Topographically Driven Groundwater Flow Through A Heterogeneous Permeability Subsurface: Implications for Surface Heat Flow Near Parkfield, California and in the Western Mojave Desert

Open Access
Author:
Popek, Margaret Ann
Graduate Program:
Geosciences
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
November 13, 2009
Committee Members:
  • Demian Saffer, Thesis Advisor
Keywords:
  • heat flow
  • California
  • groundwater flow
  • tectonics
Abstract:
Surface heat flow in the California Coast Ranges near Parkfield, CA exhibits substantial scatter, with differences as large 20 mW/m^2 over lateral distances of 5-70 km. In contrast, surface heat flow in many other parts of the California dataset displays only an ~ 10 mW/m^2 range. This scatter in surface heat flow near Parkfield has been an important limitation on interpretations of geodynamic processes, but to date has not been explained. Here, I use a numerical model of coupled fluid and heat transport to test the hypothesis that heat advection by groundwater flow through an upper crust characterized by heterogeneous permeability can generate the magnitude and spatial characteristics of the scatter in the Parkfield heat flow dataset. I also compare surface heat flow near Parkfield and in the well-studied and hydrogeologically simple western Mojave Desert to investigate relationships governing fluid and heat transport in complex geologic terrains. I find that the characteristics of the heat flow scatter near Parkfield can be generated if the Tertiary sediments that comprise the upper 2-3 km of the crust are characterized by permeability ranging from 3×10^-16 m^2 to 10^-15 m^2, allowing recharge of ~ 0.5 cm/yr or higher. Simulated surface heat flow is not sensitive to basement permeability, over a range of realistic depth-dependent permeability functions. Additionally, enhanced permeability resulting from a San Andreas Fault zone and permeability anisotropy in the Tertiary sediments both have a minimal impact on simulated surface heat flow. Although topographically driven groundwater flow through a heterogeneous permeability crust can generate the characteristics of the heat flow scatter near Parkfield, low recharge rates estimated for four springs in the Coast Ranges suggest that the permeabilities and groundwater fluxes required to cause significant advection may not be prevalent on a regional scale. In contrast, the lack of a significant topographic driving force in the western Mojave Desert results in nearly constant heat flow even with sediment permeability as high as 10^-13 m^2, which is consistent with the low degree of heat flow scatter observed in that area. Lastly, although not the focus of this study, I demonstrate that for a wide range of reasonable permeability architectures in the upper crust, topographically-driven groundwater flow would not mask a thermal anomaly associated with frictional heating on the San Andreas Fault and also generate the observed scatter in heat flow.