Jason W. Ricketts

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

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.

U-series geochronology of large-volume Quaternary travertine deposits of the southeastern Colorado Plateau: Evaluating episodicity and tectonic and paleohydrologic controls

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 reflect vent locations for mantle-derived CO 2 , which was conveyed by deeply sourced hydrothermal fluid 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 flux within confined 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 field, 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.

Synchronous opening of the Rio Grande rift along its entire length at 25–10 Ma supported by apatite (U-Th)/He and fission-track thermochronology, and evaluation of possible driving mechanisms

One-hundred and forty-seven new apatite (U-Th)/He (AHe) ages are presented from 32 sample locations along the flanks of the Rio Grande rift in New Mexico and Colorado. These data are combined with apatite fission-track (AFT) analyses of the same rocks and modeled together to create well-constrained cooling histories for Rio Grande rift flank shoulders. The data indicate rapid cooling due to extension from ca. 28 to 5 Ma in the Sawatch Range, ca. 28 Ma to Quaternary in the Sangre de Cristo Mountains, ca. 25 to 5 Ma in the Albuquerque Basin, and ca. 25 to 10 Ma in the southern Rio Grande rift in southern New Mexico. Rapid cooling of rift flanks followed the Oligocene ignimbrite flare-up, and the northern section of the Rio Grande rift in Colorado exhibits semicontinuous cooling since the Oligocene. Overall, however, rift flank cooling along the length of the rift was out of phase with high-volume magmatism and hence is inferred to have been driven mainly by exhumation due to faulting. Although each location preserves a unique cooling history, when combined with existing AHe data from the Gore Range in northern Colorado and the Sandia Mountains in New Mexico, together these data indicate that extension and exhumation of rift shoulders were synchronous along >850 km of the length of the Rio Grande rift from 25 to 10 Ma. These time-space constraints provide an important new data set with which to develop geodynamic models for initiation and evolution of continental rifting. Models involving northward propagation of rifting and Colorado Plateau rotation are not favored as primary mechanisms driving extension. Instead, a geodynamic model is proposed that involves upper-mantle dynamics during multistage foundering and rollback of a segment of the Farallon plate near the Laramide hinge region that extended between the Wyoming and SE New Mexico high-velocity mantle domains. The first stage of flat slab removal accompanied ca. 40–20 Ma volcanism in the San Juan and Mogollon-Datil ignimbrite centers, which initiated asthenospheric upwelling and circulation. A second stage involved a ca. 30–25 Ma detachment of remaining fragments of the Farallon slab, intensifying asthenospheric upwelling and focusing it along a N-S trend beneath Colorado and New Mexico. By 25 Ma, the North American lithosphere had become weakened critically along this narrow zone, so that extension was accelerated, resulting in the observed 25–10 Ma cooling indicated by the thermochronologic data. This developed a central graben with increased fault-related high strain rates and resulted in maximum sediment accumulation in the Rio Grande rift. Our geodynamic model thus involves Oligocene removal of parts of the Farallon slab beneath the ignimbrite centers followed by a major Oligocene–Miocene slab break that instigated the discrete N-S Rio Grande rift through focused upper-mantle convection beneath the southern Rocky Mountain–Rio Grande rift region.

Embryonic core complexes in narrow continental rifts: The importance of low-angle normal faults in the Rio Grande rift of central New Mexico

