Search

Apatite – Ca5(PO4)3(OH,F,Cl) – is a common accessory mineral in many terrestrial rocks and has a low (~75˚C) closure temperature for He retention[1]. As such the (U-Th-Sm)/He thermochronometer has ...

New 40Ar/39Ar thermochronological and deformation temperature analyses in the Scandian (c. 435–420 Ma) orogenic retrowedge of northern Scotland demonstrate accelerated cooling during late syn- to post-orogenic exhumation of the high-grade orogenic core. Initial cooling rates of 10–30°C myr−1 immediately following peak orogenesis transitioned to rapid rates of 45–90°C myr−1 during final exhumation of the Naver thrust sheet in the orogenic core. The flanking ductile thrust sheets exhibit a similar, albeit less pronounced, acceleration of cooling, with rates increasing by c. 150–300% following peak orogenesis. Closer to the foreland, the Moine thrust sheet did not experience increased cooling rates. Calculated unroofing rates of 3.75 mm a−1 in the high-grade Naver thrust sheet suggest increasing, rapid exhumation in the orogenic core during a presumed collapse phase of orogenesis. This is contrary to the expectation of decreasing erosional efficiency as topography is diminished and is interpreted to suggest that unroofing of the Scottish Caledonides may have been partially enhanced by upper crustal extensional deformation during ductile flow of the infrastructure of the orogenic core. Similar processes have been interpreted in the East Greenland Caledonides, which form the northern extension of the Scandian retrowedge. Supplementary material:40Ar/39Ar analytical data for muscovite (Supplementary Data Table 1), 40Ar/39Ar analytical data for amphibole (Supplementary Data Table 2), and electron microprobe analytical data for amphibole samples (Supplementary Data Table 3) is available at: https://doi.org/10.6084/m9.figshare.c.5087057

Models of the exhumation histories of mountain ranges are often quantified through thermal-kinematic modeling of thermochronologic datasets. The Himalaya correspond with one of the world’s starkest rain shadows, with data from NASA’s Tropical Rainfall Measurement Mission (TRMM) suggesting that regions on the southern flank of the Himalaya may receive up to four meters of rainfall per year, while areas of southern Tibet north of the range crest obtain only about half a meter of rainfall annually. While the precise position of this transition is only generally defined by the spatial resolution of the TRMM data to within a few kilometers, the data clearly indicate that the transition occurs south of the range crest. New multichronometer datasets (40Ar/39Ar muscovite and (U-Th)/He zircon and apatite) for bedrock samples collected along a transect from north of Mount Everest, on the Tibetan Plateau, to the southern flank of the Himalaya in the Khumbu region of Nepal, show a dramatic break in cooling histories at a position well south of the range crest and within spatial uncertainty of the TRMM-derived position of the rainfall transition. 1D inverse thermal-kinematic and erosion modeling of the thermochronometer data for the range of possible transient denudation histories indicates that the thermochronometric transition marks a shift between two regions with dramatically different exhumation histories within the late Miocene-Pliocene, with high, presumably erosional exhumation of post-Miocene age constrained to the region of high modern precipitation related to the South Asian summer monsoon. These results support a model in which the current precipitation patterns across the rain shadow, which clearly correlate with exhumation rate patterns today, had developed by at least the late Miocene. Our results do not support the model of Carrapa et al. (2016, Geology), who used their thermochronologic data for samples collected north of Mount Everest, together with pre-existing datasets from farther south to argue for a continuous northward migration of the orographic precipitation front since the Miocene from far south of Everest to its modern position at the range crest.

The purpose of this study is to design and develop educational technology tools, primarily using the World Wide Web, for effectively recruiting young scientists to Vanderbilt University School of Medicine. We have taken the position that technology would offer major advantages in administration, teaching and learning, and have embraced this approach wholeheartedly over the last two years. The results appear to support our optimism in utilizing this approach. The primary functions of this office are recruiting, finding resources, educating and providing career development assistance for young scientists. We have used the Web and its database capabilities extensively in almost every aspect of our mission. The Educational Technology Unit designs and develops interactive Web-based applications that provide user-specific information for young scientists, investigators and communities.