ISSN 00168521, Geotectonics, 2010, Vol. 44, No. 1, pp. 45–59. © Pleiades Publishing, Inc., 2010. Original Russian Text © S.D. Sokolov, K.A. Krylov, 2010, published in Geotektonika, 2010, Vol. 44, No. 1, pp. 52–68.
Structured Serpentinite Melanges of the Koryak–Kamchatka Fold System S. D. Sokolov and K. A. Krylov Geological Institute, Russian Academy of Sciences, Pyzhevskii per. 7, Moscow, 119017 Russia email:
[email protected] Received May 21, 2009
Abstract—The Kuyul, Taigonos, and Upper Khatyrka structured serpentinite melanges at the Pacific conti nental margin of Northeast Asia considered in this paper are diverse in tectonic setting, age, and geological history. They are characterized by an ordered internal structure expressed in regular arrangement of rock blocks different in composition. These blocks make up mappable tectonic sheets deformed into complex, napperelated folds. Difference in the block composition of particular sheets is often combined with different composition of serpentinite cement. The structured melanges of Cape Povorotny (Taigonos Peninsula) and the Kuyul Terrane were formed during accretion of oceanic complexes to a suprasubduction zone and their subsequent juxtaposition. The Upper Khatyrka melange comprises oceanic, islandarc, and marginalsea complexes. The study of structured melanges makes it possible to ascertain the history of continental accre tion and transformation of the oceanic crust. The combination of ordering and chaotic structure allows us to consider structured melanges as manifestations of nonlinear geodynamics. DOI: 10.1134/S0016852110010048
regular alternation of melange sheets consisting of fragments differing in composition. Melanges with ordered structure were called structured melanges [29]. It was noted that their formation cannot be explained only by disintegration of the ophiolitic sec tion; thrusting and folding contributed to their origin as well.
INTRODUCTION Among the various chaotic geological complexes, tectonic melanges are perhaps the best studied. Melange is a mappable body of rocks composed of foliated, finegrained, commonly pelitic groundmass with incorporated local and exotic fragments, blocks, or sheets; such a body may be a few kilometers in extent and fragments are centimetersized [14]. Most authors have emphasized the chaotic structure of melanges and the homogeneity of their matrices [2, 10, 22, 24, 33, 35, 37, 40]. Melanges differ in com position of cement (terrigenous, serpentinite, etc.), vari ety of inclusions (mono and polymictic), and genesis (accretionary, subductionrelated, protrusive, etc.). Tectonic melanges are a characteristic element of foldbelts often associated with ophiolites. Serpentinite melanges are commonly regarded as products of disin tegration of ophiolitic sections. Indeed, the develop ment of ophiolitic allochthons is accompanied by tec tonic delamination into sheets of ultramafic rocks, gabbroids, dike, and volcanosedimentary complexes [10, 22, 24, 31]. Melanges are formed at the bottom of allochthons and at the boundaries of separate sheets. The ability of serpentinite to plastic flow ensures the development of largescale protrusive phenomena. Sub sequent folding and faulting facilitate stirring and mixing of rocks with the formation of chaotic structure. The detailed mapping of serpentinite melanges in accretionary structural elements of the Koryak High land allowed us to reveal the elements of their compo sitional and structural ordering [7, 9, 29] expressed in
The most typical examples of the structured melanges widespread in the Koryak–Kamchatka Fold System are considered in this paper to attract attention to the necessity of further revealing and studying them in other regions. The Kuyul serpentinite melange characterized in detail in [29] is regarded as a tectono type. Its brief description is given below. THE KUYUL MELANGE Geology. The Kuyul ophiolitic terrane is located at the junction of the Mesozoides of the Russian Northeast and the Koryak–Kamchatka Fold System (Fig. 1). Ophiolites crop out as a tract extending for more than 140 km and having a width of up to 20 km. According to the results of satellite magnetic survey, this tract extends further to the northeast for more than 100 km. In the southeast, ophiolites are overlapped by Paleo gene volcanic and sedimentary rocks of the Koryak– Kamchatka volcanic belt and Neogene–Quaternary sediments of the Parapol’sky Depression and border in the northwest on the Upper Jurassic and Lower Creta ceous flyschoid terrigenous rocks of the Ainyn Ter rane. Along with other terranes of the Penzhina dis 45
46
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Fig. 1. Tectonic scheme of the Koryak Highland. (1) Paleogene–Quaternary cover; (2–6) terranes with predominance of the fol lowing complexes: (2) islandarc and marginalsea, (3) ophiolitic, (4) oceanic, (5) accretionary wedge, (6) terrigenous; (7) com posite terranes with complexes pertaining to various geodynamic settings; (8) faults: (a) strikeslip and normal and (b) thrust; (9) studied area of the Emravaam–Pikas’vayam Nappe System (Fig. 9). Terranes (abbreviations in Figure): AV, Al’katvaam; AL, Algan; AM, Ainyn–Main; VL, Velikorechensky; GA, Ganychalan; KU, Kuyul; MA, Mainits; OL, Olyutor; UB, UstBelaya; UK, Ukelayat; EK, Ekonai; YA, Yanranai.
trict, they are constituents of the MidCretaceous accretionary continental margin of Asia [25, 26]. The Kyyul Terrane is composed of ophiolites, which are strongly disintegrated and transformed into polymictic serpentinite melange [1, 2, 9, 16, 25, 29, 33, 41] consisting of fragments of ultramafic rocks, gabbro, dike complex, plagiogranite, basalt, lime stone, cherty, tuffaceous terrigenous, and metamor phic rocks (greenschist, amphibolite). At least three types of ophiolites—suprasubduc tion, oceanic, and withinplate—are combined in the Kuyul Terrane [2, 16, 29, 33]. They occur as extended mappable strips; blocks of corresponding composition are contained in the serpentinite cement (Fig. 2). The internal structure of the blocks, their spatial position, and their relationships with other blocks and the
matrix show that these strips are actually tectonic sheets disintegrated to varying extents [29]. Ophiolites of the Gankuvayam sheet belong to the first type. The most complete section is composed of mantle peridotite (harzburgite), gabbro–troctolite– wehrlite cumulative complex and isotropic gabbro, plagiogranite, complex of parallel dikes, and pilllow lavas. The Bathonian–Callovian and Tithonian radi olarians were determined in chert segregations among pillow lavas [6]. The recently determined zircon age of amphibole–pyroxene pegmatite (157.0 ± 4.0 Ma) and plagiogranite (146.3 ± 3.7 Ma) [12] supports the Late Jurassic age of suprasubduction ophiolites. It was commonly accepted that the Kuyul serpentinite melange was formed owing to disintegration of the Gankuvayam section [1, 2, 13]. Subsequent study of the blocks contained in the melange has shown, how GEOTEKTONICS
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А Talovka R.
