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date 30/09/2009 |
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Yakovlev
Fedor Home-page |
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
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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. |
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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 // |
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© 2009, ÈÔÇ ÐÀÍ, ßêîâëåâ |