Density inhomogeneity of the lithosphere in the southeastern periphery of the North Asian craton

Russian Geology and Geophysics 48 (2007) 442–455 www.els\e\vier.com/locate/rgg\

Density inhomogeneity of the lithosphere in the southeastern periphery of the North Asian craton A.M. Petrishchevsky * Institute of Complex Analysis of Regional Problems, Far Eastern Branch of the RAS,4 ul. Sholom Aleikhema, Birobidzhan, 679016, Russia Received 9 June 2005

Abstract The study addresses the space distribution of lithospheric density contrasts in 3D and 2D surface (spherical) sources of gravity anomalies to depths of 120 km below the geoid surface and their relationship with shallow deformation and Archean, Early Paleozoic, and Late Mesozoic geodynamic environments. The lithospheric section in northeastern Transbaikalia and the Upper Amur region includes two layers of low-density gradients attendant with low seismic velocities and low electrical resistivity. The lower layer at depths of 80–120 km is attributed to an asthenospheric upwarp that extends beneath the North Asian craton from the Emuershan volcanic belt and the Songliao basin. The concentric pattern of density contrasts in the middle and lower crust beneath the Upper Amur region may be produced by the activity of the Aldan-Zeya plume, which spatially correlates with the geometry of the asthenospheric upwarp as well as with the regional seismicity field, magnetic and heat flow anomalies, and stresses caused by large earthquakes and recent vertical crustal movements. The relationship between shallow and deep structures in the crust and upper mantle bears signature of horizontal displacement (subduction) of the lower crust of the Baikal-Vitim and Amur superterranes beneath the North Asian craton. © 2007, IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: Gravity modeling; tectonosphere; the Southeastern Russia

Introduction The southeastern periphery of the North Asian craton from Baikal in the west to the Sea of Okhotsk in the east is of extremely complicated geologic structure. Archean and Proterozoic subcrystalline formations predominant in the crust were formed there during 1500 Myr, in several (3–4) tectonomagmatic stages (cycles) that differ in contents and intensity (Evolution...1987; Larin et al., 2002). In the Paleozoic and Mesozoic, these formations repeatedly underwent tectonic and magmatic alterations (Geology..., 1984; Geological Map..., 1986; Menaker, 1986; Gusev and Khain, 1995; Parfenov et al., 1996; Krasny et al., 1999). Now, active geodynamic processes accompanied by orogeny (Map of modern..., 1983; Gorkusha et al., 1999), high seismicity (Seismotectonics..., 1982; Gorkusha et al., 1999; Nikolaev and Semenov, 2004), heat-flow (Nikulin and Baryshev, 1977; Geology..., 1984; Krasny, 1988) and electric conductivity anomalies (Nikulin and Baryshev, 1977; Geology..., 1984; Kaplun, 2002; Pospeev, 2004) continue to occur there. Pa* Corresponding author. E-mail address: [email protected] (A.M. Petrishchevsky)

leodynamic reconstructions of the lithosphere in Transbaikalia and at the Amur headwaters (Gusev and Khain, 1995; Parfenov et al., 1996, 1999; Suvorov et al., 1999) suggest that the ancient tectonic plates (terranes) were highly mobile in the Paleozoic and Mesozoic under the conditions of repeated collisions of the Amur and Baikal-Vitim superterranes with the North Asian craton. Geological surveys have revealed there numerous accretionary structures and thrusts, with attributes of differently oriented (at different times) subduction of the paleo-oceanic crust (Gusev and Khain, 1995; Sorokin, 2001). The close structural adjacency and alteration of heterochronous tectonic and rather heterogeneous magmatic complexes (from ultramafic complexes up to plagiogranites) combined here with attributes of modern geodynamic activity is a strong complicating factor in the gravity modeling of deep structures, which, with rare exceptions (Malyshev, 1998; Zorin et al., 1990), is restricted to the crust in this region. The basic method of gravity modeling for density discontinuities in this area (first of all, at the crust bottom) is correlating between “regional” components of Bouguer anomalies and depths of seismic velocity discontinuities (Menaker, 1986; Tuezov et al., 1993), or heights of the Earth’s surface relief (Geology..., 1984; Zorin et al., 1990), which in

1068-7971/$ - see front matter D 2007, IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.rgg.200 7. 05.003

A.M. Petrishchevsky / Russian Geology and Geophysics 48 (2007) 442–455

Transbaikalia are corrected for intermediate layer density. The gravity contribution of the upper mantle density inhomogeneities to the observed Bouguer anomalies is not estimated unambiguously. According to Zorin et al. (1990), it can be rather significant (60% together with inhomogeneities of the lower crust), whereas other authors (Bryansky, 1995; Lyubalin, 1990) believe that the upper mantle, above which the crust blocks and plates form, transform and move, is weakly differentiated in density, and density inhomogeneities are chiefly recorded within the crust. The latter assumption, however, contradicts the measurements of seismic velocity (Nikulin and Baryshev, 1977; Geology..., 1984; Menaker, 1986; Deep structures..., 1991; Suvorov et al., 1999), electrical conductivity (Geology..., 1984; Kaplun, 2002; Pospeev, 2004), and heat flow (Nikulin and Baryshev, 1977; Krasny, 1988; Deep structures..., 1991), testifying to high tectonic (and rheological) lamination of the upper mantle and its high lateral variability. Insufficient depth of seismic sounding (DSS) and rare network of seismological, magnetotelluric and thermometric observations hinder the 3D modeling of deep structures in the lower crust and upper mantle. For this reason, spatial parameters, deep interrelations and the paleodynamics of large lithosphere segments (superterranes) and their components (tectonic blocks, plates and structures) remain rather obscure. In this situation, methods of traditional density modeling (iteration solution of a forward gravity problem based on wave velocity discontinuities) are rather ambiguous to study the lower crust and mantle structures. The new data on the crust and upper mantle structures of the Transbaikalian and Upper Amur regions obtained by the author are the result of the approximation gravity modeling of tectonosphere density inhomogeneities to a depth of 120 km. Advantages of the approximate approach, considered by Strakhov (2002) quite promising for gravity modeling, are that deep structures of a geologic space can be studied without solving a forward gravity problem and, as compared with the fitting density methods, it requires much less previous substantiation of models. Experience of similar research (Petrishchevsky, 1988, 2004a, 2004b, 2004c, 2006; Yushmanov and Petrishchevsky, 2004) proves that the crust and upper mantle deep structures can be approximately modeled without petrophysical and preliminary conceptual tectonic substantiations of models; moreover, it is possible to estimate the reliability of geological-structural and paleodynamic hypotheses based on superficial geological surveys.

