Search for: All records

ABSTRACT Field and desktop mapping studies were conducted for the stable continental region in the Western Cape Province of South Africa to characterize fault activity of four fault systems, including the Worcester, Groenhof, Piketberg-Wellington, and Colenso faults. The geologic studies presented here were in support of a Probabilistic Seismic Hazard Analysis (PSHA) for a nearby nuclear power facility site. Previous studies performed by the South African Council for Geoscience in the region suggested evidence of near-surface co-seismic deformation (De Beer, 2004; De Beer et al., 2008). The goal of this study is to re-assess the prior interpretations of these four faults and gather the required data for including them in a seismic source model for use in a PSHA. The primary aspects to include in the characterization are the recency of movement, slip rate, kinematics, and geometry. To improve the interpretation and target sites, the study used a satellite-derived digital elevation model and aerial imagery for six areas, totaling over 900 km2 of data. Limited Quaternary cover, or other late Cenozoic deposits that overlie the Precambrian and Paleozoic bedrock structures, resulted in difficulty constraining the recency of faulting. The new observations presented in this study suggest that reactivation and surface rupture along pre-Cenozoic faults of the four fault systems have not occurred in at least the last 10 ka. Further, the lack of youthful tectonic geomorphology and deformation of Quaternary stratigraphy indicate that surface faulting has not occurred in the late to middle Quaternary along any of these four structures. 

Interactions among tectonics, climate, and lithology shape the Earth’s surface. In regions dominated by tectonic quiescence and climate stability, the role of rock strength related to lithology, and its role in landscape evolution, can be most clearly perceived. We leverage these qualities in a unique field site by integrating topographic data, erosion rates, and a large suite of rock strength measurements to quantify the relationship between bedrock strength and erosion rates along a 200-km section of the southeast coast of Brazil where climatic and tectonic variability are minimal. Our findings reveal a 20-fold erosion rate variation best explained by differences in rock strength. We also demonstrate that incorporating lithological strength variability into the analysis of landscape incision substantially improves accuracy, with outcomes that better reproduce natural settings. Our results underscore the crucial role of rock strength in landscape evolution and emphasize the need for field studies to account for lithological variability and thus accurately interpret landscape dynamics. 

Abstract. Since the 1990s, analysis of cosmogenic nuclides, primarily 10Be, in quartz-bearing river sand, has allowed for quantitative determination of erosion rates at a basin scale. Paired measurements of in situ cosmogenic 26Al and 10Be in sediment are less common but offers insight into the history of riverine sediment moving down slopes and through drainage basins. Prolonged sediment burial (>105 years), a violation of assumptions underlying erosion rate calculations, is indicated by higher 26Al-based than 10Be-based erosion rates due to preferential loss of shorter-lived 26Al by decay when quartz is shielded from cosmic rays. Here, we use a global compilation of 26Al and 10Be data generated from quartz-bearing fluvial sediment samples (n = 624, including 121 new measurements) and calculate the discordance between erosion rates derived from each nuclide. We test for correlations between such discordance and topographic metrics for drainage basins, allowing us to infer the likelihood of sediment burial during transport in different geomorphic settings. We find that nearly half of samples (n = 276) exhibit discordance (> 1σ uncertainty) between erosion rates derived from 10Be and 26Al, indicating sediment histories that must include extended burial during residence on hillslopes and/or in the fluvial system after or during initial near-surface exposure. Physical basin parameters such as basin area, slope, and tectonic activity exhibit significant correlation with erosion rate discordance whereas climatic parameters have little correlation. Our analysis suggests that 26Al/10Be erosion rate discordance occurs more regularly in basins larger than 1,000 km2, particularly when such basins have low average slopes and are in tectonically quiescent terrains. Sediment sourced from smaller, steeper basins in tectonically active regions is more likely to have similar 10Be and 26Al erosion rates indicative of limited storage and limited burial during residence in the hillslope and fluvial sediment system. The data and analysis we present demonstrate that paired 26Al and 10Be analyses in detrital fluvial samples can provide a window into watershed processes, elucidating landscape behavior at different spatial scales and allowing a deeper understanding of both sediment routing systems and whether erosion rate assumptions are violated. Large lowland basins are more likely to transport detrital sediment that has experienced prolonged sediment storage and burial either on hillslopes and/or in fluvial networks; thus, erosion rates from such basins are lower limits due to nuclide decay during storage. Conversely, samples from smaller upland basins are more likely to provide reliable erosion rates. 

