West-central Arizona Research Articles

Laramide bulldozing of lithosphere beneath the Arizona transition zone, southwestern United States

Abstract The northwest-trending transition zone (TZ) in Arizona (southwestern United States) is an ~100-km-wide physiographic province that separates the relatively undeformed southwestern margin of the Colorado Plateau from the hyperextended Basin and Range province to the southwest. The TZ is widely depicted to have been a Late Cretaceous–Paleogene northeast-dipping erosional slope along which Proterozoic rocks were denuded but not significantly deformed. Our multi-method thermochronological study (biotite 40Ar/39Ar, zircon and apatite [U-Th-Sm]/He, and apatite fission track) of Proterozoic rocks in the Bradshaw Mountains of the west-central Arizona TZ reveals relatively rapid cooling (~10 °C/m.y.) from temperatures of >180 °C to <60 °C between ca. 70 and ca. 50 Ma. Given minimal ca. 70–50 Ma upper-crustal shortening in the TZ, we attribute cooling to exhumation driven by northeastward bulldozing of continental lower crust and mantle lithosphere beneath it by the Farallon flat slab. Bulldozing is consistent with contemporaneous (ca. 70–50 Ma) underplating and initial exhumation of Orocopia Schist to the southwest in western Arizona and Mesozoic garnet-clinopyroxenite xenoliths of possible Mojave batholith keel affinity in ca. 25 Ma TZ volcanic rocks.

Differences in menopausal symptoms and female sexual function by region and ethnicity in West Texas and Central Arizona: a cross-sectional survey.

To evaluate menopausal symptoms and sexual problems in Hispanic and non-Hispanic women in two Southwest areas. An anonymous survey including the Green Climacteric Scale (GCS), Female Sexual Function Index (FSFI), and demographics was distributed to English and Spanish-speaking women age 40 to 60 in Scottsdale, Arizona, and West Texas. FSFI for sexually active women and GCS scores for the Hispanic and non-Hispanic women in Texas were analyzed with multivariable analysis and compared between Texas and Arizona for Non-Hispanic participants. Predominantly non-Hispanic women (70%), average age 51.5 (SD = 7.25) completed questionnaires (199 West Texas, 163 Scottsdale). A majority of sexually active women (88%) were found to be at risk of sexual dysfunction. Within the Texas cohort, GCS score was estimated to be 3.49 points lower (less symptoms) in Hispanic versus non-Hispanic participants [95% CI -6.58 to -0.40, P = 0.03], and FSFI score was estimated to be 2.31 points lower (more symptoms) in Hispanic versus non-Hispanic participants [95% CI -4.49 to -0.14, P = 0.04]. Among non-Hispanic women, GCS scores were lower (less symptoms) in Texas versus Arizona by 10.25 points [95% CI -14.83 to -5.66, P < 0.01], while FSFI scores were higher overall (less symptoms) in Texas by 3.65 points [95% CI 0.53-6.77), P = 0.02]. All FSFI and GCS scores were adjusted for multiple variables. In a group of menopausal women from the Southwest, most reported symptoms were consistent with FSD, and the degree of sexual problems appeared to be greater in the Hispanic participants from Texas.

Evolution of the early Mesozoic Cordilleran arc: The detrital zircon record of back-arc basin deposits, Triassic Buckskin Formation, western Arizona and southeastern California, USA

