Ice-Age Cycling Enhancement of Volcanism and Geothermal Heat Flux: A Stress Modeling Study

Open Access
Author:
Stevens, Nathan Thomas
Graduate Program:
Geosciences
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
November 16, 2015
Committee Members:
  • Richard B Alley, Thesis Advisor
  • Byron Richard Parizek, Thesis Advisor
  • Sridhar Anandakrishnan, Thesis Advisor
Keywords:
  • Greenland
  • Isostasy
  • Flexure
  • Stresses
  • Ice-Age Cycling
  • Ice Sheet
  • Heat Flux Enhancement
  • Numerical Modeling
Abstract:
Borehole and geophysically inferred geothermal heat fluxes beneath the Greenland Ice Sheet are in some places much higher than suggested by current knowledge of the underlying geology, particularly at the head of the Northeast Greenland Ice Stream. Geologically rapid changes in lithospheric loading during ice-sheet growth and decay produce large changes in the effective stress state beneath and near the ice sheet. Cyclic loading will cause oscillating melt volume in deep rocks, and the nonlinear increase of melt-migration velocity with melt fraction means that extended ice-age cycling likely enhances upward melt migration over several cycles. Computationally efficient simulations resolving flexural stresses from ice-sheet/lithosphere interactions result in widespread occurrence of stresses sufficient to allow dike emplacement across all reasonable ranges of parameters in this system. Modeled stresses in the shallow crust tend to be compressive beneath an ice sheet and tensile in the forebulge, with opposite-sign anomalies beneath. The stress patterns migrate laterally with ice-sheet growth and shrinkage, with magnitudes that depend somewhat on the rate of ice-sheet change, as well as on the ice-sheet thickness and on focusing from variations in bedrock topography and crustal thickness. Magma could move upward through dike emplacement in regions of strongly tensile stresses, and the migration of the stress pattern suggests the possibility of multi-step transport upward, thereby transporting heat closer to the base of the ice sheet. Large shear stresses are modeled some places extending through most of the crustal thickness; vug waves or other shear-related transport mechanisms thus may allow magma migration from deep in the crust to near the surface. Fracturing from flexural stresses might also increase upper-crustal permeability, promoting groundwater flow that also moves heat upwards. These results are consistent with the hypothesis that the regions of anomalously high geothermal flux beneath Greenland’s ice sheet arose from ice-age cycling of the Greenland ice sheet, enhancing melt extraction from a deep source and emplacement near or at the base of the ice sheet, with correspondingly enhanced geothermal flux and possibly volcanism. The pre-glacial passage of the Iceland-Jan Mayen hotspot may have provided melt beneath Greenland, and subsequent load cycling helped move it upwards, significantly impacting heat flow.