A north–south-trending belt of amphibolite facies regional metamorphism parallels the Purcell Trench, transects the Kootenay Arc, and is, in part, fault bounded. Towards the axis of this belt progressively higher pressure metamorphic mineral assemblages are exposed in the contact aureoles of post-kinematic, mid-Cretaceous (~100 Ma) plutons and in metapelites. Contours of K–Ar biotite dates for plutonic rocks (55–95 Ma) are regular, are broadly conformable with metamorphic isograds, appear to cross internal intrusive contacts of post-kinematic plutons, and young towards the highest pressure and temperature regional metamorphic zones. Within the sillimanite zone most micas yield K–Ar dates between 40 and 55 Ma; Rb–Sr muscovite dates for deformed and undeformed pegmatites and for muscovite-bearing monzogranite and granodiorite fall between 53 and 84 Ma. U–Pb zircon dates for the Kaniksu batholith and nearby gneiss of uncertain origin yield a lower concordia intercept of 94 Ma. Micas from mid-Jurassic and mid-Cretaceous plutons yielding conventional K–Ar dates between 55 and 100 Ma also yield plateau-shaped 40 Ar/ 39 Ar age spectra that are indicative of normal closure to Ar diffusion due to cooling during this time interval.Contrasting isotopic cooling curves for plutonic rocks in the Purcell Anticlinorium and in the metamorphic infrastructure imply that these regions had different thermal histories. Combined with metamorphic mineral assemblage data and interpreted in terms of uplift and erosion, these curves support a tectonothermal model for the development of the Kootenay Arc and Purcell Anticlinorium that involves (1) mid-Cretaceous emplacement of post-kinematic plutons into a tectonically dormant supra-structure accompanying renewed heating, deformation, and metamorphism in the deepest levels of an evolving infrastructure; (2) slow cooling from mid- to Late Cretaceous time; (3) uplift and erosion of the continental terrace wedge and post-kinematic plutons and parts of the mid-Cretaceous infrastructure in latest Cretaceous–earliest Tertiary time as these rocks were thrust eastward over a steplike feature in the basement leading to the formation of the Purcell Anticlinorium; and (4) rapid uplift and cooling of the metamorphic infrastructure in Eocene time.
Abstract Compressive strain-rates in discrete layers of a sub-alpine snow cover are analyzed. Individual layers are identified according to density and the dominant type of metamorphism which contributed to their formation. Data were collected during four winter seasons at the Institute of Arctic and Alpine Research (INSTAAR) snow-study site (3 400 m), Red Mountain Pass, south-western Colorado, U.S.A. At average densities of less than 250 kg m ₋3 the influence of metamorphism on strain-rate is not apparent. However, at densities greater than 250 kg m ₋3 , two separate relationships emerge for strain as a function of crystal type and density. While two adjacent layers may exhibit comparable densities, a layer of sintered, fine grained (ET) snow indicates a strain-rate approximately one order of magnitude greater than an adjacent layer of cohesionless, coarse-grained (TG) snow.
Approximately 3,000 Ar, Sr, and Pb isotopic age determinations for Canadian Cordilleran rocks have been cataloged, categorized as to reliability and significance, and plotted on histograms, distribution maps for different time intervals, and space-time plots to show the magmatic evolution in this 2,300-km portion of the Circum-Pacific Mobile Belt. The history revealed is episodic, with stable distribution patterns within episodes and distinct lulls and changes in distribution between the episodes.
The timing, duration, and areal extent of the near‐surface soil freeze/thaw status were investigated using passive microwave satellite remote sensing data for the 1997/98 winter over the contiguous United States. A frozen soil algorithm was validated using soil temperature data at 0 cm and 5 cm depths from more than 20 sites over the study area. Results indicated that a negative spectral gradient and a cut‐off 37‐GHz vertical polarized brightness temperature of 258.2K can be used to determine near‐surface soil freeze/thaw status with confidence. The microwave freeze/thaw boundary generally agreed with −5.0°C isotherm of air temperature although frozen soils occurred sporadically between 0°C and −5.0°C isotherms. The maximum frozen soil area over snow‐free land surface was about 3.75 × 10 6 km² or about 37% of the total study area during the 1997/98 winter. The near‐surface soils often froze before snow covered the land surface, but soil freeze/thaw status under snow cover cannot be detected using this microwave technique. The onset of soil freeze mainly occurred in October and November, while the last days of soil freeze occurred in March and April, resulting in the duration of soil freezing varying from five to seven months over the majority of the study areas. The number of days of surface soil freezing varied from several days to longer than five months.
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