Abstract A shift in the depositional systems and tectonic regime along the western margin of Laurentia marked the end of the Paleozoic Era. The record of this transition and the inception and tectonic development of the Permo-Triassic Cordilleran magmatic arc is preserved in plutonic rocks in southwestern North America, in successions in the distal back-arc region on the Colorado Plateau, and in the more proximal back-arc region in the rocks of the Buckskin Formation of southeastern California and west-central Arizona (southwestern North America). The Buckskin Formation is correlated to the Lower–Middle Triassic Moenkopi and Upper Triassic Chinle Formations of the Colorado Plateau based on stratigraphic facies and position and new detrital zircon data. Calcareous, fine- to medium-grained and locally gypsiferous quartzites (quartz siltstone) of the lower and quartzite members of the Buckskin Formation were deposited in a marginal-marine environment between ca. 250 and 245 Ma, based on detrital zircon U-Pb data analysis, matching a detrital-zircon maximum depositional age of 250 Ma from the Holbrook Member of the Moenkopi Formation. An unconformity that separates the quartzite and phyllite members is inferred to be the Tr-3 unconformity that is documented across the Colorado Plateau, and marks a transition in depositional environments. Rocks of the phyllite and upper members were deposited in wholly continental depositional environments beginning at ca. 220 Ma. Lenticular bodies of pebble to cobble (meta) conglomerate and medium- to coarse-grained phyllite (subfeldspathic or quartz wacke) in the phyllite member indicate deposition in fluvial systems, whereas the fine- to medium-grained beds of quartzite (quartz arenite) in the upper member indicate deposition in fluvial and shallow-lacustrine environments. The lower and phyllite members show very strong age and Th/U overlap with grains derived from Cordilleran arc plutons. A normalized-distribution plot of Triassic ages across southwestern North America shows peak magmatism at ca. 260–250 Ma and 230–210 Ma, with relatively less activity at ca. 240 Ma, when a land bridge between the arc and the continent was established. Ages and facies of the Buckskin Formation provide insight into the tectono-magmatic evolution of early Mesozoic southwestern North America. During deposition of the lower and quartzite members, the Cordilleran arc was offshore and likely dominantly marine. Sedimentation patterns were most strongly influenced by the Sonoma orogeny in northern Nevada and Utah (USA). The Tr-3 unconformity corresponds to both a lull in magmatism and the “shoaling” of the arc. The phyllite and upper members were deposited in a sedimentary system that was still influenced by a strong contribution of detritus from headwaters far to the southeast, but more locally by a developing arc that had a far stronger effect on sedimentation than the initial phases of magmatism during deposition of the basal members.

Orocopia Schist in the northern Plomosa Mountains, west-central Arizona: A Laramide subduction complex exhumed in a Miocene metamorphic core complex

Orocopia Schist in the northern Plomosa Mountains, west-central Arizona: A Laramide subduction complex exhumed in a Miocene metamorphic core complex

Laramide subduction and metamorphism of the Orocopia Schist, northern Plomosa Mountains, west-central Arizona: Insights from zircon U-Pb geochronology

Laramide subduction and metamorphism of the Orocopia Schist, northern Plomosa Mountains, west-central Arizona: Insights from zircon U-Pb geochronology

The role of calcite-rich metasedimentary mylonites in localizing detachment fault strain and influencing the structural evolution of the Buckskin-Rawhide metamorphic core complex, west-central Arizona

The role of calcite-rich metasedimentary mylonites in localizing detachment fault strain and influencing the structural evolution of the Buckskin-Rawhide metamorphic core complex, west-central Arizona

Miocene slip history of the Eagle Eye detachment fault, Harquahala Mountains metamorphic core complex, west-central Arizona

The structural and thermal evolution of major low-angle normal faults in the Colorado River extensional corridor has been a controversial topic since the pioneering studies of metamorphic core complexes in the early 1980s. We present new geo-thermochronometry data from the Harquahala Mountains in west-central Arizona to determine the timing of extension, displacement magnitude, and slip rates along the Eagle Eye detachment fault (EED) during large-magnitude Miocene extension. Zircon and apatite (U-Th)/He data (ZHe and AHe, respectively) from 31 samples along a ~55 km extension-parallel transect indicate active slip along the EED occurred between ~21 ± 1 Ma until ~14 Ma. The spatial extent of ZHe ages and exhumation of the zircon partial retention zone indicated ~44 ± 2 km of total displacement, whereas lithologic similarity and identical U-Pb ages between correlated footwall rocks in the Little Harquahala Mountains and breccia clasts at Bullard Peak in the NE Harcuvar Mountains indicated ~43-45 km of displacement across the EED. AHe and ZHe data indicated slip rates of ~6.7 + 7.8/-2.3 km/Myr, and ~6.6 + 7.1/-2.0 km/Myr, respectively, both consistent with the duration and displacement estimates. The EED initiated as a listric fault with an ~34 ± 9° dip that decreased to ~13 ± 5° below ~7 km depth. Secondary breakaway development and footwall exposure occurred by ~17 Ma, during active EED slip. Lithologic and geo-thermochronometric offset constraints show excellent agreement and provided a rare opportunity to fully resolve the timing, rates, and total displacement magnitudes along a major continental detachment fault.