Abstract. Measurements of multiple cosmogenic nuclides in a single sample are valuable for various applications of cosmogenic nuclide exposure dating and allow for correcting exposure ages for surface weathering and erosion and establishing exposure–burial history. Here we provide advances in the measurement of cosmogenic 10Be in pyroxene and constraints on the production rate that provide new opportunities for measurements of multi-nuclide systems, such as 10Be/3He, in pyroxene-bearing samples. We extracted and measured cosmogenic 10Be in pyroxene from two sets of Ferrar Dolerite samples collected from the Transantarctic Mountains in Antarctica. One set of samples has 10Be concentrations close to saturation, which allows for the production rate calibration of 10Be in pyroxene by assuming production–decay equilibrium. The other set of samples, which has a more recent exposure history, is used to determine if a rapid fusion method can be successfully applied to samples with Holocene to Last Glacial Maximum exposure ages. From measured 10Be concentrations in the near-saturation sample set we find the production rate of 10Be in pyroxene to be 3.74 ± 0.10 atoms g−1 yr−1, which is consistent with 10Be/3He paired nuclide ratios from samples assumed to have simple exposure. Given the high 10Be concentration measured in this sample set, a sample mass of ∼ 0.5 g of pyroxene is sufficient for the extraction of cosmogenic 10Be from pyroxene using a rapid fusion method. However, for the set of samples that have low 10Be concentrations, measured concentrations were higher than expected. We attribute spuriously high 10Be concentrations to failure in removing all meteoric 10Be and/or a highly variable and poorly quantified procedural blank background correction. 

The emerging field of quartz luminescence properties in Earth-surface processes research shows promise, with optically stimulated luminescence (OSL) sensitivity proposed as a valuable tool for provenance or sediment history tracing. However, the geologic processes that lead to quartz sensitization remain under investigation. Here we study the impact of source rock and surface processes on the luminescence properties of quartz sand from bedrock and modern and Late Pleistocene alluvium generated from a mountainous catchment in northern Utah, USA. Continuous wave and linear modulated OSL are used to characterize the luminescence sensitivity and intensity of the fast-decay component. We compare the OSL sensitivity with sand-grain provenance and with proxies for surface processes such as topographic metrics, cosmogenic 10Be-derived erosion rates, chemical weathering indices, and magnetic susceptibility. Late Pleistocene sediment has low OSL sensitivity and a weak fast-decay component, similar to bedrock samples from the source area. In contrast, modern alluvium is dominated by the fast-decay component and has higher and more variable OSL sensitivity, with no clear relationship to bedrock sources in their prospective catchment areas. There is, however, an inverse relationship between OSL sensitivity and catchment-averaged erosion rates and a positive relationship with chemical weathering indices and magnetic susceptibility. These metrics suggest that the modern alluvium has experienced increased residence time in the shallow critical zone compared to the Late Pleistocene sediments. We suggest that changes in hillslope processes between the effectively wetter, cooler Pleistocene and the dryer, warmer conditions of the Holocene modulated the luminescence properties. The results suggest that climatic controls on rates and processes of chemical and mechanical weathering and sediment transport and residence within the critical zone are encoded in the luminescence properties of quartz sand. 

