Westernmost Grand Canyon incision: Testing thermochronometric resolution

The timing of carving of Grand Canyon has been debated for over 100 years with competing endmember hypotheses advocating for either a 70 Ma (“old”) or <6 Ma (“young”) Grand Canyon. Several geological constraints appear to support a “young” canyon model, but thermochronometric measures of cooling history and corresponding estimates of landscape evolution have been in debate. In particular, 4He/3He thermochronometric data record the distribution of radiogenic 4He (from the 238U, 235U and 232Th decay series) within an individual apatite crystal and thus are highly sensitive to the thermal history corresponding to landscape evolution. However, there are several complicating factors that make interpreting such data challenging in geologic scenarios involving reheating. Here, we analyze new data that provide measures of the cooling of basement rocks at the base of westernmost Grand Canyon, and use these data as a testbed for exploring the resolving power and limitations of 4He/3He data in general. We explore a range of thermal histories and find that these data are most consistent with a “young” Grand Canyon. A problem with the recovered thermal history, however, is that burial temperatures are under predicted based on sedimentological evidence. A solution to this problem is to increase the resistance of alpha recoil damage to annealing, thus modifying He diffusion kinetics, allowing for higher temperatures throughout the thermal history. This limitation in quantifying radiation damage (and hence crystal retentivity) introduces non-uniqueness to interpreting time–temperature paths in rocks that resided in the apatite helium partial retention zone for long durations. Another source of non-uniqueness, is due to unknown U and Th distributions within crystals. We show that for highly zoned with a decrease in effective U of 20 ppm over the outer 80% of the radius of the crystal, the 4He/3He data could be consistent with an “old” canyon model. To reduce this non-uniqueness, we obtain U and Th zonation information for separate crystals from the same rock sample through LA-ICP-MS analysis. The observed U and Th distributions are relatively uniform and not strongly zoned, thus supporting a “young” canyon model interpretation of the 4He/3He data. Furthermore, we show that for the mapped zonation, the difference between predicted 4He/3He data for a uniform crystal and a 3D model of the crystal are minimal, highlighting that zonation is unlikely to lead us to falsely infer an “old” Grand Canyon

Natural trace element salinization of the Jemez River, New Mexico by geothermal springs and major tributaries

The Jemez River (JR), a tributary of the Rio Grande, is in north-central New Mexico within the Jemez Mountains, which houses the active, high-temperature (≤ 300 oC), liquid-dominated Valles Caldera geothermal system (VC). This work focuses on the northern portion of the JR, spanning a reach from the East Fork JR to the town of San Ysidro. Previous decadal work during low-flow or baseflow conditions (~10-20 cfs) has identified and characterized significant major-solute contributions from two outflow expressions of the VC, Soda Dam Springs and Jemez Hot Springs, and two major tributaries, Rio San Antonio and Rio Guadalupe. There is generally a net ~500-ppm increase from below Soda Dam to the end of the study segment. The distribution of concentrations of twenty-four trace metals from recent Fall 2017 sampling are defined by range from \u27ultra-trace\u27 levels (0.1-1 ppb) to measurements as much as 1 ppm. A set of elements (e.g., As, Li, Rb, Ba, Ti) follows the same downstream behavior of major ions, which is characterized by an increase in concentrations at each inflow and the observed greatest contribution (as much as an order of magnitude) is at Soda Dam. Another group (e.g., U, Al, Fe, Mn, Se) shows complex downstream patterns, which may be a result of non-conservative processes, such as precipitation/dissolution, sorption, and complexation. We attempt to resolve these potential in-stream processes with high-resolution (regular 1-km spacing with interspersed 50-m intervals around sites with complete chemistry) spatial surveys of temperature, dissolved oxygen, pH, oxidation-reduction potential, and turbidity

Using Fill Terraces to Understand Incision Rates and Evolution of the Colorado River in Eastern Grand Canyon, Arizona

The incision and aggradation of the Colorado River in eastern Grand Canyon through middle to late Quaternary time can be traced in detail using well-exposed fill terraces dated by a combination of optically stimulated luminescence, uranium series, and cosmogenic nuclide dating. This fluvial history provides the best bedrock incision rate for this important landscape and highlights the complications and advantages of fill terrace records for understanding river long-profile evolution and incision. The use of fill terraces, as distinct from strath terraces, for calculating incision rates is complicated by the cyclic alluviation and incision they record. In the example of the Grand Canyon this has led to various rates being reported by different workers and rates that tend to be overestimates in shorter records. We illustrate that a meaningful long-term bedrock incision rate of 140 m/m.y. can be extracted from the Grand Canyon record by linking episodes when the Colorado River is floored on bedrock. Variable incision rates reported in the greater region may be, to some degree, due to inconsistent calculations. Our data also highlight that the Colorado River has been a mixed alluvial-bedrock river through both time and space and has been a bedrock river for less than half of its Pleistocene history. This strong temporal variation, combined with the varying bedrock the river encounters on its path, heightens the challenge of understanding the tectonic, climatic, and drainage integration controls on the form and evolution of the Colorado River’s long profile

