ALBERT

Description: 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.

Description: 〈span〉Although stand‐alone geophones have been used for decades within the active‐source seismic community (e.g., 〈a href="https://pubs.geoscienceworld.org/srl#rf25"〉Mooney and Brocher, 1987〈/a〉), recent technological advances in geophone instrumentation have made it possible to use them for a wide range of passive seismic studies (e.g., 〈a href="https://pubs.geoscienceworld.org/srl#rf21"〉Lin 〈span〉et al.〈/span〉, 2013〈/a〉; 〈a href="https://pubs.geoscienceworld.org/srl#rf34"〉Schmandt and Clayton, 2013〈/a〉). Compact, all‐in‐one seismic systems including a geophone, digitizer, and battery—often called nodes—are lightweight and easy to deploy, allowing large numbers of instruments (“large‐N,” typically 100s or 1000s of nodes) to be used on a single project. These stand‐alone seismic systems, especially the RefTek 125A Texan seismometer with a 4.5‐Hz geophone, have been used since the 1990s within the academic seismology community, primarily for active‐source studies (〈a href="https://pubs.geoscienceworld.org/srl#rf15"〉Harder and Keller, 2000〈/a〉) but also for passive‐source studies (〈a href="https://pubs.geoscienceworld.org/srl#rf7"〉Byerly 〈span〉et al.〈/span〉, 2010〈/a〉; 〈a href="https://pubs.geoscienceworld.org/srl#rf30"〉Quiros 〈span〉et al.〈/span〉, 2015〈/a〉, 〈a href="https://pubs.geoscienceworld.org/srl#rf29"〉2017〈/a〉; 〈a href="https://pubs.geoscienceworld.org/srl#rf37"〉Sun 〈span〉et al.〈/span〉, 2015〈/a〉; 〈a href="https://pubs.geoscienceworld.org/srl#rf44"〉Wu 〈span〉et al.〈/span〉, 2016〈/a〉; 〈a href="https://pubs.geoscienceworld.org/srl#rf4"〉Beskardes 〈span〉et al.〈/span〉, 2018〈/a〉). The oil industry has also used a wide variety of these seismic geophone systems for decades (e.g., 〈a href="https://pubs.geoscienceworld.org/srl#rf8"〉Dean 〈span〉et al.〈/span〉, 2018〈/a〉). In this focus section, we use the term “geophone array” to refer to an array of compact, stand‐alone seismic stations typically using one‐component or three‐component geophone sensors. We note that this is distinct from the historic definition of a “geophone array,” which was a geometrical arrangement of geophones with signals recorded by a single channel.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉We describe the scientific motivation and deployment strategy for the 2015 Sevilleta Socorro magma body (SMB) mixed‐mode seismic experiment in central New Mexico, U.S.A. The array consisted of seven temporary broadband, 801 short‐period geophones, and seven regional network seismic stations placed within the central Rio Grande rift and above the SMB, one of the largest known mid‐crustal continental magma bodies globally. The array recorded teleseismic, regional, and local earthquakes as well as regional explosions. Current analysis efforts include earthquake detection and location, structural imaging, and velocity model refinement along the segment of the magma body region experiencing the highest uplift rates.〈/span〉

Description: The size, frequency, and intensity of volcanic eruptions are strongly controlled by the volume and connectivity of magma within the crust. Several geophysical and geochemical studies have produced a comprehensive model of the magmatic system to depths near 7 km beneath Mount St. Helens (Washington State, USA), currently the most active volcano in the Cascade Range. Data limitations have precluded imaging below this depth to observe the entire primary shallow magma reservoir, as well as its connection to deeper zones of magma accumulation in the crust. The inversion of P and S wave traveltime data collected during the active-source component of the iMUSH (Imaging Magma Under St. Helens) project reveals a high P-wave (Vp)/S-wave (Vs) velocity anomaly beneath Mount St. Helens between depths of 4 and 13 km, which we interpret as the primary upper–middle crustal magma reservoir. Beneath and southeast of this shallow reservoir, a low Vp velocity column extends from 15 km depth to the Moho. Deep long-period events near the boundary of this column indicate that this anomaly is associated with the injection of magmatic fluids. Southeast of Mount St. Helens, an upper–middle crustal high Vp/Vs body beneath the Indian Heaven Volcanic Field may also have a magmatic origin. Both of these high Vp/Vs bodies are at the boundaries of the low Vp middle–lower crustal column and both are directly above high Vp middle–lower crustal regions that may represent cumulates associated with recent Quaternary or Paleogene–Neogene Cascade magmatism. Seismicity immediately following the 18 May 1980 eruption terminates near the top of the inferred middle–lower crustal cumulates and directly adjacent to the inferred middle–lower crustal magma reservoir. These spatial relationships suggest that the boundaries of these high-density cumulates play an important role in both vertical and lateral transport of magma through the crust.

Description: The correspondence between seismic velocity anomalies in the crust and mantle and the differential incision of the continental-scale Colorado River system suggests that significant mantle-to-surface interactions can take place deep within continental interiors. The Colorado Rocky Mountain region exhibits low-seismic-velocity crust and mantle associated with atypically high (and rough) topography, steep normalized river segments, and areas of greatest differential river incision. Thermochronologic and geologic data show that regional exhumation accelerated starting ca. 6–10 Ma, especially in regions underlain by low-velocity mantle. Integration and synthesis of diverse geologic and geophysical data sets support the provocative hypothesis that Neogene mantle convection has driven long-wavelength surface deformation and tilting over the past 10 Ma. Attendant surface uplift on the order of 500–1000 m may account for ∼25%–50% of the current elevation of the region, with the rest achieved during Laramide and mid-Tertiary uplift episodes. This hypothesis highlights the importance of continued multidisciplinary tests of the nature and magnitude of surface responses to mantle dynamics in intraplate settings.

