In the pre-Silurian lithotectonic units of the northern Vermont Appalachians, the timing of orogenesis and tectonometamorphism has traditionally been ascribed to the combined effects of the Middle Ordovician Taconian orogeny and Middle to Late Devonian Acadian orogeny. However, numerous geo chronological studies throughout the Northern Appalachians, including neighboring southern Quebec, have obtained Silurian and Early Devonian age data that document more or less continuous tectonometamorphic activity throughout the Ordovician-Devonian. The structural and metamorphic evolution of northern Vermont can be separated into three regional phases, which are characterized by distinct structures, fabrics, and metamorphic parageneses. The fi rst phase (D 1 ), associated with westward emplacement of various thrust slices leading to crustal thickening and regional metamorphism, and the second phase (D 2 ), characterized by bivergent structures and metamorphic overprint, have both been considered to be Taconian. The third phase, the structure and fabric of which are also observed in the Silurian-Devonian rocks to the east, is considered to be Acadian. We present new step-heating and spot fusion 40 Ar/ 39 Ar geochronological data on amphibole and fabric-forming muscovite from samples taken across the Green Mountain anticlinorium, which, coupled with published data, provide improved age constraints on tectonometamorphism of D 1 (latest Cambrian to Middle Ordovician), D 2 (Silurian-Early Devonian), and D 3 (Middle Devonian) events. By comparing structural and metamorphic characteristics, and now timing, these phases are interpreted to be correlative to the tripartite tectonometamorphic evolution documented in southern Quebec, and they further exemplify the along-strike diachronism of tectonism induced by the inherited irregular geometry of the Laurentian margin. Ar-Ar age Castonguay et al. (2007) Ar-Ar ageCambrian and Ordovician ophiolitic and suprasubduction zone metamorphosed magmatic and associated metasedimentary rocks from a peri-Laurentian arc system.Ordovician ophiolitic and suprasubduction zone metamorphosed magmatic and associated metasedimentary rocks from a peri-Gondwanan arc system. Proterozoic Grenville basement of Laurentia.Silurian to Devonian syn-Acadian metasedimentary rocks and associated metaigneous rocks.Neoproterozoic to Cambrian rift-related clastic metasedimentary rocks and associated metavolcanic rocks.Middle Devonian granitic rocks. Mesozoic intrusive rocks.Cambrian and Ordovician clastic and carbonate platform rocks overlain by Middle to Upper Ordovician Taconic clastic wedge rocks.Neoproterozoic-Ordovician Iapetan drift facies continental slope and rise metasedimentary and associated metaigneous rocks.

This paper presents evidence for an early Taconian, west-directed fault in northern Vermont, the Prospect Rock thrust, which represents the trace of the contact between the Taconian oceanic accretionary prism (Dunnage Zone) and less allochthonous, continental margin rocks (Humber Zone). Mapping at 1 : 24 000 over the last decade has led to reassessment of units within the Camels Hump Group, resulting in newly defined lithotectonic packages: the Green Mountain slice and the Prospect Rock slice. Rocks in these slices may be of similar age, but those in the Green Mountain slice were originally deposited as more proximal sediments and remained on the Humber side of the sub duction zone, whereas those in the Prospect Rock slice were transferred by underplating to the accretionary prism. Both slices contain ultramafics. Motion on the Prospect Rock thrust (D1) preceded Taconian garnet-grade metamorphism and subsequent east-directed back-folding. However, D1 structures were diachronous across the orogen. Late Taconian (D2) structures record a change from east-verging back-folds in northern Vermont and southern Quebec to west-verging folds farther south. The Prospect Rock thrust does not correspond exactly to Cameron's line nor to the Baie Verte – Brompton line as originally defined. However, our understanding of the geometry of the Prospect Rock thrust demonstrates that the Dunnage Zone extends farther west than previously recognized and may also explain features farther to the east.

A field investigation has been undertaken to characterize the event‐based bed load transport dynamics of a highly urbanized gravel bed stream. A combination of direct bed load and tracer particle measurements were taken over a 3 year period during which time approximately 30 sediment mobilizing events occurred. Sediment transport measurements were used to calibrate a fractional bed load transport model and combined with hydrometric data which represent four different land use conditions (ranging from rural to highly urbanized) to analyze the differences in discharge magnitude and frequency and its impact on sediment transport. Fractional transport analysis of the bed load measurements indicates that frequent intermediate discharge events can mobilize sand and fine gravel to an approximate equally mobile condition, however, the transport rates at these discharges exhibit greater variability than at discharges above the bankfull discharge. Path lengths of the coarse fraction, measured using tracer clasts, are insensitive to peak discharge, and instead transport at distances less than those reported in other gravel bed channels, which is attributed to the shorter duration discharge events common to urban streams. The magnitude‐frequency analysis reveals that the frequency, time, and volume of competent sediment mobilizing events are increasing with urbanization. Variability in effective discharges suggests that a range of discharges, spanning between frequent, low magnitude events to less frequent, high magnitude events are geomorphically significant. However, trends in the different land use scenarios suggest that urbanization is shifting the geomorphic significance toward more frequent, lower magnitude events.

Urban gravel‐bed stream channels in southern Ontario, Canada identified to be in a state of quasi‐equilibrium have been studied over the past 15 years and compared against rural gravel bed stream channels of the same hydrophysiographic region. Bankfull width and depth versus bankfull discharge were not found to increase as a function of increasing urbanization as has been found in many other studies. The observed annual frequency of bankfull discharge was typically less than a 1‐year return period with many sites ranging between 2 and 18 bankfull events per year with higher intensity and shorter duration urban flood responses, which further identified significant limitations in using annual peak discharge methods for predicting morphological forming flows in urban watersheds. The cumulative volume of bankfull and larger flood events from the urban stream channels were very similar to the same annual event volumes in the rural comparison study reaches. Bed material supply was found to decrease with increasing urbanization and the reduction in bed material supply appears to be off‐set by the smaller bankfull channel width, depth and access to floodplains during large flood events. Field evidence may also suggest a even greater reduction in channel width trajectory, relative to the rural setting, with expansive floodplains to maintain quasi‐equilibrium conditions as bed material supply continues to decrease with increased anthropogenic activity. Copyright © 2010 John Wiley & Sons, Ltd.

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