Oceanic mega-impacts and crustal evolution

Abstract

Lunar mare crater counts, the terrestrial impact flux, and astronomical observations of asteroids and comets define a consistent impact rate of 4–6 ṁ 10 −15 km −2 ṁ yr −1 within the inner solar system since the end of the late heavy bombardment ∼3.8 Ga. Coupled with the observed crater size vs. cumulative crater size frequency relationship of N ∝ D c −1.8 ( N = cumulative number of craters of diameter > D c ), these rates imply formation on Earth of more than 450 D c ≥ 100-km-diameter craters, more than 50 D c ≥ 300-km-diameter craters, and more than 20 D c ≥ 500-km-diameter craters. Geochemical and isotopic constraints require that more than 80% of the projectiles impacted on time-integrated oceanic crust since the late heavy bombardment. The injection of shock energies calculated at >10 8 Mt TNT equivalent by a D p >10-km-diameter projectile may result in propagating fractures and rift networks, thermal perturbations, and ensuing magmatic activity. Examinations of the geologic record for correlated impact and magmatic fingerprints of such events remain inconclusive in view of isotopic age uncertainties. Potential but unproven connections may be represented by the (1) Cretaceous-Tertiary boundary (ca. 65 Ma) impact(s), onset of the Carlsberg Ridge spreading, Deccan volcanism, and onset of the mantle plume of the Emperor-Hawaii chain; (2) Jurassic-Cretaceous boundary (ca. 145 Ma) impacts, onset of Gondwana breakup, including precursors of the East African rift structures; (3) Permian-Triassic boundary (ca. 251 Ma) impact(s), Siberian Norilsk traps, and Early Triassic rifting; and (4) the 3.26 Ga basal Fig Tree Group (east Transvaal) Ir-rich and Ni-rich quench spinel-bearing impact spherules and contemporaneous igneous-tectonic activity. Tests of the theory require further identification and isotopic dating of distal ejecta, impact spherule condensates, and meteoritic geochemical anomalies.