Seismic structure beneath the contiguous U.S. was imaged with multimode receiver function stacking and inversion of Rayleigh wave dispersion and ellipticity measurements. Crust thickness and ...elevation are weakly correlated across the contiguous U.S., but the correlation is ~3–4 times greater for separate areas east and west of the Rocky Mountain Front (RMF). Greater lower crustal shear velocities east of the RMF, particularly in low‐elevation areas with thick crust, are consistent with deep crustal density as the primary cause of the contrasting crust thickness versus elevation trends. Separate eastern and western trends are best fit by Airy isostasy models that assume lower crust to uppermost mantle density increases of 0.18 g/cm3 and 0.40 g/cm3, respectively. The former value is near the minimum that is plausible for felsic lower crust. Location of the transition at the RMF suggests that Laramide to post‐Laramide processes reduced western U.S. lower crustal density.
Key Points
Distinct crustal isostasy trends identified east and west of the Rocky Mountain Front
East of the RMF mean lower crust to mantle density contrast is about half that in the west
Laramide to post‐Laramide processes likely reduced the density of western U.S. lower crust
The “water corridor” of Grand Canyon (Arizona, USA) includes the Transcanyon Pipeline, which conveys water from Roaring Springs (North Rim) to Grand Canyon Village (South Rim) to supply the park’s ...5–6 million annual visitors. The North Rim water has been reclaimed at the South Rim Water Reclamation Plant (WRP) since the 1960s. This report describes a hypothesis in which the returned pipeline water infiltrates along the Bright Angel Fault and intermingles with groundwater. Geochemical tracers (major ions, stable isotopes) are used to define end members and develop mixing models for South Rim groundwater. It was found that Havasupai Gardens Spring water, discharging below the South Rim along the Bright Angel Fault (~1 km below the WRP), is ~40% North Rim water. Other South Rim springs below the rim also have 10–60% anthropogenic North Rim contribution. Similarly, Coconino Plateau groundwater wells in the town of Tusayan and the Pinyon Plain uranium mine may contain tens of percent of North Rim water. Compatible with this hypothesis, pharmaceutical and personal-care products present in discharge from the WRP, and also in Havasupai Gardens Spring and Pipe Creek Spring below the rim, were found in trace amounts. This study explains the hydrochemical variability of South Rim springs and groundwater as primarily due to anthropogenic groundwater mixing and secondarily due to variations in local recharge, as proposed by others. The hypothesis suggests that uranium mining, local groundwater pumping, and management of the pipeline and WRP infrastructure are all part of an interconnected South Rim groundwater system.
Tectonics of the Colorado Plateau and Its Margins Karlstrom, Karl E; Wilgus, Justin; Thacker, Jacob O ...
Annual review of earth and planetary sciences,
05/2022, Letnik:
50, Številka:
1
Journal Article
Recenzirano
Odprti dostop
The Cenozoic Colorado Plateau physiographic province overlies multiple Precambrian provinces. Its ∼2-km elevation rim surrounds an ∼1.6-km elevation core that is underlain by thicker crust and ...lithospheric mantle, with a sharp structural transition ∼100 km concentrically inboard of the physiographic boundary on all but its northeastern margin. The region was uplifted in three episodes: ∼70-50 Ma uplift above sea level driven by flat-slab subduction; ∼38-23 Ma uplift associated with voluminous regional magmatism and slab removal, and less than 20 Ma uplift associated with inboard propagation of basaltic magmatism that tracked convective erosion of the lithospheric core. Neogene uplift helped integrate the Colorado River from the Rockies at 11 Ma to the Gulf of California by ∼5 Ma. The sharp rim-to-core transition defined by geological and geophysical data sets suggests a young transient plateau that is uplifting as it shrinks to merge with surrounding regions of postorogenic extension.
The Colorado Plateau's iconic landscapes were shaped during its 70-million-year, still-enigmatic, tectonic evolution characterized by uplift and erosion.
Uplift of the Colorado Plateau from sea level took place in three episodes, the youngest of which has been ongoing for the past 20 million years.
Tectonism across the Colorado Plateau's nearest plate margin (the base of the plate!) is driving uplift and volcanism and enhancing its rugged landscapes.
The bowl-shaped Colorado Plateau province is defined by ongoing uplift and an inboard sweep of magmatism around its margins.
The keel of the Colorado Plateau is being thinned as the North American plate moves southwest through the underlying asthenosphere.
