The Sierra Nevada of California, USA, is an ancient mountain range, formed from the batholithic core of a continental volcanic arc during the Triassic through Late Cretaceous. Nevertheless, beginning with the earliest geological research in the mid- nineteenth century, many researchers have maintained that the range has been rejuvenated by crest uplift in the latest Cenozoic (Pliocene- Quaternary). Other researchers have maintained that the range has been stable since at least the Eocene. One archive of information on the tilting history of the range that has been only lightly investigated is the inclination of formation boundaries in the subsurface of the Central Valley, which lies to the west of the Sierra Nevada. The Central Valley sediments lie on top of the quite rigid Sierra Nevada crustal block. Tilting of the block should both deepen the Central Valley and raise the crest of the range. Although this approach has been previously used at a limited number of locations, it has not been employed systematically along the entire extent of the Sierra Nevada block. In order to improve the spatial and temporal resolution of data on Sierra Nevada block tilting, I have constructed cross sections from the west side of the Central Valley to east of the Sierra Nevada crest at seven locations, using a combination of published petroleum and hydrogeology data sources. Dips were measured from the cross sections on formation tops ranging in age from 61 Ma to 15 ka. These dips were converted to tectonic tilt estimates by subtracting estimates of depositional dip and sediment compaction. Using the width of the range at each cross section, these tilt values were used to calculate rock uplift with time. Using estimates of landscape erosion rate, range- crest elevations at specific times were reconstructed. From these reconstructions, a coherent tilting history emerged. The northern Sierra was virtually quiescent until after 5 Ma. Between 5 Ma and 1.7 Ma, a brief episode of tilting resulted in 1.0- 1.5 km uplift of the range crest. Since 1.7 Ma, tilting has slowed significantly. In contrast, the southern Sierra has been tilting throughout the Cenozoic, although quite slowly for most of it. At ca. 15 Ma, it began to tilt more rapidly, and after 5 Ma, quite rapidly, producing 2-4 km of crest uplift since 12 Ma. Data from Quaternary units indicate that this tilting is ongoing. The cause of early and middle Cenozoic slow tilting of the southern Sierra unclear, but it may be related to isostatic response to mountain erosion and valley deposition and/or plate-boundary interactions along the western side of the block. The more rapid tilting since the middle Miocene is most plausibly attributed to changes in the distribution of mantle density, and therefore crustal buoyancy, as a result of foundering (sometimes described as "delamination") and downward convection of the arclogite root of the Mesozoic volcanic arc. The location and timing of the foundering process inferred from geophysical and geochemical data correspond well with the tilt history inferred from formation dips in this study. The nature of lithospheric foundering of the northern Sierra, if any, is poorly understood, but the synchrony of timing between the major tilting events in the northern and southern Sierra (ca. 5 Ma) points to foundering as the likely cause of tilt in the north as well.

The area now occupied by the Great Ba-sin, western USA, contained paleo-fluvial systems that predated the modern-day en-dorheic (closed) basins. The areal extent of these paleo-fluvial systems within the south-western Great Basin is known mainly from isolated remnants preserved in the modern mountain ranges. We document the age, ex-tent, and tectonic disruption of Mio-Pliocene fluvial systems of the southwestern Great Basin. Synthesis of new field observations, geochemistry, and geochronology with ex-isting studies defines two latest Miocene to Pliocene east-southeast flowing drainages that predated the modern endorheic ba-sins. The drainage network was ultimately fragmented in Pliocene time (ca. 3.5-4 Ma). Fragmentation of the drainage network led to lake formation, drying of lakes, and the formation of isolated springs. The rapid envi-ronmental changes initiated by faulting and volcanism isolated previously interbreeding populations of spring-dwelling taxa and have caused divergent evolution since Pliocene time. Modern endemism within the region's springs is thus a direct consequence of intra-plate tectonism.

