Origin of the Blue Ridge escarpment along the passive margin of Eastern North America

The Blue Ridge escarpment is a rugged landform situated within the ancient Appalachian orogen. While similar in some respects to the great escarpments along other passive margins, which have evolved by erosion following rifting, its youthful topographic expression has inspired proposals of Cenozoic tectonic rejuvenation in eastern North America. To better understand the post‐orogenic and post‐rift geomorphic evolution of passive margins, we have examined the origin of this landform using low‐temperature thermochronometry and manipulation of topographic indices. Apatite (U–Th)/He and fission‐track analyses along transects across the escarpment reveal a younging trend towards the coast. This pattern is consistent with other great escarpments and fits with an interpretation of having evolved by prolonged erosion, without the requirement of tectonic rejuvenation. Measured ages are also comparable specifically to those measured along other great escarpments that are as much as 100 Myr younger. This suggests that erosional mechanisms that maintain rugged escarpments in the early post‐rift stages may remain active on ancient passive margins for prolonged periods. The precise erosional evolution of the escarpment is less clear, however, and several end‐member models can explain the data. Our preferred model, which fits with all data, involves a significant degree of erosional escarpment retreat in the Cenozoic. Although this suggests that early onset of topographic stability is not required of passive margin evolution, more data are required to better constrain the details of the escarpment's development.     

Adjustments by the Charwell River, New Zealand, to uplift and climatic changes

Major climatic changes and rapid local and regional tectonic movements were common in New Zealand during the late Quaternary and caused a diversity of adjustments in the drainage-basin and piedmont reaches of the Charwell River, which are separated by the Hope Fault. The onset of semi-arid, frigid climates during the latest Pleistocene probably greatly increased hillslope sediment yields in a periglacial environment, and the piedmont reach aggraded as much as 42 m on top of a broad strath. With the return of humid, mesic climates in the Holocene sediment yields decreased as dense forests again mantled the slopes, and the piedmont reach degraded as mush as 81 m. Dating of eleven cut-and-strath terraces by radiocarbon-calibrated weathering rind measurements on greyawake cobbles shows the degradation rates varied greatly during the last 14 ka (1 ka = 1000 yr). Initial degradation rates of < 4 m ka⁻¹ increased to 30 m ka ⁻¹ by 6 ka ago during a mid-Holocene climatic optimum. Since 4 ka ago degradation rates have been only 1.2 m ka⁻¹, comparable to uplift rates in the piedmont reach inferred from marine-terrace studies, and the river is again cutting a broad strath. Each broad strath represents equilibrium conditions attained by this powerful stream during interglacial times despite episodes of being overwhelmed by climatically induced sediment-yield increases during full-glacial climates and having to maintain a long-term degradation rate equal to the uplift rate.The 75–81 m of degradation since formation of the latest Pleistocene fill-terrace tread is the sum of the amount of late Pleistocene valley-floor aggradation and the amount of regional uplift that occurred between the estimated times of major strath formation at about 30 and 0 ka. The 39 m of tectonically induced degradation below the pre-aggradation strath is sufficiently large that post-30 ka uplift may have doubled Holocene degradation rates.Each of the eleven degradation terraces represents pauses of a few centuries in Holocene downcutting. Brief equilibrium conditions were attained by streambed armoring and concurrent growth of riparian plants; both processes progressively increased hydraulic roughness and the shear stresses needed to entrain streambed materials. Occasional floods, possibly from rare cyclones derived from tropical moisture sources, destroyed streambed armor and channel downcutting was renewed. Thus the formation of eleven equilibrium terraces can be accounted for without postulating additional tectonic perturbations or secular climatic changes.     

THE DEMOGRAPHIC CONSEQUENCES OF METROPOLITAN POPULATION DECONCENTRATION IN THE U.S.

Although the recent growth in the nonmetropolitan population of the U.S. is now well documented, little attention has been given to the consequences these trends will have on the future composition and growth of metropolitan and nonmetropolitan regions. This paper discusses the feedback effects of migration on the future age structure and population growth of both regions during the period 1975–2000.     

Lake Floor Morphology and Sediment Architecture of Lake Torneträsk,Northern Sweden

Here we present datasets from a hydroacoustic survey in July 2011 at Lake Torneträsk, northern Sweden. Our hydroacoustic data exhibit lake floor morphologies formed by glacial erosion and accumulation processes, insights into lacustrine sediment accumulation since the beginning of deglaciation, and information on seismic activity along the Pärvie Fault. Features of glacial scouring with a high‐energy relief, steep slopes, and relative reliefs of more than 50 m are observed in the large W‐basin. The remainder of the lacustrine subsurface appears to host a broad variety of well preserved formations from glacial accumulation related to the last retreat of the Fennoscandian ice sheet. Deposition of glaciolacustrine and lacustrine sediments is focused in areas situated in proximity to major inlets. Sediment accumulation in distal areas of the lake seldom exceeds 2 m or is not observable. We assume that lack of sediment deposition in the lake is a result of different factors, including low rates of erosion in the catchment, a previously high lake level leading to deposition of sediments in higher elevated paleodeltas, tributaries carrying low suspension loads as a result of sedimentation in upstream lakes, and an overall low productivity in the lake. A clear off‐shore trace of the Pärvie Fault could not be detected from our hydroacoustic data. However, an absence of sediment disturbance in close proximity to the presumed fault trace implies minimal seismic activity since deposition of the glaciolacustrine and lacustrine sediments.     

The application of GPR to barrier-lagoon sedimentation study in Boao of Hainan Island

Sedimentary successions and internal structure of the coastal barrier-lagoon system of Boao, eastern Hainan Island were studied through utilizing data from test holes and trenches and ground-penetrating-radar (GPR) profiles. During late Pleistocene, fluvial and delta plains developed over an unevenly eroded bedrock during low sea level stand, followed by the formation of littoral and lagoon facies and defined coastal barrier-lagoon-estuary system during the post-glacial uppermost Pleistocene-lower Holocene eustatic rise of the sea level, and the upper Holocene high stand. GPR results show that Yudaitan, a sandy coastal bar backed by a low-laying land (shoal) just east of the active lagoon, is a continuous, parallel and slightly-wavy reflectors indicating homogeneous sandy or sandy gravel sediments, and inclined reflectors partly caused by progradation and accumulation of beach sand and gravel. Quasi-continuous, hummocky and chaotic reflectors from the shoal of Nangang village correspond to mixed accumula