Methodological The below models analyze the spatial distributions of quasi-isometric, or “compact”, sources of gravity anomalies satisfying to the condition Z1 > 0.5D, where Z1 is the distance to the body surface above its gravity center, and D is its horizontal size in the section under study. With this condition

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fulfilled, the geometric center of a density-homogeneous mass is determined with an error not exceeding 30% of its true value (Petrishchevsky, 2004c). In case of a body with variable density, the gravity center of a source (Z0) is calculated with the same accuracy. As follows from the gravity potential theory, the approximation of a geologic space by quasi-isometric bodies is also applicable to multiply connected systems of density inhomogeneities, for which the equivalent simply connected star mass is unequivocally located in space (Zidarov, 1968; Prilepko, 1970; Nikonova, 1979). The parameters (depth center, effective mass) of a unit body that belong to a class of uniqueness (for example, a ball or horizontal cylinder) in complex systems of density inhomogeneities can be determined unambiguously only if their surfaces are not mutually intersected (Gravirazvedka, 1990, pp. 234–235). This property of the gravitational potential reveals common (spatially associated) features of distributions of unit bodies and structures of different geological origins (magmatic, metamorphic, folding etc.) but of the same tectonic nature. To compute Z0 from gravity anomaly curves, any method (Petrishchevsky, 1981; Gravity..., 1990) can be used which allows formalization without previous information about the density and size of the disturbing masses. Results of simulation (Petrishchevsky, 2004b) prove that the distribution of “compact sources” might be a proxy for the distribution of horizontal and vertical density inhomogeneities in a wide spatial range: 5 > ∆H/D > 0.1, where ∆H is the vertical width (thickness) of the body. When constructing tentative geologic boundaries of second class* running through the centers of apparent “compact” masses, it is possible to reveal deep-structure features of the crust and upper mantle without a priori information, on the basis of the character of grouping of density inhomogeneities and orientations of isolines (Z0) at different depths of the geologic space. This approach to simulation of deep structures is difficult to apply because of superimposed depressions and basins (Z1 < 0.2D) as well as autochthonous palingenic granite massifs (Petrishchevsky, 1985), for the processing and interpretation of their gravity anomalies leads to considerable overestimations of depths. Large basins, however, are absent from the most part of the area under study (Fig. 1, a), and horizontal dimensions of superimposed depressions and granite bodies (in cross sections) are much inferior to the size of examined regional gravity anomalies. The large part of extensive Mesozoic-Cenozoic depressions are displaced relative to the gravity minimums (Upper Zeya, Tok, Chul’man), or are located within maximums of gravity anomalies (Uda, Amur-Zeya). Most pre-Mesozoic granitoids and metamorphicintrusive massifs are also associated with zones of maximums or border zones of Bouguer anomalies. On the territory of northeastern Transbaikalia and the Amur headwaters (Fig. 1) we have studied the distributions of “compact” density heterogeneities in the following intervals of depths: 0–10, 5–15, 10–20, 15–30, 25–40, 30–50, 40–60,

* By geological (tectonic) boundaries of second class are meant tentative surfaces connecting special points of geologic space (Kosygin, 1983), for example, fold flexures of continuous stratified horizon (hinge surface).

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Fig. 1. Geological-structural scheme (a) (Geological map..., 1986; Gusev, Khain, 1995; Krasny et al., 1999) and gravity anomalies (b) of the southeastern framing of the North Asian craton. 1–5 — structural-material complexes (1 — Cenozoic, 2 — Mesozoic, 3 — Paleozoic, 4 — Proterozoic and Late Archean, 5 — Early Archean); 6, 7 — granitoids (6 — Paleozoic, 7 — Mesozoic); 8 — largest faults; 9–11 — elements of gravity anomalies (9 — gradation lines of field of different intensity, 10 — directions of increasing regional component of gravity anomalies, 11— marks of local gravity disturbances); 12 — borders of terranes and their designation; 13–18 — density scale of superficial rock layer (b), g /cm3 (Krasny, 1988) (13 — less than 2.50, 14 — 2.50–2.60, 15 — 2.60–2.65, 16 — 2.65–2.70, 17 — 2.70–2.80, 18 — more than 2.80). Signatures of largest faults on the scheme (a), squared figures: 1 — Uda-Vitim, 2 — Mongolo-Okhotsk, 3 — Zhuya, 4 — Dzheltulak, 5 — Stanovoy, 6 — Sutam, 7 — South Tukuringra. Depressions: M — Muya, Ch — Chul’man, T — Tok, UZ — Upper Zeya, At — Uda, AZ — Amur-Zeya. Abbreviations of terranes (b): BV — Baikal-Vitim, St — Stanovoy, MO — Mongolo-Okhotsk, A — Amur, ASh — Aldan Shield.

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A.M. Petrishchevsky / Russian Geology and Geophysics 48 (2007) 442–455 Table 1 Structural features of distributions of density inhomogeneities in the crust and upper mantle of northeastern Transbaikalia and in the Upper Amur region Depths of Number of layer Z1–Z2, km points in calculations

Dominant direction of Z0-isolines

Connection with tectonic structures

0–10

28

L, M

Increase in depth of density inhomogeneities beneath the Aldan Shield and Amur terrane

5–15

124

10–20

208

15–30

219

25–40

72

NW

Correlation of gradients of Z0-isolines with northwestern faults; collision joining of deep structures of the Baikal-Vitim terrane with the Aldan-Stanovoy terrane

30–50

53

L, M

Increase in depth of density inhomogeneities beneath the Baikal-Vitim terrane, and decrease, beneath the Aldan and Amur terranes

40–60

51

M, NW

Thickening of this layer beneath the Baikal-Vitim, Amur, and Aldan terranes and northeastern regions of the Stanovoy system

50–70

47

M, NW, NE

Rising of density inhomogeneities in the crossing block of northwestern direction beneath the Aldan Shield

60–80

38

M, NE

None

70–90

31

M in north and west; NE in southeast

Accretion of deep structures of the Stanovoy and Mongolo-Okhotsk folded systems to the Baikal-Vitim terrane

75–100

21

NE, M

Accretion of deep structures of the North Asian craton to structures of the Stanovoy and MongoloOkhotsk folded systems

80–120

24

NE

Linear rise of the lower lithosphere layer of northeastern direction, which separate deep structures of the Aldan Shield and Baikal-Vitim terrane from the Amur terrane

None NW, NE

Correlation of gradients of Z0-isolines with northeastern faults of the Aldan-Stanovoy component terrane Increase in depth of density inhomogeneities beneath the Aldan Shield, and decrease, beneath the Amur terrane

Note. Abbreviations of directions of Z0-isolines: M — submeridional, L — sublatitudinal, NE — northeastern, NW — northwestern.

