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

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 decollement syntectonic with, or postdating, middle Cretaceous intrusions, but predating and displaced similar to 430km by the Eocene-aged Tintina fault. The decollement trends N35 degrees E and shallows from a depth of similar to 15-20km beneath Selwyn basin to similar to 11km beneath the Mackenzie Mountains. Here surface faults echo the decollement geometry, linked to fold and thrust belt development between the middle Cretaceous and Paleocene. The decollement 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 decollement's northwestern extent is broadly defined by surface faults in the northern Ogilvie Mountains. However, prior to Tintina fault displacement, the decollement was likely connected to a fold and thrust belt and related decollement in east-central Alaska. Estimates of post-middle Cretaceous exhumation suggest regional tectonic modification of the decollement, possibly including the presently active detachment inferred for the weak lower crust.