Mikhail L. Bazhenov

Paleomagnetic implications on the evolution of the tethys belt from the atlantic ocean to the pamirs since the triassic

Abstract We first re-examined the apparent polar wander curves for stable Eurasia and Africa since the Triassic. These curves were then combined together with curves of North and South America according to the kinematics of the Atlantic ocean and a synthetic polar wander curve was given. Then, most of the paleomagnetic results from the Tethys mobile belt, from the Atlantic to the Pamirs, were analysed. Several groups of plates, microplates and blocks can be seen. First, relatively stable regions like Maghreb and Sicily, which have not moved much. Then we have a group formed by Iberia, Sardinia, Italy and, to a lesser extent, Corsica and the Western and Central Alps. For these blocks, movements are anticlockwise rotations chiefly driven by the anticlockwise rotation of Africa, but they are sometimes stronger. To the east, a major change takes place. The north of the Aegean Sea and the Ionian zone are clockwise rotated and these rotations are recent: Oligocene-Miocene for the first part, Pliocene to the present for the second part. A major problem arises in Turkey, Caucasus and Iran. Paleomagnetic results indicate a position far to the south of Eurasia, and, at the same time, geological evidence is in favour of a position close to Eurasia. We discuss these discrepancies.

A paleomagnetic study of Tertiary formations from the Kyrgyz Tien‐Shan and its tectonic implications

As part of a general investigation of Cenozoic deformation in Central Asia, we studied the paleomagnetism of Tertiary red beds and lava flows from intermontane basins in the Tien-Shan region of Kyrgyzstan. We collected 532 cores and hand samples from 78 sites at 12 localities and progressively demagnetized them, thermally or with alternating fields. For most sites, there are magnetic components with unblocking temperatures higher than 600°C. We infer that the magnetic carriers are mainly hematite and magnetite. For most localities, the high-temperature component appears to predate tectonic folding. For all localities, mean inclinations are shallower than expected from apparent polar wander paths. Inclination anomalies range from 16°±5° for the Issyk-Kul basin, to 26°±7° for the Fergana basin. If due to changes in latitude, these anomalies imply at least 2000 km of northward displacement of the Tien-Shan during the Tertiary, for which there is no tectonic evidence. We thus consider that the paleomagnetic reference directions cannot be directly compared with our Tertiary data. We explored other possible reasons for this anomaly, which has also been reported from other parts of the Alpine belt, but we could not find a satisfactory explanation. Absolute rotations cannot be accurately determined, because of problems with the reference direction. Nevertheless, the mean declination for the Fergana basin lies counterclockwise by 20°±11° from the mean declination of the Issyk-Kul basin. This result is consistent with the counterclockwise rotation inferred for the Fergana basin from the pattern of Cenozoic faults and folds. It suggests a Cenozoic right-lateral displacement of 110±60 km on the Talas-Fergana fault.

Paleomagnetic evidence for Cenozoic block rotations in the Tadjik depression (Central Asia)

This paper presents results of a paleomagnetic study of Oligo-Miocene red beds of the Tadjik depression in Central Asia. We sampled about 530 cores at 69 sites and six localities across the depression and along the western border of the Pamirs. Samples were thermally demagnetized and high-temperature components appear to predate folding of upper tertiary age. Throughout the depression, paleomagnetic inclinations are consistent with those observed on the stable Turan platform, at the western margin of the depression. However, they are shallower by about 30° than the inclination predicted from the reference apparent polar wander path. This appears to indicate a 23° difference in latitude, which is incompatible with paleogeographic reconstructions for the Tertiary. A sound interpretation of this anomaly would require a better-constrained Tertiary paleomagnetic reference for Asia. Inside the Tadjik depression, paleomagnetic declinations are all significantly rotated, counterclockwise with respect to those measured on the Turan platform. The eastern part of the depression is a domain of large rotation (52°±13° to 46°±15°), whereas smaller amounts of rotation have occurred in the western part (27°±14° to 14°±15°). The similarity between Tertiary and Cretaceous data available for the area shows that rotations have occurred since the Miocene. Little or no paleomagnetic rotations are observed in the ranges bordering the northern and western parts of the depression. Paleomagnetic and structural data suggest that block rotations in the Tadjik depression are associated with indentation of the Pamirs into stable Asia. At a larger scale, observed rotations are compatible with a model of regional sinistral wrenching, along a strip running from the Gulf of Oman to Lake Baikal.

Paleomagnetism of Paleogene basalts from the Tien Shan, Kyrgyzstan: rigid Eurasia and dipole geomagnetic field

Several conflicting explanations, invoking persistent non-dipole fields or tectonic motion on various scales, were suggested to account for anomalously low paleomagnetic inclinations in Cenozoic rocks of Central Asia. In order to better understand this problem we undertook a paleomagnetic study of Paleogene basalts from a part of the Tien Shan close to the China^Kyrgyzstan border. Stepwise thermal demagnetization showed that only a single-component remanence of reversed polarity is present in these rocks above 200^260‡C. The overall mean direction of this remanence (D = 194.6‡, I =354.0‡, K95 = 3.8‡, n = 18 sites) agrees well with the Eurasian reference directions for the Paleogene which is in sharp contrast with most earlier published Cenozoic inclinations from Central Asia which are 20^30‡ shallower than the expected values. Good agreement of the reference inclinations and those from basalts rules out models which invoke non-dipole fields and/or large-scale tectonic motions for explanation of the Cenozoic inclination anomaly. Instead, a mechanism related to natural remanent magnetization acquisition in redbeds is strongly indicated. The above conclusion also implies that the differences between the reference data and measured inclinations in Cretaceous redbeds from Central Asia are also of non-tectonic origin and a revision of our views on evolution of this region is required. fl 2002 Elsevier Science B.V. All rights reserved.

