Terrestrial Cosmogenic Nuclide Surface Exposure Research Articles

Lake Wellington and West Walker River in Great Basin of western United States: History and genesis

ABSTRACTClosed basins within the Great Basin of the western United States were home to numerous lakes during the Pleistocene. One of these paleolakes along the western edge of the Great Basin, Lake Wellington, once filled a 10 × 25‐km expanse of Smith Valley to depths approaching 90 m. This and other lakes that existed during the Pleistocene are generally considered to be pluvial, indicating contemporaneity with either or all a period of cooler climate, increased rainfall and snowmelt, and relatively reduced rates of evaporation as compared to today. Here we combine the results of 36Cl terrestrial cosmogenic nuclide surface exposure dating with soils and geomorphic observations to suggest Lake Wellington was not a pluvial lake but instead the result of a large landslide prior to ~43 ± 15 ka along the West Walker River where it exited Smith Valley. The observations collected also reveal an ancestral course of the West Walker River ~85 m above the current river grade. Attributing the elevation difference to incision caused by active 0.05 ± 0.01 mm a−1 uplift of the underlying Singatse and assuming the ancestral course followed the same path as today places the age of the paleoriver course at ~1.7 Ma.

Quaternary Map, Active Faulting, Tectonic Geomorphology, and Uplift Rate of the Humboldt Range in the Basin and Range Province of Nevada, United States

ABSTRACT Fault-truncated, uplifted, and abandoned alluvial fans are frequent along the western flank of the active normal fault bounded Humboldt range. Terrestrial cosmogenic nuclide surface exposure dating of a faulted terrace at Rocky Canyon indicates that uplift of the range has averaged about 0.12 mm/yr during the last ∼160 ka. A similar uplift rate of less than about 0.07 mm/yr over the last 35 ka or more years is estimated from offset of a younger terrace at the same site. Values of extension rate across the range are 0.04–0.07 mm/yr, if it is assumed that the range-bounding fault dips at 60°, in general accord with very low values of contemporary geodetic strain reported for the area. Geomorphology at Rocky Canyon records a period leading up to about 160 ka ago that erosion and aggradation of sediment along the rangefront were dominant over fault generated uplift, at which time faulting again became dominant, leading to uplift, incision, and preservation of ∼160-ka-old alluvial terraces here and elsewhere along the rangefront.

Late Quaternary Intraplate Deformation Defined by the Las Chacras Fault Zone, West‐Central Argentina

AbstractSeveral major (up to MW 7.5) earthquakes over the past 320 years have shaken the thick‐skinned Sierras Pampeanas region of Argentina, despite exhibiting much lower GPS‐shortening rates than across the thin‐skinned Precordillera region to its west. Whether geodetic shortening rates indicate an actual long‐term shortening gradient, and whether shortening rates translate to higher uplift rates due to steeper faults in the Sierras Pampeanas, remain uncertain due to the limited spatio‐temporal coverage and the inherently large error in the vertical component of deformation of GPS measurements. We measure geomorphic offsets and use 10Be terrestrial cosmogenic nuclide surface exposure dating to determine slip rates on the Las Chacras Fault Zone (LCFZ)—an ∼30 km long, NNW‐trending, steeply dipping (55–65°E) reverse fault that branches off of the longest, westernmost, thick‐skinned Valle Fértil range‐front fault in the western Sierras Pampeanas. Average shortening and uplift rates measured on the LCFZ are ∼0.2 and ∼0.3–0.4 mm/yr, respectively. Despite an uplift rate similar to most other faults in the region, the LCFZ shortening rate is lower than faults to its west; this is in agreement with the inferred west‐east decrease in shortening rates from GPS data, indicating consistent regional deformation patterns since the Late Pleistocene. The decrease in shortening to the east coincides spatially with the termination of the flat portion of the subducted Nazca plate between 67 and 68°W. From scaling relationships among magnitude, slip rate, and fault length, the LCFZ is capable of generating earthquakes of MW 6.7–7.1.