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Fig. 2. Sheets of serpentinite melange in the Talovka basin. (1–4) Sheets of serpentinite melange of the Kuyul Terrane: (1) Vesely sheet with blocks of basalt, limestone, and chert (Triassic–Middle Jurassic); (2) Udachny sheet with amphibolite and greenschist; (3) Tylpyn tykhlavayam sheet with blocks of siltstone and claystone, tuffaceous silicite, and olistostromes; (4) Vstrechny sheet with blocks of basalt and chert; (5, 6) Ainyn Terrane: (5) turbidites (Upper Jurassic–Lower Cretaceous), (6) metamorphic schists; (7) Neogene–Quaternary sediments; (8) tectonic contact; (9) folds.
ever, that the observed variety of mafic–ultramafic rocks and basaltic–cherty complex cannot be deriva tives of a single ophiolitic section [9, 29, 33]. Oceanic ophiolites are separated into particular tectonic sheets composed of ultramafic rocks and gab bro (Unnavayam sheet) and Kingiveemtype basalts and cherts (Vstrechny and Vesely sheets). Mantle peri dotites are less depleted in comparison with the Gankuvayam rocks and are similar to oceanic ultramfics in the composition of minerals [29]. The Vesely sheet consists of rocks pertaining to the basalt– limestone–chert association; the Vstrechny sheet con tains rocks of the basalt–chert association (Fig. 3) formed in spreading centers. In composition of basalts and character of sedimentary rocks, these are oceanic GEOTEKTONICS
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rocks, which substantially differ from the basalt– andesite–dacite series of the Gankuvayam sheet [29, 33]. Permian (?), Middle Triassic–Bajocian, Bathonian–Callovian, and early Tithonian radiolari ans and conodonts are contained in the limestones and platy cherts [9, 19]. Melange of the Talovka sheet is referred to the third group, where the least depleted mantle peridotites are associated with withinplate basalts. These ophiolites were arbitrarily classified as of the withinplate type [29]. In addition to ophiolitic sheets, the Udachny sheet of serpentinite melange with amphibolite and green schist blocks and the Tylpyntykhlavayam sheet of accretionary terrigenous melange with blocks of tuf faceous terrigenous and cherty rocks were identified.
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Fig. 3. Oceanic complexes of the Kuyul melange. (1) Basalt, (2) limestone, (3) chert, (4) basalt with chert lenses and interbeds.
The nature of the metamorphic rocks remains unknown; the rocks of the Tylpyntykhlavayam sheet accumulated in the deepwater trench or at a rise of the island arc [9, 29]. Internal structure. In the character of the spatial distribution of varisized blocks, the Kuyul serpentinite melange is typically chaotic, as is clearly seen from the geological map [27, Fig. 3]. At the same time, certain regularity is observed in the distribution of blocks hav ing the same composition (Fig. 2). This implies that the melange consists of tectonic sheets of disintegrated rocks formed in specific geodynamic settings (Table). The sheets are deformed into complexly built folds and complicated by strikeslip faults. The folds are seen within blocks (Fig. 4) and are restored from an arrangement of blocks with variable dip azimuths and
angles (Fig. 5) and the occurrence of blocks with nor mal and overturned bedding [29, Fig. 4]. The leftlateral strikeslip faults with offsets mea sured in tens of kilometers are oriented parallel to the terrane strike, whereas the rightlateral strikeslip faults with offsets of a few kilometers in length are ori ented across the strike of the terrane [34]. It is not a simple task to furnish direct evidence for strike slip faulting in serpentinite melange; however, the Sshaped blocks observed in some places are oriented in line with longitudinal leftlateral strikeslip faulting. Repeated deformations were accompanied by plas tic flow of serpentinite matrix and increasing disinte gration of fragments of ophiolitic sections. As a result, the outer appearance of general chaotic structure arises so that the internal ordering has escaped the attention of geologists for a long time. In addition, the material pertaining to different sheets mixed in the narrow segments of ophiolitic terranes due to late deformations and protrusive phenomena. The block diagram shown in Fig. 6 illustrates the ordered internal structure, which is characterized by a rather regular spatial distribution of blocks formed in the same geodynamic setting and mapped as separate tectonic sheets. Tectonic interpretation. The formation of struc tured melange of the Kuyul Terrane has a complex and long history. In addition to disintegration of ophiolitic sections, the spatial juxtaposition of ophiolites differ ent in their origin and age was crucial. Oceanic ophio lites were formed in spreading centers of the Paleo and Mesopacific situated at southern latitudes [19, 25, 39]. Withinplate ophiolites were formed later during traveling of the oceanic lithosphere. The Gankuvayam ophiolites arose at the subduction zone in the Batho nian–early Tithonian. As follows from the paleobio geographic data, they were localized at northern lati tudes [19], however, their transfer for a long distance cannot be ruled out. Unfortunately, the paleolatitude remains undetermined by paleomagnetic methods due to the subsequent remagnetization [39]. The subduction of the oceanic plate at the time of the origin of the Gankuvayam ophiolitic sheet was accompanied by tectonic delamination of oceanic
Lithotectonic units of the Kuyul Terrane Tectonic sheet Gankuvayam Vesely Vstrechny Unnavayam Talovka Tylpyntykhlavayam Udachnaya
Rocks
Age and geodynamic setting
Peridotite, cumulate, gabbro, plagiogranite, dike complex, and basalt–andesite–dacite association Basalt, limestone, and chert Basalt and chert Peridotite, cumulate, and gabbro Peridotite, gabbro, and basalt Sandstone, siltstone, claystone, chert, olistostrome, and terrigenous melange Amphibolite, greenschist, and glaucophane schist
Middle–Late Jurassic, 157 and 146 Ma (zircon) suprasubduction Permian–Middle Jurassic oceanic Middle Triassic–Late Jurassic oceanic Mesozoic oceanic ? Withinplate Late Jurassic–Early Cretaceous oceanic trench 139 and 92 Ma (Rb/Sr); 134, 122, and 82 Ma (K/Ar) subduction
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NW
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tone Limes
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Fig. 4. Isoclinal folds in calcareous cherty rocks of the Vesely sheet. Photo by K.A. Krylov.
