Sudbury impact event: Cratering mechanics and thermal history

Abstract

The Sudbury Igneous Complex (SIC) is interpreted as the solidified impact melt body of the 1.850-g.y.-old Sudbury impact structure. First results of cratering and thermal modeling for this ~250-km sized multi-ring structure are presented. The numerical calculations were done for the vertical impact of a stony (granite) body (cylindrical projectile, 12.5 km in diameter and height) impacting at a granite target with a velocity of 20 km s -1 . These simulations yield estimates of the transient cavity dimensions and the temperature field below the impact structure just after the modification stage. One-dimensional heat transfer modeling sets constraints for the thermal history of the impact melt. Cooling of the melt sheet, the present SIC, from the initial temperature of 2,000°K to the liquidus at 1,450°K lasted several 100 k.y, and below the solidus at 1,270°K, about 300 k.y. to 2 m.y., depending on the initial melt thickness H(SIC). The cooling sequence was modeled for H(SIC) of 2.5, 4, and 6 km. Given this long duration of cooling, postimpact tectonic processes during the Penokean orogeny may well have deformed the melt sheet prior to its final solidification. Prolonged cooling as well as this large-scale deformation may explain the present structural position and the composition of the Offset Dikes, consisting of differentiated impact melt.Ivanov, B. A.

Thermal evolution and interaction between impact melt sheet and footwall: A genetic model for the contact sublayer of the Sudbury Igneous Complex, Canada

“…The order of magnitude of these cooling times is consistent with previous estimates for the SIC [e.g., Ivanov and Deutsch, 1999], although shorter than that calculated by Grieve [1994], which was on the order of a million years. The relatively simple models in this study and their strong dependence on variables such as initial temperature, thermal conductivity, and latent heats which relate to the choice of liquidus and solidus temperatures suggest that timing estimates should be treated cautiously.…”

“…Thermal and compositional equilibration and homogenization of the melt sheet apparently occurs very rapidly [e.g., Oronato et al, 1978], and the rapid attenuation of the impact-induced shock wave is not conducive to significant thermal effects (as opposed to brittle deformational ones) in the impacted target rock beyond the melt zone [e.g., Simonds et al, 1976]. Typically, the outer contact zone is viewed as an irregular surface between cold country rock and hot, clast-laden impact melt, which progressively cools by conduction, convection, and radiation [e.g., Ivanov and Deutsch, 1999]. Within the ''irregularities'' in this surface, impact melt may be injected (e.g., socalled granophyre or offset dike at Vredefort [e.g., French and Nielsen, 1990] and Sudbury [e.g., Grant and Bite, 1984], respectively) hosting sulphide liquids and dense (mafic to ultramafic) xenoliths.…”

Differentiation and emplacement of the Worthington Offset Dike of the Sudbury impact structure, Ontario

“…1a and 12a). Assimilation of host rocks is most effective immediately after emplacement of the superheated melt sheet, when its temperature is about 1800°C (Ivanov and Deutsch 1999) or at least well above the liquidus temperatures of the host rocks. Modeling of fractional crystallization of the melt sheet indicates that the volumes of its different lithologies cannot be explained by starting with a compositionally homogeneous melt sheet of average SIC composition (Ariskin et al 1999).…”

“…Thermal models by Ivanov and Deutsch (1999) and Prevec and Cawthorn (2002) suggest that the liquidus temperature of a 2.5 km thick SIC undergoing strictly conductive cooling is reached after about 200,000 to 45,000 yr. However, for a thick Fig.…”

Differentiated impact melt sheets may be a potential source of Hadean detrital zircon

“…The 2.5-3.0-km-thick impact melt sheet at Sudbury cooled over ~250,000-500,000 yr (Ivanov and Deutsch, 1999;Davis, 2008), allowing the complex to differentiate into a mafic base (norite), intermediate middle layer (quartz gabbro) and more felsic top (granophyre).…”