One of the characteristic features of the geological structure of Kyrgyzstan is the presence of two major rock complexes: pre-Mesozoic and Meso-Cenozoic. The first is represented by diverse sedimentary, igneous, and metamorphic rocks that are intensely deformed with a very complex internal structure. The rocks of the second complex consist of weakly deformed, unmetamorphosed, predominantly continental sedimentary terrigenous strata.

The rocks of the lower complex form numerous ridges of the Tien Shan, while Meso-Cenozoic sediments fill intermountain depressions. Only the Fergana and Zaalaisky ridges are composed mainly of the rocks of the upper complex.

A. Pre-Mesozoic Structural Stage

Deep differences in the geological structure of areas where the rocks of the lower complex are developed made it possible to distinguish five large tectonic elements in the general structure of the Tien Shan: Northern Tien Shan, Middle Tien Shan, Southern Tien Shan, Northern Pamir, and the northern margin of the Tarim Platform. The Talas region, which has a geological structure distinct from neighboring areas, is often separated from the Northern Tien Shan. There is little in common among the listed regions, and therefore a brief geological description of each of them is given separately below.

Northern Tien Shan

The folded system of the Northern Tien Shan occupies the territory of the northern part of the republic (Kyrgyz, Talas, Susamyr, Jumgal, Zailiysky, Kungei, and Terskey ridges). Its northern border is located in Kazakhstan. Its southern border with the Middle Tien Shan is tectonic. It was identified by V.A. Nikolaev, who named it the Most Important Structural Line of the Tien Shan (Nikolaev's Line).

The oldest rocks are Lower Proterozoic. The lower part of the section is represented by garnet-potassium feldspar-muscovite gneisses, mica schists, marbles with lenses of garnet amphibolites and eclogites. The thickness of the rocks exceeds 3000 m. The composition of the primary rocks is unclear. Relations with younger rocks are tectonic. Higher-lying metamorphic strata are somewhat more widely developed throughout the region. Two varieties of rocks are distinguished among them. One of them is characterized by a stronger alteration of the substrate: gneisses, amphibolites, quartzites, crystalline schists, migmatites, marbles, green schists. The initial rocks were sedimentary and effusive strata. The second variety is represented only by primary sedimentary rocks: graphitic-mica and garnet-amphibole schists, marbles, hornfels, quartzites, migmatites. The total thickness of the rocks is many kilometers. Contacts are generally tectonic. The age of the rocks has not been finally clarified. Some strata may have a younger age.

Strata R1-2 are widespread in the Northern Tien Shan.

The section begins with a contrasting basalt-liparite formation R1 with a thickness of up to 2000 m: quartz porphyries, dacites, tuffs, porphyrites, amphibolites, diabases, conglomerates. Phyllites, schists, limestones, sandstones, siltstones, and quartzites R2 lie above. Their thickness is more than 7 km. The absence of R1-2 rocks in the Kemin region (between Lake Issyk-Kul and the Chui Valley) is noteworthy.

R3-V rocks are also represented by two varieties. In the Talas Ridge, their section is composed predominantly of phyllites, sandstones, siltstones, schists, and limestones with a thickness of 500–5000 m, and only in the upper part of the section are liparites, dacites, and porphyrites present with a thickness of 100–700 m. In the rest of the Northern Tien Shan, the main part of R3 rocks is a thick (1000–2500 m) stratum of basic effusives, pyroclasts, and cherts, and only Vendian deposits are represented by sandstones, siltstones, schists, and conglomerates with a thickness of 500 m.

Early Paleozoic rocks are distributed almost throughout the entire territory of the Northern Tien Shan. In the Talas region, they are represented by a stratum of tabular limestones and dolomites with lenticular black cherts epsilon-O2 up to 2000 m thick, lying unconformably on Proterozoic deposits. In the rest of the territory, the section of epsilon1-O3 rocks is composed of terrigenous-volcanogenic formations. But even here, in some areas, the lower (epsilon1-O2) effusive part is more developed, while in others, the upper (epsilon2-O3) terrigenous part is prominent. The base of the section is composed of diabases, porphyrites, spilites, tuffs with interlayers of jaspers, sandstones, schists, and limestones epsilon1 with a thickness of 800–3000 m. Sandstones, conglomerates, tuffs, porphyrites, and cherts epsilon2-O1 with a thickness of 1000–4600 m are located above. Even higher is a thick (several kilometers) stratum of greenish-gray sandstones, schists, and conglomerates with interlayers of tuffs and porphyrites O1-3.