Fence Springs are the highest discharge springs of the Redwall-Muav (R-M) karst aquifer in Marble Canyon of eastern Grand Canyon, Arizona, USA. Vents on opposite banks of the Colorado River within ...the Fence fault system have similar chemistries indicating the springs are connected hydrologically within the confined karst aquifer below the river. Stable isotope data fingerprint the recharge area for both springs to be the Kaibab Plateau, west of the river. Chemical variation in nearby R-M springs indicates mixing between karst base flow (represented by Fence Springs) and fast-traveled meteoric waters (Vasey’s Paradise). A 7-year record (2012—2019) suggests the karst base flow has steady temperature (~20 °C) and specific conductance (~2,000 μS/cm) and no seasonality. A progressive decrease of ~1 °C in both springs and ~100 μS/cm in Fence East over 7 years reflects declining spring discharge accompanying declining meteoric recharge. Fortuitous high-flow experiments in the Colorado River during Glen Canyon Dam management operations provide data analogous to a “slug test” for the groundwater system. Rapid increase in river level from ~142 to 1,218 m
3
/s caused the springs to be inundated and mix with river water. Recovery curves showed rapid return of spring temperature from ~10 to 20 °C and specific conductance from 500 to 2,000 μS/cm once river stage fell below ~283 m
3
/s. After 2016, an increase in short-term fluctuations during recovery suggests declining spring discharge through the 7-year period. This multitracer hydrochemical dataset combined with spring monitoring helps establish a baseline for groundwater in eastern Grand Canyon.
Laramide tectonic models are testable with spatiotemporal constraints. The Colorado Plateau portion of the orogen, however, lacks detailed information on deformation timing. Herein, we analyze the ...Zuni Mountains, a Colorado Plateau Laramide arch, with apatite thermochronology, stratigraphic analysis, and thermal calculations to constrain and interpret its low‐temperature and tectonic history. Results show 2.9–3.6 km of pre‐exhumation burial by Late Cretaceous time that was sufficient to reset apatite (U‐Th)/He and apatite fission track dates after Pennsylvanian–Permian Ancestral Rocky Mountains exhumation. Low thermal conductivity of fine‐grained sediments was instrumental in facilitating sufficient temperatures to reset dates. Cooling histories of samples were structurally controlled. Forward and inverse models suggest ca. 80–45 Ma moderate to rapid cooling on the southwest flank and range crest of the arch, while the northeast flank displays 80–55 Ma slow cooling, very slow 55–20 Ma cooling and/or reheating, and post‐20 Ma rapid cooling. Northeast flank protracted cooling likely resulted from burial during backlimb tilting of the arch. Onset of exhumation was ca. 80 Ma, similar to 80–75 Ma onset of localized subsidence in San Juan Basin and 80–77 Ma onset of exhumation on the Nacimiento arch, but somewhat earlier than commonly considered for Colorado Plateau Laramide arches. In the regional context, our results support an easterly sweep of deformation driven by Farallon flat‐slab processes, and argues against prior suggestions for synchronous onset of Colorado Plateau Laramide deformation. Timing of post‐20 Ma cooling is most consistent with mantle‐driven epeirogenic uplift rather than Oligocene geotherm relaxation.
Plain Language Summary
The Laramide orogeny deformed the western interior of North America >1,000 km from the plate margin approximately 90–40 million years ago (m.y.a.). The tectonic process that drove Laramide deformation so far inboard remains enigmatic. Timing of tectonism is a crucial data set that allows proposed tectonic models to be validated, but there are few timing constraints on Colorado Plateau deformation. We use apatite thermochronology to interpret when the Zuni Mountains in west‐central New Mexico (southeastern Colorado Plateau) deformed. Prior to deformation, sedimentary burial heated samples and reset thermochronologic dates. Samples from the Zuni Mountains have different cooling histories based on their location in the range. Southwest flank and range crest samples cooled relatively quickly from 80–45 m.y.a. in a single stage. Northeast flank samples cooled in two stages: approximately 80–55 m.y.a. and at or after 20 m.y.a. Laramide tectonism caused cooling from 80–45 m.y.a. and started earlier than commonly considered for the Colorado Plateau. Published data in the region suggests Laramide‐style deformation swept easterly through time, and fits with Farallon flat‐slab subduction models. Timing of cooling after 20 m.y.a. is the most consistent with increased erosion due to mantle‐driven regional uplift.