The Rio Grande rift in central New Mexico provides an excellent location to study the interaction between high-angle and low-angle (15°–35°) normal faults during crustal extension. Here we evaluate the relative importance of low-angle normal faults (LANFs) in the Albuquerque basin of central New Mexico with goals of testing two conflicting models of rift geometry and producing evolutionary models for the northern and southern parts of the basin. Using physiographic relationships, field observations, structural data analysis, and thermal history modeling, we document two brittle LANF systems on salients in adjacent opposite-polarity half-grabens. These fault systems were both active ca. 20–10 Ma and are locations of maximum fault slip as indicated by thickness of sedimentary fill in adjacent sub-basins and highest elevation rift flanks. Average fault dip increases basinward, and outbound faults were abandoned while intrabasinal faults cut Quaternary units, supporting an evolutionary model where master normal faults initiated at a higher dip, were shallowed by isostatic footwall uplift in regions of highest slip, and became inactive while younger normal faults emerged basinward. These geometrical and kinematic observations are predicted by the rolling-hinge model for the formation of LANFs. This mechanism has been widely applied to core complexes in highly extended terranes (e.g., Basin and Range), regions of orogenic collapse, and mid-ocean ridges, and it is shown here to also be applicable to narrow continental rifts of modest (∼35%) extension. Similarities to core complexes include a physiographic expression of domal uplifts, evolution of a master detachment horizon that initiated as a breakaway, and isostatically rotated low-angle normal faults. Although the degree of extension was too low to juxtapose ductile footwall rocks against brittle hanging-wall rocks, if extension had progressed in the Albuquerque basin, eventually a mature metamorphic core complex would have formed, similar to those preserved in the adjacent Basin and Range Province. The Rio Grande rift, therefore, provides a snapshot of the embryonic stages of core complex formation, bridging the gap between mature core complexes and incipient extensional environments.

Surface uplift above the Jemez mantle anomaly in the past 4 Ma based on 40Ar/39Ar dated paleoprofiles of the Rio San Jose, New Mexico, USA

We combine 15 new 40 Ar/ 39 Ar ages with existing age constraints of basalts to investigate the incision and denudation history of the ∼150-km-long Rio San Jose (RSJ) of west-central New Mexico (USA) over the past 4 Ma. Temporal and spatial scales of differential incision may help evaluate the relative importance of neotectonic, geomorphic and climatic forcings. The RSJ is a southeast-flowing river that orthogonally crosses the northeast-trending Jemez volcanic lineament, which is underlain by a zone of low-velocity mantle. Preserved basalt flows along the length of the river at different elevations that directly overlie river gravels are used to construct paleoprofiles of the RSJ and give insight into the differential incision history, which can test the hypothesis that epeirogenic uplift associated with the Jemez lineament influenced differential incision of the RSJ. Observations include (1) a northeast-trending graben along the central reach of the RSJ (El Malpais valley graben) which is parallel to the Jemez lineament, (2) the present-day east tilt of the originally west-flowing 3.7 Ma Mesa Lucero flow along the eastern edge of the Jemez lineament, and (3) modern profile convexities that are colocated with ca. 3 Ma paleoprofile convexities and are centered above the Jemez lineament. The arched ca. 3 Ma paleoprofile defined by the pre–Mount Taylor strath has greater convexity than younger profiles, suggesting neotectonic bowing of ∼135 m (∼50 m/Ma) in this reach over the past ∼3 Ma relative to areas off axis of the Jemez lineament, in spite of graben subsidence and aggradational fill in this reach exceeding 100 m. Differential incision of the 184 ka Suwanee flow at the edge of the Colorado Plateau may be attributable to base-level fall in downstream reaches of the RSJ and/or headwater uplift, and more erosive climate in the past several hundred thousand years. However, these observations, when considered together, cannot be explained entirely by geomorphic or climatic forcings. Rather, they are best interpreted as resulting from surface uplift centered over the northeast-trending Jemez lineament, and our model suggests that both the faulting and broad bending may relate to mantle driven epeirogeny that caused differential river incision. Several interacting neotectonic and magmatic mechanisms may have contributed to postulated uplift. Magmatically driven geodynamic uplift forcings may include construction of the Mount Taylor stratovolcano just north of the RSJ that changed surface elevation by several kilometers at the volcanic peak itself. However, semisteady denudation and similar incision rates in other rivers in the region indicate that a regional erosional landscape was the primary driver of differential river incision over the past 5–8 Ma. Our focus on the pre–Mount Tayler RSJ paleoprofile reinforces this conclusion. Other mantle-related uplift mechanisms that may have generated mantle buoyancy include thermal buoyancy or magmatic inflation due to dike and sill networks related to the building of the Mount Taylor stratovolcano and eruption of Zuni-Bandera volcanic fields. Both could have contributed to uplift, but their relative importance is unknown. Broad epeirogenic uplift is also possible due to small-scale upper mantle convection beneath a thin elastic plate and resulting dynamic topography.

