Esci 203, Earth Structure and Deformation

Heat flow and faulting (2)

John Townend

EQC Fellow in Seismic Studies

john.townend@vuw.ac.nz

Outline

• Recap on the last lecture– Conductive heat flow and the heat flow equation– Measuring surface heat flow– The brittle-ductile transition

• Characteristic times

• Interpreting heat flow maps

• Inferring paleoclimates from heat flow data

• Shear heating and the San Andreas paradox

Shallow geotherm

Photo: D.L. Homer

Temp. gradient: 63 ± 2°C km–1

Heat flow: 158 ± 69 mW m–2

Characteristic time/length scales

• When’s breakfast?

L

Characteristic time, t = L2/

So, if the rock is 20 cm thick, and =10–6 m2 s–1, then it takes

~40,000 s (or ~11 hours) for substantial heat to be

conducted through the slab

Age of oceanic lithosphere

Muller et al., 2008. Geo3

Ocean depth is related to the age of oceanic lithosphere, which cools and sinks as it propagates from a MOR

dkm ~ 2.6 + 0.36tMyr

Why?• We can relate water depth and age (√t) using

models of cooling– Half-space model (lithospheric thickness defined

by temperature)– Plate model (lithospheric thickness specified)

• The models differ in their boundary conditions

Thermal structure Density structure Elevation below sea-level

Assuming

isostatic eqbm.

Kelvin’s model of the Earth

• Assumptions:– The Earth is flat– The Earth’s surface

temperature has always been 0°C

– The Earth’s interior temperature was initially 4000°C

• Answer:– 20–400 Ma, with a

final preference for ~24 Ma

020406080

100120140

0 100 200 300 400 500

Time since the Earth formed (Ma)

Geo

ther

mal

gra

die

nt

(°C

km

–1)

It never rains but it pours...

• “[T]he inexorable physicist [has] remorselessly struck slice after slice for his allowance of geological time.”

— Sir Archibald Geikie, 1892

• “[T]he geologist who ten years ago was embarrassed by the shortness of time allowed to him for the evolution of the earth’s crust is now still more embarrassed by the superabundance with which he is confronted.”

— Arthur Holmes, Nature, 1913

Blackwell, D. D., and Richards, M. 2004. Geothermal Map of North America. American Assoc. Petroleum Geologist (AAPG), 1 sheet, scale 1:6,500,000.

Australian heat flow provinces

Beardsmore and Cull, 2001

Heat flow from seismic data

Paleoclimate research

• Changes in temperature at the ground surface propagate downwards over time

• If that’s the case, then we should be able to deduce what ground surface temperature changes have occurred in the past using borehole measurements of temperature vs. depth

• This is called an inverse problem

Background

Temperature fluctuations of longer period propagate to

greater depth

The depth at which the amplitude of the perturbation

is 0.37the value at the surface is known as the “skin

depth”

If P is the period of the perturbation, then the skin

depth d is

P

d ~Pollack and Huang, 2008

Borehole temperature profiles

The task is to relate T(z,t=0) to T(z=0,t)Pollack and Huang, 2008

Undisturbed background geotherms

Perturbed shallow sections

A global compilationP

olla

ck a

nd H

uang

, 20

08

Analysis of global borehole

measurements yields a record that can be

used to extend instrumental records

back in time

Shear heating

• Just like when you rub your hands together, rocks sliding past each other along a fault are frictionally heated

• The amount of heat generated depends on:– How fast the fault is slipping

– How much frictional stress is resisting slip

Do big faults generate much

heat?• The San Andreas slips at

20–30 mm/yr and we’d expect it to generate substantial shear heating ... unless the frictional stresses were low

• So, can we measure a temperature anomaly across the San Andreas fault?

The source of the controversy

Fulton et al., GRL, 2004

Is there some heating we’re

missing?Perhaps shallow groundwater flow

washes out the shear heating signal

But, most plausible groundwater

scenarios mean that we should see shear heating if it’s there

Fulton et al., GRL, 2004

The San Andreas Fault Observatory at Depth

(SAFOD)

IODP Expedition 343Japan Trench Fast Drilling Project (JFAST)

Science team• 28 scientists from 10 countries (Japan, US, UK, Canada,

Germany, France, Italy, China, India, NZ)• Geologists, geophysicists, geochemists, one microbiologist

Suggested reading material

• Fowler (2004)– Chapter 7, particularly §7.1, (7.2–7.3), 7.5.1, 7.8

• Mussett and Khan– Chapter 17, particularly §17.1, 17.2, 17.4

• Beardsmore and Cull (2001)– Any or all of chapters 1–3

• Turcotte and Schubert (1982)– Section 4.1

Not on

reserve (see me)