Abstract The Laramie Peak shear system (LPSS) is a 10 km-thick zone of heterogeneous general shear (non-coaxial) that records significant tectonic regeneration of middle-lower crustal rocks of the Archean Wyoming province. The shear system is related to the 1.78–1.74 Ga Medicine Bow orogeny that involved the collision of an oceanic-arc terrane (Colorado province or Green Mountain block or arc) with the rifted, southern margin of the Wyoming province. The style and character of deformation associated with the LPSS is distinctive: a strong, penetrative (mylonitic) foliation commonly containing a moderately steep, SW-plunging elongation lineation. In mylonitic quartzo-feldspathic gneisses of the Fletcher Park shear zone, shear-sense indicators indicate southside-up, and this interpretation is supported by metamorphic and geochronological studies across the LPSS. We argue that distributed general shear (non-coaxial) involving high-strain zones and multiple folding events yielded a broad, en-masse uplift (Palmer Canyon block) during the late stages of the Medicine Bow orogeny. The LPSS is thus an excellent example of how crystal-plastic strain is distributed in sialic crust during an oceanic arc-continental margin collision. As magmatism (and attendant thermal softening) did not occur in the Wyoming province during its partial subduction beneath the oceanic-arc terranes of the Colorado province, the crystal-plastic strain manifested within the Wyoming province is mechanical in nature and was concurrent with crustal thickening. Strain is localized into discrete shear zones separated by weakly deformed rocks. These high-strain zones are commonly located along contacts between differing rock types and we propose that mechanical and chemical weakening processes may have contributed to strain localization.

Abstract Britain is littered with active and disused military airfields, arguably the most distinctive feature of its twentieth-century defenses. More than 850 airfields were active during World War II, covering about 162000 hectares or 0.7% of the national land area. Today, fewer than 50 remain active, the majority having been transferred to ground defense roles, civil flying, agriculture, and other land uses. There is a strong spatial association between the leading air bases and well-drained Middle Jurassic limestone, Upper Cretaceous chalk, and Quaternary fluvioglacial sands and gravels. Peacetime consolidation at a dwindling number of key bases has intensified impacts on these permeable lithologies, which are vulnerable to groundwater contamination, soil degradation, and erosion of landscapes rich in ecological and archaeological heritage. Demolition for agriculture has been uncoordinated and incomplete. Alternative uses include laboratories, prisons, and motor racing, many introduced without environmental controls. Asset stripping of valuable infrastructure has become a public issue since the ending of the Cold War. Disused airfields are now the foci of comprehensive regeneration schemes. Options include mineral extraction, afforestation, new towns, and runway conversion to airports. Decision making involves careful assessment of geological resources, geologic hazards, and available remediation technologies within the framework of a systematic environmental audit. Britain’s “airfield problem” is the unique product of a densely settled countryside, a rich aeronautical history, and an imperfect planning system. Experience gained trying to solve this problem has many international applications.