Branching river networks are one of the most widespread and recognizable features of Earth's landscapes and have also been discovered elsewhere in the Solar System. But the mechanisms that create these patterns and control their spatial scales are poorly understood. Theories based on probability or optimality have proven useful, but do not explain how river networks develop over time through erosion and sediment transport. Here we show that branching at the uppermost reaches of river networks is rooted in two coupled instabilities: first, valleys widen at the expense of their smaller neighbours, and second, side slopes of the widening valleys become susceptible to channel incision. Each instability occurs at a critical ratio of the characteristic timescales for soil transport and channel incision. Measurements from two field sites demonstrate that our theory correctly predicts the size of the smallest valleys with tributaries. We also show that the dominant control on the scale of landscape dissection in these sites is the strength of channel incision, which correlates with aridity and rock weakness, rather than the strength of soil transport. These results imply that the fine-scale structure of branching river networks is an organized signature of erosional mechanics, not a consequence of random topology.

Soil‐mantled pole‐facing hillslopes on Earth tend to be steeper, wetter, and have more vegetation cover compared with adjacent equator‐facing hillslopes. These and other slope aspect controls are often the consequence of feedbacks among hydrologic, ecologic, pedogenic, and geomorphic processes triggered by spatial variations in mean annual insolation. In this paper we review the state of knowledge on slope aspect controls of Critical Zone (CZ) processes using the latitudinal and elevational dependence of topographic asymmetry as a motivating observation. At relatively low latitudes and elevations, pole‐facing hillslopes tend to be steeper. At higher latitudes and elevations this pattern reverses. We reproduce this pattern using an empirical model based on parsimonious functions of latitude, an aridity index, mean‐annual temperature, and slope gradient. Using this empirical model and the literature as guides, we present a conceptual model for the slope‐aspect‐driven CZ feedbacks that generate asymmetry in water‐limited and temperature‐limited end‐member cases. In this conceptual model the dominant factor driving slope aspect differences at relatively low latitudes and elevations is the difference in mean‐annual soil moisture. The dominant factor at higher latitudes and elevations is temperature limitation on vegetation growth. In water‐limited cases, we propose that higher mean‐annual soil moisture on pole‐facing hillslopes drives higher soil production rates, higher water storage potential, more vegetation cover, faster dust deposition, and lower erosional efficiency in a positive feedback. At higher latitudes and elevations, pole‐facing hillslopes tend to have less vegetation cover, greater erosional efficiency, and gentler slopes, thus reversing the pattern of asymmetry found at lower latitudes and elevations. Our conceptual model emphasizes the linkages among short‐ and long‐timescale processes and across CZ sub‐disciplines; it also points to opportunities to further understand how CZ processes interact. We also demonstrate the importance of paleoclimatic conditions and non‐climatic factors in influencing slope aspect variations. Copyright © 2017 John Wiley & Sons, Ltd.

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New England and Maritime Canada host two major suites of Mesozoic diabase dykes. The oldest is the Coastal New England dykes that were emplaced between 225 and 230 Ma. These rocks are dominantly alkaline with trace element and isotopic compositions indicative of a high-238 U/ 204 Pb mantle (HIMU) source. The oldest of the -200 Ma Mesozoic rift magmas is represented by the Talcott basalt of the Hartford basin and its feeder dykes. External to the basin is the compositionally equivalent Higganum dyke. The extension of the Higganum, the Onway dyke in New Hampshire, is identical in major and trace element and isotopic compositions indicating that the dyke system represented a feeder to flows of flood basalt proportions. The Talcott system rocks have some trace element similarities with arc basalts and have been interpreted as representing melts of a subduction zone modified mantle beneath the LaurentianGondwanan suture. Incompatible trace element ratios and Ba, Th, and U values are, however, unlike arc basalts and are more indicative of crustal contamination of the primary magma. The coastal New England magmas have oceanic island basalt signatures that are generally thought to represent plume-tail magmatism, which is antithetic to a plume-head origin for the younger eastern North America magmas. However, coastal New England rocks have the same trace element signatures as the alkaline rocks of the Loihi seamount, which represent the pre-shield stage to the voluminous tholeiitic magmatism in Hawaii.Résumé : La Nouvelle-Angleterre et les provinces maritimes canadiennes sont les hôtes de deux suites majeures de dykes de diabase datant du Mésozoïque. La plus ancienne est la suite de dykes de la côte de la Nouvelle-Angleterre; les dykes ont été mis en place il y a 225 à 230 Ma. Ces roches sont surtout alcalines avec des éléments traces et des compositions isotopes indiquent une source HIMU (rapport élevé 238 U/ 204 Pb). Le plus ancien des magmas de rift (~200 Ma, Mésozoïque) est représenté par le basalte Talcott du bassin Hartford et ses filons nourriciers. À l'extérieur du bassin on retrouve le dyke Higgamun de composition équivalente. L'extension du dyke Higganum, le dyke Onway au New Hampshire, est identique en ce qui concerne les compositions des éléments majeurs, des éléments traces et des isotopes, indiquant que le système de dykes représentait un filon nourricier pour des écoulements de basalte à l'échelle de plateaux. Les roches du système Talcott ont des similitudes avec les basaltes d'arc en ce qui concerne les éléments traces; elles représenteraient des fusions d'un manteau modifié d'une zone de subduction sous la suture Laurentia -Gondwana. Les rapports d'éléments traces incompatibles et les valeurs Ba, Th et U sont toutefois peu semblables aux basaltes d'arc et indiquent plus une contamination du magma primaire par la croûte. Les magmas de la côte de la Nouvelle-Angleterre ont des signatures de basaltes d'îles océaniques, lesquels représenteraient un magmatisme de queue de panache, ce qui est antithétique à un...

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