ophiolites and the formation of melange sheets during accretion and underplating. This stage of accretion corresponded to the time of accumulation of rocks pertaining to the Tylpyntykhlavayam Complex (Oxfordian–Berriasian ?). The lower age limit is con strained by the age of the youngest (early Tithonian) accreted fragments of the oceanic crust. In the Early Cretaceous, the sheets of the Kuyul serpentinite melange together with the Gankuvayam ophiolites were already incorporated into the Ainyn accretionary wedge at the front of the Uda–Murgal island arc [22, 26]. The ultimate accretion of the Kuyul Terrane to the continental margin took place at the end of the Early Cretaceous. Afterwards, in the late Albian, the extended Andeantype Okhotsk– Chukotka volcanic belt arose along the Asian margin. The structural rearrangement of the convergent boundary of the northwestern Pacific and Asian conti nent coincides in time with the global Austrian oro genic phase [11]. SERPENTINITE MELANGE OF CAPE POVOROTNY Geology. The islandarc complexes widespread in the Taigonos Peninsula were formed along the conver gent boundary of the northwestern Pacific and Asian continent [15, 18, 25, 27, 32]. Three islandarc sys tems of different ages are distinguished in the tectonic history of the interaction between these plates: the Late Paleozoic–Early Mesozoic Koni–Taigonos, the Late Jurassic–Early Cretaceous Uda–Murgal, and the Okhotsk–Chukotka marginal continental volca nic belt. A lateral series—volcanic arc–forearc zone– accretionary wedge—is reconstructed for the Late Jurassic and Early Cretaceous [27, 28]. The structural elements of the accretionary wedge are retained along the southern coast of the Taigonos Penin GEOTEKTONICS
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Fig. 5. Panorama of the Kuyul melange. Photo by K.A. Krylov. In the foreground, a block of the Vesely sheet (from bottom to top): basalt (turfcovered slope), limestone (light), and chert (dark). On the distant slope, blocks are incorporated into serpentinite melange. The variable dip angles outline a synform with gently dipping southeastern limb.
sula. They were studied in detail on Cape Povorotny, where the diverse and complexly built rock association comprises serpentinite melanges, islandarc volcanic and terrigenous complexes, oceanic cherts, as well as turbid ites and olistostromes [5, 28, 41]. The largest outcrops of melange are located in the central part of the accretionary wedge shown in the map as the Main serpentinite melange (Fig. 7) that separates the islandarc complexes in the north from the volcanic–cherty rock association and turbidites, which make up a system of accreted tectonic sheets in the south of Cape Povorotny. The Kingiveem 1, 3, and 4 sheets are composed of basalts and varicolored cherty and clayey rocks [5, 28]. The sheets have imbricate internal structure. Duplex type detachments along the chert members and tight, often overturned, and southeastverging folds are characteristic. The cherts correspond to the strati graphic interval from the Upper Triassic to the Titho nian and Berriasian. The geochemistry of the cherts indicates that they were formed in an oceanic basin. The appearance of terrigenous material in the upper part of the section testifies to their subsequent conver gence with the continental margin. In the sheet of polymictic melange, coherent (undisturbed) turbid ites are combined with units of terrigenous melange. The Bajocian–early Callovian radiolarians are con tained in cherty and clayey interbeds. The Lagerny Complex and the Vitaetgl Sequence are exposed to the north of the Main serpentinite melange (Fig. 7). The Lagerny Complex is a system of tectonic slices composed of volcanic and terrigenous rocks and mixtite (olistostrome association). The vol canic rocks correspond to boninite and basalt in com position [5, 28, 36, 41]. Terrigenous rocks comprise volcanomictic sand stone, siltstone, and claystone. Turbidites, lavas, lava
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Fig. 6. Block diagram of the Kuyul serpentinite melange, after [26]. Tectonic sheets composed of melange: (1) Ganguvayam sheet with blocks and fragments of section of suprasubduction ophiolites; (2) Vesely sheet with blocks of oceanic basalt, limestone, and chert; (3) Udachny sheet with blocks of amphibolite and greenschist; (4) Unnavayam sheet with blocks of oceanic ultramafic rocks and gabbro; (5) Vstrechny sheet with blocks of oceanic basalt and chert; (6) Tylpyntykhlavayam sheet with blocks of terrig enous rocks and olistostromes formed along oceanic trench.
