PALEOMAGNETISM OF SEDIMENTARY-METAMORPHIC ROCKS OF THE AMUR PLATE

Bretshtein Yu.S.

Institute of Tectonics and Geophysics, Far Eastern Branch, Russian Academy of Sciences, Khabarovsk, 680000, Russia, e-mail: yurybr2007@yandex.ru

Paleomagnetic study of terrigenous-metamorphic rock assemblages in Primorye, Priamurye and Zabaikalye disclosed a complicated kinematic pattern of tectonic-stratigraphical terranes within the Amur Plate during the Phanerozoic. Low (near-equatorial) paleolatitudes are established for the formation of rocks of the major part of geoblocks and their “intraplate” rotations without sig-nificant latitudinal relative displacement are revealed. Their clearly observed scatter in declination along the arc of a great circle took place about the rotation center (Euleur pole) which coincides with average coordinates of the area under paleomagnetic study. The general rotation trend of an entire collage of terranes of the Amur Plate (AP) was directed counterclockwise, the rotation angle attaining about 120º during the Phanerozoic. The most latitudinal displacement of AP relative the Siberian plate is observed in the Midpaleozoic. In accordance with the direction of polarity chosen the apparent polar wander path of pole (APWP) for the AP appears to be more distant from APWP for the Siberian Craton and closer to APWP for the North China Plate (NCP). This may indicate that some terranes of the Amur Plate could correspond to the passive continental margin of NCP during the Early- and Middle Paleozoic.

In the marginal parts of the basement and collisional zones of the Siberian (SP) and North China (NCP) mega plates the different-aged and geologically non-uniform structural-facial zones are widespread which form a complicated mosaic of tectonic blocks (domen), namely the Central Asia Orogenic Belt, the eastern part of which is known as the Amur Plate (AP) [1]. The latter is represented by a system of differently oriented orogenic belts, composed of a collage of tectonostratigraphic terranes of different age and genesis, which formed the Eurasian supercontinent during the Late Jurassic-Cretaceous collision of the North China continent with the Siberian Plate and the Proterozoic terranes of its outer framing [2 and others]. It is a fact of common knowledge that the lithospheric plate of the “Paleoasian” Ocean included microcon-tinents and terranes of the Gondwana origin, which had formed the Amur Plate as a result of amalgamation during Mid- and Late Phanerozoic (Fig. 1).

Fig. 1. Sketch-map of location of major geotectonic plates of Eurasia Including SP, АP, NCP, SCP corresponding respectively to the Siberian, Amur(ian) ,

North-China and South China plates.

In the recent decade paleomagnetic study has been carried out in the area covering South-West Primorye, Priamurye and East Zabaikalie. A large number of the geological sections composed of different sedimentary

and metamorphic rocks forming the Early- and Middle Phanerozoic tectonostratigraphic terranes have been studied. As a result of the study it has been disclosed a complicated pattern of development (geokinematics) of isolated intraplate geoblocks having been amalgamated in the entire Amur plate before the Mesozoic. It has been established relative stability of paleolatitudinal positions and “intermediate” location of terranes of the Amur plate between the Siberian Craton and the North China Plate during the Late Ruphean and Early Cambrian. Relatively compact location of the above geoblocks (for the adopted polarity version) could be determined by their belonging to the single supercontinent Rodinia during the entire Late-Proterozoic – Early Cambrian [3]. Paleomagnetic study of Middle Paleozoic (Silurian and Devonian), and, also, Middle Mesozoic (Jurassic) terrigenous-carbonaceous rocks which was conducted during 2008-2010, made it possible to widen the scope of the investigated objects and refine the previously obtained data. Based on the results of field and laborato-ry paleomagnetic investigations (stepwise temperature demagnetization to 690° С and demagnetization using alternating magnetic field to 100 mT) performed at the most of the geological sections, it has been distin-guished prefolding high-temperature ChRM in the rocks (In carrier –magnetite predominantly). For the ChRM distinguished in these rocks gentle inclinations are characteristic (units and first tens of degrees) of the direct and inverse sense in SSW and NNE directions of declination. The predominantly SW, more rarely S- and SE declinations in stereographic projection were chosen as directions of direct (positive) polarity. The corresponding graph-analytic tests which determine a degree of retention of prefolding ChRM in the rocks [4-6] are positive. R-test [7] performed in the major terranes for average directions of N and R rocks studied in the terrane localities, is positive in the majority of cases and corresponds (more often) to category C (more rarely – to categories A and B). As a result of investigation, it has been established that the general trend of the latitudinal displacement of terranes from the Proterozoic to Middle Paleozoic was restricted by near-equatorial latitudes of both hemis-pheres [8] (Fig. 2).

