A cosmogenic view of erosion, relief generation, and the age of faulting in southern Africa

Southernmost Africa, with extensive upland geomorphic surfaces, deep canyons, and numerous faults, has long interested geoscientists. A paucity of dates and low rates of background seismicity make it challenging to quantify the pace of landscape change and determine the likelihood and timing of fault movement that could raise and lower parts of the landscape and create associated geohazards. To infer regional rates of denudation, we measured 10Be in river sediment samples and found that south-central South Africa is eroding ∼5 m m.y.-1, a slow erosion rate consistent with those measured in other non-tectonically active areas, including much of southern Africa. To estimate the rate at which extensive, fossil, upland, silcrete-mantled pediment surfaces erode, we measured 10Be and 26Al in exposed quartzite samples. Undeformed upland surfaces are little changed since the Pliocene; some have minimum exposure ages exceeding 2.5 m.y. (median, 1.3 m.y.) and maximum erosion rates of \u3c0.2 m m.y.-1 (median, 0.34 m m.y.-1), consistent with no Quaternary movement on faults that displace the underlying quartzite but not the silcrete cover. We directly dated a recent displacement event on the only recognized Quaternary-active fault in South Africa, a fault that displaces both silcrete and the underlying quartzite. The concentrations of 10Be in exposed fault scarp samples are consistent with a 1.5 m displacement occurring ca. 25 ka. Samples from this offset upland surface have lower minimum limiting exposure ages and higher maximum erosion rates than those from undeformed pediment surfaces, consistent with Pleistocene earthquakes and deformation reducing overall landscape stability proximal to the fault zone. Rates of landscape change on the extensive, stable, silcretized, upland pediment surfaces are an order of magnitude lower than basin-average erosion rates. As isostatic response to regional denudation uplifts the entire landscape at several meters per million years, valleys deepen, isolating stable upland surfaces and creating the spectacular relief for which the region is known

Transport through an impurity tunnel coupled to a Si/SiGe quantum dot

Achieving controllable coupling of dopants in silicon is crucial for operating donor-based qubit devices, but it is difficult because of the small size of donor-bound electron wavefunctions. Here we report the characterization of a quantum dot coupled to a localized electronic state, and we present evidence of controllable coupling between the quantum dot and the localized state. A set of measurements of transport through this device enable the determination of the most likely location of the localized state, consistent with an electronically active impurity in the quantum well near the edge of the quantum dot. The experiments we report are consistent with a gate-voltage controllable tunnel coupling, which is an important building block for hybrid donor and gate-defined quantum dot devices.Comment: 5 pages, 3 figure

Structural analysis of the San Simeon fault zone, California : implications for transform tectonics