Despite the ecological significance of desert springs, little is known about relationships between spring hydrogeochemistry and ecology, particularly over multiple trophic levels. Here, we surveyed microbial communities (bacteria and archaea) and benthic macroinvertebrate (BMI) communities in springs that recharge through granitoid rocks in Owens Valley, CA, to determine whether subtle geochemical differences imparted by Paleozoic roof pendant weathering in the source area affects spring ecosystems. Relative to other springs, roof pendant-recharged springs were characterized by elevated: (i) Ca2+/Na+, Ca2+/Mg2+, and divalent/monovalent cation ratios, (ii) relative abundance of benthic aerobes/facultative anaerobes and holdfast/stalked bacteria, and (iii) abundance and diversity of shredder and collector-gatherer BMIs. These BMI feeding groups graze on biofilm communities and stimulate bacterial degradation of particulate organic matter, consistent with extensive bacteria-BMI connections in co-occurrence networks of these springs. Springs not impacted by roof pendants were instead enriched with anaerobes and chemolithotrophs and low-diversity BMI communities with poor bacteria-BMI network connectivity. We speculate that excess Ca2+ derived from roof pendant weathering plays a key role in biofilm formation on coarse granite substrates, with subsequent synergy between benthic microbial biofilms and diverse BMI shredders/collector-gatherers to degrade allochthonous organic material.

Carlsbad Cavern and Lechuguilla Cave are large limestone caves located within Carlsbad Caverns National Park, southeastern New Mexico, USA. Both caves feature numerous pools that enable sampling of locally perched water bodies at different depths within the thick vadose zone. Isotopic analysis of multiple water samples from both caves revealed clear evidence of bomb-pulse tritium (up to 6.1% bomb-pulse maximum, BPM) and Cl-36 (up to 5.2% BPM) in many pools. In all cases, isotope concentrations were well below peak bomb-pulse levels, indicating complex mixing processes between the ground surface and the cave pools. The deepest bomb-pulse detections in the caves indicate vertical transport velocities of up to 6-9 m/yr for the fastest flowpaths. Peak tritium levels in Lechuguilla are lower than those in Carlsbad, potentially due to mixing and dilution via vapor phase exchange through the high relative humidity cave atmosphere, processes that may be less effective in Carlsbad's drier atmosphere. Peak Cl-36/Cl ratios in Carlsbad are lower than those in Lechuguilla, possibly due to dilution by introduced stable Cl- associated with tourist developments in the cave. Stable isotope results demonstrate the different evaporative regimes of the two caves, caused by Carlsbad Cavern's large natural entrance, in contrast to Lechuguilla's complete lack of a present-day natural entrance. Our results bolster the use of speleothem records for paleoclimate reconstruction, demonstrating the effective transmission of the bomb pulse signal to depths of hundreds of meters within a few decades. At the same time, the pronounced and variable impact of evaporation on some samples shows the potential importance of local conditions on specific sample results.

Hildreth et al. (2021) analyzed a set of table mountains near the San Joaquin River that are capped by a 9.3 Ma trachyandesite lava flow and concluded that, since the deposition of the volcanic rocks, the table mountains have been tilted 1.07° due to uplift of the central Sierra Nevada. While Gabet (2014) suggested that, under a limited set of conditions, the size of fluvial gravels under the table mountains would support the hypothesis of postdepositional uplift, the authors claimed that their evidence is more definitive. In addition, the authors proposed that the central Sierra Nevada tilted as a rigid block. However, their analyses rely on inferences and assumptions that are not supported by field evidence.

