Colorado Plateau Transition Zone Research Articles

Cenozoic drainage reversal on the southern margin of the Colorado Plateau, east-central Arizona, USA

Laramide northeast-flowing streams from the ancestral Mogollon highland beveled gently northeast-dipping Late Proterozoic to Cretaceous strata across the southern Colorado Plateau Transition Zone. Late Eocene renewed uplift rejuvenated northeast-flowing streams incising paleocanyons. Apache paleocanyon was incised into the Mogollon highland, including the north-trending Laramide Apache uplift bounded by major reverse faults/monoclines. The Mogollon Rim sequence aggraded in Apache paleocanyon and on a broad alluvial plain to the east. Middle Cenozoic tectonic subsidence of the Transition Zone, aridification, and volcanism combined to aggrade Apache paleocanyon with sedimentary and volcanic rocks from 37.6 to 18.63 Ma. Emplacement of the Mogollon-Datil caldera complex and Chuska erg on the southeastern Colorado Plateau forced streamflow to northwest-dispersal of fluvio-eolian sediment from 34 to 26 Ma. Following erosion by northwest-flowing streams on the southern Colorado Plateau from ~26 to 16 Ma, lake sediments of the lower Bidahochi Fm were deposited. Southwestward reactivation of Laramide faults was underway by ~25 Ma coeval with extensive 25 to 20 Ma Natanes Plateau basalt flows and extreme crustal thinning southwest of the Transition Zone. As northeastward streamflow gradually diminished, a Superstition field ash flow tuff ended northeastward flow at 18.63 Ma and was followed by a period of sluggish southwest stream flow and ponding until after ~14.84 Ma. Southwestward structural subsidence and possible spillover from ancestral Lake Hopi on the Colorado Plateau southern margin caused incision of the southwest-directed Dagger Canyon paleovalley after 14.84, which followed the path of the earlier Apache paleocanyon and possibly to the Sespe delta on the California coast before opening of Gulf of California. Structural collapse of the Tonto Basin to the west induced deposition of the Dagger Canyon Conglomerate in the Dagger Canyon paleovalley before the modern Salt River incised all previous deposits and became integrated with the Gila River during the Plio-Pleistocene.

In search for the missing arc root of the Southern California Batholith: P-T-t evolution of upper mantle xenoliths of the Colorado Plateau Transition Zone

Xenolith and seismic studies provide evidence for tectonic erosion and eastward displacement of lower crust-subcontinental mantle lithosphere (LC-SCML) underlying the Mojave Desert Region (i.e. southern California batholith (SCB)). Intensified traction associated with the Late Cretaceous flattening of the subducting Farallon plate, responsible for deforming the SW U.S., likely played a key role in “bulldozing” the tectonically eroded LC-SCML ∼500 km eastwards, to underneath the Colorado Plateau Transition Zone (CPTZ) and further inboard. The garnet clinopyroxenite xenoliths from two CPTZ localities, Chino Valley and Camp Creek (central Arizona), provide a rare glimpse of the material underlying the CPTZ. Thermodynamic modeling, in addition to major and trace element thermobarometry, suggests that the xenoliths experienced peak conditions of equilibration at 600-900°C and 12-28 kbar. These peak conditions, along with the composition of the xenoliths (type “B” garnet and diopsidic clinopyroxene) strongly suggest a continental arc residue (“arclogite”), rather than a lower plate subduction (“eclogite”), origin. A bimodal zircon U-Pb age distribution with peaks at ca. 75 and 150 Ma, and a Jurassic Sm-Nd garnet age (154 ± 16 Ma, with initial εNd value of +8) overlaps eastern SCB pluton ages and suggests a consanguineous relationship. Cenozoic zircon U-Pb ages, REE geochemistry of zircon grains, and partially re-equilibrated Sm-Nd garnet ages indicate that displaced arclogite remained at elevated PT conditions (>700°C) for 10s of Myr following its dispersal until late Oligocene entrainment in host latite. With a ∼100 Myr long thermal history overlapping that of the SCB and the CPTZ, these assemblages also contain evidence for late-stage hydration (e.g. secondary amphibole), potentially driven by de-watering of the Laramide slab.In light of these results, we suggest that the CPTZ arclogite originates from beneath the eastern half of the SCB, where it began forming in Late Jurassic time as mafic keel to continental arc magmas. The displacement and re-affixation of the arclogites further inboard during the Late Cretaceous flat slab subduction, might have contributed to the tectonic stability of the Colorado Plateau relative to adjacent geologic provinces through Laramide time and likely preconditioned the region to Cenozoic tectonism, e.g. present-day delamination beneath the plateau, high-magnitude extension and formation of metamorphic core complexes.

