A review of existing geophysical information and new data presented in this special section indicate that major changes in crustal properties between the Basin and Range and Colorado Plateau occur in, or directly adjacent to, the region defined as the Arizona Transition Zone. Although this region was designated on a physiographic basis, studies indicate that it is also the geophysical transition between adjoining provinces. The Transition Zone displays anomalous crustal and upper mantle seismic properties, shallow Curie isotherms, high heat flow, and steep down-to-the-plateau Bouguer gravity gradients. Seismic and gravity studies suggest that the change in crustal thickness, from thin crust in the Basin and Range to thick crust in the Colorado Plateau, may occur as a series of steps rather than a planar surface. Anomalous P wave velocities, high heat flow, shallow Curie isotherms, and results of gravity modeling suggest that the upper mantle is heterogeneous in this region. A relatively shallow asthenosphere beneath the Basin and Range and Transition Zone contrasted with a thick lithosphere beneath the Colorado Plateau would be one explanation that would satisfy these geophysical observations. INTRODUCTIONThe state of Arizona has been divided into several tectonic/physiographic terranes: the Colorado Plateau, the Basin and Range, and the transition zone between the two ( Figure 1). The geophysical signature, tectonic style, and surface geology are distinctly different in each of these terranes. In order to better understand the geologic evolution and current crustal structure of the region, the U.S. Geological Survey is engaged in a program called the Pacific to Arizona Crustal Experiment (PACE) that includes detailed geologic and geophysical (seismic, gravity, magnetic, electrical, and heat flow) surveys.The purpose of this paper is to review the existing geophysical work dealing with the Transition Zone in Arizona and to suggest geophysical models for the structure of the crust and upper mantle which are consistent with those data. As the detailed information from the PACE Program becomes available, these models should provide a basis for regional interpretations. GENERAL CHARACTER OF THE REGION Physiographic Setting Arizona straddles two of the major physiographic subdivisions of the western United States: the Basin and Range and Colorado Plateau provinces [Fenneman, 1931]. The southern boundary of the Colorado Plateau in Arizona is generally thought of as the escarpment of the Mogollon Rim in the east and central parts of the state and the physiographically similar, but unconnected, series of structurally controlled cliffs to the west. South and southwest of this boundary is a region that is physiographically distinct from either the Basin and Range or Colorado Plateau. This region of diverse topography, climate, and geology was included as part of the Mexican Highland by Ransome [1903] and has been called the Central Mountain Belt or Transition Zone [Wilson and Moore, 1959]. The boundary between the Transit...
Temperature logs were made repeatedly during breaks in drilling and both during and after flow tests in the Salton Sea Scientific Drilling Project well (State 2–14). The purpose of these logs was to assist in identifying zones of fluid loss or grain and to characterize reservoir temperatures. At the conclusion of the active phase of the project, a series of logs was begun in an attempt to establish the equilibrium temperature profile. Initially, we were able to log to depths below 3 km, but beginning in late May of 1986, it was impossible to log below about 1.8 km owing to casing failure. Our best estimates of formation temperature below 1.8 km are 305° ± 5°C at 1890 m and 355° ± 10°C at 3170 m. For the upper 1.8 km the latest temperature log (October 24, 1986), using a digital “slickline” (heat‐shielded downhole recording) device, was within a few degrees Celsius of equilibrium, as confirmed by a more recent log (July 31, 1987) to a depth of ∼ 1 km. As in most other wells in the Salton Sea geothermal field, there is an impermeable, thermally conductive “cap” on the hydrothermal system; this cap extends to a depth of more than 900 m at the State 2–14 well. Thermal conductivities of 19 samples of drill cuttings from this interval were measured at room temperature. The conductivity values were corrected for in situ porosity as determined from geophysical logs and for the effects of elevated temperature. Thermal gradients decrease from about 250 mK m−1 (same as degrees Celsius per kilometer) in the upper few hundred meters to just below 200 mK m−1 near the base of the conductive cap. Using one interpretation, thermal conductivities increase with depth (mainly because of decreasing porosity), resulting in component heat flows that agree reasonably well with the mean of about 450 m W m−2. This value agrees well with heat flow data from shallow wells within the Salton Sea geothermal field. A second interpretation, in which measured temperature coefficients of quartz‐ and carbonate‐rich rocks are used to correct thermal conductivity, results in lower mean conductivities that are roughly constant with depth and, consequently, systematically decreasing heat flux averaging about 350 mW m−2 below 300 m. This interpretation is consistent with the inference (from fluid inclusion studies) that the rocks in this part of the field were once several tens of degrees Celsius hotter than they are now. The age of this possible disturbance is estimated at a few thousand years.
Variations in the Earth’s magnetic field arising from local concentrations of shallow subsurface magnetite were mapped in the Arctic National Wildlife Refuge and elsewhere in northern Alaska. The anomalies were delineated with a magnetic horizontal gradiometer mounted on a low‐flying (300 ft or ∼90 m above ground) fixed‐wing airplane. Limited data from stable carbon‐isotope and remanent magnetism measurements of rock cores from the Cape Simpson region strongly suggest that the magnetic anomalies result from the chemical reduction of iron oxides in the presence of seeping hydrocarbons. The magnetic contrast between sedimentary rocks of normally low magnetic susceptibility and those locally enriched with this epigenetic magnetite results in distinctive high‐wavenumber and low‐amplitude total‐field anomalies. Magnetometers extended from each wing tip and in a tail stinger permit calculation of the resultant horizontal gradient vector relative to the flight path. This calculation allows more meaningful interpolation of data for the unsurveyed area between adjacent flight lines spaced at 1.0 mile (1.6 km), thereby allowing generation of accurate computer‐enhanced images or maps. Problems related to diurnal variations and solar storms at high magnetic latitudes are largely overcome because changes in total magnetic field do not significantly affect the magnetic gradient. Analysis of an experimental survey, covering 2745 line‐miles (4418 line‐km), reveals numerous anomalies, the most prominent of which parallels the Marsh Creek anticline. The data provide further evidence that the Marsh Creek anticline is prospective for oil and/or gas. Although the effect of permafrost on epigenetic processes has not been investigated, the data suggest that special‐purpose aeromagnetic surveying may be a useful and inexpensive way to explore for oil and gas in this harsh environment.
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