Info

block. The realization that plate tectonics changed what was then the conventional view of the tectonic history of Europe to that outlined above led to the first experiments in the British Isles to try to delineate terrane boundaries during the Paleozoic. Paleomagnetic traverses across the British Isles produced a series of papers culminating in the review by Briden et al. (1973) that was the benchmark for paleomagnetic studies in Europe. Since that time further important reviews include those of Briden and Duff (1981), Torsvik et al. (1990), Torsvik and Trench (1991), and Trench and Torsvik (1991) and Torsvik et al. (1992).

Using the tectonic constraints described above, mean poles for "Stable Europe" for successive time intervals through the Phanerozoic are listed in Table 6.6 and the resulting APWP is illustrated in Fig. 6.14. For times >425 Ma the data are split into separate sets for North Britain and Baltica. As pointed out by Van der Voo (1990b, 1993), the distribution of poles for the Late/Middle Jurassic of Europe is strongly bimodal for reasons that are not understood, so they have been averaged to produce the mean at 160 Ma. Note that for times prior to 425 Ma, data from North Britain and from Baltica are listed separately. The data from North Britain are relevant when its reconstruction with Laurentia is being considered and those from Baltica are relevant to the past relationship between Baltica and Laurentia to be discussed in §7.4. The analysis of 289 studies representing 4840 sites is summarized in Table 6.6, a substantial data set that is somewhat larger than that from North America and Greenland.

The mean pole positions listed in Table 6.6 contain many data for the Russian Platform as summarized in the GPMDB. A point of concern has always been how these data may be compared with their western counterparts. Russian data have generally not been published in full and are only reported in summary reports and catalogs compiled by A.N. Khramov at the VNIGRI Institute in St. Petersburg. It is the data from these compilations that are included in the GPMDB and in this form it is not easy to assess their quality index Q according to the seven criteria set out in Table 6.1 following Van der Voo (1990a). Therefore, in attempting to assess these data, the important criterion first considered has been the presence of reversals (criterion 6 of Table 6.1). Many of the results are derived from the use of bulk demagnetization procedures, so the presence of reversals becomes a strong criterion for inferring a primary magnetization. The results from such data are then compared with others where reversals are not present, and when there is general agreement these are also deemed to be acceptable. Such data are then considered to be at least at the level of the minimum acceptance criterion of Q = 3. Many of the Russian results only indicate the number of samples collected rather than the number of sites, which is required for the method of combining data (§6.4.2). In these cases a conservative estimate of 10 samples per site has been used to determine the number of sites investigated in each study.

The Permian of Europe is the period which contains the most data and between the Early and Late Permian apparent polar wander is only 2.0°. Therefore, to

Fig. 6.14. Phanerozoic APWP for Stable Europe (separating North Britain and Baltica for >425 Ma) using the mean pole positions given in Table 6.6. Each mean and its circle of 95% confidence is labeled with the mean age in millions of years.

obtain a good data set enabling a comparison of data from the Russian platform and western Europe it is convenient to combine the data for the whole of the Permian. As is evident from Table 6.6, the mean pole positions differ by only 1.7°. These means are not discernibly different at the 95% confidence limit (angle of separation would need to exceed 2.4°). Therefore, the large amount of Russian data has been included in this analysis with some confidence. As a result, for pre-Permian times the number of pole positions averaged is now more than double that listed by Van der Voo (1993).

The separated paths for North Britain and Baltica prior to 420 Ma became widely separated in the Middle Ordovician (470- and 465-Ma poles, respectively). For the Ordovician and Cambrian of Baltica it appears that many of the data from Russia have been remagnetized. Therefore, those poles for Baltica identified by Torsvik et al. (1992) as being reliable have been mainly used in determining the extension of the APWP for Europe to Baltica for pre-425 Ma. Note that when the number of groups m = 1, A95 cannot be determined.

6.5.3 Asia

Asia is a composite continent made up of blocks that have accreted during most of the Phanerozoic (Hamilton, 1970; Zonenshain et al., 1990). The major cratonic nucleus is Siberia (Fig. 6.15). To the west the early Paleozoic Kazakhstan sub-continent consists of a mosaic of displaced terranes that, together with Siberia, collided with Baltica during the late Paleozoic along the site of the Urals (Urals Orogenic Belt). Between the Kazakhstan sub-continent and Siberia lies the Central Asia Orogenic Belt formed during the Paleozoic collisions between Siberia, Kazakhstan in the north, and Junggar, Tarim and North China to the south. To the north lies the Precambrian Kara block separated from Siberia by the Taimyr Orogenic Belt. To the southeast of Siberia the Mongol-Okhotsk Orogenic Belt formed through the late Paleozoic and early Mesozoic during the collision of Mongolia with Siberia. Mongolia, which is referred to as Amur by Zonenshain et al. (1991), is probably made up of several terranes. The Mongol-Okhotsk suture extends to the east along the Okhotsk-Chukotka Orogenic Belt for a combined length of 5000 km into far-eastern Asia. Further to the south, South China amalgamated with North China during the Mesozoic.

