Tectonic History Of The Margins Of The Southern Pacific Ocean

The Pacific Ocean margin of Gondwana (South America, Antarctica and Australasia), as noted earlier, had been an active orogenic zone from the Precambrian to the Late Cretaceous (Scotese, 1987). It formed the boundary zone between the Pacific Ocean and the stable shield area comprising the Precambrian meta-morphic rocks which occur in Brazil, Uruguay, northeastern Argentina, Falkland Islands, Falkland Plateau, East Antarctica and Australia. Knowledge of the area is highly variable reflecting the limited information and rock exposure, particularly in parts of Antarctica.

The margin falls into two major regions; one has a more or less continuous history of convergent plate tectonics (South America and Antarctic Peninsula), the other is strongly affected by the extensional rifting tectonics of the past 80-100 Ma B.P. (Southwestern Pacific). The geology of coastal Marie Byrd Land and Ellsworth Land covers the transition between these two geological environments.

The continental margin of the ocean/continent convergent plate boundary has three major tectonic components. These are, from ocean to continent, a fore-arc sedimentary province which includes an accretionary sedimentary pile and fore-arc basin, a magmatic arc consisting mainly of calc-alkaline volcanic rocks and underlying granitoids, and a back-arc sedimentary basin province. Rifted margins, in contrast, are associated with initial uplift and erosion, major faulting and the extrusion of basic volcanics.

South America and Greater Antarctic Peninsula

The development of the Pacific Ocean margin of South America and Antarctic Peninsula falls into two main phases represented by (a) the present Pacific Ocean margin cordillera, initiated in middle-late Jurassic times with widespread extrusion of calc-alkaline (dominantly silicic) volcanic rocks and the site of an active subducting plate boundary between the Pacific Ocean (Farallón) and American plates, and (b) a previously deformed and metamorphosed basement complex separated from the overlying younger rocks by a major unconformity (Dalziel, 1982). This major unconformity represents uplift and erosion of the basement complex in Late Triassic-Jurassic times.

In southern South America, basement comprises a central late Palaeozoic-early Mesozoic magmatic arc, an eastern back-arc epicratonic sedimentary province, which corresponds to the Samfrau Geosyncline of Du Toit (1937), and a fore-arc province to the west, (Fig. 5.9) (Forsythe, 1982). Dalziel (1982) considered that the basement geology of the Antarctic Peninsula represents a portion of the fore-arc and main magmatic arc of a pre-Middle Jurassic subduction zone (Fig. 5.9). A Palaeozoic-early Mesozoic subduction complex therefore existed along the South American-Antarctic Peninsula margin of Gondwana and extended to New Zealand.

A major unconformity occurs between this basement complex and the overlying Late Jurassic-Cretaceous arc sequences. Dalziel (1982) concluded that the uplift and erosion of the basement, following deformation and metamorphism, which resulted in this unconformity was related to the initial phases of the breakup of Gondwana. Overlying the major middle Jurassic unconformity are primarily volcanic rocks interlayered with volcaniclastic sandstone and shales indicative of a shallow marine environment. Deposition of these rocks accompanied widespread extension in the latest Jurassic-Early Cretaceous, probably associated with Gondwana fragmentation (Dalziel, 1982) when vast quantities of extrusive and intrusive igneous rocks were associated with extensional tectonics in the southern continents. Convergent margin tectonics continued in southern South America, where a marginal (back-arc) basin of Late Jurassic-Cretaceous age formed behind the calc-alkaline volcanic chain that was developing along the South American continental magin (Dalziel, 1981). Central and southwestern Antarctic Peninsula was the site of a narrow, more or less continuous, volcanic arc terrane by Late Jurassic times. A fore-arc basin was developing in the Alexander Island area, while along the eastern Antarctic Peninsula, from James Ross Island to Hope Bay and South Orkney Islands in the north, and along the southeastern Antarctic Peninsula, sedimentation was occurring in a back-arc basin environment.

The continental margin of South America underwent considerable deformation and uplift in the mid-Late Cretaceous (Andean Orogeny). These processes resulted in the initiation of the present southern Andean Cordillera (Bruhn and Dalziel, 1977). This compressional deformation coincided closely in time with the period of fast seafloor spreading in the southeastern Pacific (Larson and Pitman, 1972) and a change in relative plate motion between the South American and African plates (Rabinowitz and LaBreque, 1979). No fore-arc sediments of Late Cretaceous age are known in the Antarctic Peninsula and the uplift and deformation of the late Jurassic-Early Cretaceous Fossil Bluff formation of Alexander Island possibly commenced at this time. Widely scattered volcanic centres erupted chiefly basalts during the late Cenozoic in southern Patagonia and Tierra del Fuego (Dott, 1976). Volcanism continued into the early Cenozoic in South Shetland Islands and Alexander Island but late Cenozoic volcanism is restricted to South Shetland Islands and east of northern Antarctic Peninsula, reflecting the cessation of subduction further to the southwest.

