As usual, the most important parameter is the one rarely recorded in the rocks: strain rate. The boundary between litho- and asthenospheres is often defined as the (gradational?) transition between elastic and plastic behavior, on the timescale of interest (so not necessarily the same depth for post-glacial rebound and horizontal plate motion.)

I always enjoy reading student essays on this topic in my tectonics course.

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On 2011-08-29, at 11:32 PM, Nick Timms <[log in to unmask]> wrote:

Hello

'The brittle-ductile transition is not a place'!

I think that it is well established that 'brittle' and 'ductile' behaviors are rate- and state-dependent, and, to some extent, dependent on the scale of observation. So, after I carefully define the concepts of brittle and ductile deformation to undergraduate students and give examples of each,  I tell them (with moderate emphasis) that 'the brittle-ductile transition is not a place'. The 10-15 km depth frequently quoted in undergraduate textbooks is a gross simplification, and that in reality the variety of rock types, strain rates, strain partitioning, fluids, etc., need to be considered.

In more detailed lectures, I find it useful to summarize some of the good work that has been done over the years to try and quantify the boundary conditions of brittle and ductile behavior of geological materials, and acknowledge their assumptions, merits and limitations. As always, I am prepared to be corrected.

Regards,

Nick

Dr Nick Timms

Curtin Applied Geology

Perth, Western Australia

http://www.geology.curtin.edu.au

On 30/08/2011, at 6:57 AM, "Malcolm McClure" <[log in to unmask]> wrote:

Hello Scott

It seems likely that the brittle-ductile transition occurs at the depth at which the combination of temperature and pressure is sufficient for interstitial water of various compositions (salinity etc) to become super-critical.

I have not, obviosly, performed the experiments necessary to prove this, as they could be extremely dangerous.

Some of the deeper geothermal wells in Iceland have approached the supercritical water threshold, so far without disaster.

Cheers

-Malcolm

On 29 Aug 2011, at 17

On 29 Aug 2011, at 17:56, Scott T. Marshall wrote:

Hello Geo-Tectonics Folks,

I have run across a conundrum of sorts concerning my understanding of the lithosphere, asthenosphere, and the brittle-ductile transition. I hope that folks on this discussion list can be of help. Hopefully, I haven't said something foolish in here...

In my intro geology course here at Appalachian State University, I teach about the layers of the Earth (crust, mantle, and core). I also discuss the lithosphere and asthenosphere and the brittle-ductile transition. I define the lithosphere as the layer of the earth that undergoes brittle behavior and the asthenosphere as the layer that undergoes ductile flow. Later on in the course I talk about the LVZ and stuff like that, but in the beginning, I just want them to realize that there is more than one way to subdivide the layers of the earth and that the tectonic plates are lithospheric not crustal.

I then talk about the brittle-ductile transition because this marks the approximate depth of the base of seismicity (and the strongest portion of the lithosphere/crust) and it therefore controls the magnitude potential of a given active tectonic region. E.g. subduction zones can make the biggest earthquakes because they have the largest potential rupture area. I think this is a useful concept to teach intro students because I often hear the students saying things like..."I heard that the next earthquake in California could be a M9.5 or more."

This is of course not possible (unless we are way off in our understanding of basic earthquake physics). I think that teaching about the brittle-ductile transition is therefore useful, even for intro-level students. The problem is that, as I have defined lithosphere, the entire lithosphere should all undergo brittle failure.

So, here is my question (Finally!): If the brittle ductile transition lies within the lithosphere, does this not contradict the definition of lithosphere? How do folks on this list define these terms to intro-level students? Should I define lithosphere/asthenosphere as Fowler's text does (i.e. non-convecting vs. convecting)?

Any thoughts or advice list members may have on this would be very enlightening.

Cheers,

-Scott

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<><><><><><><><><><><><><><><>

Scott T. Marshall

Department Of Geology

Appalachian State University

572 Rivers St.

Boone, NC 28608

http://www.appstate.edu/~marshallst/

ftp://pm.appstate.edu/pub/prog/marshallst/