Natalia M. Levashova

Paleomagnetism of mid-Paleozoic subduction-related volcanics from the Chingiz Range in NE Kazakhstan: The evolving paleogeography of the amalgamating Eurasian composite continent

The tectonic and paleogeographic evolution of the Ural-Mongol belt between the cratons of Baltica, Siberia, and Tarim is key to the formation of the Eurasian composite continent during Paleozoic time, but the views on this complicated process remain disparate and sometimes controversial. A study of three volcanic formations of mid-Silurian, Lower to Middle Devonian, and Middle Devonian age from the southwestern boundary of the Chingiz Range (NE Kazakhstan) yields what are interpreted as primary paleomagnetic directions that help clarify the evolution of the belt. A single-polarity characteristic component in mid-Silurian andesites yields a positive intraformational conglomerate test, whereas dual-polarity prefolding components are isolated from the two Devonian collections. Post-folding, reversed-polarity overprint directions have also been isolated and are likely of Permo-Triassic age. These new data can be evaluated together with previously published paleomagnetic results from Paleozoic rocks in the Chingiz Range, and allow us to establish with confi dence the polarity of each result, and hence to determine the hemisphere in which the area was located at a given time. We conclude that NE Kazakhstan was steadily moving northward, albeit with variable velocity, crossing the equator in Silurian time. These new paleomagnetic data from the Chingiz Range also agree with and reinforce the hypothesis that the strongly curved volcanic belts of Kazakhstan underwent oroclinal bending between Middle Devonian and Middle Permian time. A comparison of the Chingiz paleolatitudes with those of Siberia shows, insofar as the sparse data allow, similarities between the northward motion of the Chingiz unit and that of Siberia, which imposes important constraints on the evolving paleogeography of the Ural-Mongol belt.

Palaeomagnetism of Palaeogene volcanic series of the KamchatskyMys Peninsula, East Kamchatka: the motion of an active island arc

Abstract The Paleocene Tarkhov and Eocene Baklan Formation are the lower and upper members, respectively, of a continuousisland-arc sequence at the Kamchatsky Mys Peninsula on the Pacific side of the Kamchatka Peninsula. We studied Paleocene tuffs at five localities and Eocene tuffs and basalts at nine localities from beds of various attitude. Thermal demagnetization revealed consistent directions of the characteristic component (ChRM) of natural remanent magnetization in both formations. Normal and reversed ChRM directions in the Paleocene rocks are roughly antiparallel thus constituting a positive reversal test; at the same time, the fold test ( McFadden and Jones, 1981 ) is not straightforward as locality-means display a banana-shaped distribution, most probably because of local rotations. Inclination-only data, however, show considerable decrease in dispersion after tilt correction thus constituting a positive fold test and implying a prefolding and most prprobably primary age of the ChRM in these rocks. The formation-mean inclination of 57.5 ± 3.8° calculated with the aid of inclination-only statistics corresponds to a palaeolatitude of 38.1 ± 4.1°N. ChRM directions isolated from the Eocene Baklan Formation are well grouped and pass both the fold and reversal tests; the formation-mean inclination of 65.0 ± 4.9° corresponds to a palaeolatitude of 47.0 ± 6.4°N. Geological data point to uninterrupted accumulation of the studied island-arc complex and, hence, to continuous activity of the subduction zone and related island arc. The difference between the Paleocene and Eocene palaeolatitudes of 8.9 ± 6.3° is statistically significant, thus implying an absolute motion of the active subduction zone and related island arc. During the Paleocene to Eocene, no other active subduction zone existed between the studied area and Eurasia. We think that the studied island arc belonged to the Pacific plate and was moving northwestward, while the oceanic part of the Eurasian (or North American) plate was subducted southward underneath this arc; this island arc became inactive in the Late Eocene and was transported with the Pacific plate which started subducting under the Eurasia margin since that time.

Oroclinal bending of the Middle and Late Paleozoic volcanic belts in Kazakhstan: Paleomagnetic evidence and geological implications

Paleomagnetic data on Middle- and Late-Paleozoic rocks from the central part of the Ural-Mongolian Belt in Kazakhstan are considered. The primary remanences in the Permian rocks and secondary magnetization components of the same age in pre-Permian rocks of central and northern Kazakhstan are not rotated relative to the East European Platform. In southern Kazakhstan adjoining the Tien Shan almost all data point to large, up to 90°, counterclockwise rotation of blocks. These rotations, related to the regional wrench fault zone, must be subtracted from older paleomagnetic data to ensure their correct interpretation. The paleomagnetic declinations in Upper Carboniferous rocks coincide more or less over all of Kazakhstan, whereas the Silurian and Early Devonian declinations in the north and south of Kazakhstan differ approximately by 180°. It can be suggested that the Devonian volcanic belt, having a horseshoe outline, was initially an almost rectilinear NW-trending feature. Its oroclinal bending took place in the Devonian and Early Carboniferous and completed by the Late Carboniferous. We compared the model of the Kazakh Orocline based on paleomagnetic data with the geological events in this territory. It turned out that a slow bending of an initially rectilinear subduction zone is consistent with lateral migration of active volcanism and folding inside a developing loop, whereas extension outside the loop was accompanied by subsidence and rifting. In general, the proposed model connects the main tectonic events in Kazakhstan with the movements established from paleomagnetic data.

