HR: 13:55h
AN: T32C-02    [PDF]
TI: Thermal Perturbations Caused by Large Impacts and Consequences for Mantle Convection
AU: * Watters, W A
EM: watters@mit.edu
AF: Department of Earth, Atmospehric and Planetary Sciences, 54-812 Masachusetts Institute of Technology, Cambridge, MA 02139-4307 United States
AU: Zuber, M T
EM: zuber@mit.edu
AF: Department of Earth, Atmospehric and Planetary Sciences, 54-812 Masachusetts Institute of Technology, Cambridge, MA 02139-4307 United States
AU: Hager, B H
EM: brad@chandler@mit.edu
AF: Department of Earth, Atmospehric and Planetary Sciences, 54-812 Masachusetts Institute of Technology, Cambridge, MA 02139-4307 United States
AB: We consider the manner in which large impacts, with projectile radii in the range of 50-500 km and incident velocities of 10-20 km/s, may perturb circulation in the mantles of solid planets with dimensions comparable to those of the Earth and Mars. In particular, we address the possibility that such impacts may initiate or disrupt deep mantle plumes. We consider three mechanisms whereby these impacts may initiate instabilities in a thermal boundary layer (TBL) at the core-mantle boundary (CMB) and lead to the formation of plumes: (1) direct heating of the CMB-TBL; (2) the lifting of TBL isotherms by relaxation of impact-related thermal perturbation; and (3) local or global disruption of circulation patterns in the CMB-TBL, causing motion to slow or stagnate and instabilities to grow. In order to evaluate the merits of mechanism (1), we first determine under what circumstances the CMB-TBL is heated significantly by large impacts. The projectiles are assumed to impinge upon a chemically homogeneous layer with an STP-centered shock equation-of-state (EOS) that is reasonable for lower-mantle materials in the Earth and Mars. We make several estimates of waste heat with increasing distance from the impact's isobaric core, using a range of peak-pressure decay laws, and also considering the effects of gravity: i.e., of increasing density and pressure with depth. The consequences of gravity are addressed by calculating re-centered Hugoniots and impedance match solutions for a shock wave propagating through a stack of layers whose densities and interface pressures increase with depth. The consequences of adding a chemically distinct upper-mantle layer with a different shock EOS is also considered. In order to complete our evaluation of mechanism (1) and in order to assess the merits of (2) and (3), we estimate the two-dimensional structure of thermal perturbations caused by large impacts, and add these to the temperature-field solutions of 2-D finite-element calculations of mantle convection. We then use this code to simulate the subsequent evolution. Our perturbations have the geometry of a squashed inverted hemisphere with a two-fold structure: (i) an inner hemisphere of partial melt; and (ii) an outer region characterized by the power-law decay of shock-related temperatures. The subsequent evolution may be summarized as follows: (a) the volume affected by the perturbation relaxes by rising and flattening; (b) the lateral motion of this relaxation locally stabilizes the mantle-lithosphere, inhibiting the initiation of downwellings and disrupting the local circulation pattern; and (c) the relaxation stalls and a new pattern is established. For the models considered, plume initiation by mechanism (3) appears to be far more important than (1) and (2). We find a range of Rayleigh numbers and impact energies for which the pattern reorganizes significantly, causing the extinction and initiation of plumes and even periods of chaotic global reorganization during which volcanism may intensify dramatically.
DE: 1213 Earth's interior--dynamics (8115, 8120)
DE: 5420 Impact phenomena (includes cratering)
DE: 6225 Mars
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8125 Evolution of the Earth
SC: Tectonophysics [T]
MN: 2003 Fall Meeting