Key Points
Thermochronologic data record ca. 80–45 Ma (Laramide) and post‐20 Ma cooling in the Zuni Mountains (west‐central New Mexico)
Cooling histories were structurally controlled; pre‐exhumation heating was aided by low thermal conductivity units
Laramide exhumation started earlier than considered for Colorado Plateau arches, and supports a flat‐slab driven regional eastward sweep
Between 1.8 and 1.0 Ga (Grenville-age), a series of subparallel accretionary orogens were added progressively to the southern edge of Laurentia. These belts now extend from Greenland/Labrador to ...southern California and are truncated at late Precambrian passive margins, suggesting that they once extended farther. We propose that Australia and Baltica contain their continuations. Together they comprise a long-lived orogenic system, >10 000 km long, that preserves a record of 800 million years of convergent margin tectonism. This tectonism culminated during Grenvillian continent–continent collisions in the assembly of the supercontinent Rodinia. Our reconstruction of the Australia–western US part of this assembly (AUSWUS) differs from the SWEAT reconstruction in that Australia is adjacent to the southwestern US rather than to northern Canada. The AUSWUS reconstruction is supported by a distinctive ‘fingerprint’ of geologic similarities between Australia and the southwestern US from 1.8 to 1.0 Ga, by numerous possible piercing points, and by an arguably better agreement between 1.45 and 1.0 Ga paleomagnetic poles between Australia and Laurentia. Geologic and paleomagnetic data suggest that separation between Laurentia and Australia took place ∼800–755 Ma and between Laurentia and Baltica ∼610 Ma. The proposed association of Australia, Laurentia, and Baltica, and the long-lived convergent margin they expose, provide a set of testable implications for the tectonic evolution of these cratons, and an important constraint for Proterozoic plate reconstructions.
Deeply-derived fluids influence river and groundwater hydrochemistry at the Socorro “constriction” in the central New Mexico Rio Grande rift. New hydrochemical transects in the Rio Grande rift ...provide three-dimensional perspectives on sources contributing to surface and groundwaters. In addition to abrupt salinity increases in the Rio Grande, major and trace ion chemistry, δ18O, δD, δ13C, 87Sr/86Sr, 3H and 3He/4He variations in springs and wells across the rift are best explained by mixing of deeply-derived endogenic fluids, containing both mantle and crustal inputs, with shallow groundwater. Endogenic fluids ascend along rift-bounding and intra-rift faults, and can cause increased salinity accompanied by an increase in metals and overall degradation of water quality. Our data reinforce models that attribute a downstream increase in salinity to geological, not agricultural, influences. Hydrochemical mixing of endogenic waters along faults in shallow aquifers has a major influence on groundwater and surface water quality and variability in arid tectonically active regions, with important ramifications for water management.
•We developed a model for water quality degradation in the Rio Grande rift.•Models show degradation due to tectonically controlled endogenic fluid inputs.•Geochemistry is best explained by mantle and crustal fluid inputs.
•Tectonic context for widespread 1340–1035Ma basins throughout southwest Laurentia.•Link between intracratonic sedimentation and Grenville Orogeny in southern Laurentia.•Revised tectonic model for ...southern Grenville Orogeny.
Late Mesoproterozoic sedimentary sequences in southwest Laurentia range in age from 1340 to 1035Ma and record regionally extensive intracratonic sedimentation before and during the Grenville Orogeny in southern Laurentia. This paper examines the specific links between intracratonic sedimentation and orogenesis and develops a new tectonic model for basin formation throughout southwest Laurentia during the Late Mesoproterozoic. New detrital zircon, muscovite, and biotite ages refine the provenance, depositional age, and regional correlations of Late Mesoproterozoic strata throughout southwest Laurentia. These data provide a new view of the Grenville orogen in southern Laurentia as it evolved from a continental margin arc to a continent-continent collision based on the record of inboard sedimentation. Deposition of all Late Mesoproterozoic sequences in southwest Laurentia was facilitated by far-traveled, continental-scale rivers that flowed north off the developing Grenville orogen. Sediment accumulation took place during four discrete basin forming events that can be directly linked to changes in the style of convergence and collision along the southern margin of Laurentia. The oldest basin system (lower Apache and Pahrump Groups) records distal back arc basin sedimentation at 1340–1320Ma, synchronous with widespread magmatism along the southern margin of Laurentia. Unconformably overlying shallow marine carbonate-bearing sequences (lower Unkar Group and correlatives) were deposited between 1255 and 1230Ma in a regionally extensive retroarc basin with clastic sediment sourced in part from an active continental arc along the southern margin of Laurentia. Shallow marine and terrestrial siliciclastic sequences (upper Unkar Group and correlatives) were deposited at 1140–1100Ma and were sourced from unroofing thrust nappes in the orogen to the south in an extensive foreland basin system during continent-continent collision. Finally, coarse-grained siliciclastic deposits of the 1060–1035Ma Hazel Formation represent a proximal foreland basin recording the final stages of the Grenville Orogeny in southern Laurentia.
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