A terrigenous-effusive stratum D1-3 lies with a sharp angular unconformity on PZ1 deposits. Its lower part is composed of andesites, dacites, andesite-basalts, tuffs, clayey and siliceous schists D1 with a thickness of 500–600 m. Liparites, dacites, felsites, and tuffs D1-2 (500–1500 m) lie upon them. Reddish sandstones D2 (100–300 m); trachyandesite and andesite-basalt porphyrites and their tuffs D2 (700–1100 m); basalts, andesites, tuffs D2 (1200 m); liparites, dacites, felsites, and their tuffs D2-3 (300–600 m) have been mapped higher up.

Reddish terrigenous formations D3-P lie with erosion and a slight unconformity on D1-3 volcanics or with a sharp angular unconformity on the Ordovician. They begin with reddish polymictic and arkosic sandstones with interlayers of conglomerates and schists D3-C1 with a thickness of 300–600 m. They are conformably replaced by conglomerates, sandstones, siltstones, with interlayers of limestones and tuffs C1-3 (1500–5000 m). The section is completed by trachybasalt and andesite porphyrites, dacites, tuffs, clayey schists, sandstones, and siltstones C3-P1 with a thickness of up to 1500 m.

There is no single section of the stratified strata listed above in the Northern Tien Shan. Separate parts of it with tectonic contacts are scattered throughout its territory.

Intrusive magmatism in the Northern Tien Shan is developed very intensely.

Its oldest representatives are plagiogranites, granites, granodiorites, and gneiss-granites R, developed in the Talas and on the western flanks of the Kyrgyz Ridge. They have not become widespread, and their genesis has not been deciphered.

Diorites, gabbro, gabbro-diorites, and serpentinized harzburgites of the Cambrian, which often form a chain of small lenses in the Susamyr, Jumgal, Djetymbel ridges, and the Karakatty Mountains or ribbon-like concordant deposits in Terskey-Alatoo, are closely associated with Lower Paleozoic terrigenous-volcanogenic strata.

Granodiorites, tonalites, and quartz monzonites O1-2, synchronous with the surrounding terrigenous-volcanogenic rocks, are more widely distributed. They form intrusive bodies up to 150 km long and up to 50 km wide in the Kyrgyz, Susamyr, and Terskey ridges. In the composition of many massifs, four phases of their formation are often distinguished: from gabbro and gabbro-diorites to granodiorites and granites.

However, O3-S granitoids, which form large batholiths up to 2500 km² in size, are most widely distributed in the Northern Tien Shan. All massifs are elongated in a sublatitudinal direction. The intrusions pierce folded Ordovician deposits, recording the collisional phase of Caledonian tectogenesis. Intrusive activity begins with diorites, quartz diorites, and through adamellites and granodiorites ends with granites, leucocratic granites, and even alkaline granites. The Silurian leucocratic phase is often mapped separately.

 

Hercynian intrusions are less developed. Small bodies of leucocratic, aplitic granites, and granophyres P are known in the eastern flank of the Chui Valley. At the eastern end of the Kyrgyz Ridge, there is the Ortotokoi massif of syenites, nepheline syenites, and granosyenites P with an area of about 200 km². Smaller similar massifs of alkaline rocks are present in the Jumgal and Susamyr ridges.

 

The general strikes of the Northern Tien Shan caledonides form an arc convex to the south. In the west of the republic, the predominant strikes of rocks and structures are north-western, in the central part they are sublatitudinal, and in the east, north-eastern. In the hypothetical vertical section of rocks, several structural stages can be distinguished: pre-Riphean (PR1), Issedonian (R1-2), Baikalian (R3-V), Caledonian (PZ1), epi-Caledonian (D-P), and Alpine (MZ-KZ).

