Crustal structure of the Innuitian region of Arctic Canada and Greenland from gravity modelling: implications for the Palaeogene Eurekan orogen

Abstract

SUMMARY New gravity observations collected over Ellesmere Island and Axel Heiberg Island have been integrated with existing Canadian and Danish data sets to produce a comprehensive regional compilation over the Innuitian Region of the Canadian and Greenland High Arctic. This compilation has provided quantitative assessment of the geometry of the plate boundary between northern Greenland and Ellesmere Island and crustal structures across the Cretaceous‐Palaeogene Eurekan Orogen. A large amplitude linear gravity low—Nares Strait Gravity Low (NSGL) (<‐160 mGal)— extends obliquely across Nares Strait from northern Greenland to Ellesmere Island. This feature closely correlates with the distribution of the Palaeozoic Franklinian Margin sequences and is cross-cut by the Cenozoic Eurekan Frontal Thrust (EFT), which represents the mappable western limit of the undeformed Greenland Plate associated with the Eurekan Orogen. Newly identified linear gravity features occur north of the NSGL: the Hazen Plateau Gravity High (HPGH), corresponding with the low-lying topography of the Hazen Trough and the Grantland Gravity Low (GGL), over the elevated topography of the Grantland Uplift. Gravity models for profiles crossing the NSGL, the HPGH and the GGL indicate that the long-wavelength component of the gravity anomalies is produced by systematic variations in Moho depth. Although significant Eurekan-age thrusting and thickening of low-density Palaeozoic strata is observed on Ellesmere Island, locally contributing to the mass-deficit generating the NSGL, equivalent strata on Greenland are undeformed. The NSGL is interpreted to be primarily the signature of the remnant (Early Palaeozoic) margin with the downwards flexure of the crust beneath a northwards thickening sedimentary wedge rather than purely the result of crustal thickening from the Eurekan Orogeny. Digital bathymetry and sediment thickness data were used to determine a residual ‘crustal’ gravity field, which in turn was used to calculate depth-to-Moho and crustal thicknesses. These have been interpreted in terms of crustal affinity and crustal thinning and thickening processes associated with the Late Cretaceous‐Palaeogene plate tectonics of the area. Significant crustal thinning is observed beneath the Lancaster Basin, between Baffin Island and Devon Island, corresponding with 40 km of separation. This is interpreted to be a failed rift-arm of the Eocene spreading system in Baffin Bay. A Fourier-domain transfer function analysis (Q) determined an intermediate average crustal strength (flexural rigidity of 10 22 N m) over the Innuitian region. Comparisons with theoretical models, based on a simple thin elastic plate model, suggest that the crust is relatively thin (30 km) and in near-isostatic equilibrium—that is, not flexurally supporting the existing sedimentary or topographic load. A clearly defined anisotropy is identified, correlating with the direction of Eocene plate convergence, suggesting that the Eurekan collisional forces have not yet dissipated and are at least partially supporting large-scale orogenic structures.