Geology Reference
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in gravity load on plastic Miocene rocks by 23-25 MPa. Pore pressure in
these rocks (about 50-60 MPa) as they turned out at a depth of 1,500-2,000
m was much over the lithostatic pressure after the erosion. This caused
the emergence of quite significant AHPP gradients (about 23-25 MPa)
directed from the bottom upward.
Affected by these gradients, the load alleviation occurred in the com-
pressed Miocene-Paleogene clays similar to the load alleviation of the
ground at the bottom of construction pits. Initially, vertical motions of the
underlying plastic rocks caused the growth of local highs. That resulted
in their even deeper erosion. Subsequently, as the static equilibrium was
disrupted between AHPP in the elastic clay and lithostatic pressure of the
overburden, the overburden's integrity was broken by intense invasion of
the Miocene-Paleogene plastic clay into the dislocation of the overburden
(Pliocene).
The manifestation of this process was paroxysmal. It caused further sig-
nificant development of local highs and tectonic belts, deeper sediment
denudation, the formation of lengthwise and crosswise faults and diapir
folds, intense mud volcanism, etc.
Another mechanism of regional orogenesis is possible. This mecha-
nism is defined by cumulative relaxation of the internal energy resources
in clay sequences with AHPP. Thick undercompacted incompetent inter-
vals of deposits with high water-saturation and lowered density, when
overlain by a sequence of much denser competent deposits, are an ele-
ment of the geo-fluid-dynamic system with the Archimedes buoyancy
force. Such a system tends to reach energy equilibrium and mass redis-
tribution according to its bulk density. This situation can and must result
in the general inversion of folding. Its intensity should be proportionate
with the thickness of the clay series, the extent of its undercompaction
and the density difference between the clay and the overlying compe-
tent beds. In general, the correspondence of maximum upheaval in com-
petent rocks with maximum thickness of the underlying clay (i.e., with
troughs in the incompetent series or regional gravity minima) appears to
be mandatory.
Realistic nature of this mechanism is clearly illustrated in the South
Caspian and Irrawaddy-Andaman Depressions, Carpathian, Indolo-Kuban
and Tersk-Caspian Troughs. There, in the intervals positioned under a
thick (3-5 km) undercompacted clay Paleogene-Miocene sequence are
common high-amplitude rootless local highs forming long en echelon
anticlinal zones and belts (Koronovsky, 1987; Mekhtiyev, Grigoriants and
Buniat-Zadeh, 1987). Thus, thick uncompacted Paleogene-Miocene clay
