Armorica

Armorica refers to the collection of Paleozoic blocks presently located within Hercynian Europe as was discussed in §6.5.2 and illustrated in Fig. 6.13. Although these blocks may have been separate units within the Iapetus Ocean prior to their Hercynian collision with Baltica, their Precambrian and Cambrian geology shows more similarities than differences. There is also some paleomagnetic evidence for considering these as a single Paleozoic plate (Van der Voo, 1982, 1988).

The rock sequences of the Armorican, Bohemian and Iberian Massifs all exhibit a pronounced late Precambrian to Cambrian unconformity associated with the Cadomian orogeny that is of the same age as the Pan-African orogeny of Gondwana. Hagstrum et al. (1980) and Perigo et al. (1983) analyzed the paleomagnetic data from Armorica (mainly from the Armorican Massif) for the time interval 650-500 Ma. Their comparisons of the APWPs during this interval for Armorica and for what was then considered to be Gondwana showed remarkable similarities. Both APWPs suggested rapid apparent polar wander during this time interval and appeared to have the same shape. These similarities extended to results from the East Avalon terrane. Although these comparisons appear impressive, it should be noted that the Pan-African orogeny (-550 Ma) is now generally regarded as the time at which the various cratons of Gondwana amalgamated. Therefore, it is not clear that a "Gondwana" APWP compiled for times older than 550 Ma has any meaning.

Armorica was certainly amalgamated with Baltica at -300 Ma but the question remains as to where it came from; Gondwana is a prime candidate for its origin. Thus, in Fig. 7.15 the paleolatitudes observed from Armorica for the time interval 550-300 Ma have been compared with those expected from Baltica and Gondwana during the same interval. These have been calculated from the mean poles listed in Tables 6.6 and 7.3, respectively, using a present-day reference point at 47N, 0E. The general agreement with the expected Gondwana high paleolatitudes during the Cambrian and Ordovician is excellent. The paleomagnetic data for Gondwana and Baltica show similar predicted paleolatitudes for Armorica for post-Ordovician times, so the precise timing of the amalgamation of Armorica and Baltica is unclear.

550 500 450 400 350 300

Fig. 7.15. Paleolatitudes (open circles) observed from rocks in Armorica for a present-day location at 47N, 0E. Paleolatitudes predicted using the results from Baltica (solid circles) and Gondwana (solid squares) are shown for comparison.

550 500 450 400 350 300

Fig. 7.15. Paleolatitudes (open circles) observed from rocks in Armorica for a present-day location at 47N, 0E. Paleolatitudes predicted using the results from Baltica (solid circles) and Gondwana (solid squares) are shown for comparison.

During Ordovician times Armorica and East and West Avalon all exhibited the high paleolatitudes to be expected for locations close to northwest Africa (Figs. 7.14 and 7.15). On the basis of the paleomagnetic data, Torsvik et al. (1996) proposed a reconstruction of these terranes adjacent to northwest Gondwana as shown in Fig. 7.16 for Early Ordovician times (490 Ma). The Iapetus Ocean between Gondwana and Laurentia had reached its maximum extent in Late Cambrian and Early Ordovician (Fig. 7.16). Torsvik et al. (1996) propose that Armorica and the East and West Avalon terranes rifted away from Gondwana in early Middle Ordovician (Llanvirn) times. By the close of the Ordovician (440 Ma), East and West Avalon had drifted into low latitudes close to Baltica and Laurentia. In the Middle Silurian (430 Ma), Baltica, East and West Avalon, and Laurentia had amalgamated. Finally, by the Late Silurian, Armorica was sutured to Baltica, but there are few paleomagnetic constraints on this timing (Torsvik et al., 1996).

A detailed paleogeographic scenario for the Ordovician evolution of the Iapetus Ocean has been proposed by Mac Niocaill et al. (1997). They suggest there was an extensive peri-Laurentian arc that collided with Laurentia in the Early and Middle Ordovician and produced the Taconian orogenic pulse. By the Middle Ordovician a peri-Avalonian arc had developed on the southern margin of the Iapetus Ocean. Paleomagnetic data from the Robert's Arm, Sommerford,

Fig. 7.16. Reconstruction of the positions of the Armorica (Ar), East Avalon (EA), and West Avalon (WA) terranes in relation to Gondwana for the Early Ordovician (490 Ma) after Torsvik et al. (1996), with permission from Elsevier Science.

and Chanceport groups of central Newfoundland indicate the presence of a third arc system, the Exploits arc, situated in the middle of the Iapetus Ocean. Convergence of Baltica and Avalon with Laurentia continued through the Middle and Late Ordovician, and final closure of the Iapetus Ocean seems to have been completed in the Silurian.