breccias, and tuffs occur as local units. Finds of mac rofauna (Kimmeridgian–Valanginian) and radiolari ans (Tithonian–Hauterivian) cover a rather wide stratigraphic interval. The mixtite association includes an olistostrome sequence with Kingiveemtype basaltic and cherty olistoliths, Lagernytype volcanosedimentary rocks, amphibolite, greenschist, marmorized limestone, ultramafic rocks, gabbro, and plagiogranite. The Vitaetgl Sequence is composed of volcanomic tic and tuffaceous turbidites, which accumulalated at the front of the island arc. The faunal finds indicate the Hauterivian–Valanginian age of this sequence [15]. Internal structure. In the composition of inclu sions, the Main Serpentinite melange is referred to the polymictic type [5, 36]. A certain ordering in the block distribution is established. The Lagernytype blocks of volcanic and terrigenous rocks, as well as harzburgite (Coastal massif) and plagiogranite, are concentrated in the northern part of the belt. The blocks of the Kingi veemtype basalt–chert association, the large Mount Greben lherzolite massif, and metamorphic rocks domi nate along the southern flank of the melange. Two geochemical type of peridotites—suprasubduction depleted harzburgite and slightly depleted llherzolite, which are considered to be oceanic [7, 41] or supra subduction [4]—are distinguished. Amphibolite and gabbroamphibolite blocks with isotopic age of 121.6–139.0 Ma are established in the central part of the melange [15]. The geochemical sig natures and relics of primary structure indicate that they are metamorphosed plutonic and volcanic rocks of the oceanic crust. The depth of their formation is estimated
at 20 km and the temperature at 420–800°C in agree ment with the model of “warm” subduction [41]. At first glance, such a distribution of blocks differ ing in composition would be explained by incorpora tion of surrounding rocks in the melange and by expo sure of the most deepseated rocks in the central part of the melange. The occurrence of blocks of the Lagerny Complex in the central part of the melange (Fig. 7) is inconsistent with this suggestion. In addi tion, blocks of metamorphic rocks appear at the con tact with the Kingiveemtype rocks and amphibolite and plagiogranite are unknown among the surround ing rocks. The detailed mapping of separate blocks allows us to suppose that they are disintegrated tectonic sheets. The Kingiveemtype rocks are spatially related to out crops of lherzolite, whereas the Lagernytype rocks, to harzburgite. Unfortunately, it is difficult to determine the initial composition of the ultramafic rocks from the foliated serpentinite matrix and small serpentinite inclusions. Moreover, as in the Kuyul melange, pro trusive phenomena and fluidity of the serpentinite matrix result in mixing of ultramafic material. The sheets of tuffaceous sedimentary rocks belong ing to the Lagerny Sequence and the location of melanges with apoharzburgite and apolherzolite matrices are shown in Fig. 8. The extended narrow eastern sheet with steep dip angles (60–80°) is traced for a few kilometers. To the southwest, this sheet gives way along the strike to a large, gently dipping synform. In the northeastern direction, the sheet is broken down into a series of small blocks that imparts chaotic appearance to the melange. GEOTEKTONICS
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The character of the block contacts, their internal structure, the direction of foliation, and the composi tion of the matrix indicate the possible development of folding (Fig. 7). It is important to note that the sequence of deformations and the spatial orientation of the compression and extension axes in the Kingi veemtype sheets and serpentinite melanges are simi lar and indicate that they are constituents of the same structural element [5]. In addition, the broken forma tion, blockinmatrix, and other textures typical of accretiontype terrigenous melanges are noted in the mixtite association and polymictic flysch [28]. Tectonic interpretation. The complex system of tectonic sheets and slices exposed on Cape Povorotny may be interpreted as a deformed accretionary wedge [28] which comprises mixtite association and turbid ites; accreted fragments of ensimatic arc and oceanic crust; and serpentinite melanges with exhumed high pressure metamorphic rocks. The complex of the aforementioned rocks was probably formed according to the following scenario. The Kingiveemtype rocks and lherzolites indicate formation of the oceanic lithosphere. Judging from the paleomagnetic data, it was the Izanagi Plate [3, 38]. The Lagerny ensimatic arc is suggested to arise within this plate and at the boundary of the Izanagi and Far allon plates [3]. Another subduction zone plunged beneath the Asian continent with the formation of the Uda–Murgal islandarc system. The accretionary structural assembly comprising the oceanic and islandarc complexes was formed along the continen tal margin. After cessation of subduction, the accre tion wedge was deformed with exhumation of high pressure complexes [41]. The appearance of serpentinite melanges distin guishes ancient accretionary complexes from their presentday counterparts, which are well studied by geophysical methods and deepsea drilling. The appearance of melanges and deepseated rocks was caused by late deformations of older wedges during collision and accretion events and the subsequent exhumation. As a result, the accretionary wedges could have been deformed to such an extent that their typical structural attributes as a system of synthetic thrust sheets and slices with characteristic younger age of lower sheets in comparison with the upper sheets would disappear. The geological map of Cape Pov orotny with the field of the Main serpentinite melange and smaller melange zones penetrating and separating the lithothectonic complexes (units) of the oceanic crust and the ensimatic arc illustrates a “chilled” stage of the transformation of the accretionary wedge into serpentinite melange. While looking at the map, one can readily imagine that more intense deformation could have led to merging of serpentinite melange into a common field, where disintegrated sheets would be transformed into separate blocks. In this regard, the melange of Cape Povorotny may be compared with the GEOTEKTONICS
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Kuyul melange, where deformation and destruction of the accretionary wedge advanced further and com pleted with the formation of melange. THE UPPER KHATYRKA MELANGE Geology. The Upper Khatyrka melange is localized in the central Koryak Highland at the headwater of the Khatyrka River. This is a separate tectonic sheet of the allochthonous Emravaam–Pikas’vayam Nappe Sys tem [7, 25, 35]. The tract of Maastrichtian–Paleogene rocks, which most likely continues the Al’katvaam Terrane, bounds this system in the north. In the south, lithotectonic complexes of the nappe system are thrust over the flysch sequence of the Ukelayat Terrane (Fig. 9). The localization of the Upper Khatyrka melange at the boundary of the large tectonic units of the Koryak Highland and the insufficient exploration of this region compel us to dwell on its geology and the main lithotectonic complexes. The Emravaam–Pikas’vayam Nappe System has a complex structure (Fig. 9). In its southern segment, the Temdynyn terrigenous–cherty–volcanic sequence is the lower structural element [7, 25]. This sequence consists of volcanomictic and polymictic sandstones with members of basalts and cherts. From radiolari ans, its age is determined as Oxfordian–Neocomian. The basalts correspond to oceanic tholeiites in geochemistry and are correlated to the Lower Chirynai sheet and the upper sheet of the Yanranai Terrane of the accretionary wedge [8, 25]. The similar ity of the Temdynyn Sequence with the complexes of the Yanranai wedge is confirmed by findings of cherts with Santonian–Maastrichtian radiolarians in the section to the south of the region under consideration (personal communication of Vishnevskaya, 2001), where they make up a system of southward verging sheets. In contrast to the Yanranai wedge, this territory is not mapped in the detail necessary to elucidate the relationships between terrigenous rocks with basalts and radiolarites and to establish the number and suc cession of tectonic sheets. In the northern segment, volcanic, cherty, and ter rigenous rocks of the Middle–Upper Triassic and the Kimmeridgian–Valanginian tuffaceous, cherty, and terrigenous rocks (variegated sequence) are the lower structural element (relative autochthon) [7, 25]. The lower part of the Triassic sequence consists of volca nomictic sandstone with an admixture of tephroid material. The sandshale members contain thin inter beds and lenses of cherts. The upper part of the section is composed of phtanites with interlayers of light mas sive flint up to 0.7 m thick, sandstone, siltstone, and claystone. Small tectonic wedges of ultramafic rocks and gabbro, as well as intrusive plagiogranite bodies are spatially associated with the Triassic rocks; strongly altered volcanics are noted as well. The Trias sic rocks are deformed into isoclinal and recumbent folds overturned to the northwest. This complex can
52
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Main serpentinite melange
Vitaetg
l B ay
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yC
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icti c
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Fig. 7. Geological map of Cape Povorotny. (1) Lherzolite; (2) harzburgite; (3) gabbro; (4) serpentinite melange with (2a) apol herzolite and (b) apoharzburgite matrices; (5) metamorphic rocks: (a) greenschist and (b) amphibolite; (6, 7) Kingiveemtype com plexes: (6) basaltic–cherty association and (7) basaltic–cherty–terrigenous association; (8) polymictic turbidite; (9–11) Lagernytype complexes: (9) boninite and basalt, (10) terrigenous rocks, (11) mixtite; (12) tuffaceous and volcanomictic turbidites (Vitaetgl forearc com plex); (13) plagiogranite; (14) gabbro; (15) axes of (a) antiforms and (b) synforms; (16) Fig. 8. KN, Kingiveem sheets 1–4.