Rapid sediment accumulation rates (SAR) in a fan delta situated on the rapidly uplifting footwall of the Taormina normal fault in NE Sicily preserves a rare record of earthquakes and base level change for a tightly coupled source to sink system. We use this sedimentary archive to reconstruct the kinematics and slip history of the fault and further an understanding of how tectonic forcing across various scales are encoded in stratigraphy. A revised luminescence-based age model indicates that ~82 m of the Pagliara fan-delta foreset facies was deposited in ~11 ka at a mean SAR of ~0.74 cm/yr during MIS 7. Syn-depositional terrestrial cosmogenic nuclide (TCN) determined paleoerosion rates of 0.91±0.12 mm/yr and 1.31 ±0.61 mm/yr are similar to published modern erosion rates for the Pagliara basin of 0.97 ±0.11 mm/yr. At the stratigraphic scale, a time series of magnetic susceptibility (c) sampled at 1 m intervals in the foresets displays four ~2,800 yr / 20 m-thick cycles of growing c, bounded by sharp decreases that do not coincide with changes in sediment texture. The c of the low-grade metamorphic bedrock in the source is 20-100 times weaker than the c of rubified soils mantling the hillslopes, which is comparable to the c of the delta sediments. We propose that large, bedrock-cored landslides quasi-periodically deliver weak c sediment to the delta that dilutes a c signal otherwise dominated by the stripping of soil-mantled hillslopes. We propose that centennial-scale recurrence interval earthquakes are most capable at triggering a basin-scale landslide only after channel incision has increased relief of hillslopes to the threshold condition, which requires millennia to achieve. At the landscape scale of delta geometry and location, the Pagliara delta accumulated in a hanging wall basin that has since been inverted. We reconstruct the history of base level fall for the delta from an inversion of fluvial topography and apportion that record to its rock uplift, delta deposition, and eustatic components. We show that footwall uplift has been unsteady over the past 600 ka ranging from -1 to 3 mm/yr. The integration of our stratigraphic- and landscape scale observations furthers our understanding of the natural hazards related to normal fault earthquakes and their impact on sediment dynamics in this steep, active tectonic setting. 

Abstract. To understand the erosivity of the eastern portion of the Laurentide Ice Sheet and the isotopic characteristics of the sediment it transported, we sampled buried sand from deglacial features (eskers and deltas) across eastern Canada (n = 10), a landscape repeatedly covered by the Quebec-Labrador Ice Dome. We measured concentrations of 10Be and 26Al in quartz isolated from the sediment and, after correcting for sub-surface cosmic-ray exposure after Holocene deglaciation, used these results to determine nuclide concentrations at the time the ice sheet deposited the sediment. To determine what percentage of sediment moving through streams and rivers currently draining the field area was derived from incision of thick glacial deposits as opposed to surface erosion, we used 10Be and 26Al as tracers by collecting and analyzing modern river sand sourced from Holocene-exposed landscapes (n = 11). We find that all ten deglacial sediment samples contain measurable concentrations of 10Be and 26Al equivalent on average to several thousand years of surface exposure – after correction, based on sampling depth, for Holocene nuclide production after deposition. Error-weighted averages (1 standard deviation errors) of measured 26Al/10Be ratios for both corrected deglacial (6.1 ± 1.2) and modern sediment samples (6.6 ± 0.5) are slightly lower than the production ratio at high latitudes (7.3 ± 0.3) implying burial and preferential decay of 26Al, the shorter-lived nuclide. However, five deglacial samples collected closer to the center of the former Quebec-Labrador Ice Dome have much lower corrected 26Al/10Be ratios (5.2 ± 0.8) than five samples collected closer to the former ice margins (7.0 ± 0.7). Modern river sand contains on average about 1.75 times the concentration of both nuclides compared to deglacial sediment corrected for Holocene exposure. The ubiquitous presence of 10Be and 26Al in eastern Quebec deglacial sediment is consistent with many older-than-expected exposure ages, reported here and by others, for bedrock outcrops and boulders once covered by the Quebec-Labrador Ice Dome. Together, these data suggest that glacial erosion and sediment transport in eastern Canada were insufficient to remove material containing cosmogenic nuclides produced during prior interglacial periods both from at least some bedrock outcrops and from all glacially transported sediment we sampled. Near the center of the Quebec-Labrador Ice Dome, ratios of 26Al/10Be are below those characteristic of surface production at high latitude. This suggests burial of the glacially transported sediment for at least many hundreds of thousands of years and the possibility that ice at the center of the Quebec-Labrador Ice Dome survived many interglacials when more distal ice melted away. 

The persistence and size of the Greenland Ice Sheet (GrIS) through the Pleistocene is uncertain. This is important because reconstructing changes in the GrIS determines its contribution to sea level rise during prior warm climate periods and informs future projections. To understand better the history of Greenland’s ice, we analyzed glacial till collected in 1993 from below 3 km of ice at Summit, Greenland. The till contains plant fragments, wood, insect parts, fungi, and cosmogenic nuclides showing that the bed of the GrIS at Summit is a long-lived, stable land surface preserving a record of deposition, exposure, and interglacial ecosystems. Knowing that central Greenland was tundra-covered during the Pleistocene informs the understanding of Arctic biosphere response to deglaciation.