GEOPHYSICAL INVESTIGATION AND RECONSTRUCTION OF THE LAURENTIAN MARGIN AND ITS INFLUENCE ON CORDILLERAN GEOLOGY, STRUCTURE AND MINERALISATION

Following the application of reconstruction of the Tintina Fault to regional gravity, aeromagnetic and topographic data, a set of east-west trending crustal lineaments are interpreted to be oblique to the dominantly NW-trending structure of the tectonostratigraphic terranes. The lineaments, which exhibit a range of geophysical and geological signatures, are interpreted to be associated with lower crustal and/or lithospheric mantle structures, and to be related to the Laird transfer zone, which divided lower and upper plate rifting of the Laurentian margin. 3-D gravity inversion models show a small increase in the density of the lower crust and mantle lithosphere to the north. The transfer zone also spatially divides bimodal mantle xenolith suites to the south from unimodal suites to the north.

The Liard transfer zone is interpreted to have been continuous at least as far west as the Denali Fault, and its western continuance is speculated in the region of the Talkeetna suture zone, SE Alaska. These conclusions suggest that hyper-extended North American basement related to rifting of the Laurentian margin, which would have brought rocks of the mantle lithosphere to a shallow depth, continuously underlies western Yukon and eastern Alaska. This may in-part explain the prevalence of ultramafic rocks of oceanic crustal affinity in the overlying terranes, readily entrapped during later tectonic events. The continuity of North American basement also implies that if oroclinal bending of the Yukon-Tanana terrane occurred, then it was prior to its emplacement upon the rifted North American basement.

Examination of the spatial relationships between mineral occurrences and lithospheric lineaments, from their manifestation in gravity, aeromagnetic, geological, structural, and topographic data, suggests that the lineaments influenced the development of the shallow crustal structure, intrusion, uplift and erosion, and the focussing of mineralisation in the North American basement and overlying terranes.