50–70, 60–80, 70–90, 75–100 and 80–120 km from the geoid surface (Table 1). The calculated profiles are directed meridionally. Distance between profiles makes 50 km, step of the field record is 5 km. The modeling is based on 510 individual determinations of (Z0) obtained from the multiple automated interpretation of quasi-symmetric gravity anomalies of both signs. Each elementary anomaly was quantitatively interpreted without determination of the geological nature of a particular source. It is accepted that any anomaly results from a number of sources and has only a physical and mathematical sense adequate to the definition only of first harmonic moments of gravity disturbances (M, M×Z0), whose random distribution in a formalized geological space contains objective and unequivocal (within the framework of fomalized procedure) information about a deep structure of the lithosphere. Methodology and technical characteristics of the computing procedure are described in detail elsewhere (Petrishchevsky, 1981, 1988, 2004c). The principles and examples of geological interpretation of spatial distributions of the density inhomogeneities in the crust and upper mantle in various geologic areas of the Russian Far East are described in (Petrishchevsky, 1987, 1988, 2004a, 2004c, 2006; Yushmanov and Petrishchevsky, 2004). Gravitational equivalent of a volumetric source (ideally, a ball) is a sphere, on the surface of which the mass (M) of this source is dispersed. Above the center of a source the mass

may be unequivocally calculated from the expression (Gravity..., 1990, page 258): M=

Vzm Z20 , K

where Vzm is the amplitude of gravity anomaly; K is the gravity constant (6.673⋅10−11 m3/kg⋅s2). The identity of superficial and volumetric gravitational potentials allows the superficial density (µ) of equivalent sphere to be described by µ=

2 M Vzm Z0 , = S 4πKr2

where S = 4πr2 is the area of sphere; r is the radius of sphere. Vertical gradient of superficial density of a spherical mass at the points Hc = Z0 – r above the center of the sphere will correspond to µz =

Vzm Z0 4πK(Z0 − Hc)2

.

(1)

The use of µz-parameter enhances the capabilities of the approximate modeling for deep structures as it takes into account the material properties of density inhomogeneities (Vzm, M). As a result of the simple automated procedure by algorithm (1), the earlier calculated array of Z0(x, y) was

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A.M. Petrishchevsky / Russian Geology and Geophysics 48 (2007) 442–455

Table 2 Structural-material inhomogeneity of the crust and upper mantle of northeastern Transbaikalia and Upper Amur region Hc. km

Depths of layer Number of points Dominant directions in calculations of µz-isolines Z1–Z2, km

Connection with tectonic structures

5

7–15

119

Not expressed

Ring megastructure (700×700 km ) with the center in the interfluve of the Zeya and Timpton headwaters

10

12–25

244

NE, NW

Northeastern µz-anomalies beneath magmatic blocks of the Baikal-Vitim terrane and northwestern ones in the eastern Aldan Shield

15

17–30

175

NE, NW

Vast areas (500×500 km) of high µz-values beneath the Baikal-Vitim and Amur terranes

20

22–40

107

Not expressed

Ring megastructure (700×700 km) with the center in the interfluve of the Zeya and Timpton headwaters

25

27–45

68

NW, M, L

Jumpwise increase in superficial density gradient of this layer to the east

30

32–50

46

NW, M, L

µz-maximums beneath the Aldan Shield, Amur terrane, and southeastern flank of the Baikal-Vitim terrane

35

37–55

43

Not expressed

Concentric location of granite massifs in the northeastern Baikal-Vitim terrane around µz-maximum

40

42–60

44

M, L, NW

µz-maximum beneath the Stanovoy system; collision joining of deep structures of the Stanovoy folded system and Aldan Shield with the Baikal-Vitim terrane

50

52–70

41

M

Meridional zones of the lower density gradients on western and eastern bounds of the Aldan Shield

60

62–80

23

NE, L

Not expressed

70

72–90

22

M, L

Vast area of decreased density gradient separating the Baikal-Vitim and AldanStanovoy terranes

80

82–120

16

NE

Decreased density gradients beneath the Aldan-Stanovoy and Amur terranes

90

100–150

9

L

Gently rising density gradients southward off the North Asian craton

Abbreviations of directions of µz-isolines: M — submeridional, L — sublatitudinal, NE — northeastern, NW — northwestern.

transposed to 3D matrices: µz(x, y, Hc = 0, 5, 10, 15, 20, 25, 30, 35, 40, 50, 60, 70, 80, 90 and 100 km). On modeling, the discontinuities of function (1) at points Z0 ≈ Hc are blocked by the condition (Z0 – Hc) > 2 km, i.e. the surfaces of “condensation” of volumetric sources were always located 2 km above the layer for which µz was calculated (Table 2). Peak values (µz) were smoothed up to the level 10µz (minimum). The physical sense of the tectonosphere gradient models reflected by vertical sections µz(x, Hc) in Fig. 3 and horizontal sections µz(x, y, Hc = const) in Fig. 4 differs from the traditional understanding of density gradient (Kosygin and Isaev, 1985; Gravity..., 1990; Bryansky, 1995; etc.). However, like the gradient of volumetric masses (kg/m3/km), the parameter µz (kg/m2/km) is a measure of presence (or absence) of sources of intense gravity anomalies, in some range of tectonosphere and in a more general sense reflects the degree of contrast of the geologic space. The internal unambiguity of the above procedures means that one distribution of gravity anomalies (step of field record and profile direction) allows for constructing only one distribution of the calculated parameters (Figs. 2–4). However, our results do not suggest that the models are geologically unambiguous, for this is true only when Z1 > 0.5D. As in the case with any other gravity models for complex geological media, the “absolute” accuracy of the approximate models (in our case, at a level of 30% before processing) can be estimated

only by means of the external checking, by comparison of the formalized model density distributions with known geological and geophysical data (ideally, by inspection of the elements to be modeled on drilling). Comparison shows a satisfactory correlation of the non-a priori formalized distributions of density inhomogeneities in a geologic space with seismic velocity discontinuities (Fig. 3), heat-flow (Fig. 3, section 3-3; Fig. 4, d), magnetic (Fig. 4, e), and geoelectrical (Pospeev, 2004) anomalies. Hence, the density inhomogeneities and the above anomalies can have common geologic origins. The task for interpreter of such gravity models is to find and explain these origins by a complex analysis of the available geological and geophysical information. Examples of geological interpretation of models µz(x, y, Hc) are given in works (Petrishchevsky, 2003, 2004a, 2006, Petrishchevsky and Zlobin, 2004).