A bstrAct — The sides of the Rio Grande Valley north and south of Socorro, New Mexico, exhibit readily apparent, sloped, quasi-planar surfaces (“terraces”). These have only recently been systematically mapped, characterized, and dated. The dating confirms that the terraces decrease in age with decreasing height above the Rio Grande. The highest terrace, the Las Cañas surface, represents the land surface at the time of maximal aggradation of the Rio Grande, at about 800 ka. At this time, the top of the basin fill was about 100 m higher than the modern Rio Grande. The surface about 20 m below it, the Tio Bartolo, stabilized at ca. 610 ka, near the 621 ka termination (Lisiecki and Raymo, 2005) of a very intense global glaciation (MIS 16). Forty meters below it, the Valle de la Parida surface formed ca. 135 ka, also at the end of an exceptionally intense glacial interval. Below the Valle de la Parida are three more terraces, two of which formed due to fluvial incision at the start of a glacial paleoclimate and the youngest within an interglacial. This pattern supports a model originally advanced by John Hawley, which proposed glacial/interglacial transitions as a control on terrace formation. The Rio Grande rift in the vicinity of Socorro is currently undergoing uplift as a result of injection of magma into a mid-crustal sill. A longitudinal profile of the terraces along the river shows two features that may be related to earlier magma injections. The first is an apparent ‘bulge’ of about 50 m in the two highest terraces, near San Acacia, the current area of most rapid uplift. The second is a more subtle rise in terrace elevations, amounting to 10 m, across the southern boundary of the current magma body. These both suggest that magma injections in the geological past may have produced upward topographic deflections, but additional data and examination of alternative explanations are needed before they can be considered solid evidence of such events.

Processes controlling the formation of continental whole-lithosphere shear zones are debated, but their existence requires that the lithosphere is mechanically coupled from base to top. We document the formation of a dextral, whole-lithosphere shear zone in the Death Valley region (DVR), southwest United States. Dextral deflections of depth gradients in the lithosphere-asthenosphere boundary and Moho are stacked vertically, defining a 20–50-km-wide, lower lithospheric shear zone with ~60 km of shear. These deflections underlie an upper-crustal fault zone that accrued ~60 km of dextral slip since ca. 8–7 Ma, when we infer that whole-lithosphere shear began. This dextral offset is less than net dextral offset on the upper-crustal fault zone (~90 km, ca. 13–0 Ma) and total upper-crustal extension (~250 km, ca. 16–0 Ma). We show that, before ca. 8–7 Ma, weak middle crust decoupled upper-crustal deformation from deformation in the lower crust and mantle lithosphere. Between 16 and 7 Ma, detachment slip thinned, uplifted, cooled, and thus strengthened the middle crust, which is exposed in metamorphic core complexes collocated with the whole-lithosphere shear zone. Midcrustal strengthening coupled the layered lithosphere vertically and therefore enabled whole-lithosphere dextral shear. Where thick crust exists (as in pre–16 Ma DVR), midcrustal strengthening is probably a necessary condition for whole-lithosphere shear.

Rates of soil development provide critical information about the types and rates of geomorphic and landscape evolutionary processes. Soil development in arid and semiarid regions of the southwestern United States is predominantly controlled by influx of eolian dust, yet our ability to quantify the rates of morphologic development and of dust and carbonate incorporation is limited by available age control. We describe 10 pedons in the Socorro area of central New Mexico and analyze their silt, clay, and carbonate contents. These soils have well-established direct or indirect age control that we used to estimate average rates of dust and carbonate accumulation over the past ~0.5–800 ka. We also computed the profile development index (PDI) for these soils using 10 common morphologic properties and compare our resulting PDI chronofunction to those from northern and southern New Mexico. We find that the net silt-and-clay content increases in progressively older soils at rates similar to the profile-mass carbonate contents, presumably sourced primarily from eolian dust. Our chronofunction comparison indicates that soil development occurs more rapidly in higher latitude regions of New Mexico than in the drier warmer climates of the Socorro area and southern New Mexico. We interpret the N–S regional trend of soil development considering a regional climate gradient. We conclude that greater mean annual precipitation and cooler mean annual temperatures and/or slower rates of eolian dust accumulation into the soil profile at higher latitude northern sites cause the observed differences in regional soil development. This would promote greater mobility of available silt and clay, and also increase rates of soil formation, as indicated by the presence of argillans in late-Pleistocene soils of northern New Mexico.

