小行星撞击对地球的上地幔对流的影响

Perturbations of Earth′s upper mantle convection caused by asteroid impacts

本文分别基于数值结果和地质学模型,在假定地球的上地幔存在稳定Rayleigh-Bénard对流的基础上,模拟了直径为10km(陨石坑直径约180km,以Chicxulub为例)和直径为100km(陨石坑直径约1000km)的小行星撞击对地球的上地幔对流格局的影响.本文将直径10km小行星的撞击效果等效为热异常,将直径100km小行星的撞击效果等效为热异常和速度异常(主要指陨石坑底部的回弹)的叠加.计算结果表明,当小行星的直径在10km左右时,撞击对上地幔对流的影响十分微弱,热扰动时间仅2—3Ma;而当小行星的直径达到100km时,撞击就会对上地幔对流产生强烈影响.这时,对流从扰动到新的稳态有一定模式可循(依次为:调整、多个对流环、调整、稳定),扰动的持续时间受黏度和撞击点位置影响,同时稳定后地幔热柱会向着撞击点的方向产生一定的位移.

Large impacts by asteroids may have significantly affected the evolutionary history of Earth. Based on geological models and numerical simulations, we have studied the effects of large impacts caused by asteroids of different diameters （10 km and 100 kin） on Earth＇s upper mantle convection. Suppose that the upper mantle convection is initially in a stable Rayleigh-Benard convection regime. The impacting effects caused by an asteroid with a diameter of 10 km （form an impact crater of diameter about 180 km, e.g. Chicxulub Crater） are considered as an abnormal temperature field. And the impacting effects are considered as an abnormal temperature field plus an abnormal velocity field （caused by rebounding after impact） when the diameter of the asteroid is increased to 100 km （with a diameter of about 1000 km for the impact crater）. Our results show that when the diameter of the impact asteroid is 10 kin, perturbations of upper mantle convection are small, and the corresponding abnormal surface heat flux lasts only 2--3 Ma; but when the diameter increased to 100 km, the perturbations are very strong, special transient convection patterns will appear （i. e. ： adjustment, several convective rings, adjustment, and stable again）. The duration of the whole process can be affected by viscosity of mantle and impact positions. After convection into a new steady state, the hot plume may slightly move towards the impact points.