The transition from large-magnitude extension to distributed dextral faulting in the Buckskin-Rawhide metamorphic core complex, west-central Arizona

Brittle fault data from the Buckskin-Rawhide metamorphic core complex, west-central Arizona, document the Miocene transition from large-magnitude, NE directed extension to distributed E-W extension and dextral faulting. The Buckskin-Rawhide detachment fault locally records a clockwise rotation of the slip direction from dominant top-NE directed slip to ENE and E directed slip during the last stages of activity. Postmylonitic brittle deformation is dominated by normal and dextral slip along moderately to steeply NE dipping faults. Relative timing relationships indicate that most postmylonitic normal faulting was coeval with middle Miocene detachment fault slip, whereas dextral and oblique dextral faulting postdates detachment fault slip (post ~11 Ma). Based on the persistent SW dip of lower plate fabrics and NE dip of abundant normal faults, the amount of NE-SW brittle extension across the mylonitic lower plate is estimated to be 20–30% in several areas. The end of detachment faulting in the Buckskin-Rawhide metamorphic core complex was marked by a transition to dominantly E-W extension and associated dextral-normal faulting. At least 10 NE dipping postdetachment faults record ~0.1–1 km dextral or oblique dextral displacement. The cumulative amount of dextral shear across the core complex is probably 7–9 km, which is the amount needed to restore the topographic trend of lower plate corrugations into alignment with the dominant extension direction. Postdetachment dextral/transtensional faulting across the core complex reflects the increasing influence of the diffuse Pacific-North America transform plate boundary toward the end of the middle Miocene.

John Beach Rinaldo: Quintessential Culture Historian

During an archaeological career that spanned four decades, John Beach Rinaldo (1912-1999) made substantive contributions to the delineation and definition of the Mogollon Culture, the culture history of west-central New Mexico and east-central Arizona, and the identification of material relationships between precolumbian cultures and modern-day Zuni. For a variety of reasons, Rinaldo is overshadowed by his Field Museum collaborator Paul Sidney Martin. As a result, historians of archaeology have failed to critically evaluate Rinaldo's career and contributions. This paper offers a controlled analysis and comparison of data in unpublished archives, artifact collections at the Field Museum, and the published record to illuminate previously unrecognized but important aspects of Rinaldo's many contributions to archaeological knowledge, method, and theory.

Timing, rate, and magnitude of slip on the Buckskin‐Rawhide detachment fault, west central Arizona