To the east of Siberia the Verkhoyansk-Kolyma Orogenic Belt formed mostly in the Early and mid-Cretaceous when numerous displaced terranes were accreted to the Asian margin during rapid convergence of the Kula, Izanagi and Eurasian plates (see Fig. 5.25). The Kolyma block to the east and Sikhote Alin terrane to the southeast (Fig. 6.15) formed during this amalgamation. The Kolyma block is a loose grouping of several terranes, notably the Omolon and Chukotka terranes, which, together with Sikhote Alin, are discussed in §7.3.5.

Fig. 6.15. Simplified tectonic map of Asia showing the major blocks (shaded) whose paleomagnetic data are analyzed in the text (§6.5.3 or §7.2.2 and §7.3.5). The positions of other smaller terranes are indicated by names in italics. Outlines in northern Asia are after Zonenshain et al. (1991).

India, which collided with Asia during the Cenozoic, is considered in the context of the Gondwana continents in §6.5.4. The associated Lhasa and Qiangtang terranes to the north are discussed further in §7.3.5. Iran is here considered as a composite terrane comprising several stable blocks of Iran and central Afghanistan (e.g., Alborz-Great Kavir, Lut and Sistan-Helmand), which most workers argue have a common paleogeographic setting. Iran may have been a northern part of Gondwana as discussed in §7.3.5. The tectonic evolution of this region has been summarized by Boulin (1991).

Siberia

Russian scientists have carried out extensive paleomagnetic studies in Siberia since the early 1960s. Data for the former Soviet Union were regularly summarized in catalogs prepared by A.N. Khramov and made available through the World Data Centers. Reviews of these data in English in terms of APWPs for the various blocks in eastern Europe and Asia have been given by Khramov et al. (1981) and Khramov (1987). Besse and Courtillot (1991) derived a reference APWP for Eurasia (meaning Siberia in the present context) on the basis of worldwide data and rotation parameters derived from ocean magnetic anonmalies. Smethurst et al. (1998) reviewed paleomagnetic data for the Neoproterozoic and Paleozoic to construct an APWP.

Gurevich (1984) and Pavlov and Petrov (1996) have proposed that Siberia can be divided into two parts referred to as Anabar (northern Siberia) and Aldan (southern Siberia) in Fig. 6.15 and separated by the Viljuy Basin. The basin encloses the Ygyatta and Viljuy grabens which began to form in the Middle Devonian resulting in an increasing amount of extension toward the northeast where buried oceanic crust has been found (Zonenshain et al., 1990). Pavlov and Petrov (1996) have proposed that the northern Anabar block rotated counterclockwise with respect to the southern Aldan block about a pole at 60N, 100E during Siluro-Devonian times. Therefore, it is necessary to rotate results from the Anabar block clockwise for pre-Devonian times to bring them into agreement with the Aldan block. Smethurst et al. (1998) conclude that the paleomagnetic data sets from the two blocks are insufficient to determine a rotation angle but suggest that a rotation of -20° (clockwise) is a good approximation.

The method for judging the reliability criteria for Russian data for Europe given in §6.5.2 has been used to compile the data set for Siberia giving the mean pole positions listed in Table 6.7. For pre-Devonian times the five mean poles in Table 6.7 were analyzed by rotating results from the northern Anabar block through angles, of -10, -15 and -20° (clockwise) about an Euler pole at 60N, 100E. Four of the five mean poles in Table 6.7 improve their grouping with a rotation of -10° compared with three for a rotation of -15° and only two for -20°. Therefore, in determining an APWP for Siberia, data from the Anabar block have been rotated about an Euler pole at 60N, 100E through an angle of -10° (clockwise) for pre-Devonian times. Data from the Taimyr Orogenic Belt have been included for post-Permian times. These data are those for the Siberian Traps that also occur on the Siberian craton and this is the reason why the Early

Was this article helpful?

0 0

Post a comment