Pliocene–Holocene deformation in the southern Rio Grande rift as inferred from topography and uplifted terraces of the Franklin Mountains, southern New Mexico and western Texas

In most extensional terrains such as the Rio Grande rift, alluvial fans and bajadas cover faults and terraces as extension progresses, thus limiting the faults and terraces as useful records of uplift. However, in the Franklin Mountains of western Texas and southern New Mexico (USA), rapid aggradation of basin floors by extensive playa lakes and floodplain deposits of the Rio Grande during the Pliocene buried the irregular mountain-front fans, thus creating a low-gradient surface. This originally planar surface was subsequently uplifted and deformed during faulting, providing a record of the Pliocene–Holocene extensional deformation in the southern Rio Grande rift. Deformation and uplift of the Franklin Mountains in the southern Rio Grande rift was estimated by measuring the elevation of late Pliocene terraces that are adjacent to range-bounding faults. The uplifted terraces are exposed along both sides of the Franklin Mountains, and lie as much as 130 m above their original elevation. Together the uplifted terraces form an anticlinal arch that mimics the profile of the range crest of the mountains. Three important conclusions can be drawn from the similarity of profiles among the terraces and mountain crest. First, the observation that the terraces mimic the range crest implies that the present-day topography of the mountains is likely tectonic in origin. Second, the east-side terraces are higher than the west-side terraces, suggesting rotation of the mountains during deformation. Estimated rotation since the Pliocene is ~5% of the total rotation. Third, fault throw rate calculations indicate differential slip along the length of the eastern boundary fault zone. The fault profile and throw rate calculations along the eastern margin of the range are skewed to the south, suggesting that the southern segment of the Franklin Mountains has accumulated a majority of the slip during this time frame. These observations, coupled with geophysical data highlighting buried faults beneath the El Paso (Texas)–Juárez (Mexico) metropolitan region, suggest that normal faults related to uplift of the Franklin Mountains have been growing in length toward the south over the last several million years. INTRODUCTION Normal fault systems develop through incremental slip events (earthquakes) that, over time, result in the final displacement profile of the fault. Extended terranes are also characterized by multiple normal fault systems that overlap to form relay ramps or intersect as extension progresses and individual faults grow in length (e.g., Peacock and Sanderson, 1991; Childs et al., 1995; Peacock, 2002; Nicol et al., 2010). These characteristics are true in cases of simple lithology and sandbox experiments (e.g., McClay and Ellis, 1987; McClay, 1990; Childs et al., 1995) but are also applicable to regions of diverse rock types and heterogeneous crustal features (e.g., Nicol et al., 2005). Although these relationships appear to be generally true for a wide range of extensional faults regard less of fault size, amount of offset, or lithology (e.g., Schlische et al., 1996), geologic evidence documenting sequential fault growth is commonly lacking. In models of idealized, isolated normal faults, displacement profiles should preserve maximum displacement at the center of the fault, and displacement should decrease to zero at the fault tip lines (Barnett et al., 1987; Walsh and Watterson, 1987). However, displacement patterns are almost always segmented and irregular, and are also commonly asymmetrical, where maximum fault throw is centered closer to one fault tip than the other (e.g., Walsh and Watterson, 1987; Childs et al., 1995; Mansfield and Cartwright, 1996; Schlagenhauf et al., 2008). As slip accumulates, fault tips typically propagate in two directions and fault length increases. This process results in a power-law relationship, where fault length scales linearly with fault displacement (e.g., Pickering et al., 1995; Schlische et al., 1996). As fault length increases, adjacent faults eventually overlap and link, which has been shown to be an important fault growth mechanism in extensional settings (e.g., Cartwright et al., 1995; Finch and Gawthorpe, 2017; Whipp et al., 2017). Alternatively, some fault systems develop through a constant-fault-length model, where they approach their maximum length early and fault tips become fixed in space as fault displacement continues to accumulate (e.g., Nicol et al., 2005; Amos et al., 2010; Mouslopoulou et al., 2012; Curry et al., 2016). GEOSPHERE GEOSPHERE; v. 14, no. 4 https://doi.org/10.1130/GES01572.1