However, normal spatial and temporal relationships between structural stages are rare. A block structure is ubiquitous, where laterally adjacent areas differ sharply from each other in rock composition and age. Such differences are already detected in the pre-Paleozoic basement. Thus, the Talas Ridge is composed only of sedimentary rocks with relatively simple folded forms, while medium and basic effusives with a more complex internal structure are widely represented in the rest of the area. But the area of the Northern Tien Shan is especially differentiated by the distribution of PZ1 formations. Based on which part of the PZ1 rock section (lower-effusive or upper-terrigenous) is located in one place or another, a multitude of tectonic zones and subzones were previously distinguished: early Caledonian, late Caledonian, eugeosynclinal, miogeosynclinal, geoanticlinal. According to their chemistry and composition, PZ1 terrigenous-volcanogenic rocks are typically island-arc. Therefore, from the standpoint of plate tectonic theory, the entire region now belongs to the Lower Paleozoic island-arc system. Within it, one can distinguish elements of volcanic arcs, inter-arc troughs, and accretionary prisms in complex tectonic relationships among themselves and with the sialic and simatic basement. In places, it is discovered that the PZ1 ophiolite complex is in an allochthonous occurrence (Karatty Mountains). Therefore, two options are possible for deciphering the geological position of the PZ1 island-arc strata: either they are in place of their formation as a result of subduction processes, only weakly deformed by subsequent folding and fault tectonics; or they are thrust (obducted), allochthonous, rootless, which explains the disordered spatial arrangement of various elements of the former island arc and the presence of limestones and shales of the passive continental margin under effusives and ultrabasites. The second option is more likely.

Middle-Upper Paleozoic deposits were formed in the process of tectonic activation (continental rifting) of the Caledonian folded basement. They also have a block structure; continuous sections are rare. Thrusting processes also affected these rocks. But they more often form folded structures with relatively calm rock occurrence.

Thus, pre-Paleozoic metamorphic and Lower Paleozoic island-arc sedimentary-volcanogenic formations are predominantly developed in the territory of the Northern Tien Shan. A subordinate role belongs to Middle-Upper Paleozoic volcanogenic and terrigenous rocks. Ordovician-Silurian granitoids occupy about half of the region's surface area. The main folding is Caledonian. The structure is mostly block-type. The Talas region, for example, can be considered as a separate structural element having nothing in common with the rest of the Northern Tien Shan. Sublatitudinal (longitudinal) steep faults are the most developed, but north-western and north-eastern (transverse) disturbances are not far behind in development. The role of Caledonian, Hercynian, and Alpine thrusts is obscured, but obviously great.

Middle Tien Shan

The folded system of the Middle Tien Shan can be traced in a sublatitudinal strip 20–100 km wide across the Central Tien Shan. It is divided into two isolated parts by the Talas-Fergana transverse fault: the Naryn (eastern) and Chatkal (western) parts. Its northern border is Nikolaev's Line, and its southern border is the Atbashi-Inylchek (in the Naryn sector) and Karasu (in the Chatkal sector) faults.

The oldest rocks are gneisses, crystalline schists, amphibolites, and marbles PR1? They form tectonic blocks, separating from younger rocks only by faults.

R-V deposits form large monoclines in the eastern part of the region. The base of the section is composed of arkosic sandstones, liparite porphyries, tuffs, tuff-sandstones with covers of porphyrites R with a thickness of 2000–2500 m. The upper part of the section is represented by tillite-like conglomerates with horizons of hematite and magnetite ores, variegated sandstones, schists, and siltstones V with a thickness of 750–3500 m. The lower and upper contacts are tectonic.

epsilon1-S1 rocks are distributed in both sectors of the Middle Tien Shan. Carbonaceous-siliceous and siliceous-clayey schists with interlayers of limestones epsilon-O2 (100–1600 m) begin the section. Variegated siliceous and clayey schists O1-2 (250–670 m) lie upon them, and higher up is a thick stratum of greenish-gray sandstones, schists, siltstones, conglomerates with interlayers of porphyrites (strata O2, O2-3, S1) – 100–7000 m. The presence of the Lower Silurian is proven only in the Chatkal region, while in the Naryn region, the section ends with the Upper Ordovician.

