Geophysical Investigation and Reconstruction of Lithospheric Structure and its Control on Geology, Structure and Mineralisation in the Cordillera of Northern Canada and Eastern Alaska

Abstract:

A reconstruction of the Tintina fault is applied to regional gravity, aeromagnetic and topographic data, facilitating the definition of E-W trending lineaments within the lower crust and/or mantle lithosphere, oblique to the dominantly NW-trending structure of the Cordilleran terranes. The lineaments, which are continuous to the Denali fault, exhibit a range of geophysical and geological signatures. They are interpreted to be related to the Liard transfer zone that divided lower and upper plates during late Proterozoic-Cambrian rifting of the Laurentian margin.

Density models derived from the 3-D inversion of reconstructed gravity data reflect a change from NW-trending structure in the upper to middle crust, to E-W trending structure below. These deeper structures are associated with a small increase in the density of the lower crust and mantle lithosphere to the north. The transfer zone also divides bimodal mantle xenolith suites, derived from late Tertiary to Recent alkaline basalts, to the south from unimodal suites to the north.

These conclusions suggest that extended North American basement, related to Laurentian margin rifting, that would have brought mantle lithosphere rocks to a shallow depth, continuously underlies a thin carapace of accreted terranes in western Yukon and eastern Alaska. The interpreted continuity of North American basement reaffirms that if oroclinal bending of the Intermontane terranes occurred, then it was prior to its emplacement upon the rifted basement.

Examination of the spatial relationships between mineral occurrences and post-accretionary, Cretaceous lithospheric lineaments, from their manifestation in geophysical, geological, and topographic data, suggests that the late Proterozoic lineaments influenced Mesozoic mineralization through influence on the development of the shallow crustal structure, intrusion, and exhumation and erosion.