FINITE NEUTRAL SURFACE
The neutral surface can be incremental or infinitesimal when established soon after the inception of folding process and may change its orientation as the folding Progresses. Thus through a series of surfaces of no incremental longitudinal surfaces
F
the neutral or finite neutral surface or surface of no finite longitudinal
strain is formed. Along the neutral surface the original length of the layer
prior to folding is maintained. Most of the folds formed by tangential
longitudinal strain depict either subclass 1A or 1B geometry. Normally
tangential longitudinal strain does not operate alone but most buckle folds
are formed by a combination of this process and flexural or interlayer slip.
Since the outer arcs suffer extension, tensile cracks are commonly formed at
the outer arcs (see Fig). Since the inner arcs are compressed, thrusts are
formed in the initial stage of buckling which give rise to high angle reverse
faults in advanced stages of folding.
Normally it is found that in naturally developed folds the neutral surface
shifts towards the inner are boundary. This implies that progressive fold
development is accompanied by greater addition of strain at the outer than at
the inner arc. Needless to say, if the position of neutral surface in the
natural fold can be exactly located, the original length before deformation
can be easily computed. But this requires detailed strain analysis through the
buckled layer. If homogeneous layer shortening precedes the buckling by tangential
longitudinal strain, then no neutral surface can exist in such a fold since
the material was already strained by layer shortening prior to buckling. In
these layer shortened strata, the effect of tangential longitudinal strain is
to reduce the principal strain ratio at the outer arc and increase it at the
inner arc. The internal deformation in this case is partly by slip and partly
by tangential longitudinal strain. The strain at the outer arc being
extensional, it slightly compensates for the earlier compressional effect of
layer Shortening.
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