No abstract

Three independent data sets from northwestern India and Pakistan suggest initial displacement along >1000 km of the Main Boundary thrust prior to 10 Ma, at least 5 m.y. earlier than previously reported. Regionally extensive changes in the depositional characteristics and rates of the foreland-basin fill between 11 and 9.5 Ma are interpreted to reflect new hinterland loading due to the formation of the Main Boundary thrust. Sediment-accumulation rates, sandstone-siltstone ratios, and thickness and amalgamation of individual sandstone bodies all substantially increase after 11 Ma in well-dated stratigraphic sections from Pakistan to Nepal across the Indo-Gangetic foreland basin. In the Himachal Pradesh reentrant of northwestern India, a newly discovered 8.7 Ma conglomerate derived from the hanging wall of the Main Boundary thrust indicates that source-area uplift and denudation must have occurred prior to 9 Ma and probably prior to 10 Ma, assuming a gravel progradation rate of 3 cm/yr. Three apatite fission-track ages from structures at the leading edge of the Main Boundary thrust in the Kohat region of northwest Pakistan indicate that rapid cooling below ϳ105 ؇C between 8 and 10 Ma followed bedrock uplift and erosion that began ϳ1-2 m.y. earlier. These data indicate that the Main Boundary thrust in the western Himalaya formed synchronously along strike in the middle-late Miocene, has a displacement rate of ϳ10 mm/yr, and has a displacement history that is coeval with late displacement on the Main Central thrust.

Landsat-7 ETMþ scenes were acquired for the western Arctic Coastal Plain of Alaska extending from 152 to 162 W longitude. A segmentation algorithm was used to classify lakes and drained thaw lake basins (DTLBs) exceeding 1 ha in size. A total of 13,214 lakes and 6539 DTLBs were identified. Several indices were obtained from the image processing software and used for a comparative analysis of lakes and basins including object size, goodness of elliptic fit, shape complexity, shape asymmetry, and orientation of the major axis. Nonparametric statistical analyses indicate that lakes and basins share similar orientation only. Three subregions of the western Arctic Coastal Plain were identified based on landscape age, as demarcated by ancient shorelines. The surfaces become progressively older inland and include the Younger Outer Coastal Plain, the Outer Coastal Plain, and the oldest Inner Coastal Plain. Lakes and basins in all subregions have statistically similar orientation, indicating that summer wind direction has not changed appreciably over the past several thousand years. Basin orientation is less clustered than lake orientation. Lakes are highly elliptical, while basins have more complex shapes. Lake coverage (%) is fairly constant across the three subregions, while DTLB coverage decreases on older surfaces. Lake and basin size decreases on progressively older surfaces, but the number of features per unit area increases. It is uncertain if surface age is responsible for differences in regional metrics as an analysis of the Inner Coastal Plain demonstrates significant internal variation. Distance from the coast, ground ice content, surficial sediments, and local relief may also influence lake morphometry.

Thousands of lakes are found on the Arctic Coastal Plain of northern Alaska and northwestern Canada. Developed atop continuous permafrost, these thaw lakes and associated drained thaw lake basins are the dominant landscape elements and together cover 46% of the 34,570 km2 western Arctic Coastal Plain (WACP). Lakes drain by a variety of episodic processes, including coastal erosion, stream meandering, and headward erosion, bank overtopping, and lake coalescence. Comparison of Landsat multispectral scanner (MSS) imagery from the mid‐1970s to Landsat 7 enhanced thematic mapper (ETM+) imagery from around 2000 shows that 50 lakes completely or partially drained over the approximately 25 year period, indicating landscape stability. The lake‐specific drainage mechanism can be inferred in some cases and is partially dependant on geographic settings conducive to active erosion such as riparian and coastal zones. In many cases, however, the cause of drainage is unknown. The availability of high‐resolution aerial photographs for the Barrow Peninsula extends the record back to circa 1950; mapping spatial time series illustrates the dynamic nature of lake expansion, coalescence, and drainage. Analysis of these historical images suggests that humans have intentionally or inadvertently triggered lake drainage near the village of Barrow. Efforts to understand landscape processes and identify events have been enhanced by interviewing Iñupiaq elders and others practicing traditional subsistence lifestyles. They can often identify the year and process by which individual lakes drained, thereby providing greater dating precision and accuracy in assessing the causal mechanism. Indigenous knowledge has provided insights into events, landforms, and processes not previously identified or considered.