Description: As the Pacific–Farallon spreading center approached North America, the Farallon plate fragmented into a number of small plates. Some of the microplate fragments ceased subducting before the spreading center reached the trench. Most tectonic models have assumed that the subducting oceanic slab detached from these microplates close to the trench,...

Description: Measurements and mechanical models of heterogeneous bedload transport in rivers remain basic challenges for studies of landscape evolution and watershed management. A 700 m reach of the Trinity River (northern California, USA), a large gravel-bed river, was instrumented with an array of 76 seismographs during a dam-controlled flood and gravel augmentation to investigate the potential for out-of-stream monitoring. The temporal response to gravel augmentation during constant discharge provides strong evidence of seismic sensitivity to bedload transport and aids in identification of the seismic frequencies most sensitive to bedload in the study area. Following gravel augmentations, the seismic array reveals a period of enhanced transport that spans most or all of the reach for ~7–10 h. Neither the duration nor the downstream extent of enhanced transport would have been constrained without the seismic array. Sensitivity to along-stream transport variations is further demonstrated by seismic amplitudes that decrease between the upper and lower halves of the reach consistent with decreased bedload flux constrained by time-lapse bathymetry. Insight into the magnitude of impact energy that reaches the bed is also gained from the seismic array. Observed peak seismic power is ~1%–5% of that predicted by a model of saltation over exposed bedrock. Our results suggest that dissipation of impact energy due to cover effects needs to be considered to seismically constrain bedload transport rates, and that noninvasive constraints from seismology can be used to test and refine mechanical models of bedload transport.

Description: We present a new 3-D seismic model of the western United States crust derived from a joint inversion of Rayleigh-wave phase velocity and ellipticity measurements using periods from 8 to 100 s. Improved constraints on upper-crustal structure result from use of short-period Rayleigh-wave ellipticity, or Rayleigh-wave H/V (horizontal to vertical) amplitude ratios, measurements determined using multicomponent ambient noise cross-correlations. To retain the amplitude ratio information between vertical and horizontal components, for each station, we perform daily noise pre-processing (temporal normalization and spectrum whitening) simultaneously for all three components. For each station pair, amplitude measurements between cross-correlations of different components (radial–radial, radial–vertical, vertical–radial and vertical–vertical) are then used to determine the Rayleigh-wave H/V ratios at the two station locations. We use all EarthScope/USArray Tranportable Array data available between 2007 January and 2011 June to determine the Rayleigh-wave H/V ratios and their uncertainties at all station locations and construct new Rayleigh-wave H/V ratio maps in the western United States between periods of 8 and 24 s. Combined with previous longer period earthquake Rayleigh-wave H/V ratio measurements and Rayleigh-wave phase velocity measurements from both ambient noise and earthquakes, we invert for a new 3-D crustal and upper-mantle model in the western United States. Correlation between the inverted model and known geological features at all depths suggests good resolution in five crustal layers. Use of short-period Rayleigh-wave H/V ratio measurements based on noise cross-correlation enables resolution of distinct near surface features such as the Columbia River Basalt flows, which overlie a thick sedimentary basin.

Description: 〈span〉〈div〉Abstract〈/div〉The advent of low‐cost continuously recording cable‐free autonomous seismographs, commonly referred to as nodes, enables dense spatiotemporal sampling of seismic wavefields. We create virtual source reflection profiles using 〈span〉P〈/span〉 waves from five teleseismic events recorded by the Sevilleta node array experiment in the southern Albuquerque basin. The basin geology records a structurally complex history, including multiple Phanerozoic orogenies, Rio Grande rift extension, and ongoing uplift from a midcrustal magma body. The Sevilleta experiment densified the long term, regionally sparse seismograph network with 801 single channel vertical‐component 10 Hz geophone nodes deployed at ∼300  m spacing for 14 days in February 2015. Results show sediment‐basement reflections at 〈5  km depth and numerous sub‐basin structures. Comparisons to legacy crustal‐scale reflection images from the Consortium for Continental Reflection Profiling show agreement with structural geometries in the rift basin and upper crust. Comparisons of the teleseismic virtual reflection profiles to synthetic tests using 2D finite‐difference elastic wave propagation show strong 〈span〉P〈/span〉‐to‐Rayleigh scattering from steep basin edges. These high‐amplitude conversions dominate the record sections near the western rift margin and originate at the Loma Pelada fault, which acts as the primary contact between rift‐bounding basement‐cored fault blocks and rift basin sediments. At near offsets, these signals may interfere with interpretation of upper crustal structure, but their relatively slow moveout compared to teleseismic 〈span〉P〈/span〉‐wave multiples provides clear temporal separation from sediment‐basement reflections across most of the array. The high‐signal‐to‐noise ratio of these converted Rayleigh‐wave signals suggests that they may be useful for constraining short‐period (∼1  Hz) dispersion with strong sensitivity in the uppermost ∼1  km of the rift basin sediments.〈/span〉