Gravity anomalies and tectonic complexes During the last three billions years the tectonic-formation complexes of the southeastern periphery of the North Asian craton repeatedly experienced crushing, heaping, differently directed displacements, intense magmatic and metamorphic processing (Krasny, 1986; Evolution..., 1987; Gusev and Khain, 1995; Parfenov et al., 1996, 1999; Krasny et al., 1999; Sorokin, 2001; Larin et al., 2002). As a result, the gravity

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Fig. 2. Spatial distributions of density inhomogeneities in the lithosphere of northeastern Transbaikalia and Upper Amur region: a — middle crust layer (Z0 = 10–20 km); b — lower crust layer (Z0 = 25–40 km); c — lower lithosphere layer (Z0 = 75–100 km). 1–4 — tectonic complexes (b, c): 1 — Cenozoic and Mesozoic, 2 — Paleozoic, 3 — Proterozoic and Late Archean, 4 — Early Archean; 5 — largest faults (a); 6, 7 — borders of terranes: 6 — on scheme (b), 7 — on scheme (a); 8 — isolines of equal depths of density inhomogeneity centers (Z0), km. Abbreviations of terranes follow Fig. 1.

anomalies in this region are ambiguously correlated with borders of pre-Mesozoic structures and density of superficial complexes. A weak connection of regional gravity minimums with Paleozoic intrusions is recorded only in northwestern, southern and extreme eastern parts of the Baikal-Vitim superterrane (Fig. 1). Most of large pre-Mesozoic intrusive and intrusive-metamorphic massifs in the study area are located within maximums or in the border (gradient) zones of Bouguer anomalies. The linear zones of intense gradients of gravity, in turn, only incidentally coincide with the location of regional sutures, the terrane borders (Fig. 1, b); more often they are displaced from the sutures for a distance of 60–80 km (Mongolo-Okhotsk, Dzheltulak and Stanovoy deep faults) or more (Zhuya fault), which indicates horizontal motions of superficial complexes. Thus, the assumed (Gusev and Khain, 1995) thrusting of the Proterozoic Stanovoy complexes upon the Aldan Shield is well expressed in gravity anomalies: The deep border of the shield expressed in a gravity gradient zone (Fig. 1, b) is displaced to the south for 120–150 km from the superficial position of the Stanovoy fault (Fig. 1, a). Close estimates were obtained by autocorrelation gravity sounding of the crust in this area (Petrishchevsky, 2004b). As inferred from the character of gravity anomalies (Fig. 1, b) the eastern part of the Baikal-Vitim superterrane is also thrust upon (or intruded into) the Aldan Shield and the Stanovoy folded system.

Among the Mesozoic structures, the spatial connection of the sublatitudinal gravity minimum with granitoids of the Stanovoy system is expressed best of all. The crossing orientation (relative to the minimum extension) of granite bodies testify to its formation in conditions of extension connected with strike-slip deformations in zones of the Stanovoy, Dzheltulak and South Tukuringra faults accompanying “oblique collision of terranes” (Gusev and Khain, 1995; Parfenov et al., 1996, 1999; Sorokin, 2001). Similar correlations of the granite bodies and volcanic structures with strike-slip deformations were considered in detail by Utkin (1989) in zones of the Central and Eastern (Coastal) strike-slip faults in the Sikhote-Alin. Largest (50×100, 100×300 km) Mesozoic and Cenozoic superimposed intermountain basins of the Aldan Shield and the Stanovoy terrane (Chul’man, Upper Zeya, Tok) are accompanied with sharply asymmetric minimums of Bouguer anomalies (with amplitudes of 15–30 mGl), displaced to the southern flanks of the depressions (Fig. 1, b). Late Mesozoic and Cenozoic complexes of the Baikal-Vitim and Amur terranes, and the Mongolo-Okhotsk folded-accretionary system have no clear connection with gravity anomalies (Fig. 1) because of small capacity and inhomogeneous composition of these complexes. The arrangement of some Mesozoic-Cenozoic depressions (Uda, Amur-Zeya, Muya) to Bouguer maximums accompanied with reduction of the crust depth, suggest

Fig. 3. Distributions of density gradients (µz) in crust and upper mantle sections at seismic profiles. 1, 2 — contrast seismic velocity discontinuities (Geology..., 1984; Bulin et al., 1972; Zolotov and Rakitov, 2000) including Moho (1); 3 — zones of lower seismic velocity (waveguides) (Geology..., 1984; Zolotov and Rakitov, 2000); 4 — partial melting zone in upper mantle (Tectonosphere..., 1992); 5 — isolines of density gradient (10–2 kg/m2/km); 6 — location of profiles; 7 — DSS profiles (a) and EWS profiles (b); 8 — borders of tectonic structures in sections. Abbreviations of structures: NAC — North Asian craton, ASh — Aldan Shield; terranes: BV — Baikal-Vitim, A — Amur, St — Stanovoy; MO — Mongolo-Okhotsk suture.

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Fig. 4. Sections of 3-D model at depths of Hc = 5 km (a), 20 km (b), 35 km (c), and 70 km (d); magnetic and heat-flow anomalies, and vectors of tectonic stresses (e). 1 — isolines of spherical density gradients (10–2 kg/m2/km); 2 — contours of central-type structures on schemes a–c; 3–5 — epicenters of earthquakes (Tectonics..., 2001) with magnitudes: >6 (3), 4–6 (4), <4 (5); 6 — vectors of mechanical stresses of compression measured in a field of strong earthquakes (Seismotectonics..., 1982; Nikolaev end Semenov, 2004); 7 — axis of arc negative magnetic anomalies (Krasny, 1988); 8 — heat-flow anomalies with intensity 50 mW/m2 and more (Krasny, 1988); 9 — Early Archean crystal complexes (d).

that these depressions are linked to modern rifting and mantle diapirism. Thus, except for intermontane troughs of the Aldan Shield, Mesozoic granitoids of the Stanovoy folded system, and ophiolite complexes of the Mongolo-Okhotsk suture, there is no unequivocal relationship of gravity anomalies with density of superficial complexes (Fig. 1, b). For instance, the largest gravity minimum in northeastern Transbaikalia related by some authors (Geology..., 1984) to a Paleozoic areal pluton (Barguzin or Angaro-Vitim batholith), correlates with a field of high (!) density east of the 166th meridian. Moreover, this

minimum correlates with anomalies of seismicity and heat flow (Nikulin and Barishev, 1977; Krasny, 1988; Geology..., 1985), but in this case it is linked to Cenozoic rather than Paleozoic tectono-magmatic processes. The extensive area of ultrametamorphic complexes of the Aldan Shield with a density of 2.80–2.85 g/cm3 is characterized by a mosaic pattern of gravity anomalies (Fig. 1, b), among which minimums prevail, and the northern flanks of the shield are crossed at a steep angle by gravity anomalies of internal areas of the North Asian craton. The level of a regional component of the anomalous gravity field decreases in both directions

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from the Stanovoy and Baikal-Vitim minimums. The extensive maximum of gravity coinciding with the eastern flank of the Amur terrane is located in a field of lower density (Fig. 1, b). Thus, the interpretation of the gravity anomalies at southern borderlands of the North Asian craton can little contribute to the study of deep structures of the crust and upper mantle by fitting density inhomogeneities, and main obstacles for solving the forward gravity problem are as follows: 1. Insufficient a priori information, first of all, because of a rare network of seismic soundings. 2. A discrepancy in outlines of tectonic complexes and structures with gravity anomalies; tectonically crushed ancient complexes and their heterochronous magmatic processing. The inverse type of interrelations between density characteristics of superficial rock layer and the sign of gravity anomalies (northeastern areas of the Baikal-Vitim superterrane, Aldan Shield, and Amur superterrane). 3. Difficulties in separating the gravity effects of the pre-Mesozoic deep structures from the Cenozoic geodynamic effects on the crust and upper mantle.

