Geodynamics OF external LATERAL compression

and structurization of THE PrimorYE’S Paleozoic granitic mASSIFS

Nevolin P.L., Utkin V.P., Mitrokhin A.N.

Far East Geological Institute FEB RAS, Vladivostok, Russia, nevpeter@yandex.ru

The formation of intrusions being usually synchronous with tectonic and/or geodynamic activation is postulated to take place in the conditions of intensive (in comparison with inert periods) stress fields. It is enough to glance just over a geological map of the Primorye Territory so as to draw the conclusion that lengthening of the majority of granite massifs is concordant with strike of folding. At the same time, the massifs’ structural geometry (with rare exception) is not investigated and so the information about markings of external dynamic actions upon intrusion geometry is extremely small. Some of heterogeneities of the massifs are explained a priori by formation of the magmatic focus per ce, mostly reasoning from the point of view of direct or counter zonality relative to the focus or contacts. However, the world practice has adequately accumulated the information about inconsistency of real structural patterns of magmatic bodies with ideas of their focal self-development (Pons, Brun, 1979). This is also confirmed by our geostructrual research of the Cretaceous intrusions within the southern Prymorye (Nevolin et al., 2003).

The object of the present study, whose results are given below, is geostructural patterns for a number of the Paleozoic granite massifs sitting within south and west of Primorye.

The regular structural geometry is peculiar in full measure to the Tafuinsky massif of muscovite granites experiencing maximal quantity of tectonic impulses. The Tafuinsky granite massif is located on Livadiysky Peninsula (a northern piece of a shore of Peter the Great Bay). The massif consists of light grey, frequently pegmatoid granites. Their age is dated as Ordovician – 491-493 Ma (Khanchuk, 1993).

In addition to well-known structural methods we also used our in-house techniques to recognize asymmetry and vergence of folded forms. Geological maps, plans, and results of observations as well as dip direction measurements for structural elements were analyzed.

Four basic structural patterns forming architecture of the Tafuinsky granite massifs are fixed. The 1st pattern is presented by trajectories of protolith foliation; the 2nd one includes shadow granitic lineation. As both are not synchronised with active influence of external stresses, they are attributed by us to a passive type. The 3rd pattern is formed by configuration of granitic bodies’ contacts with protoskeleton relics, and the 4th one consists of aplite bodies of the second phase. Two last patterns are regarded as an active type because they were formed under the lateral compression being synchronous with the intrusion. Both pattern types are essentially characterized by the fold-shape structural outline consisting of the folds of four orders, which are genetically in relationship according to the experimental model of H. Ramberg (1963).

The active fold-related pattern is organized by systems of the conjugate shears, kinematically – counter-dipping thrusts of 4 orders, whose spatial combination forms fold-shaped polyhedrons. The genetic hierarchy in such systems of the conjugate shears consists in the point that, probably, further movements involve activation of the part of shears that falls into zones of tangential displacements. To put it differently, there was a tectonic stratification of host rocks along the entire system of the low-angle shears (thrusts), whose being under the further long compression caused their crumpling that was essentially similar to contorting of layer packet under a longitudinal compression. As a result, the system of fold-and-thrust forms of several orders was formed with storage even of the vergence type, i.e. it is available the shear analogue of longitudinal folding that significantly influenced on distribution of granitic substance along with replacement of the protoskeleton structures by it. Therefore, it is nonrandom the fact that orientation diagrams for elements of the passive and active patterns are similar.

In the same way the structures defining distribution of the aplites of the 2nd phase were formed. They are marks the next impulse of compression. Azimuthal reorientation of axes of normal compression paleostresses is recognized. There occurred two impulses of compression after the protoskeleton formation, as followed: syngranitic and synaplitic. Both active patterns are products of the NNE (10-15^o) longitudinal lateral compression (axes σ₁, and σ₂ are horizontal, σ₃ is vertical): the former resulted from ductile deformations, the latter arisen from brittle-and-ductile ones. There taken place the 10-15^о step-by-step turn of compression from syngranitic to synaplitic impulses, thus, the total turn of compression came to 20-30^о ckw.

The postgranitic deformations of two tectonic impulses taking place in Late Permian and Mesozoic, accordingly, are shear-and-fold related too. The Late Permian impulse (being recognized within the western Primorye (Utkin et al., 2003)), is characterized by a latitudinal compression. It wrinkled hinges of the preexisting WNW-trending folds into crossing folds with development of the NS-trending systems of counter-dipping thrusts. The Mesozoic impulse is characterized by dominance of the NS- and NNE-trending sinistral faults which was formed under the NNW lateral compression.

We also dwell slightly on other intrusive massifs. Being westerly contiguous with the Tafuinsky massif, the Annensky gabbroid massif was formed under the near-latitudinal compression. It doesn’t have signs of diversification. Nevertheless, because of intensive squeezing the gabbroids got definite the NS-trending foliation as well as taxitic structure, and there vividly occur also the meridional shear-and-fold related forms with horizontal hinges resulting from the same compression. At that, hinge parts of antiforms are filled by the Late Permian Rudnevsky pinkish granites. The Late Permian latitudinal compression resulted also in structural position and geometry of the Gamovsky granite intrusion keeping the distinctly ordered NS orientation of its protoskeleton relics that is similar with the Tafuinsky massif.

There are at least ten examples of the Silurian and Late Permian (mainly) as well as Cretaceous intrusions of the western part of Primorye, whose structures are studied by us with a different minuteness. We emphasize that all of them have the same structurally-geodynamic signs as the Tafuinsky massif. In other words, they were formed chiefly via replacement of host structural protoskeleton by endogenic substance without force influence on environment. Structurization of the intrusions was defined by external longitudinal compression.

References

Khanchuk A.I. Geological Structure and Development of a Continental Frame of the Northwest Pacific Ocean: Abstract of Doct. Thesis. Moscow: GIN RAS, 1993. 31p. (in Russian)

Nevolin P.L, Utkin V.P, Mitrohin A.N., Kovalenko S.V., Kutub-Zade T.K. Cretaceous Intrusions of the Southern Primorye: Tectonic Position, Structures, and Dynamics of their Formation // Tikhookeanskaya Geologiya. 2003. V. 22, No. 5. P.73-86. (in Russian)

Pons J., Brun J.P. Les Structures Internes des Granitoides: Tectonique et Ecoulement Magmatique? // 7e Reun. Annu. Sci. Terre, Lyon, 1979. Paris. 379p.

Ramberg H. Evolution of Drag Fold // Geol. Mag. 1963. V. 100, No 2.

Utkin V. P., Nevolin P. L., Mitrokhin A. N. Two Deformation Patterns at the Eastern Flank of the Jilin-Laoelin Fold System // Doklady Earth Sciences. 2003. V. 389, N 2. P.171-174.