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Wanket, Sean

University of British Columbia

The rate of change of orbitally-driven variations in continental ice mass modulates volcanism by inducing variations in the compressional (lithostatic) stress regime of the wall rock surrounding hot magma reservoirs. At West Antarctica, as crustal temperatures rise in response to the delivery and storage of melt since 36 Ma, and to the insulating effects of a thick West Antarctic Ice Sheet emplaced ∼34 Ma, the effective viscosity of wall rocks decreases. Pressures in excess of lithostatic related to the injection of magma are increasingly relieved through wall rock creep at the expense of volcanism and stress changes imparted by orbitally-forced deglaciation are consequently reduced in magnitude and delayed in time. Theoretically, maximally rapid creep and a negligible volcanic response to orbital forcing corresponds to a time lag of 1/4 of the period of the glacial forcing. Clusters of tephra layers near Mount Berlin corresponding to peaks in the rate of change of volcanism occur at eight and 110 ka, but only the eight ka cluster lags a maximum in the rate of change of deglaciation by less than 1/4 of an obliquity-driven glacial cycle (∼6 kyr), suggesting the crust underwent a rheological transition before eight ka. Using a 1D numerical model of heat diffusion we study the effects of ice sheet insulation and time-dependent magma supply on the viscoelastic response of the crust to glacial cycles. Long-term ice insulation increases the crust’s proclivity for viscous creep by increasing the temperature by ∼50 K. However, a time-dependent magma supply causes the crust to alternately cool and warm. Through the exponential dependence of crustal viscosity on temperature, thermal oscillations and long-term insulation force the crust into transient behavior in which magma is preferentially stored during warmer intervals and erupted during cooler periods. We argue that the change in the response of the crust between 110 and eight ka is a consequence of such oscillations. Our data analysis applied with inferred geothermal heat fluxes and peak magma production rates in Marie Byrd Land constrain the magnitude and period of oscillation for mantle magma production to 0.01-0.1 km³/year and ∼10⁶-10⁸ years, respectively.

Text

2024-04-18

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/87880

Geophysics

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/