7.3.4 The Western Mediterranean

Some of the earliest paleomagnetic studies in Europe were made in the Mediterranean region in the early 1960s. In the western Mediterranean the most significant discovery from these studies was the observation that the paleomagnetic declinations from late Paleozoic and younger rocks of Iberia, Corsica-Sardinia, and Italy deviated counterclockwise from those expected from stable Europe. Therefore, it was proposed that each of these regions had suffered counterclockwise rotation with respect to Europe. In the eastern Mediterranean region the situation is much more complex and there have been many studies made in Greece, Turkey, Bulgaria, and the Carpathians. An excellent review of the current situation with respect to paleomagnetic studies in this region has been given by Van der Voo (1993). Therefore, only the results from Iberia and Italy will be mentioned here as examples of terranes where rotations have been the important feature determined from paleomagnetism.

Iberia

Clegg et al. (1957) reported the first paleomagnetic results from Iberia that indicated such rotations. These were from Triassic rocks in northern Spain. Because of difficulties with structural control outside the Iberian Meseta (see Fig. 6.13), it is convenient to restrict any analysis to data from the Meseta itself. Paleolatitudes from the Iberian Meseta tend to agree with those predicted either from Europe or from Gondwana (Van der Voo, 1993). It is the paleomagnetic declinations that differ, as illustrated in Fig. 7.17a. The declinations predicted from stable Europe have been calculated using the mean poles from Tables 6.6 (Europe pre-Triassic) and 7.2 (Eurasia for Triassic and younger) using a present-day reference point at 40N, 4W. Those from Gondwana have been calculated using the data from Tables 6.11 (Africa for <140 Ma) and 7.3 (Gondwana).

The declinations observed from Iberia show no systematic agreement with those expected from either Europe or Gondwana (Fig. 7.17a). The declinations change by an angle of about 35° from those predicted from Gondwana during the Jurassic and Early Cretaceous to those expected from Europe during the Late Cretaceous and Tertiary. This corresponds with the opening of the Bay of Biscay through a similar angle. Kinematic analyses based on sea-floor spreading, sedimentary basin evolution, and structural geology favor the view that Iberia was probably an independent plate (Malod, 1989; Srivastava et al., 1990), even though it may have been associated with Gondwana during the Jurassic and Early Cretaceous.

Italy {Adria)

In the Italian Peninsula some of the regions along the Adriatic coast, in the southern Alps and in northeast Sicily, are thought to be underlain by major detachment zones and have been referred to collectively as Adria (Channell et al., 1979). However, it should be noted that they may not necessarily have together formed a single rigid block. As with the results from the Iberian Meseta, the paleolatitudes determined from Adria show correspondence with those expected both from stable Europe and from Gondwana (Van der Voo, 1993). The observed paleomagnetic declinations (Fig. 7.17b) are compared with those predicted from stable Europe and Gondwana using the same mean poles as in Fig. 7.17a for a present-day reference point at 42N, 12.5E. The observed declinations show strong affinity with those predicted from Gondwana since the Late Carboniferous. More detailed work in Umbria suggests that there are differences between the declinations observed in North and South Umbria that

Fig. 7.17. Paleomagnetic declinations (open circles) observed from (a) the Iberian Meseta for a present-day location at 40N, 4W, and (b) Adria for a present-day location at 42N, 12.5E. The declinations, predicted using the results from Europe (solid circles) and Gondwana (solid squares), are shown for comparison.

Fig. 7.17. Paleomagnetic declinations (open circles) observed from (a) the Iberian Meseta for a present-day location at 40N, 4W, and (b) Adria for a present-day location at 42N, 12.5E. The declinations, predicted using the results from Europe (solid circles) and Gondwana (solid squares), are shown for comparison.

are related to thrust sheet rotations (Channell et al., 1978, 1984; VandenBerg et al., 1978; Hirt and Lowrie, 1988; Jackson, 1990).

On the basis of the paleomagnetic agreement with Gondwana, Channell et al. (1979) proposed that Adria was a fixed promontory of Africa. As a consequence, the kinematics of the collision of Africa with Europe are directly related to the relative movements between them. However, if there was rotation of the Umbrian (and other) basement with respect to Africa and the southern Alps, then Italy consisted of many small blocks that were not coherent (VandenBerg, 1983; Lowrie, 1986). In that case there was no Adriatic promontory or certainly not a permanent one. This issue is at present unresolved, but the correspondence between the paleomagnetic data of Adria and Gondwana in Fig. 7.17b appears on the face of it to support the idea of the fixed Adriatic promontory. For a more comprehensive review of paleomagnetic data from the Mediterranean region, see Van der Voo(1993).