The San Gregorio-Hosgri fault zone (SGH), located in the Southern Coast Ranges of California is a 420 kilometer long right-lateral strand of the San Andreas fault system. The San Simeon fault zone is a segment of the SGH that cross-cuts the Nacimiento block which is primarily composed of Franciscan Complex accretionary prism. The Nacimiento block is juxtaposed against the Salinian block, a portion of the Sierra Nevada batholith, by the Nacimiento Fault. The Nacimiento and Salinian blocks have been displaced from the south in a right lateral sense as part of movements within the San Andreas fault system. The San Simeon segment juxtaposes mid-Jurassic Coast Range Ophiolite with Cretaceous Franciscan accretionary prism material. These units are locally overlain by the Oligocene Lospe Formation and Miocene Monterey Formation. To better understand the movement history near the San Simeon fault zone, 33 kilometers of outcrop were examined along the sea-cliff between Ragged Point in the north and Pico Creek to the south. Of this transect, 4 kilometers were buried under marine terrace and sand dunes. No data was collected along 1 kilometer of transect due to the presence of elephant seals. The 28 kilometers of bedrock examined include: 7 kilometers of ophiolitic material, 16 kilometers of Franciscan Complex, 2 kilometers of Lospe Formation, and 3 kilometers of Monterey Formation. In all, 466 minor faults and 254 major (≥0.5 meters exposure length) faults were mapped, and 22 of these major faults juxtapose different formations (n=8) or different units within the ophiolite (n=14). Slickenlines were measured on 517 faults, of which 237 record sense of slip. Of the faults measured, 199 are strike-slip (0-30° rake), 179 are dip-slip (60-90° rake), and 139 are oblique-slip (31-59° rake). Sense of slip indicators record a wide range of movements: 49 right-lateral, 47 left-lateral, 40 normal, 38 reverse, 18 reverse left-lateral, 17 normal left-lateral, 15 normal right-lateral and 13 reverse right-lateral faults. The study transect was divided into structural domains based on fault kinematic patterns. Movement recorded in these data resulted from transform-related faulting. Fault kinematics that differ from the regional N35W strike of the San Simeon fault zone are explained by local variations in movement patterns near the San Simeon fault zone. This variations include local bends and splays off of the fault zone. The Lospe and Monterey Formations that make up 18% of the mapped transect contain 12% of the faults. These formations only experienced transform-related deformation. Faults in the Monterey Formation are parallel to the regional San Simeon fault zone. Faults in the Lospe Formation to the north primarily strike E-W. Ophiolite material contains 25% of the mapped transect and 37% of the faults. These faults primarily indicate right-lateral movement; however, reverse and normal faulting near perpendicular to the regional NW fault trend is common. The Franciscan Complex along 57% of the mapped transect contains 51% of the faults. Faults in the Franciscan Complex and the ophiolite potentially record subduction-related faulting, but evidence from fault kinematics from this study indicates transform-related faulting. Reverse and right-lateral faulting along the splays is indicated. East of San Simeon Point, a 1 kilometer wide San Simeon fault zone is indicated by a cluster of faults between the San Simeon Pier and Broken Bridge Creek, the eastern boundary of the fault zone. The complexity of fault patterns and kinematics in and near the San Simeon fault zone record a long and complex history of transform faulting.Geological Science

Mapping the Timescale of Suicidal Thinking

Suicide is one of the most devastating aspects of human nature and has puzzled scholars for thousands of years. Most suicide research to date has focused on establishing the prevalence and predictors of the presence or severity of suicidal thoughts/behaviors. Surprisingly little research has documented the fundamental properties of suicidal thoughts/behaviors, such as: when someone has a suicidal thought, how long do such thoughts last? Documenting the basic properties of a phenomenon is necessary to understand, study, and treat it. This study aims to identify the timescale of suicidal thinking, leveraging novel real-time monitoring data and a number of different novel analytic approaches. Participants were 105 adults with past week suicidal thoughts who completed a 42-day real-time monitoring study (total number of observations=20,255). Participants completed two forms of real time assessments: traditional real-time assessments (spaced hours apart each day) and high-frequency assessments (spaced 10 minutes apart over one hour). We found that suicidal thinking changes rapidly. Both descriptive statistics and Markov-Switching models indicated that that elevated states of suicidal thinking lasted on average 1 to 3 hours. Individuals exhibited considerable heterogeneity in how often and for how long they reported elevated suicidal thinking, and our analyses suggest that different aspects of suicidal thinking operated on different timescales. Continuous-time autoregressive models suggest that current suicidal intent is predictive of future intent levels for 2 to 3 hours, while current suicidal desire predictive of future suicidal desire levels for 20 hours. Multiple models found that elevated suicidal intent has on average shorter duration than elevated suicidal desire. Finally, our ability to capture within-person dynamics of suicidal thinking was improved using high-frequency sampling. For example, traditional real-time assessments alone estimated the duration of severe suicidal states of suicidal desire as 9.5 hours, whereas, the high-frequency assessments shifted the estimated duration to 1.4 hours. The high-frequency assessments identified 19% more participants with a high-risk response than the traditional real-time assessment, and high frequency measurements were shown to capture considerable levels of variation across consecutive measurement occasions. These results provide the most detailed characterization to date of the temporal dynamics of suicidal thinking. Furthermore, these findings highlight the importance of sampling frequency in capturing the dynamics of a phenomenon