date

 

30/09/2009

 

 

Yakovlev Fedor

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to first page *: yak@ifz.ru

 

 

FOLDING formation research

 

Guide:

 

about the study of folded structures of several scale levels into thin-layered flysch-like sediments in a Hinterland

II.                SEPARATE FOLDS AND AN ESTIMATION OF STRAIN VALUE

 

5. Fold morphology classification based on mechanical properties of layering (rank 2).

Description. The type of fold morphology and the sizes of the structures depend on the mechanical properties of flysch-like deposits. The most important parameter is the ratio of the competent layer thickness (of sandstone, for example) to the total thickness of the flysch rhythm (slate or shale plus sandstone). Based on this aspect, the four kinds of similar type folds are offered.

1. If this ratio is smaller then ~0.1, the larger folds have gentle shape of relatively wide hinges (the width is larger then 10-100 thicknesses of the rhythm) and relatively short flanks. However, the lesser-level folds are SINGLE VISCOUS LAYER FOLDS (SVLF, this is first type), which are mainly formed exactly in the hinge space of larger folds (Fig. 1.). The especial methods based on mechanisms depended model are used for measurement of shortening value.

2. Usual MULTILAYER FOLDS are formed in case, when the competent layer thickness is ~0.3-0.7 of the rhythm thickness. The folds do not possess gentle shapes of wide hinges (their width is smaller then ~10 thicknesses of the rhythm) and relatively long flanks. If the slates and sandstones have a cleavage, the fan of the cleavage is opened up for competent layers in anticlines and opened down for incompetent layers (Fig. 2.). The especial methods based on mechanisms depended model are used for measurement of shortening value.

3. CHEVRON-TYPE FOLDS are formed in cases when competent layers occupy more then ~0.8-0.9 of the rhythm thickness and if the viscosity contrast of the layers is high. The folds possess sharp shape of hinges (~1.0 thicknesses of rhythm and smaller) and the longest straight flanks. A usual situation is when diagenetic cracks in a competent layer being exactly perpendicular to bedding, and the cleavage/shistosity of slates being almost parallel to the bedding. Thrust-type sliding of competent layers in an incompetent layer can also take place (Fig. 3.). Because almost pure buckling (rotation of flanks) is active, the dip angle of flank regarding of axial plain may be used for measurement of shortening value.

4. If slates and sandstones have no cleavage, and the viscosity contrast of layers is low (~2 and smaller), the especial "STACK" FOLDS (or COUNTER THRUSTS FOLDS) are forming due to numerous counter thrusts on the opposite flanks of the folds. The space between two neighbored thrusts can have 1 to 10 rhythms. The folds have sharp shape of hinges (~1 - 2 thicknesses of the layer between neighboring thrusts) and long flanks. The necessary increase of the thickness of layering in the hinge ("similar" h/H ratio, class 2) exists due to doubling of inter-thrust units (Fig. 4.).Because almost pure buckling (rotation of flanks) is active, the dip angle of flank regarding of axial plain may be used for measurement of shortening value.

 

It is necessary to note, that usual types of folds which are described in main text-books are related to Foreland structure. There are large simple parallel types. Similar dependence between shape of folds (similar type?) and relation of thicknesses of competent/incompetent layers was described briefly in ([Ramsay, Huber, 1987] p. 413).

 

Publications. Full text in Russian was published in (Yakovlev F.L. Quantitative methods of analysis of natural formations mechanisms for folds and for systems of linear folding // Problems of tectonophysics. To fortieth anniversary of foundation by M.Gzovsky the Tectonophysics Laboratory in the Institute of physics of the Earth RAS // Moscow. Publishing of IPE RAS. 2008. pp. 149 188. [in Russian] PDF-R N 32 in List-En). Previously this information was announced in (Yakovlev, F. Similar folds in theory and in nature the comparison of two models by their application to the study of hinterland folds of Greater Caucasus. http://www.cosis.net/abstracts/EGU2007/05391/EGU2007-J-05391-1.pdf - PDF; N 32 in List-En). Full information exists in the poster (SlovTec-08) also. (see download materials)

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