AbstractWe present thermochronologic and geochronologic data that constrain the slip history of the Buckskin‐Rawhide detachment fault in west central Arizona, one of the largest extensional fault systems in the North American Cordillera. (U‐Th)/He zircon and apatite thermochronology, integrated with 40Ar/39Ar geochronology of postdetachment volcanic rocks, indicate that large‐magnitude extension associated with the detachment fault initiated at ~21–20 Ma and continued until ~12–11 Ma in the southwestern portion of the Buckskin‐Rawhide metamorphic core complex. (U‐Th)/He footwall cooling ages from the breakaway zone in the western Bouse Hills to upper greenschist‐facies mylonites in the southern Buckskin Mountains indicate that the slip rate on the detachment fault was 3 + 1.5/−1 km/Myr during the early Miocene. Space‐time patterns of hanging wall tilting suggest that at 17–16 Ma, a secondary detachment fault breakaway developed ~12 km northeast of the primary detachment fault breakaway. Proximal conglomerates deposited in a supradetachment basin adjacent to the secondary breakaway scarp were displaced 6–11 km northeast in the middle Miocene by the Buckskin‐Rawhide detachment fault at a slip rate of 1.2–2.7 km/Myr. The total displacement across the detachment fault in the southwestern portion of the core complex is 24 ± 10 km, well short of the previous estimate of 66 ± 8 km across the entire core complex. Based on these data and new observations, we propose that total displacement on the Buckskin‐Rawhide detachment fault system increases in the slip direction to ~40–50 km at the northeastern end of the exposed footwall, corresponding to time‐averaged slip rates that ranged from ~2 km/Myr to ≤6 km/Myr across the entire core complex.

Development of extension-parallel corrugations in the Buckskin-Rawhide metamorphic core complex, west-central Arizona

Extension-parallel detachment fault corrugations are one of the defining features of metamorphic core complexes, yet the origin of these corrugations remains disputed. Structural data from the Buckskin-Rawhide metamorphic core complex in west-central Arizona demonstrate that prominent NE-trending detachment fault corrugations are folds produced by extension-perpendicular (NW-SE) shortening during the middle to late stages of core complex extension (ca. 18–10 Ma). Kinematic data indicate that dominant NE-directed slip on the Buckskin-Rawhide detachment fault was locally overprinted by NW- and SE-directed slip associated with corrugation folding. The last motion recorded along several NW- and SE-dipping flanks of the detachment fault is reverse slip directed toward the corrugation antiform hinges. Orientation patterns of upper-plate bedding across the corrugations are compatible with folding about a NE-trending axis. Extension-perpendicular shortening in the lower plate is recorded by synmylonitic constriction and folding. Constriction and grain-scale y -axis shortening were most important during the later stages of mylonitization in the synextensional Swansea Plutonic Suite, which typically form L > S tectonites. Upright meter- and kilometer-scale lower plate folds parallel the detachment fault corrugations and are best developed in well-layered pre-Tertiary rocks. Most lower plate folding occurred by postmylonitic flexural slip that was coeval with detachment faulting. The total amount of NW-SE shortening across the lower plate is ∼10%, but the amount of NW-SE shortening recorded by the younger detachment fault is only ∼1%. The relatively late-stage development of corrugations in the Buckskin-Rawhide metamorphic core complex suggests that the constriction and corrugation folding developed primarily in response to crustal thinning and tectonic denudation.

Evidence for an Historical Occurrence of the Meadow Jumping Mouse (Zapus Hudsonius Luteus) in the Verde River Watershed, Arizona

The meadow jumping mouse (Zapus hudsonius luteus) is a riparian obligate with a fragmented distribution in the American Southwest. Because of recent rapid declines in populations, it is a candidate for federal protection under the Endangered Species Act. In Arizona, its known distribution has been restricted to the White Mountains region in the east central part of the state. I present evidence for an historical occurrence of jumping mice in the Verde River watershed, Yavapai County, in west central Arizona. Evidence includes record of a specimen collected by Edgar Mearns in the 1880s, a 1944 report of parasites collected from jumping mice in Yavapai County, and a phylogeographic history that is consistent with an occurrence in the region, including an ecological niche model that predicted suitable conditions for Z. h. luteus in the upper Verde River watershed during the last glacial maximum. Field surveys should be conducted to determine if Z. h. luteus currently exists within the Verde River watershed.

