A Discussion on volcanism and the structure of the Earth - The evolution of the Scotia Ridge and Scotia Sea

Abstract

Marine geophysical surveys over the Scotia Ridge show it to be composed of blocks mainly of continental origin. Major structures found on the blocks are in many cases truncated at block margins and their existence is also inconsistent with the present isolated situation of the blocks. The evidence suggests post-Upper Cretaceous fragmentation of a continuous continental area. Complementary marine geomagnetic studies over the deep water of the Scotia Sea have dated two areas as younger than 22 million years (Ma) and have indicated the direction of spreading in others. A model of present plate motions, based on the magnetic anomalies, explains the active volcanism of the South Sandwich Islands as being caused by consumption of Atlantic crust at the associated trench at a rate of 5.5 cm/year for the past 7 to 8 Ma at least. An Upper Tertiary episode of plate consumption at 5 cm/year at the South Shetland trench, suggested by the magnetic lineations, with a secondary slow extensional widening of Bransfield Strait is used to explain similarly the contemporaneous volcanism of the South Shetland Is. Making the reasonable assumption of a Tertiary formation of the undated parts of the Scotia Sea by spreading in the directions indicated by the magnetic lineations, a tentative reconstruction of the component blocks of the Scotia Ridge is made. The attempt is only partly successful in matching structural patterns across adjacent margins of reconstructed blocks, South Georgia being most obviously wrongly situated. It is suggested that the misfits result from minor errors in the initial assumptions and the modification of structures during fragmentation and drift. South Georgia may have formed on the Atlantic rather than the Pacific side of the compact continental region which is thought to have joined South America and west Antarctica for much of the Mesozoic at least. A Gondwanaland reconstruction is presented which is consistent with the Scotia Ridge reconstruction, in which the Antarctic Peninsula lies alongside the Caird Coast of east Antarctica. Upon break-up of Gondwanaland, the Antarctic Peninsula remained rigidly attached to South America, east Antarctica rotating clockwise to open the Weddell Sea, until early Tertiary times when the Peninsula transferred to east Antarctica which continued rotating clockwise to open the Scotia Sea.

Tectonic reconstructions for paleobathymetry in Drake Passage

“…The Scotia Arc came with time to be considered as the product of various ideas of a mobile crust, and ultimately of plate tectonics (Barker and Griffiths, 1972;Hawkes, 1962;Matthews, 1959;Wilson, 1966). Its constituents continued to be interpreted as having dispersed from the Antarctic Peninsula and Tierra del Fuego, which by now were presented as the products of convergent margin processes acting since Jurassic times.…”

Cenozoic Paleoceanography in the Southwest Pacific Ocean, Antarctic Glaciation, and the Development of the Circumantarctic Current

“…Hence, the time that the Drake Passage region opened is crucial, but is still not known. Barker and Griffiths (1972) and Dalziel and Elliot (1971) indicate opening sometime during the Paleogene at the time that the Scotia Sea region was formed. Relationships discovered between the structural history in the Australasian region and major sedimentary distribution changes disclosed by Legs 21 and 29 indicate that the Circum-Antarctic Current developed south of Australia when the constriction formed by Tasmania and the South Tasman Rise had moved north away from Antarctica.…”

On the Tectonic History and Origin of the Northeast Georgia Rise (Synthesis)

“…Crustal fragments, such as South Georgia and Shag Rocks (Fig. 11), were dispersed along a northeast-trending predominantly strike-slip boundary parallel to the direction of North Scotia Ridge (Barker and Griffiths, 1972;Barker and Hill, 1981). The southern edge of the North Scotia Ridge probably represents a shear zone whereas the northern flank is composed of a thick wedge of deformed sediments (Ludwig et al, 1978).…”