not be compared with any sequence in the adjacent districts. The set of rocks allows us to suggest their pos sible similarity with the islandarc complexes, includ ing the lavas, tuffs, and volcanomictic clastic rocks of the Ekonai Terrane [25] or the Lozovsky Sequence of the Mainits Terrane [30]. Sandstone and siltstone dominate in the lower part of the variegated sequence, whereas siltstone with sandstone, tuff, and tuffite interbeds prevail in the upper part. Cherts occur as interlayer and members up to tens of meters thick. Some of them are autochtho nous and others, especially associated with volcanic rocks, have tectonic contacts with terrigenous rocks. In the western part of the subautochthon, the Emravaam Sequence (Fig. 7) is readily mapped and interpreted in aerial photographs owing to the light color of outcrops. The relationship with the underly ing variegated sequence and its subautochthonous attitude remains rather unclear. As in most previously prepared maps, stratigraphic contact is shown in Fig. 9, although tectonic contacts were observed in outcrops [7]. The sequence is composed mainly of vol canomictic sandstone with numerous silty and clayey fragments and biotite flakes. The age of the sequences is accepted to be Albian on the basis of a single find of radiolarians in the interlayer of black chert. The allochthonous complex consists of three nappes. The lower and upper nappes are composed of Upper Jurassic–Lower Cretaceous volcanic, cherty, and terrigenous rocks, which were previously corre lated with the Chirynai Group of the Mainits Terrane [35]. Along with the general lithologic similarity of both sheets, basalts from the lower sheet are oceanic tholeiites, whereas islandarc tholeiites occur in the upper sheet [7]. The composition of the volcanic rocks of the lower sheet makes its correlation with the Chinyrai Group equivocal. The tectonic parallel with the Temdydyn Sequence and hence with the Yanranai accretionary wedge is possible. Unfortunately, the available data are insufficient for definite correlation. The sheet of the Upper Khatyrka melange is sand wiched between two nappes of the Chirynai Group. This is a polymictic serpentinite melange [7, 26, 35] consisting of blocks of ultramafic rocks, gabbro, pla giogranite, basalt, and dolerite; Upper Paleozoic and Triassic cherts and limestones; Middle Jurassic– Lower Cretaceous rocks of the Chirynai Group; Upper Jurassic–Lower Cretaceous terrigenous rocks; and metamorphic rocks (amphibolite, greenschist, and blue schists). Sporadic blocks of Upper Triassic and Hauterivian volcaniclastic rocks and Coniacian terrigenous rocks are noted. Melange and the sur GEOTEKTONICS
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rounding rocks are deformed into southward verging folds. Internal structure. Outcrops of melange make up a semiring that surrounds the exposed subautochthon (Fig. 9). On the left bank of the Khatyrka River, the melange in the hinge of the antiform is gently dipping and thus occupies a large area. In the northwestern direction, the field of melange is reduced and retained as separate tectonic outliers overlying the rocks of the lower Chirynai sheet on the northern limb of the anti form. In the southern limb of the antiform, the melange is steeply dipping and occupies a narrow tract with numerous offsets caused by folding. The scattered small outcrops and veins of melange and serpentinite in the upper Chirynai sheet are related to protrusive phenomena and complicating faults. Despite the chaotic distribution of varisized blocks (a few meters to a few hundred meters) in the melange, a rather regular alternation of blocks different in com position was established during fieldwork. The follow ing succession of rocks is observed from the limbs to the cores of the antiforms: (1) melange with blocks of upper Chirynai sheet; (2) melange with blocks of ter rigenous rocks with abundant Buchia shells; (3) melange with blocks of Carboniferous, Permian, and Triassic volcanic rocks, cherts, and limestones with Tethian fauna; (4) melange with blocks of gab broic and ultramafic rocks; (5) melange with blocks of gabbroamphibolite, amphibolite, and garnet amphib olite; (6) melange with blocks of rocks from the under lying lower Chirynai sheet. It should be noted that such a succession is observed in large, deeply eroded antiforms; only fragments of the general sequence are retained elsewhere. Such an ordered structure testifies to the initial existence of tectonic sheets and slices deformed into complexly built folds, including those with nearly ver tical hinges. The internal folded structure of the sheets is disharmonic with respect to the late anti and syn forms. As a result, an intricate block distribution is misapprehended as chaotic. This impression is increased owing to mixing of material due to protru sions and slumps. In addition, the formation of late anti and synforms was accompanied by transfer of plastic serpentinite matrix from the limbs to cores of folds. Nevertheless, the elements of general succes sions of sheets can be restored, especially in deeply eroded anti and synforms. Tectonic interpretation. Tectonic elements wide spread in the adjacent terranes are established in the Emravaam–Pikas’vayam Nappe System [7, 23, 25]. The Temdynyn sheet as a deformed accretionary
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SOKOLOV, KRYLOV
Apoharzburgite melange
ge me lan lite her zo
Sheet of the Lagerny Complex
Apo l
Massif of the Lagerny Complex
Ap oh ar zb ur git em ela ng e
Greben lherzolite massif
Fig. 8. Elements of ordered structure: tectonic sheets of melange and rocks of the Lagerny Complex.