Seismicity of the rocky mountains and Rio Grande Rift from the EarthScope Transportable Array and CREST temporary seismic networks, 2008–2010

AbstractWe developed a catalog of small magnitude (ML −0.1 to 4.7) seismicity across Colorado and New Mexico from the EarthScope USArray Transportable Array and CREST (Colorado Rocky Mountains Experiment and Seismic Transects) seismic networks from 2008 to 2010 to characterize active deformation in the Rio Grande Rift. We recorded over 900 earthquakes in the Rio Grande Rift region, not including induced earthquakes and mine blasts, and find that the rift is actively deforming both broadly and in distinct regions. Seismic events that are likely induced, mostly in the Raton Basin, make up 66% of the catalog (1837 earthquakes). Neogene faults in the northern rift in north central Colorado are seismically active in the North Park Basin and northwestern Colorado. The central rift from the San Luis Basin (southern Colorado) to south of the Socorro Magma Body is the most seismically active rift region, and seismicity delineates the deformation in the Colorado Plateau transition zone, which is spatially correlated with volcanic vents, dikes, and faults within the western Jemez Lineament. The eastern Jemez Lineament is nearly aseismic and surrounded by a halo of seismicity culminating in boundaries defined by recent moderate (Mw 3.9 and Mw 3.3) earthquakes. The southern rift is characterized by diffuse seismicity in Texas and Mexico. This study provides an updated seismic catalog built with uniformity in seismometer coverage and low epicentral uncertainties (~2 km) that allows for regional evaluation of seismicity. During this time period, clusters of seismicity and moderate magnitude earthquakes characterize deformation in a low‐strain rate extensional environment.

Magmatic modification of the uppermost mantle beneath the Basin and Range to Colorado Plateau Transition Zone; Evidence from xenoliths, Wikieup, Arizona

Upper mantle xenoliths from Wikieup, AZ, provide abundant evidence for magmatic modification of the uppermost mantle beneath the Transition Zone between the Colorado Plateau and the southern Basin and Range province. Upper mantle lithologies in this xenolith suite are represented by spinel peridotite, wehrlite, plagioclase peridotite, and Al-augite group pyroxenites. Isotopic data for these xenoliths yield relatively uniform values and suggest a common petrogenesis. Al-augite-bearing gabbro and pyroxenite xenoliths from this locality are interpreted to have formed by crystal fractionation processes from parent alkali basalts similar to the Wikieup host basalt. Mineral and whole rock compositions show consistent trends of increasing incompatible element contents (Fe, Al, Ca, Na, K, LIL, and LREE), and decreasing compatible element contents (Mg, Cr, Ni) from spinel peridotite to wehrlite to plagioclase peridotite to the host basalt composition. These compositional trends are interpreted as resulting from varying degrees of magma-mantle wall rock interaction as ascending mafic magmas infiltrated upper mantle peridotite. Small degrees of melt infiltration resulted in slightly modified spinel peridotite compositions while moderate degrees metasomatized spinel peridotite to wehrlite, and the highest degrees metasomatized it to plagioclase peridotite. Whole rock compositions and clinopyroxene, plagioclase, and whole rock isotopic data suggest that the infiltrating magmas were the same as those from which the gabbros and pyroxenites crystallized, and that they were alkalic in composition, similar to the Wikieup host alkali olivine basalts. Relatively uniform 143Nd/144Nd for the mineral separates and whole rocks in spite of the significantly wide range in their 147Sm/144Nd (0.71–0.23 in clinopyroxene) suggests that the Wikieup xenoliths including gabbro, pyroxenite, peridotite, wehrlite, and plagioclase peridotite, are all relatively young rocks formed or metasomatized by a relatively recent magmatic episode.

