Geology Reference
In-Depth Information
stress on clays from the overburden causes their local horizontal-vertical
motion accompanied by neotectonic manifestations. On the other hand,
the emerging local tectonic stress per se serves as an additional factor in
the consolidation (compaction) of clay sequences.
Spatial interconnection and interdependence between the stated phe-
nomena results in sequential involvement in the geodynamic process of
the adjacent areas. That, in turn, by way of sequential (in time and direc-
tion) crushing of incompetent clay sequence into various folds (over-
turned, harmonica-like, disharmonious, etc.), converts into a regional
effect.
Thus, the nappe and diapir folding, cleavage, mud volcanism and
intense faulting of local highs are found exclusively in zones of regional
development of thick unconsolidated (uncompacted) clay sequences
with AHPP at the lithostatic level. That reveals one of the major probable
causes for a much more complex structure of the mobile belt regions and
individual structures within them compared with the platform ones. The
platform regions are typical in their limited development of clay facies
in their geologic section and for this reason in the domination of plica-
tive dislocations, low dip angles, low manifestation of faulting, relatively
unstressed folding, etc.
The conceptual review of the discussed issues began way back by
I.M.Gubkin. He noticed substantial role in the formation of diapir folds
over the southeastern plunge of the Caucasus Major of the so-called “ver-
tical pressure” in the Miocene-Paleogene plastic clay rocks. This pressure
caused their invasion into the dislocations of the Middle Pliocene over-
burden and the formation of “special tectonic forms”. The concepts were
further developed by Mirchink, Khodkov, Khain, Agabekov, Hubbert,
Laubscher, Gretener, Berry and many others (Agabekov, 1963; Durmishyan,
Muradian and Rachinsky, 1982; Mirchink, 1935; Khain, 1950; Khodkov,
1968; Gretener, 1969; Gretener-372, Hubbert and Rubey, 1959).
Following is a view of terms of reference, adequate models, schematics
and the mechanism of the described processes. Severe restrictions (and
sometimes total absence) of the pore water outflow from the compact-
ing clays, beside generating AHPP, causes another exceptionally impor-
tant feature - their plasticity accompanied in some cases by fluidity. Lab
experiments and field observations showed that clay rocks, albeit quite
brittle in the subsurface under normal conditions, develop plasticity under
abnormally high pressure. This property is caused not only by a high pore
pressure whose role in the formation of plasticity is well known. To a sub-
stantial extent, it is caused by abnormally high content of pore water. The
pore water is a reason why the clay rocks acquire the capacity of restoring
