Gondwana

South America today includes several "terranes" that were probably elsewhere in early Paleozoic time. Their positions on figures 2.4a-e are intended only to suggest a possible evolution. Other models suggesting differing possible extents, shapes, and evolution are reviewed by Dalziel (1997). Three terranes are shown on the maps: Patagonia, the Precordillera terrane (or Occidentalia), and Chilenia (Ramos et al. 1986; Ramos 1988). Patagonia is considered here to have collided with the rest of South America in Carboniferous time, but because there are no ophiolites and no obvious suture, this orogenic belt may not reflect a collision (Dalziel, pers. comm.). The Pre-cordillera may have been an elongate fragment that started life on the edge of southeastern Laurentia (Dalla Salda et al. 1992a,b; Dalziel et al. 1994; Astini et al. 1995; Dalziel and Dalla Salda 1996; Thomas and Astini 1997; Keller 1999) and may have collided with South America during Arenig-Llanvirn time (Astini et al. 1995). However, alternative models, reviewed by Dalziel (1997), exist. Chilenia is a second elongate fragment that collided with the Precordillera in the later Carboniferous (Astini et al. 1995; Pankhurst and Rapela 1998). Apart from the geometric requirement of avoiding overlap, there are few constraints on the relative positions of Patagonia and Chilenia prior to collision. For simplicity it is assumed that they were joined together as a single fragment that lay somewhere off Antarctica, where they have been parked for most of the Vendian period.

Several large continental fragments that now lie in eastern North America were originally attached to northwestern Gondwana (mostly Africa). They constitute the Avalon zone, named after the Avalon Peninsula in southeastern Newfoundland (Williams et al. 1974; Williams 1995). Here, late Precambrian sediments and volcanics are overlain by Cambro-Ordovician shales and sandstones, rather than the platform carbonates of Laurentia. Similar rocks can be recognized in the Appalachians as far south as the Carolina slate belt and across the Atlantic in Wales, Brittany, Iberia, northwestern Africa, and parts of central Europe (Pickering and Smith 1995). Avalonia is used here as an informal name for the entire belt, which is up to 750 km wide in its type area and several thousand kilometers long. In reality it is made up of a number of fragments, as indicated on figures 2.4a-e, several of which behaved independently (Nance and Thompson 1996). Its treatment here as a single unit is simply a convenience.

Avalonia's present tectonic position against the Laurentian and Baltica margins is the result of a tectonic process that appears to be common when an ocean basin closes. The available evidence suggests that the Avalonian fragments originally bordered one continent (here northwestern Gondwana), were split from it, and migrated across the ocean (Iapetus), though not necessarily as the single fragment shown in figure 2.4a, colliding with the opposing continent (eastern North America and southern Baltica) before the collision of the two major continents. Eastern Avalonia may have separated from Gondwana some time in the Late Cambrian to Early Ordovician interval (Prig-more et al. 1997), although Landing (1996) argues that Avalon was a unified, isolated continent by latest Precambrian time that had no relationship to Gondwana or Baltica at that time. It resembles a one-way tectonic windshield wiper, which sweeps a preexisting ocean away (the Iapetus), creating a new one behind it (part of which was the Rheic Ocean), and leaving the "wiper" on the opposite continent (the Avalonian fragments). The process has no formal name nor is the underlying physics of it well understood. A simplified model that conveys the gist of the evolution of Avalonia is shown in figures 2.4a-e.

The kinematic evolution of Avalonia is analogous to that of Sibumasu (Metcalfe 1992), discussed below, and its continuation westward as far as Greece, as the Cimmerian continent (Sengor et al. 1984): a long, narrow, more or less continuous continental sliver that migrated across the Mesozoic Tethys, sweeping away Paleotethys and inaugurating Neotethys behind it. "Avalonias" are important paleontologically because they may act as rafts that transport floras and faunas across an ocean in a time span during which significant evolution can take place en route. The Precordillera may provide another possible example of the splitting off of a fragment and its migration across a small ocean.

Late Precambrian and early Paleozoic poles determined for fragments such as North China, South China, and elsewhere show that the paleolatitudes of all three fragments are more or less in the latitude range one might expect, i.e., close to the latitude range of the edge of East Gondwana. However, in a discussion of the Cambro-Ordovician poles from these three fragments, Kirschvink (1992b) concludes that although all three were close to one another and joined to East Gondwana, North and South China were both inverted at the time.

There is good stratigraphic evidence, reviewed by Burrett et al. (1990) and Metcalfe (1992), suggesting the Cambrian-Early Ordovician evolution of these and other Southeast Asian fragments. Although most of the boundaries of the fragments in central and Southeast Asia on figures 2.4a-e are close to those suggested by Metcalfe (1992: figure 1) and Yin and Nie (1996: figure 20.1), some differences exist. In par ticular, Metcalfe subdivides what is western Southeast Asia on figures 2.4a-e into two fragments: Sibumasu (northwestern Burma + Burma + Malaysia + Sunda) and East Malaya. In essence, the following fragments were probably contiguous with Australia and India in Cambrian to Early Ordovician time: Indo-Burma, western Southeast Asia, South China, Indochina, and North China, together with smaller fragments to the north such as the North and South Tarim fragments in China. South China is believed to have been close to Pakistan at the time. The post-460 Ma evolution is complex and not relevant to the maps.

Alternative views exist about the placing of North and South China in the period 620-460 Ma. For example, Yin and Nie (1996: figure 20.18) regard the North Tarim and North China fragments as having been joined together from about 630-438 Ma and to have been bordered on all sides by passive margins, rather like present-day Madagascar. Li et al. (1996) show possible earlier Cambrian to Vendian positions for an isolated North China and also for an isolated South China.

Because of the considerable uncertainties involved, it is assumed for simplicity that all these fragments remained attached to Gondwana in their likely Cambrian positions until at least 620 Ma (figures 2.4a-e), which are essentially those of Kirschvink (1992b), slightly modified.

Still further east there are many continental and igneous fragments lying off northern and eastern Australia, but it is difficult to assess what proportion of them consists of material that existed in the 620 -460 Ma interval. Some, like the Tonga-Kermadec arc, may be entirely Cenozoic in age. For completeness, they are shown on the maps.

Finally, although Antarctica has been divided into several fragments, the only ones that have been moved relative to the others on the maps are the Antarctic Peninsula and Thurston Island. These have been visually repositioned to conform to the Jurassic reconstruction (not shown) of Lawver et al. (1992), and then moved with Patagonia and Chilenia to avoid overlap. There has been relative motion between East Antarctica and other Antarctic fragments, but on a global scale this can be (and has been) neglected. The configuration of West Antarctica and its position relative to other continents in pre-Mesozoic time is not known.

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