Paleomagnetic and 40Ar/39Ar age constraints on the chronologic evolution of the Rio Puerco volcanic necks and Mesa Prieta, west-central New Mexico: Implications for transition zone magmatism

The Rio Puerco volcanic necks and Mesa Prieta, located in the transition zone between the Rio Grande rift and southeastern Colorado Plateau, New Mexico, are part of the larger Mount Taylor volcanic field. The comparatively small volume, but numerous, eruptions represented by the volcanic necks provide geochronologic insight to the larger magmatic system at Mount Taylor and in a broader sense to regional late Cenozoic volcanism of the southeastern Colorado Plateau transition zone. Whole-rock 40 Ar/ 39 Ar ages from 17 basaltic plugs, flows, and dikes range in age from 4.49 to 2.05 Ma and overlap the 3.73 to 1.57 Ma volcanism observed at Mount Taylor. Volcanic eruptions in the Rio Puerco Valley were few and sporadic prior to 3.17 Ma; after this time, eruptions progressively increased to a zenith at ca. 2.70 Ma and ceased altogether after 2.56 Ma. Volcanism at Mesa Prieta (2.36–2.05 Ma) is significantly younger than that recognized for the volcanic necks and is considerably more voluminous and chemically evolved. Paleomagnetic data from the volcanic necks show that basalts carry reliable thermoremanent magnetizations; 9 of 13 sites exhibit well-grouped statistical characteristics. Polarity results provide an independent check of the 40 Ar/ 39 Ar analytical accuracy and are useful in resolving problems associated with excess Ar and/or Ar loss. The magmatic evolution of the Rio Puerco necks differs from the rest of the Mount Taylor volcanic field in duration of volcanism, erupted volume, and composition. These differences are believed to be related to the nonuniform character of the lithosphere9s structure and dynamic thermal construct across the Jemez zone in the vicinity of the Mount Taylor volcanic field. Our model proposes a zone of localized high heat flow and higher magma production beneath Mount Taylor volcano from 3.73 to 1.57 Ma to produce a suite of voluminous basalt-rhyolite compositions. However, east of Mount Taylor in the Rio Puerco Valley where the lithosphere is proposed to be cooler and more rigid throughout the same time interval, small-volume, xenolith-bearing, mafic eruptions are more typical. A cooler lithosphere is consistent with both low magma production rates and the observed small-volume eruptions of the Rio Puerco necks. A preexisting structural trend in the Rio Puerco Valley is believed to have assisted the ascent of volatile-rich, Rio Puerco magmas, particularly through the upper crust.

Southern Basin and Range Province crust‐mantle boundary: Evidence from gabbroic xenoliths, Wikieup, Arizona

Gabbro xenoliths in Tertiary alkali olivine basalts near Wikieup, Arizona provide samples from the crust‐mantle boundary beneath the Basin and Range to Colorado Plateau transition zone. The xenoliths consist of two pyroxenes+plagioclase+spinel (gabbronorite) and olivine+clinopyroxene+plagioclase+spinel (olivine gabbro). Pyroxenes and spinel are aluminous; plagioclase is Ca‐rich (An59–71), and olivine is Mg‐rich (Fo69–79). Major element compositions suggest that the gabbros are cumulates formed by fractional crystallization from an alkalic parent magma. Thermobarometry indicates high equilibration temperatures of 900–1050°C. Abundant composite xenoliths of gabbro intruded into spinel and plagioclase peridotite constrain the depth of origin to the uppermost mantle or lowermost crust (about 30 km depth, 0.8 GPa pressure) and suggest that these rocks represent fragments from a crust‐mantle transition zone formed by basaltic underplating during extension‐related magmatism. This transition zone consists of peridotite intruded by veins of gabbro. The amount of peridotite decreases and gabbro increases upward until, in the lower crust, there is only gabbro. Elevated lower crust and upper mantle temperatures estimated from the xenoliths are in accord with the elevated geothermal gradient determined by surface heat flow (>80 mW m−2). Estimated compressional seismic velocities (Vp), corrected for temperature and pressure, are about 6.4 to 7.2 Ion s−1 for these gabbros. Geophysical data may be compatible with the presence of a thin, <2‐km thick, crust‐mantle transition zone beneath Wikieup. The Wikieup gabbro xenoliths are samples from this thin transition between crust and mantle. There is no evidence that these gabbros are representative of the lower crust as a whole.

