, with an emphasis on the different kinds of volcanic activity represented and the petrological variations. Hazards assessment indicates that is it possible for future eruptions to affect Flagstaff, but the probability is low. As information in this guide indicates, hazard assessments need to be improved to encompass a wide range of eruption types, and additional data are needed to improve models of the rate of volcanic activity and how the locus of activity has shifted over time.

Depth-dependent variations in the structure and composition of continental crust can be studied via integrated investigations of isobaric terranes. In this contribution, we summarize three isobaric terranes in Archean to Proterozoic crust. In western Canada, 35–45-km-deep lower crust is exposed over an area of more than 20,000 km 2 . The Upper Granite Gorge of Grand Canyon, Arizona, provides a transect of 20–25-km-deep middle crust. The Proterozoic basement of central Arizona represents an isobaric exposure of 10–15-km-deep middle crust. Isobaric terranes yield a conceptual image of continental crust that can be compared to seismic images, xenolith data, and drill core data to clarify rheology, coupling/decoupling of crustal levels, and the interplay between deformation, metamorphism, and plutonism. General observations include: (1) The crust is heterogeneous at all levels and cannot be accurately modeled as a simple progression from quartz-rich to feldspar-rich lithologies or from felsic to mafic bulk compositions. (2) The crust is segmented into foliation domains that alternate between steeply dipping and shallowly dipping. (3) Magmatism is expressed differently at different depths due to different background temperatures and a general upward distillation from mafic to felsic composition, and may be the most important control on crustal architecture and rheology. The strength of continental crust (and its potential for low-viscosity flow) is not simply a function of temperature, depth, and compositional layering, but is controlled by the size and distribution of rheological domains. The rheological character of a particular layer can vary in space and time at any crustal level.

Two generations of folds affected the folded unconformity between the 1,750-Ma Brady Butte Granodiorite and overlying Proterozoic Texas Gulch Formation metagraywackes and slates in the Brady Butte area of central Arizona. F 1 folds are isoclinal and were predominantly northwest-verging recumbent folds prior to F 2 folding. They occur at all scales, with macroscopic fold amplitudes exceeding 500 m. F 1 folds were temporally associated with transposition of bedding and mesoscopic thrusting in metasedimentary rocks, and development of mylonitic foliation in both metasedimentary rocks and granodiorite. Both style of structures and asymmetry of fabrics suggest F 1 folding accompanied northwest-directed thrusting. F 2 folds are open to tight and have northeast-striking, steeply dipping, axial plane foliation. F 2 folds are gently northeast plunging in the Brady Butte area, coaxial with F 1 folds, and the enveloping surface of F 2 folds is subhorizontal. New structural data may have important regional implications. A subhorizontal enveloping surface for F 2 folds implies that stratigraphic units and the basal Texas Gulch Formation unconformity are repeated across strike in F 2 fold hinges. Thus, the metasedimentary rocks of the Crazy Basin area may correlate with the Texas Gulch Formation, and parts of the Spud Mountain Volcanics may correlate with parts of the Iron King Volcanics. However, transposition during F 2 and rotation of fold hingelines toward the subvertical F 2 finite stretching axis in areas of high shortening strain complicate structural geometry such that rootless intrafolial folds are common. Furthermore, F 2 folds affected an already transposed S 1 tectonic layering formed by recumbent folds and thrusts. The complex overprinting of F 1 and F 2 on a regional scale make stratigraphic interpretations tenuous until more detailed structural, geochronologic, and geochemical data are available.