Mylonitization in the lower plate of the Buckskin-Rawhide detachment fault, west-central Arizona: Implications for the geometric evolution of metamorphic core complexes

In metamorphic core complexes it is commonly unclear whether lower plate mylonites formed as the down-dip continuation of a detachment fault, or whether they represent a subhorizontal shear zone that was captured by a more steeply dipping detachment fault. Detailed microstructural, fabric, and strain data from mylonites in the Buckskin-Rawhide metamorphic core complex, west-central Arizona, constrain the structural development of the lower plate shear zone. Widespread exposures of ∼22–21 Ma granitoids of the Swansea Plutonic Suite enable us to separate Miocene strain coeval with core complex extension from older deformation. Mylonites across the lower plate consistently record top-to-the-NE-directed shear. Miocene quartz and feldspar deformation/recrystallization mechanisms indicate ∼450–500 °C mylonitization temperatures that were relatively uniform across a distance of ∼35 km in the extension direction. Quartz dynamically recrystallized grain sizes do not systematically vary in the extension direction. Strain recorded in the Swansea Plutonic Suite is also relatively uniform in the extension direction, which is incompatible with models in which lower plate mylonites form as the ductile root of a major detachment fault. Altogether these data suggest the mylonitic shear zone initiated with a ≤4° dip and was unroofed by a more steeply dipping detachment fault system. Lower plate mylonites in the Buckskin-Rawhide metamorphic core complex thus represent a captured subhorizontal shear zone rather than the down-dip continuation of a detachment fault.

Paleomagnetic quantification of upper‐plate deformation during Miocene detachment faulting in the Mohave Mountains, Arizona

Paleomagnetic data from Miocene (∼20 Ma) volcanic rocks and dikes of west central Arizona reveal the tilt history of Proterozoic crystalline rocks in the hanging wall of the Chemehuevi‐Whipple Mountains detachment fault. We obtained magnetization data from dikes and flows in two structural blocks encompassing Crossman Peak and Standard Wash in the Mohave Mountains. In the Crossman block the dike swarm records two components of primary magnetization: (1) CNH, a normal polarity, high‐unblocking‐temperature or high‐coercivity component (inclination, I = 48.5°, declination, D = 6.4°), and (2) CRHm, a reversed polarity, high‐temperature or high‐coercivity component (I = −33.6°, D = 197.5°). Argon age spectra imply that the dikes have not been reheated above 300°C since their emplacement, and a baked‐contact test suggests that the magnetization is likely to be Miocene in age. CRHm deviates from the expected direction of the Miocene axial dipole field and is best explained as a result of progressive tilting about the strike of the overlying andesite flows. These data suggest that the Crossman block was tilted 60° to the southwest prior to intrusion of the vertical dike swarm, and the block continued to tilt during a magnetic field reversal to normal polarity (CNH). Miocene dikes in the Crossman block are roughly coplanar, so the younger dikes with normal polarity magnetization intruded along planes of weakness parallel to the earlier reversed polarity swarm. An alternative explanation involves CNH magnetization being acquired later during hydrothermal alteration associated with the final stages of dike emplacement. In the Standard Wash block, the primary component of magnetization is a dual‐polarity, high‐temperature or high‐coercivity component (SWHl, I = 7.2°, D = 0.7°). To produce agreement between the expected Miocene magnetic direction and the SWH component requires (1) correcting for a 56° tilt about the strike of flow bedding and (2) removing a counterclockwise vertical‐axis rotation of 20°. The two rotations restore the Standard Wash dikes to vertical, make parallel the dike layering in the Crossman and Standard Wash blocks, and align the strikes of bedding in both blocks. Geologic mapping, geochemical evidence, and paleomagnetic data are consistent with the upper plate of the Mohave Mountains having tilted in response to formation of the underlying detachment fault.

