Re-evaluating ambiguous age relationships in Archean cratons: implications for the origin of ultramafic-mafic complexes in the Lewisian Gneiss Complex

This is the author accepted manuscript. The final version is available from the publisher via the DOI in this record.Archean ultramafic-mafic complexes have been the focus of important and often contentious geological and geodynamic interpretations. However, their age relative to the other components of Archean cratons are often poorly-constrained, introducing significant ambiguity when interpreting their origin and geodynamic significance. The Lewisian Gneiss Complex (LGC) of the northwest Scottish mainland – a high-grade, tonalite-trondhjemite-granodiorite (TTG) terrane that forms part of the North Atlantic Craton (NAC) – contains a number of ultramafic-mafic complexes whose origin and geodynamic significance have remained enigmatic since they were first described. Previous studies have interpreted these complexes as representing a wide-range of geological environments, from oceanic crust, to the sagducted remnants of Archean greenstone belts. These interpretations, which are often critically dependent upon the ages of the complexes relative to the surrounding rocks, have disparate implications for Archean geodynamic regimes (in the NAC and globally). Most previous authors have inferred that the ultramafic-mafic complexes of the LGC pre-date the TTG magmas. This fundamental age relationship is re-evaluated in this investigation through re-mapping of the Geodh’ nan Sgadan Complex (where tonalitic gneiss reportedly cross-cuts mafic rocks) and new mapping of the 7 km2 Ben Strome Complex (the largest ultramafic-mafic complex in the LGC), alongside detailed petrography and spinel mineral chemistry. This new study reveals that, despite their close proximity in the LGC (12 km), the Ben Strome and Geodh’ nan Sgadan Complexes are petrogenetically unrelated, indicating that the LGC (and thus NAC) records multiple temporally and/or petrogenetically distinct phases of ultramafic-mafic Archean magmatism that has been masked by subsequent high-grade metamorphism. Moreover, field observations and spinel mineral chemistry demonstrate that the Ben Strome Complex represents a layered intrusion that was emplaced into a TTG-dominated crust. Further to representing a significant re-evaluation of the LGC’s magmatic evolution, these findings have important implications for the methodologies utilised in deciphering the origin of Archean ultramafic-mafic complexes globally, where material suitable for dating is often unavailable and field relationships are commonly ambiguous.We thank both the Society of Economic Geologists (Graduate Fellowship Award) and Geological Society (Timothy Jefferson Fund) for extremely generous monetary contributions in 2016 and 2017 respectively, which facilitated the (lead author’s) fieldwork that formed basis for this paper

Effects of edge restraint on slab behavior

Typescript (photocopy).This study was performed in conjunction with a Federal Emergency Management Agency program to plan, design, and construct keyworker blast shelters. The keyworker shelters are box-type structures in which damage is much more likely to occur in the roof slab than in the walls or floor. In this part of the program, the effect of edge restraint on slab behavior was investigated. The primary objective was to determine the effects of partial edge restraint on the strength, ductility, and failure mechanism of the slab. Sixteen one-way, reinforced concrete plate elements were tested in a specially designed reaction structure under uniform static water pressure. Facilities were designed and constructed to measure the slab end actions, including thrusts, moments, and rotations. The primary parameters which were investigated included the span-thickness ratio, reinforcement ratio, and degree of edge restraint. Most of the slabs were loaded until significant tensile membrane forces had been developed. Rotational restraint was found to have a significant influence on the overall behavior of the slabs. If a slab is to develop significant compressive membrane forces, both lateral and rotational restraint must be provided. Small rotational freedoms were found to have a relatively small effect on the compressive membrane response of the slabs. However, when large rotational freedoms were permitted, the slabs generally snapped through to the tensile membrane stage before significant thrusts were developed. Comparisons between the slabs with different thicknesses revealed that the thin slabs generally exhibited a much better tensile membrane response and were able to undergo larger deflections before collapse. Based on the results of this study, design recommendations were made for implementation in the keyworker blast shelter program. Also, an analytical procedure which considers nonlinear geometric behavior was proposed for investigating the total response of slabs

Traffic accident simulation using interactive computer graphics. Final report.

Louisiana Department of Transportation and Development, Baton RougeLouisiana Transportation Research Center, Baton RougeFederal Highway Administration, Washington, D.C.Mode of access: Internet.Author corporate affiliation: Louisiana Technical University, RustonReport covers the period 1988 - 1990Subject code: JLMSubject code: WNBFSubject code: X