wedge is correlated with the Yanranai Terrane. The upper Chirynai sheet corresponds to the complexes of the Mainits Terrane. As was mentioned above, the tec tonic affiliation of the lower Chirynai Terrane is insuf ficiently clear. In the outer appearance, character of deformation, and secondary alteration, this sheet reveals similarity with sections of the Mainits Terrane. In composition of basalts, the upper Chirynai sheet is close to the fragments of oceanic crust of the Yanranai accretionary wedge. The elements of the Ekonai and Al’katvaam ter ranes are readily identified in the Upper Khatyrka melange [7, 23, 25, 35]. For example, ophiolites, vol canic rocks, cherts, and limestones with Late Paleo zoic–Triassic Tethian fauna are similar to the rocks of the Ekonai allochthon. Terrigenous and tuffaceous terrigenous rocks with Buchia fossils are widespread in the Nakypyilyak Complex of the Ekonai and Al’katvaam terranes. The Upper Triassic and Hau terivian volcaniclastic rocks are characteristic of the Al’katvaam Terrane as well. The Late Triassic island arc complexes are also known from the Ekonai Ter rane. Thus, the mapped melange sheets with blocks of specific composition are fragments of the lithotectonic zones formerly existing in the northeastern Koryak Highland. As a result of deformations, they have been eliminated and are retained only as relict sheets in the Upper Khatyrka melange.
Let us turn to the regional geology. The location of the Upper Khatyrka melange among the main tec tonic units of the Koryak Highland is shown in Fig. 10. As can be seen, the Ekonai and Al’tatvaam terranes disappear in the western direction. At the same time, the rock complexes characteristic of these terranes occur in the Upper Khatyrka melange. This implies grandiose tectonic destruction and subsequent or syn chronous juxtaposition of these structural elements, which turned out to be concentrated as a system of sheets in the structured melange. The fragments of the Ekonai and Al’katvaam terranes along with the com plexes of the Mainits (Upper Chirynai sheet) and the Yanranai (Lower Chirynai and Tendynyn sheets) ter ranes turned out to be combined together, making up a new complex nappe system [25] with violated spatial relationships between the separate terranes (Fig. 10b). Such a structure is a consequence of tectonic delami nation of the lithosphere at the convergent plate boundaries during continental accretion. DISCUSSION The considered examples of serpentinite melanges in the Koryak–Kamchatka Fold System are charac terized by an ordered internal structure, which is determined by the regular arrangement of blocks of a certain composition. These blocks cluster into mappa ble tectonic sheets deformed into complexly built napperelated folds different in age. In this regard, GEOTEKTONICS
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P
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P P III
II I′
III′ III I
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I P
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9
4
II′
8 km
2
3
4
5
6
10
11
12
13
14
a
b
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16
Fig. 9. Geological map of the Emravaam–Pikas’vayam Nappe System, prepared by S.D. Sokolov, V.N. Grigor’ev, and K.A. Kry lov. (1) Neogene–Quaternary sediments; (2) Paleogene; (3–5) terrigenous rocks of the Koryak Highland: (3) Maastrichtian, (4) Campanian, (6) Senonian; (6) Koryak Group (Santonian–Maastrichtian), the Ukelayat Terrane; (7) Emravaam Sequence (Lower Cretaceous); (8–10) Chinyrai Group: (8) unspecified, (9) upper sheet, (10) lower sheet; (11) Upper Khatyrka serpen tinite melange; (12) Temdynyn Sequence; (13) variegated sequence (Kimmeridgian–Valanginian); (14) cherty sequence with members of volcanic and terrigenous rocks (Middle–Upper Triassic); (15) faults: (a) thrust and (b) normal and strikeslip; (16) stratigraphic boundary.
they differ from tectonic melanges with chaotic distri bution of blocks. In structured melanges, the chaotic state is expressed only in the random distribution of blocks having various dimensions. At the same time, the blocks of the same composition are combined into a regular system of sheets. Another distinction is related to the traditional idea of the homogeneous matrix. The cement composition of the structured melanges of the Kuyul Terrane and GEOTEKTONICS
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accretionary structural units of the Taigonos Peninsula shows that in addition to different composition of blocks, the separate melange sheets have different matrices. In the Kuyul melange, the matrix is com posed of serpentinites formed after oceanic, supra subduction, and withinplate ophiolites. On Cape Povorotny, the lherzolites and apolherzolite melange are spatially related to blocks of oceanic rocks, whereas the harzburgites and apoharzburgite melange
1 2 3 4 5
(a)
T2–K1
J2–K1
6 7 8 9 10
J3–K1
62°
64°
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a б
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56 SOKOLOV, KRYLOV
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Fig. 10. Main structural elements of the eastern Koryak Range and structural position of the Upper Khatyrka melange. (a) Tec tonostratigraphic column of the Emravaam–Pikas’vayam Terrane. (1) Serpentinite melnge; (2) amphibolite and greenschist; (3) gabbroic rocks; (4) chert and tuffaceous silicite; (5) sandstone; (6) internediate tuff; (7) acid tuff; (8) basalt, (9) siltstone and claystone; (10) Upper Paleozoic–Triassic basalt, platy chert, and limestone. (b) Tectonic scheme. (1) Algan Terrane; (2) Mainits Terrane; (3–5) Emravaam –Pikas’vayam Nappe System: (3) upper Chinyrai sheet, (4) lower Chinyrai sheet, (5) Triassic–Lower Cretaceous cherty–terrigenous–tuffaceous rocks (relative autochthon); (6) Upper Khatyrka serpentinite melange; (7) Al’katvaam and Velikorechensky terranes; (8) Ekonai Terrane; (9) Yanranai Terrane along with Temdynyn sheet; (10) Uke layat Terrane; (11) Olyutor Terrane; (12) faults: (a) normal and thrust, (b) strikeslip; (13) Neogene–Quaternary sediments of the Anadyr Basin.