The 1.75-Ga Iron King Volcanics in west-central Arizona: a remnant of an accreted oceanic plateau derived from a mantle plume with a deep depleted component

Two lithologic assemblages are recognized in the 1.75-Ga Iron King Volcanics in west-central Arizona: an arc assemblage composed of pillow basalts, intermediate and felsic volcanics and associated volcaniclastic sediment, and an oceanic plateau assemblage composed chiefly of pillow basalts and mafic hyaloclastic breccia. During collision of the Iron King oceanic plateau with Laurentia 1.7 Ga, plateau and arc components were tectonically interleaved. Iron King arc volcanics have subduction-related geochemical signatures with affinities to continental margin arcs. The Iron King plateau basalts include two groups: enriched and nonenriched in very incompatible elements. The enriched group may reflect smaller degrees of melting in the outer, cooler part of a plume head. Incompatible element distributions indicate that the plateau basalts came from a mantle source with mixed depleted and recycled components and a small contribution of an enriched component. Nb/Y–Zr/Y relationships suggest the depleted component came from the deep mantle, rather than being entrained at shallow depths. Our results suggest that recycled, depleted and enriched components were available in the deep mantle by 1.75 Ga.

RediscoveringRana onca: Evidence for Phylogenetically Distinct Leopard Frogs from the Border Region of Nevada, Utah, and Arizona

Remnant populations of leopard frogs within the Virgin River drainage and adjacent portions of the Colorado River (Black Canyon) in northwestern Arizona and southern Nevada either represent the reportedly extinct taxon Rana onca or northern, disjunct Rana yavapaiensis. To determine the evolutionary distinctiveness of these leopard frogs, we evaluated mitochondrial DNA (mtDNA) restriction site variation (RFLP), mtDNA control region sequences, randomly amplified polymorphic DNA (RAPD) markers, and morphological characters. Individuals from the Virgin River drainage and Black Canyon represented a single RFLP haplotype and were identical for nucleotides along a portion of control region sequence. Evaluations of RAPD data demonstrated high levels of similarity among individuals and populations from this region. Leopard frogs from the Virgin River drainage and Black Canyon differed from R. yavapaiensis from west-central Arizona and northern Mexico in maximum parsimony and distance analyses of RFLP and control region sequence data and in maximum-likelihood analysis of the sequence data. Multidimensional scaling of RAPD data provided a similar and congruent indication of this separation. Analysis of principal component scores demonstrated significant morphological differentiation between leopard frog specimens from the Virgin River drainage and R. yavapaiensis. Parallel patterns of divergence observed in the mtDNA, RAPD, and morphological analyses indicate that leopard frogs from the Virgin River drainage and adjacent portions of the Colorado River are phylogenetically distinct. These leopard frogs should be recognized as a lineage separate from southern populations of R. yavapaiensis and classified as the species R. onca.

Abstract: Stratigraphy and Correlation of Triassic and Jurassic Rocks in West-Central Arizona and Southeastern California&nbsp;

Abstract: Stratigraphy and Correlation of Triassic and Jurassic Rocks in West-Central Arizona and Southeastern California&nbsp;

Mid- to Late Cretaceous ductile deformation and thermal evolution of the crust in the northern Dome Rock Mountains, Arizona

Mid- to Late Cretaceous basement-involved deformation in the Maria Tectonic Belt of southeastern California and west-central Arizona varies in style from low-angle overthrusts to km-scale fold nappes. To establish the relationship between tectonite fabrics and thermal evolution of the rocks, structural and petrographic analysis was conducted in the northern Dome Rock Mountains, where the two different styles converge. A penetrative northeast-dipping foliation forms the most conspicuous fabric in an ∼5 km wide zone of distributed ductile deformation. Within the broader zone of ductile deformation, northeast-dipping shear zones contain evidence for both southwest- and northeast-directed deformation. Textural analysis of mylonitic rocks reveals that southwest-directed deformation occurred under prograde conditions, whereas northeast-directed deformation occurred under retrograde conditions. Southwest-directed deformation resulted in tubular (extreme sheath) fold development on a kilometer scale in the footwall of a reverse-sense shear zone. Northeast-directed deformation cuts the map-scale tubular fold and is inferred to have developed as a manifestation of gravitational collapse of rocks mechanically weakened by fluid infiltration and upper greenschist- to lower amphibolite-facies metamorphism. We propose that the mode and degree of fluid infiltration, rather than temperature or structural level alone, controlled the style of deformation in the Maria Belt.