The mechanics of continental extension in western North America: implications for the magmatic and structural evolution of the Great Basin

A finite element model of continental extension within a rheologically stratified lithosphere is used to examine the structural and magmatic consequences of extensional collapse of thickened crust within the Early Tertiary North American Cordillera. Crustal thickening creates a weakness in the upper mantle in the model which focuses strain within the Great Basin during extension. Marginal highlands, corresponding to the Sierra Nevada and Colorado Plateau, develop near the edges of the weakened region, where abrupt changes in the strength of the lithosphere create large gradients in the amount of strain. The great thickness of the crust results in widespread ductile flow in the lower crust within the interior of the Great Basin, favoring the formation of metamorphic core complexes during the early stages of extension. Ductile flow becomes inhibited as the crust thins and cools, possibly contributing to the change in the style of extension from low-angle detachment faulting to high-angle normal faulting during the Middle Miocene. As the lithosphere cools, the strength of the uppermost mantle increases. After about 10 m.y. the upper mantle becomes stronger beneath the Great Basin than beneath adjacent unextended regions. Strain then begins to migrate outward from the interior of the Great Basin onto the marginal highlands, and their interior edges begin to collapse and are incorporated into the widening Great Basin. After 40 m.y. of extension the highest rates of strain are localized at the edges of the Great Basin, in positions corresponding to the Walker Lane tectonic belt of western Nevada and the Colorado Plateau transition zone. The weight of the marginal highlands in the model is partially supported by flexure of the strong layer in the uppermost mantle. Portions of the uppermost mantle which have been flexed upward cool more rapidly than deeper portions, creating periodical variations in the strength of the lithosphere which eventually develop into crustal and lithospheric boudinage with a wavelength similar to that observed in regional gravity studies. Rapid thinning of the model lithosphere during the first 10 m.y. of extension results in nearly adiabatic decompression of the lithospheric mantle. Partial melting of the lower lithospheric mantle is likely during this period if volatile phases or mafic lithologies were present. This mechanism may account for widespread silicic magmatism in the Great Basin during the Middle Tertiary. As the region undergoing extension widens, the rate of lithospheric thinning decreases and the mantle beneath the Great Basin begins to cool. This leads to a cessation of melting within the lithosphere and waning of silicic volcanism 15–20 m.y. after the onset of extension. The appearance of predominantly basaltic and bimodal volcanic suites in the Great Basin during the last 5–10 Ma correlates well with the onset of decompression melting in the asthenosphere if the mantle potential temperature is greater than about 1350°C (approximately 70°C warmer than typical mantle potential temperature).

Implications of low‐temperature cooling history on a transect across the Colorado Plateau‐Basin and Range Boundary, west central Arizona

