Quaternary extension in the Rio Grande rift at elevated strain rates recorded in travertine deposits, central New Mexico

Abstract

Calcite-filled 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 234U model ages from calcite infillings 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 finite extensional strain in the rift and with the global positioning system (GPS)–constrained deformation field. 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 s1 and s2 » s3, and (3) horizontal veins that are interpreted to reflect times of highest pore fluid 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) finite 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 CO2-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 CO2 interacting with regional tectonism, and they emphasize that rift maturation is a highly dynamic process, both spatially and temporally.