>> Date: Mon, 26 Oct 1998 14:34:19 +0200
>> Subject: sheath folds
>> From: JOHANNES VAN BEVER DONKER <[log in to unmask]>
>> To: [log in to unmask]
>> Reply-to: [log in to unmask]
>
>> Dear All,
>>
>> I am looking for some help with the following problem. We
>> are dealing with a sequence of calc silicate-rich
>> quartzites, overlain by at least a 100 m thick pack of
>> crossbedded quartzo-feldspathic sandstones. Deformation
>> resulted in an isoclinal fold, adjacent to large, dome-like
>> structures. Total length of the feature is about 50km along
>> the axes and about 20 km across! The shape of the entire
>> unit resambles that of a sheath fold. Sheath folds have
>> been encountered elsewhere in the area, always in the
>> calc-silicates and near clearly identifyable shearzones, but
>> of the "normal" size of up to about 1 - 2 metres. We use
>> them as indicators for the direction of movement in such
>> cases. Although it looks like a sheath, I have a problem
>> with the rheology and the size. I have not found any large
>> scale sheath folds in the literature and wonder whether
>> there is any one who can point me in the right direction?
>>
>> Thanks
>>
>> Jan van Bever Donker
>>
Jan,
See Oliver GJH (1994) Mid-crustal detachment and domes ete...... Jl Af
Earth Sci v19, pp331-344.
Oliver GJH (1995) The Central Zone of the Damara Orogen, Namibia,
as a deep metamorphic core complex. Communs geol. Surv. Namibia 10, 33-41.
Oliver GJH & Kinnaird (1996) The Rossing-SJ Dome, Damara Belt,
Namibia.....ete. Communs geol. Surv. Namibia. 11, 53-64.
These domes are km scale sheaths
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Grahame Oliver, Crustal Geodynamics Group, School of Geography and
Geosciences, Purdie Building, The University, St Andrews, Fife, Scotland,
KY16 9ST.
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Home address: 5, St Leonards Rd., St Andrews, Fife, Scotland, KY16 9ST
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