Late Cretaceous paleomagnetism of the East Ranges island arc complex, Kamchatka: Implications for terrane movements and kinematics of the northwest Pacific

A Campanian-lower Paleocene island arc complex was sampled for paleomagnetic studies at 12 sites in the East Ranges tectonic zone of Kamchatka. After thermal demagnetization, a reversed polarity characteristic remanent magnetization (ChRM) was isolated from most volcanoclastic and basaltic units as well as from lava debris from intraformational conglomerates. The fold and conglomerate tests are positive, and the ChRM in the studied rocks is likely primary. The formation-mean inclination of 66.3° ± 3.7° corresponds to a paleolatitude of 48.7° ± 5.0°N which is about 20° lower than the Late Cretaceous North American reference values. Because northward displacement of the studied terrane is indicated by the paleomagnetic data, we examine several models of intraoceanic transport with the Pacific and/or Kula plates and coastwise transport after terrane accretion, far to the south of the present-day position of Kamchatka. Our preferred interpretation is that the studied island arc complex accumulated at about 83-79 Ma; the island arc, to which the studied terrane had originally belonged, was active between this time and 65-60 Ma. According to geological data, the docking time nearly coincided with cessation of volcanic activity, and northward movement of the island arc took place simultaneously with the volcanic activity. The absolute motion of a subduction zone should have the same direction as the overriding plate; therefore, the subduction zone related to the East Ranges island arc is inferred to have moved northward with the Kula plate or with the Kula and Pacific plates, successively, consuming either the oceanic periphery of a continental plate or some unknown minor oceanic plate. This process went on until 65-55 Ma when the island arc and related subduction zone approached the continental margin and became extinct. The proposed models also place additional constraints on kinematics of the Kula-Pacific transform plate boundary.

Permian and Triassic paleomagnetism of the southwestern Tien Shan: timing and mode of tectonic rotations

Abstract Volcanic rocks of the Lower Permian Luchob Formation and sedimentary rocks of the presumed Upper Permian Hanaka and Middle-Upper Triassic Madighen formations were studied in three localities in the southwestern Tien Shan. Primary magnetizations were isolated in Lower Permian and Triassic rocks, whereas the remanence of reversed polarity in the Hanaka Formation acquired during the Kiaman superchron may be slightly younger than the rock age. Inclinations in all three formations agree with the European reference data, but declinations are deflected westward. We argue that these deflections are due to the Late Permian-Late Triassic (but pre-Rhaetian) counterclockwise rotations and partly due to Late Cenozoic counterclockwise rotations. Inclinations in Permian and Lower Triassic rocks throughout the Tien Shan foldbelt, the Junggar basin and north Tarim are also in agreement with the reference data, implying that, within the error limits, this part of Central Asia was already welded together with Kazakhstan and the European plate in the Permian. Westward-deflected declinations throughout this region are accounted for not by movements of large blocks such as the Tarim but by domino-fashion Late Permian-Triassic rotations by strike-slip.

Paleomagnetism of a Late Cretaceous island arc complex from South Sakhalin, East Asia: Convergent boundaries far away from the Asian continental margin?

The Hokkaido-Sakhalin fold system stretches for ∼1500 km along the eastern coast of Asia and consists of several N-S trending tectonic belts. Studies in South Sakhalin show that the northern part of the Tonino-Aniva Peninsula (Ozersk unit) is a counterpart of the Tokoro belt on Hokkaido, Japan. In the eastern part of the Ozersk unit, 195 hand samples were sampled at 20 sites from Campanian-Maastrichtian tuffaceous siltstones and sandstones of the Chayka Formation of island arc affinity. Stepwise thermal demagnetization isolates a postfolding low- to intermediate-temperature component (B) of normal polarity. A high-temperature component (A) is isolated from about half the samples. Because of strong overlap of unblocking temperature spectra of these two components in other samples, direct observations and remagnetization circles were combined for calculation of site-mean directions. Component A is mostly of reversed polarity; a few samples of normal polarity are found at three sites. The presence of two polarities with approximately antipodal directions and the positive fold test imply a prefolding, and most probably primary, origin of component A. A formation-mean inclination of 45.0°±6.4° calculated with the aid of inclination-only statistics corresponds to a latitude of 26.6°±5.2°N. A similar inclination is derived from a late Cretaceous island arc complex from the Tokoro belt on Hokkaido. Since both mean inclinations are ∼30° lower than the coeval Eurasian reference value, a large-scale northward transport of the entire Tokoro island arc is inferred. We exclude the possibility of displacement with the Kula plate and coast-parallel transport; instead, intra-oceanic motion with the Pacific plate and docking at the Eurasian margin at circa 30 Ma are inferred. Combined paleomagnetic data from the Tokoro belt, the Nemuro belt and Kamchatka region imply that a system of intra-oceanic island arcs existed in the northwest Pacific in Late Cretaceous time.