Tectonic implications of rapid cooling of lower plate rocks from the Buckskin-Rawhide metamorphic core complex, west-central Arizona

Lower plate rocks exposed in the Buckskin-Rawhide metamorphic core complex, west-central Arizona, underwent a pronounced increase in cooling rate 7–12 m.y. after the onset of extensional tectonic denudation at ca. 27 Ma. A detailed 40 Ar/ 39 Ar study of lower plate rocks exposed near Planet Peak, in the southwest part of the core complex, indicates that the highest-level mylonitic rocks exposed in the complex cooled below 350 °C prior to 20 ± 1 Ma, at an average rate of ≤15 °C/m.y. Thereafter, the cooling rate of these rocks increased progressively, peaking at 280–80 °C/m.y. prior to initial exposure of the core between 15.1 and 13.3 Ma. Published K-Ar, 40 Ar/ 39 Ar, and fission-track ages for the most recently unroofed lower plate rocks, exposed at the northeastern end of the core complex, are consistent with a similar cooling history, although the abrupt increase in cooling rate occurred as many as 5 m.y. later in this area. Because the increase in the cooling rate appears to have occurred progressively later toward the northeast, it is unlikely to reflect changes in slip rate. It appears to have coincided with cooling below 350–300 °C, and thus is interpreted to reflect steepening of the detachment system above the brittle-ductile transition. Thermochronological data from lower plate rocks in the Planet Peak area, together with constraints on their initial exposure at the surface, are consistent with an increase in the dip of the detachment from le;5° at midcrustal depths (where it was represented by a shear zone) to At temperatures below 200–150 °C, lower plate rocks at both ends of the core complex probably cooled at rates >130 °C/m.y., which can not be reconciled with the average dip (le;25°) and slip rate (6–12 mm/yr) for the detachment fault, given the likely geothermal gradient (∼25 °C/km) during the early stages of extension. This implies that cooling did not keep pace with denudation at upper crustal levels. The very high cooling rates are interpreted to have primarily reflected the development of a pronounced thermal discontinuity across the detachment fault (possibly >100 °C), as hot lower plate rocks were transported to shallow crustal levels. Rapid cooling of the lower plate at these levels may have been enhanced by interaction with fluids derived from the upper plate, consistent with models that link the shallow-level (

Distribution of heat‐producing elements in the upper and middle crust of southern and west central Arizona: Evidence from the core complexes

The amount and distribution of crustal heat production are a central component of all estimates of continental thermal structure, yet they remain in most cases an assumption rather than a constraint. This study utilizes the Catalina and Harquahala metamorphic core complexes as large exposures of the recent (&lt;30 m.y. ago) upper and middle crust of the Arizona Basin and Range to gather primary heat production data. Samples were measured for their heat production, and their positions in the predetachment crust were reconstructed by restoring all major deformation associated with core complex formation, including subsequent block faulting. The restored part of the Catalina core complex spans a depth range from 3 to 13 ± 1 km, and the Harquahala core complex encompasses a depth range from the surface to 14 ± 3 km. Mylonitization probably has little systematic effect on the distribution of heat production in these core complexes. The depth distributions of heat production obtained in this study do not follow a smooth or systematic function but rather reflect a primary control exerted by local structural or magmatic history. Variogram analysis indicates that the scale of autocorrelation of heat production is variable but in most cases under 10 km. In each core complex, the amount of heat production observed plus that inferred to reside in the deeper crust is approximately 30 ± 5 μW/m3, roughly 50% higher than previous estimates. The difference can cause overestimates of 75–300°C in deep crustal temperatures if surface heat flow is assumed to be known. The use of a temperature‐ and pressure‐dependent function for thermal conductivity instead of a constant value causes an upward revision in estimated average geothermal gradients of approximately 2.5–4°C/km.