Spatial distributions of density inhomogeneities in the crust and upper mantle The spatial distribution of density inhomogeneities adequate, in cross sections, to quasi-symmetric gravity anomalies is characterized by a thin-bedded structure, which is displayed in sharp, but regular, alterations of prevailing directions of isolines Z0 in the adjacent layers of the lithosphere. More often, isolines of density inhomogeneity depths (Z0) are of submeridional orientation. These orientations with insignificant interruptions are traced from the surface to a depth of 90 km (Table 1). The NW orientations are more typical of the middle and lower crust layers (Z0 = 15–40 km) and of the upper mantle inhomogeneities to a depth of 70 km. The NE orientations begin to clearly manifest themselves from a depth of 60–70 km, and at depths of more than 80 km (the lower lithosphere) they prevail. The observable variations of the spatial distributions of density inhomogeneities in different layers of the tectonosphere can be linked to changes of the trend of tectonic stresses during interactions (collisions) of large lithosphere segments (plates, microplates and superterranes) in the east of the Eurasian plate in the Archean, Early Proterozoic, Paleozoic, and Late Mesozoic. The submeridional orientation in the distribution of density inhomogeneity centers of the subcrystalline sialic layer to a depth of 15 km (Table 1) is drastically discordant to the strike of superficial tectonic structures on the largest part of the area under study (Fig. 1). On the Earth’s surface the meridional orientations of Z0-isolines correspond to the orientations of fractures, magmatic bodies, migmatite and laminated zones, and fold axes in the western Aldan Shield, in the region of granite-greenstone complexes of the Olekma series (AR31) (Evolution..., 1987; Kosygin and Parfenov, 1978; Krasny et al., 1999). In the central areas of the Aldan Shield, fragments of submeridional structures broken by later northwestern and

northeastern structural elements are observed in the field of the Timpton series (AR21), which occupies the middle stratigraphic position in the Archean section of the Aldan Shield (Geology of USSR, 1972). In other areas of eastern flanks of the Eurasian plate and its Pacific borderland the submeridional orientations of internal elements of tectonic structures are well shown in the Archean and Proterozoic complexes exposed in rises of the “granite-metamorphic” layer of the crust (northern flank of the Khanka, Okhotsk, Kolyma, and Tas-Khayakhtas craton-type terranes; Proterozoic structures of the Sette-Daban and Maly Khingan; the mid-Kamchatka metamorphic rise (Kosygin and Parfenov, 1978; Tectonics..., 2001)). The global character of the submeridional orientations of fractures and folded structures in most ancient complexes of northeastern Asia suggests that the meridional orientation of deep structures of the Aldan-Stanovoy composite terrane and the bordering Amur and Baikal-Vitim terranes reflects ancient (pre-Paleozoic) deformations of the crust and upper mantle of the southeastern periphery of the North Asian craton, whose traces are kept till now owing to the rigidity acquired in the Archean. These deformations were disturbed or inherited (differently in different blocks) at subsequent tectonic deformations (“activities”) of southeastern flanks of the craton. Most intense of them were accompanied by the formation of extensive fields of granitoids in the Middle Paleozoic (Baikal-Vitim and Amur superterranes) and Late Mesozoic (Stanovoy folded system). The middle (Fig. 2, a) and lower (Fig. 2, b) crust layers of northeastern Transbaikalia and the Upper Amur region are characterized by dominant northwestern orientations of Z0isolines, concordant with fractures of this direction, western and eastern borders of the Early Archean complexes of the Aldan-Stanovoy composite terrane as well as with the mainly northwestern direction of faults, and the fold axes, and layered zones (Geology of USSR, 1972; Kosygin and Parfenov, 1978) in the granulite-gneiss complexes of the crossing Central Aldan block (Gusev and Khain, 1995) of the Aldan Shield. Granitoid massifs of Stanovoy system are oriented in the same northwestern direction (Fig. 1, a). On intruding, they entrained pre-Mesozoic structures of crushing and layering. The data obtained (Fig. 2, a) show that the deep structures of the crossing block extend to the southeast from superficial boundaries of the Aldan Shield for a distance of 300 km. Therefore, it is quite possible that Paleozoic and Early Mesozoic turbidate complexes of this area (Tukuringra-Dzhagdy, and the Un’ya-Bomsky terranes (Sorokin, 2001)), deformed in structures of the same northwestern direction (Kosygin and Parfenov, 1978), could be thrust on the Late Paleozoic margin of the North Asian craton by its collision with the Amur superterrane. On the vast territory of northeastern Asia (southern block of the Khanka superterrane, eastern flanks of the MongoloOkhotsk, and Verkhoyansk-Kolyma accretion-folded systems (Kosygin and Parfenov, 1978)) the northwestern orientations of structural elements in the Paleozoic complexes are connected with the northeastern (or southwestern) vectors of paleotectonic stresses. The data obtained suggest that the deep rearrangement of the transverse block of the Aldan Shield