Fig. 2. Paleolatitudinal positions of the Amur (1), North China (2) and Siberian (3) plates on a geological time scale. А, В, С denote the average contemporary geographical latitudes of the corresponding Plates. The calculated paleomagnetic pole positions for the AP terranes in the Proterozoic to Cambrian timespan, with some minor exceptions, do not differ within the range of statistical error and form a swarm of directions attributed to South East Asia and the contemporary Indian Ocean southwest of Australia (for the chosen ChRM polarity in the west compass points of the stereographic projection). Practically, a similar pattern has been retaining for the Silurian and Devonian, paleomagnetic pole positions are only westward displaced toward the present-day Africa [Fig. 3].

Fig. 3. Paleomagnetic pole positions for the Amur (AP) and North China (NCP) plates. Red (non-filled) symbols and red (black) lines denote paleopole positions and (APWP) for the AP (NCP). Dashed (solid) lines are APWP projections on the Lower (Upper) Hemisphere. The fill color of the confidence ovals corres-ponds to the Legend. The non-filled thickened confidence oval contours correspond to the paleopole posi-tions for the rocks from those sections, whose paleomagnetic age, presumably, reflects syn- and post-fold remagnetization, otherwise the geological age was determined with a lack of reliability. In the Figure the shadows of colors for the Triassic and Jurassic (missing in the Legend) correspond to the epoch of these geological periods. Table 1 shows the main paleomagnetic data for the Cambrian, Silurian, Devonian and Jurassic rocks of the Amur, Siberian and North China Plates. It is not expelled that if the age of rocks was rather approximately (controversially) estimated within some sections, their ChRM could be formed within a broad timespan range (during which the geomagnetic field polarity could change), and owing to this, the problem of precise age correlation of paleomagnetic data still persists. This refers either both to our data and not numerous, in part contradictory foreign data (for example [9, 10]). It is quite probable that in separate cases, particularly, for the most ancient (Proterozoic and Cambrian) rocks, for which the magnetostratigraphic scale is devel-oped to a relatively less degree, the “different-pole” positions of the corresponding terranes and plates can be compared. When choosing positive polarity in ENE compass points of the stereographic projection the dis-tinctions in the pole positions for the separate AP and NCP terranes are more obviously visually explained by clockwise and counterclockwise some geoblocks rotations relative to sampling localities within these plates. In this case the proximity (up to a complete coincidence) of these positions occurs for the NCP with her Late Paleozoic-Mesozoic segment of APWP, when the Early and Middle Paleozoic poles are “superim-posed” on the Mesozoic ones. That is, in the case if we assume the NE directions of the distinguished ChRM-components to be the positive polarity, then the APWP path becomes a rather “complicated” one though somewhat shorter [11-13]. Giving favor to WSW compass points of location of the ChRM vector projections (for AP and NCP) in the capacity of the direction of the positive polarity yields in a more “con-secutive” (without different-aged pole coverage), though to a certain degree more lengthened APWP trend [14].

TABLE 1

PALEOMAGNETIC DATA FOR LATE PROTEROZOIC, EARLY CAMBRIAN, DEVONIAN AND JURASSIC ROCKS OF THE AMUR, SIBERIAN AND NORTH CHINA PLATES

NOTE: N is the number of poles, λ, φ denote the calculated average coordinates for plates including the lon-gitude and latitude; φm, Λ, Ф, A95 denominate the geomagnetic latitude (paleolatitude), longitude and lati-tude, the confidence oval radius for the average pole at confidence level 1-p=0.95. The average coordinates and paleolatitudes for the AP are calculated from the paleomagnetic pole coordinates of the terranes and sampling localities. When calculating paleomagnetic parameters for the SP and NCP the Global Paleomag-netic Database GPMDB Version 4.6 was used (till the year 2005). All the digital data are given in the de-grees of spherical coordinates. The predominant negative gentle inclination of our Cambrian rocks in SW compass points of the stereo-graphic projection may indicate that the AP was located either the Southern Hemisphere during the dominat-ing N-polarity time span or in the Northern Hemisphere during R-polarity time span. The latter case “prede-termines” the paleomagnetic pole positions in the Southern Hemisphere (during the epoch of reversed polari-ty of the geomagnetic field) whereas positions of the AP and NCP terranes, as well, in the Northern Hemis-phere, respectively. The estimation of relative displacements and rotations of isolated geological blocks shows the lack of the statistically significant latitudinal drift of most of the geological objects of the Amur plate with respect to the North China Plate since the Devonian. It is also recorded permanently increasing significant displacement of both plates with respect to the Siberian Plate (Table 2, Fig. 4). When comparing paleomagnetic pole positions for the Amur and North China Plates during different time-spans in the Phanerozoic, a clearly observed scatter by declination along the arc of a great circle takes place about the rotation center ,which coincides approximately with the study areas. The rotatiojn parameters of the AP terranes are significant in terms of statistics only in the Devonian (with tespect to the NChP) and in the Jurassic (with respect to the SP).