LATE CRETACEOUS PALEOMAGNETIC DATA FROM THE MEDIAN RANGE OF KAMCHATKA, RUSSIA : TECTONIC IMPLICATIONS

Abstract A Late Cretaceous island arc complex was sampled for paleomagnetic study in the northern part of the Median Range of Kamchatka. In most samples, thermal demagnetization in 12–18 steps revealed either a reversed characteristic remanent magnetization (ChRM) or a remagnetization circle. Combining these yielded a positive fold test ( K s / K g =5.8) and we interpreted the ChRM to be primary. The tilt corrected overall mean direction of dec. = 105.5°, inc. = −67.0°, α 95 =4.1° for these rocks corresponds to a paleolatitude of 49.7° ± 5.6°N. This value is 20° to 22° lower than the Late Cretaceous North American reference values, and northward transport of the subject tectonostratigraphic terrane and island arc to which it had originally belonged, is strongly indicated. The observed paleolatitudes are approximately similar for all Late Cretaceous island arc complexes in Kamchatka and the southern part of the Koryak Highlands. We conclude that these complexes were originally part of the same island arc which originated 20° to 25° south of the continental margin in the Campanian, and then was transported northward until its emplacement in the Paleocene. Northward drift of the island arc coincided with its activity, and the related subduction zone also had to drift with a rapidly moving oceanic plate, most probably the Kula plate. This relationship implies that the subduction zone dipped southward and was consuming oceanic lithosphere, presumably the oceanic periphery of the North American plate.

Paleomagnetism and geochronology of the Late Cretaceous-Paleogene island arc complex of the Kronotsky Peninsula, Kamchatka, Russia: Kinematic implications

The remnants of ancient island arcs are exposed in the Achayvayam-Valagina and East Peninsulas tectonic zones of the Olutor-Kamchatka region, northeast Russia. Geochemically, Late Cretaceous to Paleogene island arc complexes of the East Peninsulas zone are similar to intraoceanic island arcs, and the East Peninsulas zone is regarded as the extinct Kronotsky island arc. Paleomagnetic and biostratigraphic studies of Late Cretaceous and Paleogene island arc rocks were carried out at 21 sites in the eastern part of the Kronotsky Peninsula, which belongs to the East Peninsulas zone. Characteristic remanent magnetizations of both polarities isolated from most sites pass reversal and fold tests; the Cretaceous result is also supported by a positive conglomerate test on lava boulders from an intraformational conglomerate. Mean inclinations of Late Cretaceous, Ypresian and Bartonian rocks correspond to paleolatitudes of 44.8° ± 8.0°N, 38.6° ± 3.5°N, and 45.1° ± 7.0°N, respectively. These values are ∼20° lower than the corresponding reference values for the North American plate for the Cretaceous and Ypresian and ∼14° lower for the Bartonian. Therefore northward transport of the Kronotsky island arc is indicated. Kinematic evolution of the Kronotsky island arc in the Late Cretaceous-Paleogene was reconstructed using published kinematic parameters. In addition, we incorporated into analysis the geological and published paleomagnetic data for the Late Cretaceous and Tertiary island arc complexes of the Cape Kamchatka Peninsula from the East Peninsulas zone and the Late Cretaceous island-arc complexes of the Achayvayam-Valagina zone. As a result, two alternative scenarios of kinematic evolution for the Kronotsky island arc are proposed. According to both of them, the Kronotsky island arc was moving with the continental plate in Late Cretaceous time and with the Pacific plate from the beginning of the Paleocene until docking with the Eurasian margin in Miocene-Pliocene time.

Precambrian microcontinents of the Ural-Mongolian Belt: New paleomagnetic and geochronological data

The knowledge on the early stages of evolution of the Ural-Mongolian Belt (UMB) (Late Neoproterozoic-Cambrian) is a key for understanding of its evolution in the Paleozoic. Unfortunately, this stage remains poorly studied. The tectonic reconstructions of the UMB for this time primarily depend on the views on the kinematics and tectonic evolution of numerous sialic massifs with Precambrian basement in the structure of the Tien Shan, Kazakhstan, Altai, and Mongolia. At present, the concept of the origin of these massifs is largely based on the lithostratigraphic similarity of the Neoproterozoic and Lower Paleozoic sections of the Tarim, South China, and Siberian platforms with coeval sections of Precambrian massifs within the UMB. New paleomagnetic and geochronological data can serve as additional sources of information on the origin and paleotectonic position of the microcontinents. In this paper, we present new isotopic datings and a new paleomagnetic determination for the Neoproterozoic volcanic rocks of the Zabhan Formation from the Baydrag microcontinent in central Mongolia. It is established that 805−770 Ma ago (U-Pb LA-MC-ICP-MS age of zircon) the Baydrag microcontinent was situated at a latitude of 47 ± 14° in the Northern or Southern hemisphere. These data provide new insights into the possible origin of the Precambrian micro-continents in the UMB. Analysis of paleomagnetic data and comparison of the age of the basement beneath various plates allow us to state rather confidently that ∼800 Ma ago the micro-continents of the UMB belonged to one of the North Rodinian plates: Indian, Tarim, or South China; their Australian origin is less probable.