are related to islandarc blocks. Unfortunately, the composition of the cement in the Upper Khatyrka melange remains unstudied. Since fragments of oce anic, islandarc, and marginalsea complexes are rec ognized in this melange, it can be suggested that the corresponding differences will be established at the level of the matrix. The available data on the matrix composition are limited by two reasons. First, when melanges were mapped in the past, the composition of the blocks attracted primary attention. The first evidence for ordered distribution of blocks was revealed in the Upper Khatyrka melange and bore preliminary char acter. Only when the Kuyul melange had been studied and reliable data had appeared, did we pay attention to the composition of the matrix. The second reason is connected to the difficulties related to identification of the initial composition of the completely serpenti nized matrix, where relict peridotites are exceptionally rare or absent altogether. A special study is necessary to resolve this problem. It is known that serpentinite melanges had a long history of their formation, and the study of them makes it possible to restore the tectonic deformation of ophiolites. Deformation started under oceanic con ditions [24, 25, 31], reached a maximum at the stage of development of ophiolitic allochthons [10, 16, 22, 31], and continued at the postcollision and postaccretion stages. In recent foldbelts, the life of serpentinite melanges is going on; exogenic factors and neotec tonic movements are crucial. The occurrence of blocks composed of metamorphic rocks, including amphibolite and garnet amphibolite, in structured ser pentinite melanges indicates that deep levels were involved in structuring. The structuring of melanges occurred at various stages, beginning from disintegration of the ophiolitic sections. At this stage, oceanic plates undergo tectonic delamination with separation of lithosheets. Alloch thons composed of dunite–peridotite, banded, gab broic, dike, and volcanic–cherty complexes arise [31]. Further disintegration of these sheets separated by plastic serpentinites led to the formation of chaotic mono and polymictic melanges. In the process of intraoceanic deformation and especially during accretion and collision, the lithosheets consisting of fragments of already disinte grated and tectonized ophiolites and complexes of continent–ocean transition zones are juxtaposed. The GEOTEKTONICS
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diverse features of the regional tectonic history are reflected in the structured melanges. In the Kuyul melange, the combination of heterogeneous ophio lites different in age at the stage that predated incorpo ration of the Kuyul Terrane into the continental mar gin was important. In this case, the term amalgamation used in terrane analysis is suitable. The juxtaposition of oceanic and islandarc complexes in the course of the formation of the accretionary wedge is character istic of the melange on Cape Povorotny. The Emravaam–Pikas’vayam Nappe System was formed as result of destruction of the tectonic elements of the continental margin with subsequent merging of their fragments in the structure of the Upper Khatyrka melange. This regional feature is explained by the complex and multistage deformational history of the melange [7, 25] in comparison with the other two examples. The Kuyul and Cape Povorotny melanges completed their evolution by the MidCretaceous accretion. The history of the Upper Khatyrka melange was longer. First, the Upper Jurassic–Lower Creta ceous complexes were juxtaposed; the Upper Creta ceous rocks served as a transitional autochthon for these complexes. Later on, the Upper Cretaceous rocks with fragments of ophiolites and Upper Juras sic–Lower Cretaceous complexes were incorporated into the melange. Finally, all these rocks were thrust in the southern direction over the Upper Cretaceous– Paleogene Ukelayat flysch. The redistribution and juxtaposition of the masses during the formation of the melanges gave rise to the origination of an ordered but unstable structure, con sisting of blocks inhomogeneous in dimensions and composition. Under conditions of gravity instability, further development was determined by striving to equilibrium with mixing of material and elimination of ordering. The composition of cement was averaged contemporaneously with the destruction of sheets, owing to the plasticity of the serpentinite material. It may be said that structuring gave rise to heterogeneity, whereas the chaotic structure of melanges is a result of homogenization. Further study of the structured melanges will make it possible to reconstruct and specify the phenomena characterizing the transformation of the oceanic litho sphere from the moment of its origin to the incorpora tion into the continental margin. The tectonic juxta position of ophiolites resumed many times and was accompanied by their disintegration. Certain chrono
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logical relationships are established between supra subduction and oceanic ophiolites. Oceanic ophiolites are commonly older (somewhat or significantly) and therefore tectonized and disintegrated to a higher degree, whereas suprasubduction ophiolites are pre served much better (Gankuvayam section). In our view, precisely this feature, and not only the consump tion of the oceanic crust in the subduction zones, explains the fact that the most complete ophiolitic sec tions (Troodos, Oman, etc.) are related to the supra subduction type. Nevertheless, the data discussed in this paper demonstrate that the search for fragments of oceanic ophiolites is most promising in the complexly built structured serpentinite melanges. CONCLUSIONS The examples considered in this paper are diverse in tectonic setting, age, and formation history. The main conclusion proceeding from the study of struc tured melanges concerns the heterogeneity of the ophiolitic zones, which requires their indepth study. In particular, it is known that the ophiolitic belts of the Koryak Highland are differently expressed in the grav ity field and were formed in