7.3.5 South and East Asia Iran

Paleomagnetic data from Iran have been acquired over the past two decades (Wensink, 1979, 1982, 1983; Soffel and Förster, 1981; Soffel et al., 1996; Besse

Fig. 7.18. Paleolatitudes (open circles) observed from Iran for a present-day location at 35N, 60E. The paleolatitudes predicted using the results from Eurasia (solid circles) and Gondwana (solid squares) are shown for comparison.

Fig. 7.18. Paleolatitudes (open circles) observed from Iran for a present-day location at 35N, 60E. The paleolatitudes predicted using the results from Eurasia (solid circles) and Gondwana (solid squares) are shown for comparison.

et ah, 1998). Van der Voo (1993) and Besse et ah (1998) provided substantive reviews of all the data and their tectonic implications. The paleolatitudes observed from Devonian and younger rocks from Iran are illustrated in Fig. 7.18. They are compared with those expected from Eurasia and Gondwana during the same time interval. These reference paleolatitudes have been calculated using the data listed in Tables 6.6 and 7.2 for Eurasia and those in Tables 6.11 and 7.4 for Gondwana using a present-day reference point at 35N, 60E.

During the Devonian and Permian the paleolatitude of Iran tracks that expected from Gondwana rather than that from Eurasia. During the Paleozoic it appears that Iran was situated in Gondwana adjacent to Arabia, close to its present relative position. During the Triassic the paleolatitude changes rapidly from that predicted for Gondwana to that predicted for Eurasia (Fig. 7.18). Therefore, at the end of the Permian Iran must have rifted away from Gondwana and drifted rapidly northwards during the Early Triassic. Iran then converged with Turan, which was then the southern margin of Eurasia, during the Middle Triassic (Besse et ah, 1998).

Lhasa and Qiangtang Terranes

Paleomagnetic data for the Lhasa and Qiangtang terranes of Tibet are confined to the Cretaceous and Tertiary and have been reviewed by Van der Voo (1993) and Chen et al. (1993). Agreement between the Cretaceous paleomagnetic directions for these terranes suggests they have been a single unit at least since then. Cretaceous paleolatitudes of these terranes are consistent with the view that they formed a stable southern margin of Eurasia with a general eastward trend at about 10°N latitude (Chen et al., 1993). However, the various poles do not coincide and the change in paleodeclinations from west to east suggests that these terranes have suffered internal deformation on different scales up to 1000 km. These declination differences indicate that the suture was more or less linear prior to collision with Eurasia and acquired its curvature subsequently (Chen et al., 1993). On the basis of paleomagnetic data, such a form of oroclinal bending was previously proposed, with larger amplitude, for the southern boundary of the Lhasa terrane as a result of the collision of India (Klootwijk et al., 1985).

During the Tertiary it has been observed from these terranes, and the Kunlun and Qaidam terranes to the north, that the paleomagnetic inclinations are about 20° shallower than would be expected from the reference APWP for Eurasia. Westphal (1993) proposed that this implies the presence of some magnetic field anomaly over this part of Eurasia causing a large departure from the geocentric axial dipole field direction. However, Cogné et al. (1999) have proposed that this large-scale inconsistency can be satisfactorily explained by a combination of factors, including the nonrigid behavior of the Eurasia plate during the Tertiary and intracontinental crustal shortening resulting from the collision of India with Asia.

Kolyma and Sikhote Alin

In the far east of Asia the Kolyma block (see Fig. 6.15) is made up of several terranes, the most important being the Omolon Massif and the Chukotka terrane. Southeast of Siberia the Sikhote Alin terrane lies on the Asian mainland adjacent to the island of Sakhalin. Khramov and Ustritksy (1990) summarized the paleomagnetic data from these terranes. In Fig. 7.19 the paleolatitudes observed for the Omolon and Sikhote Alin terranes since the Permian are compared with those expected using the mean poles for Eurasia (Table 7.2) using present-day reference points at 64N, 159E and 45N, 135E respectively. The Omolon Massif was located far south of Eurasia during the Permian and Triassic and then amalgamated with Eurasia during the Cretaceous (Fig. 7.19a). Closely associated with the Omolon Massif is the Chukotka terrane. However, the paleolatitudes observed from this terrane show good agreement with those predicted from North America throughout the Mesozoic and Cenozoic. Khramov and Ustritsky (1990) therefore propose that the Chukotka terrane may have been part of a single Chukotka-Arctic Alaska block that existed in Mesozoic time.

The Sikhote Alin terrane was also located far south of Eurasia during the Permo-Triassic but must have been closely associated with Asia by the Late Cretaceous (Fig. 7.19b). However, the final amalgamation with Asia may only have occurred sometime during the Tertiary.

(a) Omolori

(b) Sikhote Alin

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