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(northwestern direction of structures) was completed somewhat later (in the Late Paleozoic or Mesozoic) than it is inferred from superficial geological data (Late ProterozoicEarly Paleozoic (Gusev and Khain, 1995; Parfenov et al., 1996, 1999)). The deep deformations of this block in the superficial crust layer are reflected in thrusting of the ancient granulite-gneiss complexes of the Central Aldan block 3 (AR1−2 1 ) upon greenstone complexes of its frame (AR1) (Evolution..., 1987; Gusev and Khain, 1995), and later on (in the Mesozoic) the western flanks of the Stanovoy terrane were thrust upon the Aldan Shield (Gusev and Khain, 1995). The earlier calculations (Petrishchevsky, 2004b, Fig. 4) showed that the horizontal amplitude of the thrust at the southern borders of the shield makes 120–140 km, and the vertical thickness of the allochthone is about 10 km. At a depth approximately corresponding to the allochthone bottom (10– 15 km), the predominant direction of Z0-isolines drastically changed from submeridional to northwest (Table 1, Fig. 2), which in many other areas (Petrishchevsky, 1987, 1988, 2004b) is indicative of deep structural discordance. Deep structures of the Baikal-Vitim terrane are drastically discordant to structures of the Aldan Shield in a wide range of depths, from 15 to 100 km (Table 1, Fig. 2) and usually abut against them, which is visible in superficial structures (Fig. 1, a) and in gravity anomalies (Fig. 1, b). Thus, the Western Stanovoy (Geological map..., 1986) (or the SelengaStanovoy (Krasny et al., 1999)) geoblock in northeastern Transbaikalia does not belong to the Stanovoy terrane. It is more reasonable to relate it to the Baikal-Vitim terrane (Gusev and Khain, 1995), which is bordered in the east by the Zhuya and Dzheltulak fractures (Fig. 1, a). The extension of gravity anomalies (Fig. 1, b) and appropriate deep structures (Fig. 2, a, b) of the Baikal-Vitim terrane to the northeast from the Zhuya and Dzheltulak faults indicates that the Aldan-Stanovoy composite terrane was subhorizontally injected by magmatic structures of the Angara-Vitim batholith. According to some data (Gusev and Khain, 1995), this injection was accompanied by thrusting of superficial complexes. The crossing, northwestern, orientations of deep structures of the Aldan Shield are traced to a depth of 70 km (Table 1), and at depths of 80–90 km (Fig. 2, c) the ancient lithosphere platform segment with the Late Proterozoic-Early Paleozoic Baikal-Vitim and Amur superterranes is precisely outlined by Z0-isolines. In the lower lithosphere layer (Fig. 2, c) the Mongolo-Okhotsk accretionary-folded system, a suture at the depth boundaries of the Amur terrane with the Baikal-Vitim and Stanovoy terranes, clearly displays its collisional nature. Thus, the distributions of deep density inhomogeneities in northeastern Transbaikalia and the Upper Amur region are structurally related to paleogeodynamics of large lithospheric segments (superterranes). One of the reviewers of this article believe that the orientations of deep density inhomogeneities as well as deformations of superficial tectonic-formation complexes can repeat boundaries of the lithosphere plates (microplates and large terranes) at their interactions (collisions). Meridional and conjugated latitudinal orientations of the

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Archean and Early Proterozoic structures are the most ancient in the area under study. They are observed in the crystalline basement of the North Asian craton, in a wide range of depths of the tectonosphere (0–100 km) within and beyond the craton, independently of the borders of the terrane adjacent to the craton. The NW orientations of deep structures, best shown in the middle and lower crust layers of the Aldan-Stanovoy composite terrane, seem to be caused by the Early Paleozoic collision of the Baikal-Vitim terrane (active role) with the North Asian craton (Parfenov et al., 1996, 1999), with NE tectonic stresses being predominant. The northeastern orientations of density inhomogeneities dominating in the lower lithosphere and asthenosphere (at depths of 80–120 km) are probably connected with the Late Paleozoic and Mesozoic geodynamics of this region (Gusev and Khain, 1995; Parfenov, 1999), when the Amur superterrane joined the southeastern margin of the North Asian craton. Structure and composition of the tectonosphere of the southeastern margin of the North Asian craton are considered on the basis of a 3-D model for density gradients of equivalent spherical sources µz(x, y, Hc = 0, 5, 10, 15, 20, 25, 30, 35, 40, 50, 60, 70, 80, 90 and 100 km), which takes into account intensity (Vzm) and, consequently, the density properties (M, σ) of elementary gravity disturbances. In the sections to a depth of 25 km the northeastern and northwestern isoline orientations are dominating, in the depth interval of 25–40 km, the NW, NE and latitudinal-meridional orientations are combined, and in sections at depths of more than 50 km, the NE, latidudinal and meridional directions prevail (Table 2). Comparison with distributions of the mass centers (Table 1, Fig. 2) shows that the distributions of density gradients acquire new characteristics related to paleomagmatic structures and rheological properties of the crust und upper mantle. The data obtained (Fig. 3) show that the section of tectonosphere of northeastern Transbaikalia and the Upper Amur region consists of five layers. The first, superficial, layer to a depth of 10–15 km composed of heterochronous metamorphic, volcanosedimentary and intrusive complexes is characterized by lower density gradients (µz <10−2 kg/m2/km). The second layer, with high values of density gradients µz = (20−50)⋅10−2 kg/m2/km, lies at depths from 10–15 km to 35–40 km and corresponds to the lower (mafic) crust layer. The lower border of this layer in sections of seismic sounding frequently coincides with the crust bottom (Fig. 3, sections 2-2 and 3-3). The highest density gradients of the second layer are recorded beneath the Aldan Shield and Amur terrane. Beneath the Baikal-Vitim terrane the thickness of the highgradient layer is reduced to 10–15 km (sections 1-1, 4-4 in Fig. 3), which can be related to modern geodynamic processes on the east flank of the Baikal rift zone (Nikulin and Baryshev, 1977; Geology.., 1984, 1985; Zorin et al., 1990; Nikolaev and Semenov, 2004). The second layer (Fig. 3, sections 1-1, 2-2, 3-3) quite often wedges out at the borders of terranes. The third layer with lower density gradients (µz = (5−15)⋅10−2 kg/m2/km) is located in the upper mantle, less often at the base of the crust, where it is compared with seismic

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waveguides (Fig. 3, sections 1-1, 2-2 and 4-4), with attributes of partial melting (section 3-3 in Fig. 3) detected by heat-flow data (Tectonosphere..., 1992) and layer with low electrical resistance by magnetotelluric data (Pospeev, 2004). The fourth layer with higher and high density gradients is located in the depth interval of 50–90 km and corresponds to the lower lithosphere rigid layer. At the base of the studied section of the tectonosphere, at depths of more than 80 km, the fifth layer with lower density gradients lies, which coincides with the asthenosphere, recognized there from magnetotelluric, heat-flow, and seismic data (Nikulin and Baryshev, 1977; Geology..., 1984; Deep structure..., 1991; Kaplun, 2002). The existence of a rheologically weakened layer at depths of 80–100 km revealed by seismic data in many other areas of the world, is a global feature of the upper mantle of the Earth (Pavlenkova, 1995) and in the tectonic active areas this layer, as a rule, occurs at the lithosphere bottom. Thus, low density gradients indicate that the tectonosphere under study contains two layers with lower rheological properties (decreased viscosity or increased plasticity), relative to which horizontal displacements of blocks and plates might occur in over- and underlying layers. Anomalies of the seismic velocity and electrical conductivity of the subcrustal layer in Transbaikalia (Geology..., 1984; Pospeev, 2004) and other areas (Pavlenkova, 1995) are explained by an “overcritical phase condition” of the upper mantle matter (increase in the content of free fluids), while the asthenosphere layer is universally accepted as a layer of partial melting. The spatial distributions of density gradients in the crust of the Upper Amur region (Fig. 4, a, b) are characterized by the concentric-zonal subordination of local anomalies and the dividing gradient zones of µz-parameter, which is a typical feature of the central-type magmatic structures (Petrishchevsky, 1988, 2003), in particular, plumes (Petrishchevsky and Zlobin, 2004). This structure is most distinct in the interfluve of the Aldan, Olekma, and Zeya Rivers. In the superficial (Fig. 4, a) and mid-crustal (Fig. 4, b) sections it is characterized by a concentric arrangement of the maximum density gradients concerning a minimum at the center, while in the sections Hc = 35 km (Fig. 4, c) the root part of the structure is characterized by intense maximum (µz) and its sharply reduced sizes specify a mainly horizontal distribution (“spreading”) of paleomagmatic melts in the mid-crustal layer and transfer of upper crust tectonic plates above them. The outline of the central-type structure repeats arc negative magnetic anomalies, whose mutual location is characterized by concentric zonality (Fig. 4, e). These anomalies are frequently associated with counter-inclined faults resulting from the downwarping of domes of volcanotectonic structures. As inferred from a total of geological and geophysical data, the Aldan-Zeya structure corresponds to the generalized parameters of a lithosphere plume with the collapsed head (Anfilogov, 2005). In addition to concentric distributions of density gradient (Fig. 4, a–d), magnetic, and heat-flow (Fig. 4, e) anomalies, this structure is expressed in intense downwarping (10 mm per year) of the Earth’s surface (Map of modern..., 1983), and concentric-zonal distribution of

