Heat Flow

Rob Harris

Oregon State University

I am interested in understanding the energy budget associated with geologic processes.  I think this understanding leads to better insights into how the Earth works.  For example, plate tectonics – the creation, motion, and destruction of plates – reflects Earth cooling.  About 70% of the Earth’s heat loss occurs through the ocean floor which is reflected by the cooling and subsidence of oceanic plates as they move away from spreading centers.  The upper layer of these plates, the oceanic crust, is cooled efficiently by hydrothermal circulation.  It turns out that the entire volume of the global ocean circulates through the oceanic crust every few hundred thousand years.  This hydrothermal circulation is important because it leads to significant exchanges of energy, mass, and solutes between the ocean and crust.  These exchanges modify the chemistry of the ocean, the chemical and physical properties of the oceanic crust, and support a globally significant biosphere.

At many subduction zones, temperatures along the subduction thrust fault appear to play a role in governing the depth extent of seismicity.  Earthquakes represent the release of stress stored in rocks, but once rocks exceed a certain temperature (~350° C) they are too weak to store the elastic stresses required for an earthquake.  It turns out that the deeper an earthquake can nucleate the bigger it can be, so estimating the position of the 350° C isotherm is important for both understanding geologic risks due to earthquakes and also the physics of how earthquakes work.

To estimate temperature along the subduction thrust, we need to know the thermal state of the Pacific plate before it subducts beneath the Australian plate, the geometry of the fault zone, and the long-terms rates at which these plates are moving past each other.  The geometry of the fault zone is illuminated by seismic reflection data that we are collecting, combined with previously collected data.  Deeper in the earth the fault zone is illuminated by earthquakes.  The long-term rate at which these plates move past each other is estimated from geodetic data.  Finally, the thermal state of the incoming Pacific plate is estimated using the heat flow measurements we are collecting.  All of these data are necessary inputs for developing a thermal model used to estimate temperatures along the plate boundary.

F1_SubductionZone

Cross section of a subduction thrust.

 

A second interesting aspect of this system is the Australian margin that overrides the Pacific plate and how it changes along the length of the Hikurangi trench. In the northern region it appears that most of the sediment on the incoming plate is carried down into the subduction zone whereas at the southern portion of the margin some of the sediment is scraped off the incoming plate and accreted to the Australian margin.  Fluid flowing through these sediments plays an important role in their evolution, structure, and deformation, the occurrence of overpressures and also influences the nature of seismicity along the subduction thrust.  Heat flow measurements like those that we are making can be used to detect and trace the flow of fluids.

It’s very exciting to have the opportunity to investigate these questions and to be sailing on the R/V Roger Revelle.  Without ships like these, important science that we and others are doing wouldn’t be possible.

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One thought on “Heat Flow

  1. Hi Rob – Thanks for the informative blog. Nice to hear how you are all doing at sea and the science that goes with it. See you in Napier! Anne S.

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