The volcanogenic section D1-2 is most developed in the western (Chatkal) sector. It is represented by andesites, dacites, liparites, tuffs, siltstones, sandstones, conglomerates (strata D1-2, D2?) with a thickness of 50–4500 m. Small tectonic blocks of these rocks are also observed in the eastern sector of the region.

Located higher is a stratum of terrigenous-carbonate rocks D2-C3. At the base are continental redbed gravelstones, sandstones, and siltstones D2-3 characteristic only of the Middle Tien Shan, lying with visible unconformity on the Ordovician. Thickness is 350–2000 m. The section is continued by a thick (3000–4000 m) stratum of limestones D3-C1, and then by shales, siltstones, sandstones, conglomerates with interlayers of limestones C1-3 – about 3000 m.

On the southwestern slope of the Chatkal Ridge, a peculiar stratum of andesite-dacite porphyrites with interlayers of sandstones, conglomerates, and limestones C2 with a thickness of up to 3100 m is developed.

Volcanogenic-terrigenous formations C3-P2 complete the section. In the Naryn sector, they are represented mainly by terrigenous rocks: sandstones, siltstones, conglomerates, variegated schists C3 (about 1000 m) and sandstones, clayey schists P1 with a thickness of more than 1000 m. In the Chatkal region, besides redbed sandstones, schists, and conglomerates C3, andesite, dacite, trachyandesite-basalt porphyrites, tuff-conglomerates, and lava-breccias P1 with a thickness of 600–3900 m, filling young grabens, are present in large volumes.

Intrusive activity in the Middle Tien Shan was on a more limited scale compared to the northern region.

In the extreme east (upper reaches of the Naryn River, Sarydjaz River basin) and west (Sandalash River basin), Late Proterozoic porphyritic granites and granodiorites, plagiogranites, and "globular" granites are developed. Intrusions of the Sarydjaz complex reach 100 km in length and are closely associated with PR1 metamorphic strata, while massifs in the Chatkal region form small tectonic blocks.

Silurian intrusions have received little distribution. They are composed of gneissic granites and granodiorites. A narrow, elongated massif of granites along the southern border of the Middle Tien Shan on its eastern flank and the Zeksai massif of granites and granodiorites in the Kassan River basin in the Chatkal region are attributed to them. Their age is still poorly substantiated.

Hercynian intrusions are more intensively developed. Among them, C2-3 granodiorites are predominantly distributed. The largest massif (300 km²) is located near Lake Son-Kul. A large number of smaller intrusive bodies of this complex crop out in the Chatkal and Sandalash ridges. Small bodies are identified in the southern flank of Nikolaev's Line. The intrusions pierce Carboniferous deposits and are composed of a colorful gamut of rocks: gabbro, monzonites, diorites, syenite-diorites, granodiorites, granosyenites, granites, granite-porphyries, divided into 4 phases.

Granites, granodiorites, and granosyenites C3-P1 complete the intrusive activity. Within this complex structure, C3 granites and granodiorites, P1 leucocratic granites and alaskites, and P syenites, monzonites, and granosyenites are distinguished.

It should be particularly emphasized the large scope of Upper Paleozoic intrusive activity on the southwestern flank of the Chatkal region, which sharply distinguishes it by this feature from the rest of the Middle Tien Shan area.

Structurally, the eastern and western sectors of the Middle Tien Shan differ from each other.

Folded structures in the Naryn sector have a predominantly latitudinal strike. Its eastern half is composed of almost all the structural-material complexes listed above, while the western one is composed predominantly of PZ3 deposits. The relationships between rock complexes are deciphered ambiguously.

One of the possible options is this. In the Akshyrak Ridge (the narrowest place of the Middle Tien Shan), its axial (middle) part is composed of PR1 metamorphic rocks. To the north and south of the metamorphics, on both slopes of this ridge, symmetrically relative to the oldest rocks, narrow tectonic strips stretch with interruptions, first of acid effusives and tillites R-V, then cherts and sandstones epsilon-O1, then limestones, schists, and sandstones D2-C3. The dips of the northern strips are to the south, and the southern ones to the north, i.e., under the ridge, forming a negative-form structure. Contacts are only thrust-type, gentle, well emphasized by the relief. As the axis of the fold rises in a western direction, strips of rocks of different ages and different facies (scales) of opposite limbs of the fold connect on the surface among themselves, forming a typical synform.

