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 tõ 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.HAYWARD 307

Geophysical reassessment of the role of ancient lineaments on the development of the western Laurentian margin and its sediment-hosted Zn–Pb deposits, Yukon and Northwest Territories, Canada

“…The modeled décollement has an average orientation of N35°E (Fig. 4b) and shallows from a depth of ~15-25 km beneath Selwyn basin to ~11 km below the MMFTB, where it shows close agreement with geological cross-sections (e.g., Cecile and Cook 1981;Cecile et al 1982;Gordey et al 2011;Hayward 2019). East of the CDF, the model surface is generally shallow, but locally expresses the geometry of thicker, relatively lower density, sedimentary rocks.…”

“…2, Williams 1989;Meijer-Drees 1993;MacLean 1999;Cook and Maclean 2004;Hadlari et al 2013;MacLean et al 2014;Morrow 2018) may provide important constraints in the interpretation of the 3-D models. Interpretation of Lithoprobe SNORCLE seismic profiles (Cook et al 1999(Cook et al , 2004Hayward , 2019 provides details of deep crustal composition and structure locally and for direct comparisons with the 3-D models.…”

“…In order to investigate the lower crustal structure beneath Selwyn basin, gravity models include the depth to a model surface (Hayward 2019) delimited by the modeled depth of relatively lower density plutons and sedimentary rocks. West of the CDF, the model surface approximates the décollement at the base of an allocthonous wedge, associated with the MMFTB.…”

“…The gravity contribution of the layer above the model surface (Fig. 4c), forward modeled from the density model (0 to -313 kg/m 3 ) of Hayward (2019), was subtracted from the Bouguer anomaly to give a residual gravity anomaly (Fig. 4d).…”

“…In this study, a range of potential field inversion strategies are developed and employed to model the crustal character of the western Canadian craton and its rifted Laurentian margin beneath Selwyn basin. The study builds on that of Hayward (2019), which modeled the N35°E décollement beneath the Selwyn basin, related to the MMFTB. Specifically, the character of regional lineaments, interpreted to have played an important role in the development of Selwyn 2004).…”