The continental shelf of the Antarctic Peninsula is at depths of 250-300 m in the northeast, deepening to 500 m or greater in the southwest near Thurston Island. The shelf varies in width, reaching a maximum of 500 km in the Bellingshausen

Early Cretaceous Antarctica
Fig. 5.9. Major pre-Jurassic and Jurassic-Early Cretaceous tectonostratigraphic provinces of South America (A, B) (after Forsythe, 1982) and Antarctica (C) (after Dalziel, 1982).

Sea (Fig. 5.10) and possibly 400 km in Amundsen Sea (Fig. 5.13). Sediments thicken in a southeast direction under the continental rise to reach thicknesses in excess of 3 km under the continental slope and over 5 km on the shelf northwest of the South Shetland Islands (Fig. 5.11) (Ashcroft, 1972; Houtz, 1974; Tucholke and Houtz, 1976; Kimura, 1982). The age of these sediments is not well constrained but they probably are Mesozoic or younger in age.

Off central Antarctic Peninsula, where active subduction ceased after ridge-trench collision, a palaeo island arc sequence has been interpreted in the sediments of the continental margin (Fig. 5.12) (Kimura, 1982). The morphological expression and large negative free air gravity anomaly of the palaeo-trench has deteriorated through isostatic rebound since the trench-arc system was transformed into a passive margin after ridge subduction during the Oligocene. In the north of the Antarctic Peninsula, the present convergent plate boundary gives rise to the South Shetland Trench and to the back-arc basin (Bransfield Strait) which lies to the east of the South Shetland Islands. Thin crust (25 km) occurs in this back-arc basin (Guterch et al., 1985).

The similarity and presumed continuity of the pre-Cretaceous geology of South America, Antarctic Peninsula and the South Georgia and South Orkney continental blocks allow a reconstruction to be made of Gondwana for this region (Dalziel, 1983) (Fig. 5.12) and demonstrate the complex movement that has occurred since breakup.

Western Ellsworth Land and Marie Byrd Land

The older basement geology along the coastal region between Antarctic Peninsula and Ross Sea includes low grade metasedimentary rocks (quartzose flysch deposits) of possibly Precambrian-Palaeozoic age (Swanson Group) in the west (Marie Byrd Land) and possible correlatives (or older), comprising higher grade metamorphic rock sequences and igneous rock (Fosdick Complex), in western Marie Byrd Land and Thurston Island (Fig. 5.13). Sporli and Craddock (1981) suggested that these latter rocks may have correlatives along the Ruppert and Hobbs Coasts of central Marie Byrd Land (Wade and Wilbanks, 1972).

A younger basement complex of widespread granitoids and rhyolitic volcanics of calc-alkaline affinities were emplaced throughout Marie Byrd Land and in the Thurston Island-Eights Coast region of western Ellsworth Land. They are very similar to the suites of Mesozoic volcanic rocks of Antarctic Peninsula (Metcalfe et al., 1978) and may form part of a magma tic arc of Late Palaeozoic-Early Mesozoic age that was continuous with a contemporaneous arc in the Antarctic Peninsula. Since that time, the geological history of the region has differed from that of the Antarctic Peninsula, reflecting the effect of the rifting of the New Zealand region from Antarctica.

During Cretaceous times, mafic dyke swarms intruded the coastal region of Marie Byrd Land. They increase in number towards the coast and trend subparallel to the continental margin reflecting the extensional regime which existed prior to the separation of New Zealand from Gondwana.

A widespread erosional surface formed during the Late Cretaceous-Early

Antarctica Climate During Cretatous

Fig. 5.10. Antarctic Peninsula. Location of geophysical profiles (A, B and C in Fig. 5.11) across the western Antarctic Peninsula-Bellingshausen Sea margin. HFZ - Hero Fracture Zone, TFZ - Tula Fracture Zone. Bathymetry contours at 500 m and 3,000 m delineate continental slope and shelf (Polar Stereographic projection).

Fig. 5.10. Antarctic Peninsula. Location of geophysical profiles (A, B and C in Fig. 5.11) across the western Antarctic Peninsula-Bellingshausen Sea margin. HFZ - Hero Fracture Zone, TFZ - Tula Fracture Zone. Bathymetry contours at 500 m and 3,000 m delineate continental slope and shelf (Polar Stereographic projection).

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