distinct geodynamic set tings. The formation of structured melanges was a multistage process, and their multifold amalgamation and accretion are recorded in their structure. The combination of ordered and chaotic structure allows us to consider structured melanges as objects of non linear geodynamics [20, 21]. It is important to note that the complexes formed in the same geodynamic setting occupy different structural position in the course of accretion and, as a result, are located in different terranes or thrust–fold structural elements. This indicates the possibility of differentiated motion of lithosheets. In this regard, the Upper Khatyrka melange of the Emravaam– Pikas’vayam Nappe System is a good example. At the same time, the complexes formed in different geody namic settings may be combined as a result of accre tion and further occupy the common structural posi tion, as is exemplified in the Kuyul serpentinite melange and Cape Povorotny. The study of structured melanges allows us to make insights into the history and mechanisms of continen tal accretion. In addition, the occurrence of fragments of the oceanic lithosphere in the melanges is often the sole source of information available for the recon struction of ancient oceanic plates. ACKNOWLEDGMENTS This study was supported by the Russian Founda tion for Basic Research (project no. 080500547), the Council for Grants of the President of the Russian Federation for Support of Leading Scientific Schools (grant no. NSh3172.2008.5), and the Division of
Earth Sciences, Russian Academy of Sciences (pro gram of basic research no. 10). REFERENCES 1. A. A. Aleksandrov, Nappes and Imbricate Structural Ele ments in the Koryak Highland (Nauka, Moscow, 1978) [in Russian]. 2. E. S. Alekseev, “The Kuyul Serpentinite Melange and Structure of the Talovka–Main Zone,” Geotektonika 15 (1), 105–120 (1981). 3. M. V. Aleksyutin and S. D. Sokolov, “Results of Paleo magneric Studies in the Penzhina–Taigonos Segment of the Pacific Belt and Their Tectonic Interpretation,” Tikhookean. Geol. 17 (1), 38–52 (1998). 4. B. A. Bazylev, S. A. Palandzhyan, A. V. Ganelin, et al., “Petrology of Peridotites from Ophiolitic Melange of Cape Povorotny, Taigonos Peninsula, NE Russia: Man tle Processes above a Subduction Zone,” Petrologiya 9 (2), 165–184 (2001) [Petrology 9 (2), 142–160 (2001)]. 5. G. E. Bondarenko, Tektonics and Geodynamic Evolu tion of the Mesozoides in the Northern Framework of the Pacific Ocean (Moscow State Univ., Moscow, 2004) [in Russian]. 6. V. S. Vishnevskaya, A. A. Peive, and S. D. Sokolov, “Age of Ophiolites in the Kuyul Terrane, Talovsky Moun tains, the Russian Northeast,” Dokl. Akad. Nauk 327 (3), 364–367 (1992). 7. V. N. Grigor’ev, K. A. Krylov, and S. D. Sokolov, “Upper Jurassic–Lower Cretaceous Rocks in the Cen tral Koryak Highland,” in Essays on Geology of the East ern USSR (Nauka, Moscow, 1986), pp. 58–80 [in Rus sian]. 8. V. N. Grigor’ev, K. A. Krylov, and S. D. Sokolov, “Jurassic–Cretaceous Rocks of the Yanranai Accre tionary Complex, the Koryak Highland,” in Essays on Geology of the Northwestern Sector of the Pacific Belt (Nauka, Moscow, 1987), pp. 132–159 [in Russian]. 9. V. N. Grigor’ev, S. D. Sokolov, K. A. Krylov et al., “Geodynamic Typification Of the Triassic–Jurassic Volcanic–Cherty Complexes of the Kuyul Terrane, the Koryak Highland,” Geotektonics 29 (3), 59–69 (1995). 10. A. L. Knipper, Oceanic Crust in the Structure of the Alpine Fold Region (Nauka, Moscow, 1975) [in Russian]. 11. A. L. Knipper, “Movements of the Austrian Phase in the Tethian Oceanic Crust: Character of Manifestation, Consequences, and Possible Causes,” Geotektonika 19 (2), 5–27 (1985). 12. G. V. Ledneva and D. I. Matukov, “Timing of Crystal lization of Plutonic Rocks from the Kuyul Ophiolitic Terrane (Koryak Highland): UPb Microprobe (SHRIMP) Zircon Dating,” Dokl. Akad. Nauk 424 (1), 71–75 (2009) [Dokl. 424 (1), 11–14 (2009)]. 13. M. S. Markov, G. E. Nekrasov, and S. A. Palandzhyan, “Ophiolites and Melanocratic Basement of the Koryak Highland,” in Essays on Tectonics of the Koryak High land (Nauka, Moscow, 1982), pp. 30–70 [in Russian]. 14. International Tectonic Lexicon, Ed. by J. Dennis, H. Murawski, and K. Weber (Intern. Union Geol. Sci., 1979; Mir, Moscow, 1982) [in Russian]. GEOTEKTONICS
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19. I. E. Pral’nikova and V. S. Vishnevskaya, “Middle to Late Jurassic Radiolarian Associations from the Oce anic Complexes of the Kuyul Terrane (Koryak Upland, Northeast Russia) and Their Paleogeographic Affinity,” Dokl. Akad. Nauk 351 (2), 240–245 (1996) [Dokl. 351 (8),1250–1255 (1996)]. 20. Yu. M. Pushcharovsky, “Three Paradigms in Geology,” Geotektonika 29 (1), 4–11 (1995). 21. Yu. M. Pushcharovsky and S. D. Sokolov, “Nonlinear Tectonics,” in Fundamentals of General Tectonics, Ed. by Yu. M. Pushcharovsky (Nauchnyi Mir, Moscow, 2001), pp. 476–508 [in Russian]. 22. S. V. Ruzhentsev, Marginal Ophiolitic Allochthons (Nauka, Moscow, 1976) [in Russian]. 23. S. V. Ruzhentsev, S. G. Byalobzhesky, V. N. Grigor’ev et al., “Tectonics of the Koryak Range,” in Essay of Tectonics of the Koryak Highland (Nauka, Moscow, 1982), pp. 136–189 [in Russian]. 24. A. A. Saveliev and G. N. Savelieva, “Ductile Flow of Ultramafic and Mafic Rocks in AlpineType Massifs,” in Tectonic Delamination of the Lithosphere (Nauka, Moscow, 1980), pp. 147–170 [in Russian]. 25. S. D. Sokolov, Accretionary Tectonics of the Koryak– Chukotka Segment of the Pacific Belt (Nauka, Moscow, 1992) [in Russian]. 26. S. D. Sokolov, “Accretionary Structure of the Penzhina Range in Northeast Russia,” Geotektonika 37 (5), 3–10 (2003) [Geotectonics 37 (5), 345–351 (2003)]. 27. S. D. Sokolov, G. E. Bondarenko, O. L. Morozov, and V. N. Grigor’ev, “The Asian Continent–Northwestern Pacific Transitional Zone in the Late Jurassic and Early Cretaceous,” in Theoretical and Regional Geodynamics (Nauka, Moscow, 1999), pp. 30–84 [in Russian]. 28. S. D. Sokolov, G. E. Bondarenko, O. L. Morozov et al., “Paleoaccretionary Prism of the Taigonos Peninsula, Northeastern Russia,” Dokl. Akad. Nauk 377 (6), GEOTEKTONICS
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