Crustal-scale boudinage and migmatization of gneiss duringtheir exhumation in the UHP Province of Wester - sci.geo.geology

Message from discussion Crustal-scale boudinage and migmatization of gneiss duringtheir exhumation in the UHP Province of Wester

Bigdakine

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More options Jul 21 2003, 6:17 pm

Newsgroups: sci.geo.geology

From: bigdak...@aol.comGetaGrip (Bigdakine)

Date: 21 Jul 2003 08:13:35 GMT

Local: Mon, Jul 21 2003 6:13 pm

Subject: Re: Crustal-scale boudinage and migmatization of gneiss duringtheir exhumation in the UHP Province of Wester

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>Subject: Re: Crustal-scale boudinage and migmatization of gneiss duringtheir
>exhumation in the UHP Province of Wester
>From: Peter Halls p...@york.ac.uk
>Date: 7/20/03 8:36 PM Hawaiian Standard Time
>Message-id: <Pine.SGI.3.95L.1030721073552.31205I-100...@peters.york.ac.uk>

>Thanks, George, for some real geology on this group ...

>On Sat, 19 Jul 2003, George wrote:

>> I just thought some of you would find this article interesting.

>> --George

>http://216.239.33.104/search?q=cache:SWRyM7fOdxwJ:www.fys.uio.no/pgp/...

etal.pdf+boudinage&hl=en&ie=UTF-8

>> A B S T R A C T

>> A field study in the coesite province, the deepest unit of the Norwegian
>> Caledonides, gives new constraints on the rheological behaviour of the
>> continental crust during exhumation. Lithological heterogeneities and
>> differential retrogression led to crustal-scale boudinage during the
>> late-orogenic intense E-W stretching event in the footwall of the
>> Nordfjord-Sogn Detachment. The main gneissic lithologies display a modest
>> but wide-spread syn-exhumation migmatization. Textural criteria allow
>> estimation of a 30% fusion rate. Partial melting mostly post-dates
>> eclogitization and is synchronous with ductile stretching and top-to-west
>> shearing. Presented observations suggest that the melt reactions and
>> migmatization resulted in a soft rheology. During subduction to ~ 100 km
>> depth and subsequent exhumation, crustal viscosity can be reduced by up to
>> four orders of magnitude. Models are discussed that consider a transition
>> from a small internal strain of the crust to viscous flow during
>exhumation.

I've become fairly interested in such terranes as of late. I find Ultra high
pressure complexes quite curious. The Dabie Shan province in China not only has
Coesite, but diamonds as well.

That requires depths of burial of around 130-160km or so; twice the the
thickness of the Himalayas.

My thinking is that requires unusual subduction. About 10 years ago, there was
a lot of interest in the possibility of phase change induced *mantle
avalanches* where subducted material would remain trapped in the transition
zone, until a sufficient quantity accumulated and overcame the blocking effects
of the 660km discontinutiy. When that occurred, subduction would switch to high
gear. The problem is, there was no geological evidence, or smoking gun for
such an event. However, I suspect, that these ultra high pressure terranes, may
indeed be just that. Remnants of thick crust produced by anomalously vigorous
subduction.

Stuart
Dr. Stuart A. Weinstein
Ewa Beach Institute of Tectonics
"To err is human, but to really foul things up
requires a creationist"

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