Terrestrial cosmogenic surface exposure dating of moraines at Lake Tahoe in the Sierra Nevada of California and slip rate estimate for the West Tahoe Fault

Two sets of Pleistocene moraines (Tioga and Tahoe) are preserved at Cascade Lake along the west side of Lake Tahoe. The 10Be terrestrial cosmogenic nuclide surface exposure ages for two younger Tioga moraines yield an average age of 22.3±1.2ka, coincident with the global Last Glacial Maximum. The ages suggest that the Tioga glaciation may have reached its maximum several thousand years earlier in the Lake Tahoe basin than to the south along the east flank of the Sierra Nevada. The oldest 10Be age (120±8ka) determined for an additional suite of 10 boulders exhibiting significant scatter in 10Be ages is interpreted to be the minimum age of formation for older Tahoe moraines in the Tahoe basin, suggesting they were deposited during marine oxygen isotope stage 6. The moraines at Cascade Lake are displaced by the West Tahoe Fault that strikes northward for 45km along the western edge of the Lake Tahoe basin. Vertical displacements of the crests of the Tahoe and Tioga moraines are 59±10 and 32±12m respectively. Averaged over the time since the formation of the Tahoe and Tioga moraines, the average vertical separation rates are <0.5±0.1 and ~1.4±0.7mm/y respectively. The measured vertical separation across the broad graben on the Tioga moraine may be accentuated by its deposition on a preexisting scarp and, in this regard, the increase in slip rate since the Tioga glaciation may be apparent rather than real. The fault slip rate and accompanying horizontal rate of extension averaged over the time since the formation of the older Tahoe moraines are respectively 0.6±0.2 and 0.3±0.2mm/y. The slip rate averaged over the time since emplacement of the Tahoe moraine is in general accord with prior geologic studies reporting slip rate estimates elsewhere along the fault, and the horizontal extension rate is at the lower end of extension rates estimated by others with geodesy across the Tahoe basin.

Beryllium-10 terrestrial cosmogenic nuclide surface exposure dating of Quaternary landforms in Death Valley

Quaternary alluvial fans, and shorelines, spits and beach bars were dated using 10Be terrestrial cosmogenic nuclide (TCN) surface exposure methods in Death Valley. The 10Be TCN ages show considerable variance on individual surfaces. Samples collected in the active channels date from ~ 6 ka to ~ 93 ka, showing that there is significant 10Be TCN inheritance within cobbles and boulders. This suggests that the predominantly bedrock hillslopes erode very slowly and sediment is transferred very gradually in most regions within Death Valley. Comparisons of 10Be TCN ages on alluvial fan surfaces with chronostratigraphies based on soil development and optically stimulated luminescence dating show that minimum 10Be TCN ages within sample sets on individual surfaces most closely approximate to the age of landforms that are younger than ~ 70 ka. Alluvial fan surfaces older than ~ 70 ka have begun to undergo sufficient erosion such that the majority of 10Be TCN ages for datasets on individual surfaces probably underestimate the true age of the surface due to erosion and exhumation of fresh cobbles and boulders. The spread of 10Be TCN ages for beach bars near Beatty Junction and shorelines ~ 8 km south of Furnace Creek is large, ranging from ~ 119 ka to ~ 385 ka and ~ 109 ka to ~ 465 ka, respectively. New and previously published luminescence ages and soil development suggest that these landforms may have formed during marine isotope stage (MIS) 2 (~ 22–18 ka), but these younger ages may reflect elluviation of material into the bar deposit long after deposition, and hence the younger ages do not record the true antiquity of the landforms. This disparity between dates determined by different dating methods and the large spread of TCN ages suggests that the cobbles and boulders have considerable inherited 10Be concentrations, suggesting that the clasts have been derived from older shorelines or associated landforms. These results highlight the problems associated with using surface cobbles and boulders to date Quaternary surfaces in Death Valley and emphasizes the need to combine multiple, different dating methods to accurately date landforms in similar dryland regions elsewhere in the world. However, these results highlight the potential to use TCN methods, when used in combination with other dating techniques, to examine and quantify processes such as sediment transfer and denudation in drylands.

Late Quaternary glacial chronology on Nevado Illimani, Bolivia, and the implications for paleoclimatic reconstructions across the Andes

10Be terrestrial cosmogenic nuclide surface exposure ages from moraines on Nevado Illimani, Cordillera Real, Bolivia suggest that glaciers retreated from moraines during the periods 15.5–13.0 ka, 10.0–8.5 ka, and 3.5–2.0 ka. Late glacial moraines at Illimani are associated with an ELA depression of 400–600 m, which is consistent with other local reconstructions of late glacial ELAs in the Eastern Cordillera of the central Andes. A comparison of late glacial ELAs between the Eastern Cordillera and Western Cordillera indicates a marked change toward flattening of the east-to-west regional ELA gradient. This flattening is consistent with increased precipitation from the Pacific during the late glacial period.