High-Precision U-Pb Geochronology Links Magmatism in the Southwestern Laurentia Large Igneous Province and Midcontinent Rift

The Southwestern Laurentia large igneous province (SWLLIP) comprises voluminous, widespread ca 1.1 Ga magmatism in the southwestern United States and northern Mexico. The timing and tempo of SWLLIP magmatism and its relationship to other late Mesoproterozoic igneous provinces have been unclear due to difficulties in dating mafic rocks at high precision. New precise U-Pb zircon dates for comagmatic felsic segregations within mafic rocks reveal distinct magmatic episodes at ca. 1098 Ma (represented by massive sills in Death Valley, California, the Grand Canyon, and central Arizona) and ca. 1083 Ma (represented by the Cardenas Basalts in the Grand Canyon and a sill in the Dead Mountains, California). The ca. 1098 Ma magmatic pulse was short-lived, lasting 0.25^+0.67_-0.24 m.y., and voluminous and widespread, evidenced by the ≥100 m sills in Death Valley, the Grand Canyon, and central Arizona, consistent with decompression melting of an upwelling mantle plume. The ca. 1083 Ma magmatism may have been generated by a secondary plume pulse or post-plume lithosphere extension. The ca. 1098 Ma pulse of magmatism in southwestern Laurentia occurred ≁2 m.y. prior to an anomalous renewal of voluminous melt generation in the Midcontinent Rift of central Laurentia that is recorded by the ca. 1096 Ma Duluth Complex layered mafic intrusions. Rates of lateral plume spread predicted by mantle plume lubrication theory support a model where a plume derived from the deep mantle impinged near southwestern Laurentia, then spread to thinned Midcontinent Rift lithosphere over ~2 m.y. to elevate mantle temperatures and generate melt. This geodynamic hypothesis reconciles the close temporal relationships between voluminous magmatism across Laurentia and provides an explanation for that anomalous renewal of high magmatic flux within the protracted magmatic history of the Midcontinent Rift

Late Miocene erosion and evolution of topography along the western slope of the Colorado Rockies

In the Colorado Rocky Mountains, the association of high topography and low seismic velocity in the underlying mantle suggests that recent changes in lithospheric buoyancy may have been associated with surface uplift of the range. This paper examines the relationships among late Cenozoic fl uvial incision, channel steepness, and mantle velocity domains along the western slope of the northern Colorado Rockies. New 40Ar/39Ar ages on basalts capping the Tertiary Browns Park Formation range from ca. 11 to 6 Ma and provide markers from which we reconstruct incision along the White, Yampa, and Little Snake rivers. The magnitude of post-10 Ma incision varies systematically from north to south, increasing from ~ 500 m along the Little Snake River to ~1500 m along the Colorado River. Spatial variations in the amount of late Cenozoic incision are matched by metrics of channel steepness; the upper Colorado River and its tributaries (e.g., Gunnison and Dolores rivers) are two to three times steeper than the Yampa and White rivers, and these variations are independent of both discharge and lithologic substrate. The coincidence of steep river profi les with deep incision suggests that the fl uvial systems are dynamically adjusting to an external forcing but is not readily explained by a putative increase in erosivity associated with late Cenozoic climate change. Rather, channel steepness correlates with the position of the channels relative to low-velocity mantle. We suggest that the history of late Miocene-present incision and channel adjustment refl ects long-wavelength tilting across the western slope of the Rocky Mountains

The laurentian record of neoproterozoic glaciation, tectonism, and eukaryotic evolution in Death Vally, California

Neoproterozoic strata in Death Valley, California contain eukaryotic microfossils and glacial deposits that have been used to assess the severity of putative Snowball Earth events and the biological response to extreme environmental change. These successions also contain evidence for syn-sedimentary faulting that has been related to the rifting of Rodinia, and in turn the tectonic context of the onset of Snowball Earth. These interpretations hinge on local geological relationships and both regional and global stratigraphic correlations. Here we present new geological mapping, measured stratigraphic sections, carbon and strontium isotope chemostratigraphy, and micropaleontology from the Neoproterozoic glacial deposits and bounding strata in Death Valley. These new data enable us to refine regional correlations both across Death Valley and throughout Laurentia, and construct a new age model for glaciogenic strata and microfossil assemblages. Particularly, our remapping of the Kingston Peak Formation in the Saddle Peak Hills and near the type locality shows for the first time that glacial deposits of both the Marinoan and Sturtian glaciations can be distinguished in southeastern Death Valley, and that beds containing vase-shaped microfossils are slump blocks derived from the underlying strata. These slump blocks are associated with multiple overlapping unconformities that developed during syn-sedimentary faulting, which is a common feature of Cyrogenian strata along the margin of Laurentia from California to Alaska. With these data, we conclude that all of the microfossils that have been described to date in Neoproterozoic strata of Death Valley predate the glaciations and do not bear on the severity, extent or duration of Neoproterozoic Snowball Earth events

The Pattern of the Mineralization of Enamel

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/68291/2/10.1177_00220345610400050101.pd