vectors of seismotectonic pressure (Fig. 4, e) and presence of Pliocene-Quaternary alkaline basalts on the surface (Krasny et al., 1999). The largest and most intense gold dispersion halo is found in the center (Vyunov and Stepanov, 2004). Most likely, it is the result of a long and stable transfer of gold to the center of the tectonomagmatic structure. Subsidence (collapse) of the Aldan-Zeya central-type structure manifests itself in concentration of maximums of µz-parameter at flanks of the structure (Fig. 4, a, b) and downwarping of µz-isolines above its center in sections 2-2 and 3-3 (Fig. 3). The displacement of the concentral and external contour of ring µz-anomalies in the lower crust layer (Fig. 4, b) relatively upper crust anomalies (Fig. 4, a) can be the result of the transfer of the lower crust layer in northeast direction for a distance of 150–180 km, which was possible owing to Late Mesozoic and Cenozoic magmatic melts. Formal attributes of large-scale horizontal displacements of the lower crust layer of the Stanovoy system and southern regions of the Aldan Shield (Fig. 4, b) agree with ideas (Parfenov et al., 1996, 1999; Sorokin, 2001) about “oblique” (from southwest to northeast) joining of the Amur microcontinent to the North Asian craton, involved with the underthrusting of rigid tectonic plates (structural wedges or “indentors”) beneath the North Asian craton, and formation of thrusts and tectonic nappes in the superficial layer. Similar mutual relations of upper and lower crust structures are revealed in the Sikhote Alin (Yushmanov and Petrishchevsky, 2004), Magadan Region (Migursky et al., 2005) and Kamchatka (Petrishchevsky, 2006), where they characterize the Late Mesozoic and modern active margins of lithosphere plates. In the middle reaches of the Vitim River, µz-anomalies in the section Hc = 35 km (Fig. 4, c) suggest the existence of one more deep tectonomagmatic structure of the central type, which is expressed on the surface (Fig. 1) in the concentric arrangement of Paleozoic granitoids. The structure is located in the area of dome-like thickening of the first high-gradient layer of the crust (section 4-4, Fig. 3), concordantly to which contrasting seismic velocity discontinuities rise, coinciding with the bottom of the sialic crust layer (sections 1-1 and 4-4 in Fig. 3). Less precisely the structure is expressed in section 1-1, which is caused by orientation of this section along the Baikal rifting zone (Geology..., 1984, 1985). Our data (Figs. 1, a, 3, 4, b, c), suggest that the Mid-Vitim central-type structure, as compared with the Aldan-Zeya structure, has a deeper erosion section and, obviously, more ancient age of crystallization of deep magmatic melts. The maximum of density gradients in subcrustal layer (Fig. 4, c) can correspond to the root part of ancient (Middle Paleozoic?) plume, whose supradomal part was ruined completely by the Late Paleozoic and Early Mesozoic collisional and Cenozoic rifting processes. The central-type structures are embodied in the field of seismicity (Fig. 4, a–c). On the territory of the Aldan-Zeya plume the spatial distribution of earthquake epicenters, located mostly in the middle and lower crust layers (Tectonics..., 2001), conforms to the concentric zonality of µz-anomalies: The flanks of structure are more seismic, and the inverted part above the dome is less seismic. The vectors of compression

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measured in fields of strong earthquakes of this area also regularly change their orientation along the circular contour coinciding with the external borders of the plume (Fig. 4, e). The increased concentration of the earthquake epicenters within and close to the borders of µz-maximums (Fig. 4, a–c) can result from the interaction of rigid subcrystalline blocks and plates above the lower-viscosity layer at the base of the crust (Fig. 3). In a similar way in the middle and lower reaches of the Amur River the medium- and high-magnitude earthquakes (M > 4.5) are statistically related to maximums of µz-parameter at depths of 25–35 km (the lower crust layer) (Petrishchevsky, 2004a), and in eastern areas of the Sea of Okhotsk at depths of 80–120 km (subasthenosphere layer in the upper mantle) (Petrishchevsky and Zlobin, 2004). Within the Mid-Vitim central-type structure, where igneous rocks were crystallized and acquired rigidity in earlier time (Paleozoic), only deep section of structure (Fig. 4, c) correlates with the distribution of earthquake epicenters. The distributions of deep density inhomogeneities in the crust and subcrustal layer of the upper mantle (Fig. 4, a–c) explain different tectonic origins of the seismicity fields of the Baikal and Olekma-Stanovoy seismic zones. On the eastern flank of the Baikal zone the linear character of the seismicity field and heat-flow anomalies is clearly correlated with rifting processes (Krasny, 1988; Geology..., 1984, 1985), while the modern geodynamic activity in southern Yakutia and in the Upper Amur region is characterized by concentric zonality (Fig. 4), connected with structures of the Cenozoic plume. The eastward continuation of deep structures of the Baikal rift is limited by the Chara River basin (120th meridian). This feature coincides with the western boundary of the Aldan Shield (Figs. 1, 4, d). The spatial distributions of density gradients in deep layers of the tectonosphere (Fig. 4, d) suggest the existence of a large asthenosphere lens extending beneath the North Asian craton (northward) and Baikal-Vitim superterrane (westward). The spatial parameters of the roof of the asthenosphere lens are reflected on the sections of the density gradients model at a depth of 70 km (Fig. 4, d), which clearly shows that the asthenosphere rise is spatially connected with the Aldan-Zeya plume. The supradomal part of the plume (Fig. 4, a–c) is displaced northeastward for a distance of 180–200 km from the center of the asthenosphere lens (Fig. 4, d). Comparison of the schemes of density gradient distributions at different depths (Fig. 4, a–d) shows that the greatest displacement was experienced by the lower crust layer. The southern flank of the asthenosphere lens extends beyond the study area, and in northeastern China it is correlated with the Emuarshang volcanic belt and Songliao basin (Krasny et al., 1999). In the deeper section of the tectonosphere (Hc = 90 km, Z0 = 100–150 km) the sublatitudinal direction (concordant with the southern borders of the North Asian craton) of the density gradient isolines and a gentle increase in µz-parameter to the south as well as the distribution character of centers of density inhomogeneities within the lithosphere-asthenosphere transition layer (Fig. 2, c) are in agreement with the supposed underthrusting of the lower lithosphere layer of the Amur

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microcontinent beneath the Baikal-Vitim superterrane and the North Asian craton (Gusev and Khain, 1995).