Fission track ages of apatite and zircon from metamorphic, plutonic, and sedimentary rocks along a 80‐km transect across the Colorado Plateau‐Basin and Range boundary in west central Arizona show differences in the low‐temperature cooling histories between the provinces. The transect extends from Cypress Mountain in the Colorado Plateau transition zone to the eastern Buckskin Mountains in the Basin and Range. Along the northeast margin of the Basin and Range province, metamorphic rocks exposed in the footwall of a major detachment fault system yield zircon and apatite fission track ages of 16–10 Ma. These ages are similar to K‐Ar fusion ages of biotite and age minima of K‐feldspar on 40Ar/39Ar age spectra and collectively indicate rapid cooling. One K‐feldspar age spectrum has an age maximum of about 22.8 Ma, an age minimum of 11.8 Ma, and a spectrum whose shape is suggestive of reheating, possibly in middle Miocene time. The heating event was probably related to hydrothermal activity during emplacement of Cu and Mn deposits in and above the detachment fault zone. Effects of this heating are only locally detected in rocks above the detachment fault. In the Poachie Range fission track ages of apatite and zircon are 60–50 and 80–70 Ma, respectively. The disparity between the apatite and zircon ages indicates that the rocks cooled slowly in Late Cretaceous and early Tertiary time, probably due to gradual uplift and erosion. Total uplift and denudation in the area of the Poachie Range since Cretaceous time is 6 km or more. To the northeast of the range, fission track ages of apatite and zircon increase and diverge, indicating that apparent uplift decreased in that direction. The apatite ages from the Poachie Range are concordant with early Tertiary hornblende ages determined in other studies of lower plate rocks near the southwest end of the transect. The ages represent cooling of crystalline rocks after Cretaceous regional metamorphism and magmatism. Near Bagdad, 20 km northeast of the Poachie Range, 2 km or less of erosion has occurred since intrusion of high‐level plutons and dikes and caldera formation in Late Cretaceous time. Remnants of an erosion surface that developed in middle Tertiary time are preserved in the transition zone. Volcanic and sedimentary rocks at least as old as early Miocene were deposited on the erosion surface and filled valleys cut into it. Dissection of these deposits began about 8 Ma. We interpret these data combined with those from other studies to indicate that in Cretaceous time southward thrusting and later extensive magmatism in the middle crust led to thickening and heating of the crust. The Cretaceous igneous rocks at Bagdad are high‐level manifestations of this magmatism. Uplift and slow cooling occurred in Late Cretaceous and early Tertiary time. In late Oligocene and early Miocene time during northeast‐southwest extension, middle crustal rocks moved southwest put from beneath the southwest margin of the transition zone. Tectonic denudation rapidly exposed the crust that had been brought up from a depth of 10 km or more and rapidly cooled in the eastern Buckskin and Harcuvar mountains. Middle Miocene reheating occurred locally in the lower plate, along the detachment, and in nearby parts of the upper plate.

Composition of the lower crust in west central Arizona from three‐component seismic data

New P and S wave seismic data from the Basin and Range province—Colorado Plateau transition zone allow a better determination of the composition of the lower crust than would be possible from P wave data alone. The data were collected in 1987 by recording large seismic refraction shots into stationary reflection spreads during the U.S. Geological Survey's Pacific to Arizona Crustal Experiment. Significant seismic observations about the lower crust include (1) strong P wave reflections at its top, (2) moderate‐strength S wave reflections from the entire lower crust, (3) an average P wave velocity of 6.6±0.15 kms−1, and (4) an average Poisson's ratio of σ = 0.27±0.02. Using these constraints and physical property measurements from laboratory rock samples, we conclude that the average composition of the lower crust in this region is intermediate‐to‐rnafic and that mafic underpiating has been minor. Consequently, the Cenozoic‐age mafic and ultramafic xenoliths that are found in the Transition Zone must have resided in the upper mantle or in a thin transition zone at the base of the crust, since these rocks have higher P wave velocities and Poisson's ratio than observed seismically.

Lithology and evolution of the crust‐mantle boundary region in the southwestern Basin and Range Province