Quaternary glaciation in the Nubra and Shyok valley confluence, northernmost Ladakh, India

Three glacial stages (Deshkit 1, Deshkit 2 and Dishkit 3 glacial stages) are identified in the Nubra and Shyok valleys in northernmost Ladakh, northwest India, on the basis of geomorphic field mapping, remote sensing, and 10 Be terrestrial cosmogenic nuclide surface exposure dating. The glacial stages date to ∼ 45 ka (Deshkit 1 glacial stage), ∼ 81 ka (Deshkit 2 glacial stage) and ∼ 144 ka (Deshkit 3 glacial stage). A mean equilibrium line altitude depression of ∼ 290 m for the Deshkit 1 glacial stage was calculated using the area accumulation ratio, toe-to-headwall ratio, area–altitude, and area–altitude balance ratio methods. Comparison of glaciation in the Nubra and Shyok valleys with glaciations in the adjacent Central Karakoram of northern Pakistan and northern side of the Ladakh Range of northern India indicates that glaciation was synchronous on Milankovitch timescales across the region during MIS-6, but differed greatly in extent, with more extensive glaciation in the Karakoram than the morphostratigraphically equivalent glaciation on the northern slopes of the Ladakh Range. This highlights the strong contrast in the extent of glaciation across ranges in the Himalaya–Tibetan orogen, necessitating caution when correlating glacial successions within and between mountain ranges.

Quaternary glacial history of the Central Karakoram

The Quaternary glacial history of the world's highest mountains, the Central Karakoram, is examined for the first time using geomorphic mapping of landforms and sediments, and 10Be terrestrial cosmogenic nuclide surface exposure dating of boulders on the moraines and glacially eroded surfaces. Four glacial stages are defined: the Bunthang glacial stage (>0.7Ma); the Skardu glacial stage (marine Oxygen Isotope Stage [MIS] 6 or older); the Mungo glacial stage (MIS 2); and the Askole glacial stage (Holocene). Glaciers advanced several times during each glacial stage. These advances are not well defined for the oldest glacial stages, but during the Mungo and Askole glacial stages glacial advances likely occurred at ∼16, ∼11–13, ∼5 and ∼0.8ka. The extent of glaciation in this region became increasingly more restricted over time. In the Braldu and Shigar valleys, glaciers advanced >150km during the Bunthang and Skardu glacial stages, while glaciers advanced >80km beyond their present positions during the Mungo glacial stage. In contrast, glaciers advanced a few kilometers from present ice margins during the Askole glacial stage. Glacier in this region likely respond in a complex fashion to the same forcing that causes changes in Northern Hemisphere oceans and ice sheets, teleconnected via the mid-latitude westerlies, and also to changes in monsoonal intensity.

Terrestrial cosmogenic nuclide surface exposure dating of the oldest glacial successions in the Himalayan orogen: Ladakh Range, northern India

Terrestrial cosmogenic nuclide surface exposure dating of moraine boulders and alluvial fan sediments define the timing of five glacial advances over at least the last five glacial cycles in the Ladakh Range of the Transhimalaya. The glacial stages that have been identified are: the Indus Valley glacial stage, dated at older than 430 ka; the Leh glacial stage occurring in the penultimate glacial cycle or older; the Kar glacial stage, occurring during the early part of the last glacial cycle; the Bazgo glacial stage, at its maximum during the middle of the last glacial cycle; and the early Holocene Khalling glacial stage. The exposure ages of the Indus Valley moraines are the oldest observed to date throughout the Himalayan orogen. We observe a pattern of progressively more restricted glaciation during the last five glacial cycles, likely indicating a progressive reduction in the moisture supply necessary to sustain glaciation. A possible explanation is that uplift of Himalayan ranges to the south and/or of the Karakoram Mountains to the west of the region may have effectively blocked moisture supply by the south Asian summer monsoon and mid-latitude westerlies, respectively. Alternatively, this pattern of glaciation may reflect a trend of progressively less extensive glaciation in mountain regions that has been observed globally throughout the Pleistocene.