Each higher-lying tectonic scale is composed of older rocks. The lowest structural position is occupied by PZ3 terrigenous-carbonate rocks. While maintaining the uplift of the fold axis in the western direction, the uppermost-oldest rocks gradually disappear one after another from the surface, and in the western part of the Naryn sector of the Middle Tien Shan, only the rocks of the lower scale—limestones, sandstones, schists D2-C3—remain on the surface. After nappe formation (in PZ3), folding occurred, followed by repeated fracturing of structures into blocks by vertical faults.

Nikolaev's Line, which separates the Northern and Middle Tien Shan, is considered by most geologists to be ancient and long-lived (pre-Paleozoic). However, if we compare the Riphean, Vendian, Cambrian, Ordovician, and Silurian sedimentary, volcanogenic, and intrusive complexes located on both sides of the fault, it becomes clear that such sharply different rocks in terms of formation conditions could not simultaneously form next to each other, but were brought into contact much later than their origin. This fault is no older than PZ3 and is obviously post-ore with respect to deposits of gold and other minerals. The same conclusion can be made regarding the Atbashi-Inylchek fault.

The rocks of the Middle Tien Shan in composition and age are close to those of the Chinese Platform. And the entire Middle Tien Shan is a foreign structural element among the folded structures of the Northern and Southern Tien Shan.

In the Chatkal region, folded structures have a north-eastern strike, changing to south-eastern near the Talas-Fergana fault. Its north-eastern flank is composed predominantly of Paleozoic terrigenous and carbonate rocks, gathered into folds of NE direction and pierced by small intrusive bodies. The opposite (southwestern) part is more complexly arranged. The entire section of Middle Tien Shan rocks is present here: from the Proterozoic to the Permian. Upper Paleozoic magmatism is intensively manifested. The structure is fold-block: a complex mosaic of small tectonic blocks composed of various rocks and separated from each other by faults is observed. It is clear that this is not their primary position. The tectonic mechanism that led to such a complex relationship of small blocks is not entirely clear. Most likely, this is a consequence of the sequence of processes usual for the Tien Shan: nappe formation, folding, intrusive magmatism, vertical movements along a system of several steep faults, uneven denudation.

There is still no consensus on the Talas-Fergana fault—a major transverse structure cutting through the folded systems of the Middle and Southern Tien Shan. It stretches far to the southeast along the border of the Pamir with the Tarim Platform and further into the Himalayas parallel to the edge of the Indian Platform. This is clearly a right-lateral strike-slip fault with an amplitude of up to 250 km. Movements are also taking place at present. The depth of propagation according to geophysical data is 10–15 km. The time of its formation is unclear, but no earlier than the Upper Paleozoic.

Southern Tien Shan

The hercynides of the Southern Tien Shan are located south of the folded structures of the Middle Tien Shan. In the south, they border the folded system of the Northern Pamir (west of the Talas-Fergana fault) and the Tarim Platform (in the lower reaches of the Sarydjaz River).

The main role in the structure of this region belongs to PZ2 and PZ3 sedimentary and volcanogenic strata. Pre-Paleozoic rocks and intrusive bodies have a subordinate value. The main phase of folding is Upper Paleozoic.

The most characteristic feature of the geology of the Southern Tien Shan is the presence of several practically coeval, but sharply facially different rock sections, in the relationship of which lies the key to unraveling the internal structure of the region. Therefore, the description of stratified rocks is given below not by age, but by types of sections.

The Talas-Fergana fault divides the Southern Tien Shan into two folded systems: Turkestan-Alai (in the west) and Kokshaal (in the east), which have many common features, but also significant differences.

The Turkestan-Alai system has been better studied. At least eight types of pre-Mesozoic rock sections have developed on its territory.

The metamorphic (Kan, Mailisai) rock section R?+S-D2 is exposed in the southern and northeastern flanks of the Fergana Valley. It is composed of green sericite-epidote-chlorite-actinolite and siliceous-chlorite schists, cherts with interlayers of sandstones, limestones, horizons, and lenses of serpentinites. Thickness is up to 1200 m. Riphean age is assumed conditionally. Unmetamorphosed limestones, siliceous and clayey schists S-D2 with a thickness of up to 900 m conformably lie on the green schists, participating in folding together with phyllites.