N

AGE, N, COORDI-NATES λ, φ φm Λ Ф A95

A. AMUR PLATE

2 Cambrian, N = 21, λ =130.4, φ = 48.7 0.4 82.1 -30.8 12.3

3 Devonian, N = 17, λ =124.3, φ = 50.1 -1.9 34.9 -3.0 23.4

4 Jurassic, N = 5, λ =123.9, φ = 52.2 38.5 349.1 72.4 19.7

B. NORTH CHINA PLATE

2 Early- Midcambrian, N=7, λ = 114.8, φ = 38.0 19.5 143.3 -27.2 14.2

3 Devonian, N = 3, λ = 113.0, φ = 37.8 2.7 47.6 -24.3 17.1

4 Jurassic, N = 3, λ =117.2, φ = 37.2 27.8 234.4 72.4 7.7

C. SIBERIAN PLATE 2 Early-, Middle Cambrian

N=10, λ=106.4 φ = 65.8 -14.1 137.1 -35.6 3.8

3 Devonian, N = 25, λ =106.9, φ = 61.6 34.3 140.6 11.5 9.7

4 Jurassic, N = 5, λ =123.8, φ = 62.5 82.9 133.9 68.3 16.8

TABLE 2 COMPARISON OF KINEMATIC PARAMETERS

OF PLATE MOTIONS FROM THE LATE PROTEROZOIC TO THE JURASSIC

NOTE: F is the latitudinal displacement of the AP with respect to the NCP and SP, recalculated from the pole coordinates (for the NCP in the numerator and for the SP in the denominator, respectively); sign (+) or (- ) means the displacement of the calculated position (фm) relative to the position obtained northward or southward (if to suggest tectonic community of the compared plates); B, C are the results of the similar “formal” calculation for the NCP and SP from the paleopole coordinates of one another; R is the rotation angle of plates; (ΔF), (ΔR) are the confidence intervals (standard angular deviations) for the confidence level 1-p = 0.95. Statistically insignificant values of the parameters F (ΔF) and R (ΔR) are shown in italics.

Fig. 4. Palinspastic reconstruction of spatio-temporal evolution of plates in the Phanerozoic.

N

AGE, N, COORDINATES λ, φ F ΔF R ΔR

A. AMUR PLATE

1 Cambrian, N = 21 λ =130.4, φ = 48.7

13.7 5.9

13.3 9.4

51.7 45.4

11.1 9.5

2 Devonian, N = 17, λ =124.3, φ = 50.1

12.1 47.4

21.2 18.6

16.1 67.5

23.3 20.3

3 Jurassic, N = 5 [8], λ =123.9, φ = 52.2

4.6 35.2

15.0 18.0

38.9 28.7

20.1 46.5

B. NORTH CHINA PLATE 1 Early to Mid Cambrian

N = 7, λ = 114.8, φ = 38.0-

6.1 -

10.8 -

8.3 -

11.4

2 Devonian, N = 3, λ =124.3, φ = 50.1

- 56.9

- 15.0

- 73.5

- 19.1

3 Jurassic, N = 3, λ =117.2, φ = 37.2

- 30.2

- 12.2

- 6.3

- 23.9

C. SIBERIAN PLATE

1 Early-to Middle

Cambrian, N = 10, λ =106.4, φ = 65.8

6.9 -

10.1 -

7.2 -

10.9 -

2 Devonian, N = 25, λ =106.9, φ = 61.6

26.3 -

11.4 -

86.5 -

15.3 -

4 Jurassic, N = 5, λ =123.8, φ = 62.5

-31.6 -

13.0 -

3.5 -

99.6 -

The study was partially supported by the grant of the Far Eastern Branch, Russian Academy of Sciences (Project No. 09_3_А_08_442)

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