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.

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.

Injecting anthropogenic CO2 into the subsurface is suggested for climate change mitigation. However, leakage of CO2 from its target storage formation is a concern. In the event of leakage, permeability in leakage pathways such as faults may get altered due to mineral reactions induced by CO2‐enriched water, thus influencing the migration and fate of the CO2. An example of such fault permeability alteration is found in the Little Grand Wash Fault zone (LGWF), where the fault outcrops show fractures filled with calcium carbonate. To test the nature, extent, and time‐frame of such fault ‘self‐sealing’, we developed reactive flow simulations based on hydrogeological conditions of the LGWF. We measured LGWF water chemistry and conducted an x‐ray powder diffraction (XRD) analysis of the fault‐rock to constrain the model geochemistry. We hypothesized that the choice of parameters for relative permeability, capillary pressure, and reaction kinetics will have a huge impact on the fault sealing predictions. Simulation results showed that precipitation of calcite in the top portion of the fault led to a decrease in porosity of the damage zone from a value of 40% to ∼2%. Over a simulation time of 1000 years, porosity in the damage zone showed self‐enhancing behavior in the bottom portion and self‐sealing behavior in the top portion of the fault. We found that the results were most sensitive to the relative permeability parameters and the fault architecture. A major conclusion from this analysis is that, under similar conditions, some faults are likely to seal over time. © 2017 Society of Chemical Industry and John Wiley & Sons, Ltd.

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