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Accurately dating Holocene volcanic rocks poses many challenges but is critical to assessing magmatic evolution and hazard risks at highly active volcanoes. Here we use Ra/Th and 40Ar/39Ar geochronology to date very young eruptions at Changbaishan volcano, northeastern China, a recently active stratovolcano responsible for one of the most voluminous eruptions in the past ~2000 yr. For Holocene eruptions, 40Ar/39Ar ages are consistently older than those of both independently determined ages and maximum Ra/Th ages. Overall, Ra/Th ages are most consistent with historical accounts and indicate inaccurate 40Ar/39Ar ages that are due to extraneous argon in various forms. Ra/Th geochronology also confirms the highly active nature of Changbaishan and supports the continued presence of trachytic magma residing under the volcano that appeared more than ∼1100 yr ago. [ABSTRACT FROM AUTHOR]

Deep-time thermochronology by the zircon (U-Th)/He (ZHe) method is an emerging field of study with promise for constraining Precambrian rock thermal and exhumation histories. The Grand Canyon provides an opportunity to further explore this method because excellent geologic constraints can be integrated with multiple thermochronometers to address important questions about the spatial variability of basement erosion below the sub-Cambrian Great Unconformity composite erosional surface. In this study, we synthesize new ZHe results (n = 26) and published (n = 77) ZHe data with new Kfeldspar 40Ar/39Ar data and models (n = 4) from Precambrian basement rocks of the Grand Canyon, USA. We use HeFTy and QTQt thermal history modeling to evaluate the ability of the individual ZHe and Kfeldspar 40Ar/39Ar thermochronometric data sets to resolve Precambrian thermal histories and compare those results with jointly modeled data using the QTQt software. We also compare Precambrian basement thermal histories of the eastern and western Grand Canyon, where the eastern Grand Canyon has ~4 km of Grand Canyon Supergroup strata deposited and preserved, and the western Grand Canyon, where the Supergroup was either never deposited or not preserved. In all locations, models constrained only by ZHe data have limited resolving power for the past ~600 m.y., compared to models that combine K-feldspar 40Ar/39Ar and ZHe data, which extends the recorded history into the Mesoproterozoic. Our model results suggest that two regional basement unroofing events occurred. A ca. 1350-1250 Ma cooling event is interpreted to record basement exhumation from depths of ~10 km, and a second cooling episode (~200-100 °C total) records exhumation from a depth of ~3 km to 7 km to near-surface conditions between ca. 600 Ma and 500 Ma. Easternmost Grand Canyon models suggest that the preserved maximum ~4 km thickness of the Grand Canyon Supergroup (with burial heating at ~100 °C) approximates the total original Mesoproterozoic and Neoproterozoic stratal thickness. Whether these Supergroup rocks were present and then eroded in the western Grand Canyon, as suggested by regional geologic studies, or were never deposited is not constrained by thermochronological data. [ABSTRACT FROM AUTHOR]

The area now occupied by the Great Basin, western USA, contained paleo-fluvial systems that predated the modern-day endorheic (closed) basins. The areal extent of these paleo-fluvial systems within the southwestern Great Basin is known mainly from isolated remnants preserved in the modern mountain ranges. We document the age, extent, and tectonic disruption of Mio-Pliocene fluvial systems of the southwestern Great Basin. Synthesis of new field observations, geochemistry, and geochronology with existing studies defines two latest Miocene to Pliocene east-southeast flowing drainages that predated the modern endorheic basins. 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 environmental 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 intraplate tectonism. [ABSTRACT FROM AUTHOR]

This paper documents a multi-stage incision and denudation history for the Little Colorado River (LCR) region of the southwestern Colorado Plateau over the past 70 Ma. The first two pulses of denudation are documented by thermochronologic data. Differential Laramide cooling of samples on the Mogollon Rim suggests carving of 70-30 Ma paleotopography by N- and E-flowing rivers whose pathways were partly controlled by strike valleys at the base of retreating Cretaceous cliffs. A second pulse of denudation is documented by apatite (U-Th)/He dates and thermal history models that indicate a broad LCR paleovalley was incised 25-15 Ma by an LCR paleoriver that flowed northwest and carved an East Kaibab paleovalley across the Kaibab uplift. Lacustrine strata of the lower Bidahochi Formation were deposited 16-14 Ma in the LCR paleovalley in a closed basin playa or marsh with a valley center near the modern LCR. There is a hiatus in the depositional record in the LCR valley from 12 to 8 Ma followed by aggradation of the 8-6 Ma fluvial upper Bidahochi Formation. Interlayered 8-6 Ma maar basalts that interacted with groundwater mark local base level for upper Bidahochi fluvial deposits; this was also a time of increased groundwater flow to Hualapai Limestone at the western edge of the Colorado Plateau. The paleo-base level in the central LCR valley remained stable (~1900 m modern elevation) from 16 to 6 Ma. The third pulse of regional incision and denudation, most recent and ongoing, started after integration of the Colorado River (CR) through Grand Canyon. Thermochronology from Marble Canyon indicates that early CR integration took place across the Vermillion Cliffs at Lees Ferry after 6 Ma. The elevation of the paleoconfluence between the LCR and CR at 5-6 Ma is poorly constrained, but earliest CR integration is hypothesized to have reoccupied the East Kaibab paleocanyon. In the upper LCR drainage, topographically inverted basalt mesas have elevations and K-Ar dates indicating a transition from aggradation to incision ca. 6 Ma followed by semi-steady incision of 20-40 m/Ma. In the lower LCR, incision accelerated to 120-170 m/Ma after 2 Ma as indicated by 40Ar/39Ar dating of basalt, ash-fall, and detrital sanidine. A 1.993 ± 0.002 Ma sanidine age for a tuff in the White Mesa alluvium provides a breakthroughfor LCR and CR incision studies. Post-2 Ma differential incision magnitudes (and rates) in the lower LCR and at the LCR-CR confluence were 280-320 m (140-160 m/Ma), about three times greater than the 40-80 m (20-40 m/Ma) in the LCR headwaters. The proposed mechanisms driving overall post-6 Ma differential incision of the LCR involve headwater uplift associated with the Hopi Buttes and Springerville volcanic fields plus base-level fall caused by CR integration to the Gulf of California. A proposed mechanism to explain the accelerated post-2 Ma differential incision in the central and lower LCR valley, but not in the headwaters, involves mantle-driven epeirogenic uplift due to NE-migrating volcanism associated with the San Francisco volcanic field. Tectonically driven differential surface uplift mechanisms were likely amplified by changes toward more erosive climate at ca. 6 Ma and ca. 2 Ma. [ABSTRACT FROM AUTHOR]