The Rio Grande rift hosts a remarkable record of Quaternary river incision preserved in an alluvial terrace sequence that has been studied for more than a century. However, our understanding of Rio Grande incision history in central New Mexico since the end of basin filling ca. 0.78 Ma remains hampered by poor age control. Robust correlations among Rio Grande terrace sequences in central and southern New Mexico are lacking, making it difficult to address important process-related questions about terrace formation in continental-scale river systems. We present new age controls using a combination of 40Ar/39Ar, 36Cl surface-exposure, and 14C dating techniques from alluvial deposits in the central New Mexico Socorro area to document the late Quaternary incision history of the Rio Grande. These new age controls (1) provide constraints to establish a firm foundation for Socorro basin terrace stratigraphy, (2) allow terrace correlations within the rift basin, and (3) enable testing of alternative models of terrace formation. We identified and mapped a high geomorphic surface interpreted to represent the end of basin filling in the Socorro area and five distinct, post–Santa Fe Group (ca. 0.78 Ma) alloformations and associated geomorphic surfaces using photogrammetric methods, soil characterization, and stratigraphic descriptions. Terrace deposits exhibit tread heights up to 70 m above the valley floor and are 5 to >30 m thick. Their fills generally have pebble-to-cobble bases overlain by fine-to-pebbly sand and local thin silt and clay tops. Alluvial-fan terraces and associated geomorphic surfaces grade to former valley levels defined by axial terrace treads. Carbon-14 ages from detrital charcoal above and below a buried tributary terrace tread show that the most recent aggradation event persisted until ca. 3 ka during the transition from glacial to modern climate conditions. Drill-log data show widespread valley fill ∼30 m thick that began aggrading after glacial retreat in northern New Mexico and southern Colorado (ca. 14 ka). Aggradation during this transition was likely due to hillslope destabilization, increased sediment yield, decreased runoff, and reduced stream competence. Chlorine-36 ages imply similar controls on earlier terraces that have surface ages of ca. 27–29, 64–70, and 135 ka, and suggest net incision during glacial expansions when increased runoff favored down-cutting and bedload mobilization. Our terrace chronology supports existing climate-response models of arid environments and links tributary responses to the axial Rio Grande system throughout the central Rio Grande rift. The terrace chronology also reflects a transition from modest (60 m/m.y.) to rapid (300 m/m.y.) incision between 610 and 135 ka, similar to patterns observed throughout the Rio Grande rift and the western United States in general.

This paper reviews current understanding of deglaciation in North, Central and South America from the Last Glacial Maximum to the beginning of the Holocene. Together with paleoclimatic and paleoceanographic data, we compare and contrast the pace of deglaciation and the response of glaciers to major climate events. During the Global Last Glacial Maximum (GLGM, 26.5-19 ka), average temperatures decreased 4° to 8°C in the Americas, but precipitation varied strongly throughout this large region. Many glaciers in North and Central America achieved their maximum extent during the GLGM, whereas others advanced even farther during the subsequent Heinrich Stadial 1 (HS-1). Glaciers in the Andes also expanded during the GLGM, but that advance was not the largest, except on Tierra del Fuego. HS-1 (17.5-14.6 ka) was a time of general glacier thickening and advance throughout most of North and Central America, and in the tropical Andes; however, glaciers in the temperate and subpolar Andes thinned and retreated during this period. During the Bølling-Allerød interstadial (B-A, 14.6-12.9 ka), glaciers retreated throughout North and Central America and, in some cases, completely disappeared. Many glaciers advanced during the Antarctic Cold Reversal (ACR, 14.6-12.9 ka) in the tropical Andes and Patagonia. There were small advances of glaciers in North America, Central America and in northern South America (Venezuela) during the Younger Dryas (12.9-11.7 ka), but glaciers in central and southern South America retreated during this period, except on the Altiplano where advances were driven by an increase in precipitation. Taken together, we suggest that there was a climate compensation effect, or ‘seesaw’, between the hemispheres, which affected not only marine currents and atmospheric circulation, but also the behavior of glaciers. This seesaw is consistent with the opposing behavior of many glaciers in the Northern and Southern Hemispheres.