Discussion of results Analysis of the distribution of density inhomogeneities in the crust and upper mantle of northeastern Transbaikalia and the Upper Amur region modeled from only gravity anomalies without previous supplementary data and preliminary geotectonic hypotheses provided new information about deep tectonic structures of the southeastern framing of the North Asian craton in the depth interval of 10–120 km, which can be used in further studies of the relationship between superficial tectonic complexes and deep structures, metallogeny and geodynamics of this area. The applied approach to the gravity anomaly interpretation essentially differs from traditional methods of density fitting because it has no elements of “adjustment” of analytical constructions to the existing information bases of the geological-geophysical data and tectonic concepts. As a result, the final geological conclusions are more objective. With this approach, no analysis of previous geological-geophysical data is made before the interpretation as it is practiced in traditional gravity modeling of deep structures; the analysis is carried out at the final stages of research, after formalized modeling of density inhomogeneities in the geologic space. In the last case the information importance of the received models is measured not by the “accuracy” of coincidence of observable gravity anomalies with calculated ones in the iterative solution of a forward gravity problem but by degree of conformity of unequivocally reproduced formal distributions of density inhomogeneities to a complex of previous data. So, for example, non-a priori detection of two layers with low values of density gradient in the tectonosphere of Transbaikalia and the Upper Amur region is in agreement with seismic and magnetotelluric data about the presence of zones with low seismic velocity and electrical resistance in the same depths intervals. Hence, the minimums of density gradient can be related to layers of decreased viscosity. Similar results were obtained by the author in the area of the Sea of Okhotsk, where anomalies of low values of µz-parameter coincide with zones of partial melting recognized there from magnetotelluric and heat-flow data beneath the Upper Cenozoic volcanic belts (Petrishchevsky, 2006) and asthenosphere (Petrishchevsky and Zlobin, 2004), and anomalies of high values of µz-parameter coincide with contours of the seismically active plate in the subasthenosphere layer of the upper mantle (Petrishchevsky and Zlobin, 2004). The location and spatial parameters of the Aldan-Zeya plume determined when studying non-a priori distributions of density inhomogeneities in the crust and upper mantle, are clearly correlated with anomalies of heat-flow, arc magnetic anomalies, field of seismicity and vectors of mechanical pressures caused by strong earthquakes. The tectonic inversion (downwarping) of the plume roof, supposed from distributions of formal parameter (µz) in the structure section, agrees with the character of modern vertical movements of the Earth’s

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surface established by repeated altitude measurements. The listed attributes are external criteria of objectivity of the considered models and prove the existence of a plume. The fact that the pattern of spatial distributions of deep density inhomogeneities (orientations of Z0 and µz-isolines, character of grouping of anomalies) matches the dominant orientations of fractured, folded and magmatic structures of appropriate superficial tectonic complexes (axes of folds, faults, laminated zones, dikes and magmatic bodies) in many areas (Petrishchevsky, 1987, 1988, 2003, 2004a, 2004c, 2006; Yushmanov and Petrishchevsky, 2004) allows us to study deep tectonic and magmatic structures of the crust beneath thrusts, volcanic covers, and tectonic nappes. An abrupt change in the distribution of density inhomogeneities in adjacent layers of the crust and upper mantle is a stable attribute of discontinuities of structural-material (or tectonophysic) complexes (Petrishchevsky, 1987, 1988, 2004c). The new data about the deep structure of the crust and upper mantle of the southeastern framing of the North Asian craton allow the following conclusions: 1. Structural distributions of density inhomogeneities in the tectonosphere show the tendency toward a regular change (from above downward) of prevailing orientations of Z0-isolines from meridional and sublatitudinal through northwestern to northeastern ones, which corresponds to the dominating vectors of the Archean, Paleozoic, and Late Mesozoic tectonic stresses. 2. The structural features of the middle and lower crust layers of the Aldan Shield and Stanovoy terrane are characterized by the northwestern orientations coinciding with the orientation of fractures, folds and lamination planes of the granulite-gneiss complexes of the Central Aldan block, transverse relative to structures of the Olekma and Batomga greenstone series. The mutual relationships between superficial and deep structures suggest the SW-NE thrust of the Paleozoic and Mesozoic upper crust complexes of the Amur superterrane upon the Aldan-Stanovoy composite terrane. The amplitudes of mutual displacement of the upper- and lowercrust tectonic masses on the southeast flank of the Central Aldan block can measure 150–300 km. The western and southwestern flanks of the Aldan Shield and Stanovoy terrane seem to be injected by the Paleozoic granitoids of the Angara-Vitim batholith to a distance of up to 150 km east of the Zhuya and Dzheltulak faults. In the same (northeastern) direction, and for the same distance, the structures of the lower crust layer of the Aldan-Stanovoy terrane are displaced (underthrust beneath the North Asian craton). 3. The density distributions in the basement of the lithosphere (at depths of 80–90 km) agree with contours of the Baikal-Vitim, Aldan-Stanovoy and Amur superterranes. In a total of geological and geophysical characteristics the West Stanovoy (or Selenga-Stanovoy) geoblock belongs to the Baikal-Vitim superterrane. The latter abuts against structures of the Aldan Shield and Stanovoy folded system in a wide range of depths (0–100 km). 4. The tectonosphere sections of the study area contain two layers with low-density gradients (layers with no sources of

strong gravity disturbances) accompanied by lower seismic velocity and electrical resistance. This correlation allows us to relate µz-minimums to layers with lower viscosity (or higher plasticity). The existence of these layers provides (and provided in the past) the opportunity of horizontal transfer of geological bodies and structures of a various rank: blocks, plates, tectonic nappes, and large lithospheric segments (terranes). 5. The material inhomogeneity of the middle and lower crust layers of the Upper Amur region is substantially determined by paleomagmatic processes and is connected with a Cenozoic central-type structure, the Aldan-Zeya plume. The concentric-zonal structure of the crust in the supradomal part of the plume correlates with a rise of an asthenosphere lens at a depth of 70 km, field of seismicity, magnetic and heat-flow anomalies.

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