Mantle and crustal xenoliths from volcanic rocks in the southwestern Basin and Range province and Colorado Plateau Transition Zone reveal histories of episodic magmatism and deformation that have profoundly influenced the crustal structure of this region. Seismic transects in this area show a strongly reflective Moho of generally low relief, which, in the area of modern transects, consists of a thin zone (<2 km thick) of short reflectors. The upper mantle is transparent and has a Pn of 7.8–8.0 km/s similar to much of the western United States. A lower crustal zone, 2–13 km thick, has variable internal reflectivity and a relatively low velocity of 6.6–6.8 km/s. Upper mantle peridotite xenoliths show both ductile and brittle deformational features and have structures and compositions affected by magmatic intrusion; intrusions form complex dike systems and extensive zones of grain boundary infiltration in peridotite xenoliths. Whereas melt infiltration preceded and followed ductile deformation, brittle deformation, represented by closely spaced joint systems and faults, followed ductile deformation and is related to the youngest magmatic episodes. These structural characteristics and high uppermost mantle temperature (∼1000°C) may combine to explain the relatively low Pn. Alternating layers of ductily deformed and undeformed peridotites, with or without igneous intrusions, may contribute to the reflectivity of the Moho. Lower crustal xenoliths are dominantly igneous‐textured pyroxenites and mafic to intermediate gabbros identical to the dikes in peridotite xenoliths. The crustal xenoliths also commonly are jointed, and in addition many show partial melting and have abundant cavities that probably were filled with CO2‐rich fluids. These rocks are interpreted as products of underplated magmas that were fed through the mantle dike systems and may represent the lowest crustal unit identified in the seismic records. The mafic compositions and high densities of the crustal xenoliths indicate that the low velocity of the lower crust may be caused in part by fracture systems, partial melts, and high temperatures. Garnet granulite xenoliths from a locality with no mantle peridotite xenoliths probably represent crust of the region before late Miocene extension. Felsic granulite xenoliths from two localities have velocities like those of the two lower crustal units identified seismically and could be present in the modern crust as unequilibrated remnants of old crust. The preferred model for the evolution of the lower lithosphere is one in which extension affects the upper mantle as well as the crust and is overlapped in time by multiple magmatic episodes. The earliest magmatic events preceded extension, and later events accompanied and followed extension.

Seismic identification of basement reflectors: The Bagdad Reflection Sequence in the Basin and Range Province—Colorado Plateau Transition Zone, Arizona

Although seismic reflection profiling has contributed greatly to our knowledge of the crust and upper mantle, it is often difficult to unambiguously identify the geologic origin of reflections. This is particularly true of intrabasement reflectors whose geometry and origin we need to know in order to understand tectonic processes operating in the crust. We present synthetic seismogram modeling of recently collected P and S wave seismic reflection data which suggests that the intrabasement Bagdad reflectors in west central Arizona are intrusive mafic sheets. The identification is based on the determination of the phase (positive polarity) and amplitudes of the reflected arrivals. If the Bagdad reflectors are Cenozoic age mafic sheets, they may be the product of rapid intrusion into an area of weak extension where the principal stresses are approximately equal.

Chemical and isotopic evidence for lithospheric thinning beneath the Rio Grande rift

Continental rifting disrupts the lithosphere, thermally and chemically modifying the upper mantle and crust beneath a rift. The isotopic and chemical compositions of basaltic rocks associated with continental rifting reflect the physiochemical conditions of their mantle source reservoirs and can be used as probes to trace processes associated with modification of the upper mantle. Basalts of different ages can potentially record the evolution of mantle reservoirs beneath rifts. Late Cenozoic alkali basalts from the Rio Grande rift and adjacent tectonic provinces display a wide range of isotopic values that correlate with tectonic setting and upper-mantle geophysical properties. This correlation indicates that the upper mantle beneath the rift region is composed of two distinct geochemical reservoirs1 that, before lithospheric extension, corresponded to the ancient lithospheric mantle and asthenosphere, part of the convecting upper mantle. Here we discuss major-element and isotopic data for basalts in a small region of the southeastern Colorado Plateau transition zone adjacent to the central Rio Grande rift. Basalts from this area record an episode of lithospheric thinning that occurred between 8 and 4 Myr ago, concurrent with regional uplift and reactivation of rifting.

Regional gravity survey, northern Marysvale volcanic field, south-central Utah

New gravity data in south-central Utah reveal anomalies over major structural and volcanic features in the northern Marysvale volcanic field. Gravity lows are associated with the Mount Belknap and Big John calderas in the Tushar Mountains, and with the Red Hills caldera in the Antelope Range. Steep gravity gradients mark the locations of the Sevier, Elsinore, Dry Wash, and Tushar faults; gravity lows are observed over the grabens dropped down between these major normal faults. The steep gravity gradient across the Basin and Range–Colorado Plateau transition zone is not necessarily the result of deepening of the Moho, but may simply reflect the thick low-density volcanics in the Marysvale field as well as changes in the density of sedimentary rocks across the Cordilleran hingeline. East-northeast–trending gravity contours follow closely the northern edge of a series of Tertiary intrusive bodies and are generally aligned with the trend of the Wah Wah–Tushar mineral belt of southwestern Utah.