Permian paleomagnetism of the Tien Shan fold belt, Central Asia: post-collisional rotations and deformation

Permian volcanic and sedimentary rocks were sampled from eight localities in the western and central parts of the Tien Shan fold belt. High-temperature, sometimes intermediate-temperature components isolated from these rocks at seven localities after stepwise thermal demagnetization are shown either to predate folding or be acquired during deformation; the conglomerate test at some localities is positive. The observed inclinations fit rather well with the Eurasian reference data, whereas the declinations are strongly deflected westward; westerly declinations have already been observed from the other parts of the Tien Shan (from the Turan plate in the west to the northern rim of Tarim and the Urumque area in the east). Our analysis shows that a considerable counterclockwise rotation of the Tien Shan fold belt as a rigid body is geologically improbable. We hypothesize that a sinistral shear zone existed over the fold belt thus accounting for systematically westerly declinations. This zone is about 300 km wide and is traced along the Tien Shan fold belt for 2500 km. A large area of Permian alkali magmatism in the West and Central Tien Shan is interpreted as an extensional domain conjugated with the shear zone. This shear zone can be accounted for by translation of the Kara Kum and Tarim blocks along the Eurasian boundary after their oblique collision in the Late Carboniferous. Two phases of rotation are recognized in the Tien Shan. The earlier rotation took place under shear strain during the D3 stage of deformation in the Artinskian‐Ufimian. The later rotation is connected with transpression (D4 stage of deformation) and could occur from the Late Permian to Early Jurassic.

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.

Cretaceous paleomagnetism of the Fergana Basin and adjacent ranges, central Asia: tectonic implications

Abstract Paleomagnetic results have been obtained from Cretaceous red beds sampled at fifteen localities in the Fergana basin and adjacent ranges of the Tien Shan (Central Asia). The natural remanent magnetization of these rocks was accounted for by an Late Cretaceous overprint of normal polarity at fourteen localities and dual polarity, presumably primary component, at seven localities. No definite correlation was found between the declinations of the two components and structural trends; thus implying no oroclinal bending of the arcuate structures in the area studied. Inclination data are difficult to interpret due to probable tectonic movements, inclination errors of various types, and ambiguities in the Cretaceous reference data. All locality mean declinations were found to be deflected westward with respect to the Eurasian paleomeridian. The proposed counter-clockwise rotation of the Fergana block during the late Cenozoic is shown to account for the main features of the Alpine tectonics. In particular, the 200 km offset along the Talas-Fergana fault is connected with rotation, which contradicts the common idea that this displacement is of Late Paleozoic age. The horizontal movements are most probably not older than 10 Ma.

Paleomagnetism of Paleogene rocks of the Central—East Kamchatka and Komanorsky Islands: tectonic implications

Abstract Paleomagnetic investigation of Paleocene—Eocene to Miocene rocks has been carried out at five localities in the Eastern Kamchatka and Komandorsky Islands. After thermal and af cleanings, consistent paleomagnetic directions were isolated for a number of formations. Data from the Kronotsky Peninsula and Medny Island passed both the fold and reversal tests and thus can be considered as reliable; other results are of lower quality. All the results were derived from sedimentary rocks, and thus the corresponding paleolatitudes may be biased due to inclination errors. A limited collection of basalts yielded inclination values systematically lower than those in sediments; this anomaly was tentatively explained by shape anisotropy. The correctness of paleolatitude estimations was thus left unproved. Several models of the Northern Pacific tectonic evolution were suggested and analyzed, but the authors failed to find a model that was fully compatible with paleomagnetic, kinematic and geological data

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.

Fold test in paleomagnetism: new approaches and reappraisal of data

It has been shown that any modification of the fold test leads to a definite conclusion about the magnetization nature if several basic assumptions are observed and the statistical test itself is correctly formulated. It has been demonstrated that the test based on comparison of concentration parameters (McElhinny, 1964) is controversial and may lead to erroneous conclusions. Two modifications of the fold test are proposed. The first one is based on the division of the bedding poles distribution into groups, calculation of the corresponding paleomagnetic group-means and their testing by the F distribution. This test is similar to that proposed by McFadden and Jones (1981), but can be used more widely. The second modification is based on correlations of unit bedding poles and paleomagnetic vectors and can be applied to any collection, provided the number of the unit vectors is 10 or more. All three tests were compared with the aid of simulated and real collections and it was shown that the latter two are more sensitive than the first one.