A review of the regional geophysics of the Arizona Transition Zone

Abstract

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...

Physical State and Structure of the Crust Beneath the Western‐Central United States From Multiobservable Probabilistic Inversion

“…Considering therefore (1) that this region is bounded by important structural lineaments (e.g., Holbrook Lineament—HL) and (2) its velocity, density and V p / V s structures (sections 5.3 and 5.6), it seems plausible that its relative colder structure is the a direct reflection of a distinct crustal bulk composition. This interpretation is also supported by the large magnetic and density contrasts across the Holbrook lineament (Bouligand et al, ; Hendricks & Plescia, ).…”

Seismicity within Arizona during the Deployment of the EarthScope USArray Transportable Array

“…(Figure 13). Characteristics including crustal structure and thickness, stress orientations, volcanism, seismicity, heat flow, and gravity suggest that the Transition Zone represents both a tectonic and geophysical transition between the thick and relatively stable crust of the Colorado Plateau and the thin and highly-extended crust of the southern Basin and Range (Brumbaugh, 1987;Hendricks and Plescia, 1991;Thompson and Zoback, 1979).…”

“…The Colorado Plateau encompasses much of the northern and eastern portions of the state and is characterized by relatively thick (~40 km) crust that has experienced little deformation since the Mesozoic. The Arizona Transition Zone extends across central Arizona in the northwestsoutheast direction, and is characterized by extensive horst block mountain ranges, volcanic mountains, graben and half graben basins and steep canyons (Hendricks and Plescia, 1991). The Southern Basin and Range province extends across much of the southern and western portions of the state and is characterized by extensive sediment-filled basins separated by ranges formed from tilted normal fault blocks and metamorphic core complexes.…”

Synthesis of Upper Verde River research and monitoring 1993-2008

“…The Highlands coincide closely to the Arizona Transition Zone identified by ecologists, geologists, and others (Karlstrom and Bowring 1988;Hendricks and Plescia 1991;Ezzo and Price 2002). The Central Arizona Highlands have been the focus of a wide range of research efforts designed to learn more about the effects of natural and human-induced disturbances on the functions, processes, and important components of the region's ecosystems, including hydrology (Arizona State Land Department 1962;Baker 1999).…”