The 3D Geophysical Investigation of a Middle Cretaceous to Paleocene Regional Décollement in the Cordillera of Northern Canada and Alaska

Abstract

A new approach to the 3D inversion and interpretation of gravity data is applied to the crustal architecture of the northern Cordillera of Canada and Alaska. The technique models the distribution and depth extent of rocks with systematic density contrasts, such as sedimentary or intrusive rocks. In the northern Cordillera, the geometry of low‐density zones, primarily associated with middle to Late Cretaceous granitic intrusions, and Neoproterozoic and Cretaceous sedimentary rocks, is defined. Variation in the depth extent of these zones delimits a surface, interpreted herein as a regional décollement syntectonic with, or postdating, middle Cretaceous intrusions, but predating and displaced ~430 km by the Eocene‐aged Tintina fault. The décollement trends N35°E and shallows from a depth of ~15–20 km beneath Selwyn basin to ~11 km beneath the Mackenzie Mountains. Here surface faults echo the décollement geometry, linked to fold and thrust belt development between the middle Cretaceous and Paleocene. The décollement continues southward for an unknown distance into northern British Columbia, but the Liard line represents a structural discontinuity between the fold and thrust belts of the Mackenzie Mountains and northern Rocky Mountains. The décollement's northwestern extent is broadly defined by surface faults in the northern Ogilvie Mountains. However, prior to Tintina fault displacement, the décollement was likely connected to a fold and thrust belt and related décollement in east‐central Alaska. Estimates of post‐middle Cretaceous exhumation suggest regional tectonic modification of the décollement, possibly including the presently active detachment inferred for the weak lower crust.