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 The thermal and denudational history of Ireland is evaluated using an extensive new apatite fission-track (AFT) dataset derived from surface samples. Modelled thermal histories are used to construct maps of denudation for a number of time slices from Triassic time to 10 Ma using a time-dependent palaeogeotherm. The maps illustrate the spatial variability of denudation and subsidence within each time slice. The patterns of denudation are complex, showing considerable variability at the length scale of 10 l - 10 2 km , with especially high denudation rates found over known igneous centres such as the Mournes of County Down. Based on the onshore AFT data alone, there is no definitive signature of an Irish Sea Dome extending significantly across Ireland in Early Tertiary time. The cumulative amount of denudation during Tertiary time varies depending on the AFT annealing model used, but is generally in the region between 1 and 2 km and without clear spatial trends. High amounts of denudation have been mapped over the Tertiary intrusions in County Down, reflecting their unroofing since emplacement in Paleocene time. The cumulative denudation from Triassic time to 10 Ma shows relatively low amounts of denudation (<2 km) in the Irish Midlands and the extreme NE of the island, consistent with the observation that Mesozoic-Tertiary sediments and igneous products are preserved in the Ulster Basin. The western flank of Ireland and the region between Dublin and County Down show high cumulative amounts of denudation (< 4km), the latter being consistent with the high amounts of denudation interpreted for the Irish Sea region. This denudation pattern explains in part the outcrop of Precambrian and Lower Palaeozoic rocks in these areas. The spatial integration of the denudation over the entire landmass gives the average denudation rate and the sediment discharge from Ireland as a function of time. Average denudation rates are moderately high in Triassic time, falling to low values in Cretaceous time, and increasing substantially in Tertiary time. However, the total volumetric discharge of sediment in Tertiary time is an order of magnitude smaller than the preserved solid volume of Tertiary sediment in the basins offshore western Ireland.