The volcanogenic (Kirgizatin) type of S2-C1 section is represented by basalts, siliceous and clayey schists S2-D – 500 m, basalts D1-2 – 1500–3200 m, cherts with interlayers of limestones, effusives D2-C1 – 100 m.

The terrigenous-siliceous (Shalan) shortened type of S1-C2 section has a small thickness (first hundreds of meters) and consists of graptolitic siliceous and clayey schists of the Silurian, siliceous schists of the Devonian, limestones and siliceous schists of C1-2.

The limestone (Aktur, Yauruntuz) type of S2-C2 section consists of Ludlow limestones (100 m), organogenic limestones D1 (300 m), light gray limestones D2 (550 m), layered limestones D3 (up to 300 m), light massive limestones C1 (200 m), limestones, sandstones, schists C2 (100–300 m). The terrigenous (Chauvai) type of S1-C2 section is composed of variegated Llandovery schists – 400 m, quartzitic sandstones of the Wenlock – 0–250 m, siltstones, schists, Ludlow sandstones – 600 m, conglomerates, gravelstones, siltstones, Devonian sandstones – 0–1700 m, layered limestones with interlayers of clays and siliceous schists C1 – 0–400 m.

The limestone-dolomite (Alai) type of D2-C2 section is represented by black dolomites of the Givetian – up to 800 m, limestones and dolomites D2-3 – over 500 m, layered limestones and dolomites D3fr – over 500 m, dolomites D3fm – 600 m, dark limestones and dolomites C1t – over 300 m, massive limestones C1v – 300–400 m, layered limestones with lenses of cherts C1-2 – 100–200 m, mottled limestones C2b – 0–80 m, layered limestones C2m1 – up to 200 m, sandstones C2m1-2 – 50–300 m.

All the rocks listed above form narrow (1–3 km) latitudinal strips on the northern slope of the Turkestan-Alai, regularly alternating with each other. Contacts between them are only tectonic. In most cases, these are thrusts, the planes of which are concordant with the surrounding rocks. Today, few people doubt the nappe-imbricate structure of the region. The autochthon is the limestone-dolomite (Alai) rock section, which is overlain by thrust terrigenous deposits S1-C2. Limestones S2-C2 are thrust onto the latter, then the terrigenous-siliceous section S1-C1, basic effusives S2-C1, which in turn are hidden under Precambrian green schists. The presence of large (20 km across) tectonic klippen of Silurian sandstones and shales lying on Carboniferous limestones is faunistically substantiated in the Isfairam-Abshir interfluve (Tegermach nappe).

All tectonic scales are then conformably folded into large folds (antiforms and synforms) and broken by vertical (predominantly latitudinal) faults, as a result of which we observe the modern picture of the relationship of narrow tectonic strips of different facies rocks.

In the Kokshaal sector of the Southern Tien Shan, one can also see metamorphic (Atbashi), volcanogenic (Ortosui, Kainar, Tashrabat), limestone (Borkoldoi, Ulan) and terrigenous (Balyakty, Bedel, Maidantag) types of Middle Paleozoic rock sections very similar to those described above. They differ insignificantly from the Turkestan-Alai region (especially its southern flanks) in the composition and age of the constituent rocks. The distribution areas of various rock types have arc and strip-like shapes on the surface. Contacts are often thrust-type. Here, too, one can conclude about the wide development of nappe structures.

Undivided (in the attached scheme) complex strata of S2-C1 rocks in the Eastern Alai, Atbashi Ridge, and in the Sarydjaz River basin also reveal a nappe structure upon detailed study. Small scales of the same Middle Paleozoic terrigenous rocks, limestones, medium and basic effusives, cherts are crumpled together into folds and broken into small blocks by vertical Late Paleozoic faults.

Upper Paleozoic deposits C3-P1 of the Southern Tien Shan are distinguished by a predominantly terrigenous composition. They are represented by molasse-flyschoid sediments characterizing the orogenic stage of development of the hercynides: conglomerates, gravelstones, sandstones, siltstones, clayey schists with interlayers of limestones, acid effusives. The coloring is redbed. Thickness is up to several kilometers.

