No abstract

No abstract

Large-volume travertine deposits in the southeastern Colorado Plateau of New Mexico and Arizona, USA, occur along the Jemez lineament and Rio Grande rift. These groundwater discharge deposits refl ect vent locations for mantle-derived CO 2 , which was conveyed by deeply sourced hydrothermal fl uid input into springs. U-series dating of stratigraphic sections shows that major aggradation and large-volume (2.5 km 3 ) deposition took place across the region episodically at 700-500 ka, 350-200 ka, and 100-40 ka. These pulses of travertine formation coincide with the occurrence of regional basaltic volcanism, which implies an association of travertine deposits with underlying low-velocity mantle that could supply the excess CO 2 . The calculation of landscape denudation rates based on basalt paleosurfaces shows that travertine platforms developed on local topographic highs that required artesian head and fault conduits. Episodic travertine accumulation that led to the formation of the observed travertine platforms represents conditions when fault conduits, high hydraulic head, and high CO 2 fl ux within confi ned aquifer systems were all favorable for facilitating large-volume travertine formation, which was therefore controlled by tectonic activity and paleohydrology. By analogy to the active Springerville-St. Johns CO 2 gas fi eld, the large volumes and similar platform geometries of travertine occurrences in this study are interpreted to represent extinct CO 2 gas reservoirs that were vents for degassing of mantle volatiles into the near-surface system.

This paper documents a multi-stage incision and denudation history for the Little Colorado River (LCR) region of the southwestern Colorado Plateau over the past 70 Ma. The first two pulses of denudation are documented by thermochronologic data. Differential Laramide cooling of samples on the Mogollon Rim suggests carving of 70-30 Ma paleotopography by N-and E-flowing rivers whose pathways were partly controlled by strike valleys at the base of retreating Cretaceous cliffs. A second pulse of denudation is documented by apatite (U-Th)/He dates and thermal history models that indicate a broad LCR paleovalley was incised 25-15 Ma by an LCR paleoriver that flowed northwest and carved an East Kaibab paleovalley across the Kaibab uplift. Lacustrine strata of the lower Bidahochi Formation were deposited 16-14 Ma in the LCR paleovalley in a closed basin playa or marsh with a valley center near the modern LCR. There is a hiatus in the depositional record in the LCR valley from 12 to 8 Ma followed by aggradation of the 8-6 Ma fluvial upper Bidahochi Formation. Interlayered 8-6 Ma maar basalts that interacted with groundwater mark local base level for upper Bidahochi fluvial deposits; this was also a time of increased groundwater flow to Hualapai Limestone at the western edge of the Colorado Plateau. The paleo-base level in the central LCR valley remained stable (~1900 m modern elevation) from 16 to 6 Ma. The third pulse of regional incision and denudation, most recent and ongoing, started after integration of the Colorado River (CR) through Grand Canyon. Thermochronology from Marble Canyon indicates that early CR integration took place across the Vermillion Cliffs at Lees Ferry after 6 Ma. The elevation of the paleoconfluence between the LCR and CR at 5-6 Ma is poorly constrained, but earliest CR integration is hypothesized to have reoccupied the East Kaibab paleocanyon. In the upper LCR drainage, topographically inverted basalt mesas have elevations and K-Ar dates indicating a transition from aggradation to incision ca. 6 Ma followed by semi-steady incision of 20-40 m/Ma. In the lower LCR, incision accelerated to 120-170 m/Ma after 2 Ma as indicated by 40 Ar/ 39 Ar dating of basalt, ash-fall, and detrital sanidine. A 1.993 ± 0.002 Ma sanidine age for a tuff in the White Mesa alluvium provides a breakthrough for LCR and CR incision studies. Post-2 Ma differential incision magnitudes (and rates) in the lower LCR and at the LCR-CR confluence were 280-320 m (140-160 m/Ma), about three times greater than the 40-80 m (20-40 m/Ma) in the LCR headwaters. The proposed mechanisms driving overall post-6 Ma differential incision of the LCR involve headwater uplift associated with the Hopi Buttes and Springer ville volcanic fields plus base-level fall caused by CR integration to the Gulf of California. A proposed mechanism to explain the accelerated post-2 Ma differential incision in the central and lower LCR valley, but not in the head waters, involves mantle-driven epeirogenic uplift due to NE-migrating vol canism associated with the...

Calcite-fi lled extension veins and shear fractures are preserved in numerous travertine deposits along the western margin of the Albuquerque Basin of the Rio Grande rift. Calcite veins are banded and show geometries suggesting incremental cracking and calcite precipitation. U-series and 234 U model ages from calcite infi llings indicate that vein formation was active in the Quaternary, from ca. 2 Ma to ca. 250 ka. Vein orientations are systematic within each deposit and record a dominant extension direction that was horizontal and varied from E-W to NW-SE, consistent with both the regional fi nite extensional strain in the rift and with the global positioning system (GPS)-constrained deformation fi eld. Three sites contain three orthogonal vein sets that crosscut one another nonsystematically, suggesting alternating times of:(1) regional E-W horizontal extension (dominant), (2) alternating N-S and E-W vertical veins that suggest vertical σ 1 and σ 2 ≈ σ 3 , and (3) horizontal veins that are interpreted to refl ect times of highest pore fl uid pressures and subequal principal stresses. One site contains conjugate normal faults that also record the dominant E-W extensional tectonic stress. Quaternary extensional strain rates calculated from vein opening for three locations range from 3.2 ± 1.4 × 10 -16 s -1 to 3.2 × 10 −15 ± 2.7 × 10 -16 s -1 , which are up to ~40 times higher than the long-term (Oligocene-Holocene) fi nite strain rates calculated for different basins of the Rio Grande rift (8.5 × 10 -17 to 4.5 × 10 -16 s -1 ), and up to ~100 times higher than modern strain rates measured by GPS data (3.9 × 10 −17 ± 6.3 × 10 -18 to 4.4 × 10 −17 ± 6.3 × 10 -18 s -1 ). These high Quaternary rates are comparable to modern strain rates measured in the Basin and Range Province and East African Rift. Thus, this paper documents persistent E-W regional extension through the Quaternary in the Rio Grande rift that bridges geologic, paleoseismic, and GPS rates. Anomalously high strain rates in the Quaternary were facilitated by ascent of travertine-depositing CO 2 -rich waters along rift-bounding normal faults, leading to locally very high stain accumulations. These sites also provide examples of natural leakage of deeply sourced CO 2 interacting with regional tectonism, and they emphasize that rift maturation is a highly dynamic process, both spatially and temporally.