Intrusive activity in the Southern Tien Shan is not on a large scale. The oldest small ultrabasic bodies (serpentinized harzburgites) are closely associated with volcanogenic and metamorphic rocks. Due to their small size, they are not shown on the scheme.

Granitoid magmatism begins with quartz diorites, granodiorites, monzonites P, distributed only in the Turkestan-Alai. Two phases are distinguished in the complex: syenite-diorite-granodiorite and granodiorite-granite. The largest massif is the Kichikalai – 550 km².

Slightly later, granodiorites, granites, and alaskites P were emplaced. They received somewhat greater development in the eastern part of the region. These complexes were also formed in two phases: 1) granites, adamellites, granodiorites, granosyenites, and 2) leucocratic and aplitic granites. The largest massifs (Uchkoshkon, Akshyrak) crop out over an area of 200–300 km².

Intrusive magmatism ends with alkaline and nepheline syenites P-T. Complexly arranged intrusions form small (1–30 km²) bodies. They are more developed in the Turkestan-Alai sector, where they form a chain of alkaline massifs more than 150 km long along the southern border of the intrusive belt.

Thus, the Southern Tien Shan differs from the Northern and Middle ones in that it is composed of several coeval (Middle Paleozoic) rock types formed in geological environments from mid-ocean ridges to a passive continental margin. The processes of nappe tectonics and intensive Hercynian orogenesis are clearly manifested here.

Northern Pamir

Within Kyrgyzstan, in the Zaalaisky Ridge, a small part of the Northern Pamir hercynides is located, in the structure of which two rock complexes take part.

The sedimentary-volcanogenic rock stratum C1 is composed at the bottom of basalts, spilites with lenses of limestones, schists. Its middle part is represented by tuff-breccias, tuffs, less often by lavas of andesite-basalts. In the upper part of the stratum, dacites, less often rhyolites, appear. The thickness of the stratum is 1500 m.

The orogenic complex P lies with angular unconformity on Carboniferous rocks and is represented by sandstones, shales, siltstones, and conglomerates. Thickness is about 2000 m.

Due to severe mining and technical conditions, the structures of the Northern Pamir are poorly studied.

Northern Margin of the Tarim Platform

In the lower reaches of the Sarydjaz River, the marginal part of the Tarim Platform is located. Its basement is composed of intensely deformed Proterozoic metamorphic rocks and granitoids. In the cover, there are gently dipping unmetamorphosed limestones and dolomites O-O with a thickness of 300–1300 m; sandstones, argillites S-D with a thickness of 250–600 m; redbed sandstones C1 with a thickness of 300–400 m; C2 limestones; shales and limestones C3-P1. The border with the Southern Tien Shan runs along the Kipchak fault, expressed by a powerful crushing zone.

Summarizing the foregoing, the following main conclusions can be drawn. The territory of the Kyrgyz Tien Shan in the geological sense consists of several sharply different large mega-blocks of pre-Mesozoic rocks. Each block has its own complex history of development. According to paleomagnetic research data, some rocks began to form even in the southern latitudes of the planet. The closure of the blocks obviously occurred at the beginning of the Mesozoic. The practical absence of Triassic deposits may indicate major tectonic reorganizations at this time.

B. Alpine Structural Stage

The rocks of this stage unite the geological mega-blocks described above. They include two complexes: the subplatform T-P and the newest orogenic N-Q.

The subplatform T-P complex is developed mainly to the west of the Talas-Fergana fault. It is composed of the continental coal-bearing stratum T-J, the redbed terrigenous formation K1, freshwater-marine terrigenous-limestone deposits K2, and lagoon-marine variegated terrigenous-carbonate gypsum-bearing deposits P. The total thickness reaches 5 km.

The orogenic N-Q complex began to form in intermountain and intramountain depressions from the Late Oligocene, in most cases directly on the Paleozoic basement. It is represented by continental molasse. The thickness reaches several kilometers. The process of its formation continues to this day. The amplitude of vertical orogenic movements along steep faults reaches 10–15 km.