With a similar form to the Weddell Sea, the Ross embayment is one of the most striking morphological expressions of the WARS, a region of thin, and, by inference, extended continental crust whose regional boundaries are difficult to define precisely and may have been different for different episodes of extension (Fig. 7.5).
Three major episodes of extension have been proposed, with rifting starting in the Middle Jurassic, in coincidence with the onset of Gondwana break-up and the associated Middle Jurassic magmatism (Ferrar Group), and with subsequent episodes in the Cretaceous and late Cenozoic. The active rifting may continue to the present as suggested by the active volcanoes along the western margin of the Ross Sea. However, most extension and thinning of the Ross Sea crust is considered to have taken place during the Mesozoic rift period and the tectonic connection between the Ross and Weddell embayments is largely unclear (Behrendt et al., 1992; LeMasurier, 2007, and references therein).
In particular, our knowledge of the Jurassic episode is problematic since, although the location of rifting is well constrained in the South Africa -Queen Maud Land region, neither the location nor the amount of extension is well known in the Transantarctic Mountains and Ross Sea.
By comparison, the Cretaceous episode is better documented (LeMasurier, 2007, and references therein) and is considered to be closely related to the initial stage of stretching and rifting in the Weddell Sea region, where disruption of the plate margin involved the rotation and translation of the several crustal blocks forming the present West Antarctica, including the Ellsworth/Whitmore Block, which was translated to the boundary between the Ross and Weddell embayments.
Since movement of the West Antarctic crustal blocks was largely completed by 110 Ma, later tectonic activity in the WARS has been restricted to the Ross sector of the TAM and the margin of the Ellsworth-Whitmore Mountains, with the continuation of the TAM into the Weddell Sea region considered a remnant Jurassic rift (Schmidt and Rowley, 1986; LeMasurier, 2007).
The first direct evidence of rifting in the Ross embayment is documented in Marie Byrd Land where a voluminous suite of mafic dikes and anorogenic A-type granites (the so-called ''Byrd Coast Granites'') have been dated as early as 10775 to 102-95Ma (Storey et al., 1999). Considering Bradshaw's (1989) suggestion that a sea-floor spreading centre was subducted beneath Mesozoic New Zealand around 105 Ma, the production of the mafic dykes could have been linked to the subduction of such a spreading centre at 10775 Ma, followed by the emplacement of the anorogenic silicic rocks at 102-95 Ma. On the basis of the age of Anomaly 33 (83 Ma) identified off Campbell Plateau, the earliest sea-floor spreading between Campbell Plateau and Marie Byrd Land must be ~85Ma. The unequivocal reconstruction of
Figure 7.5: (A) Map of the Transantarctic Mountains and West Antarctic Rift System (after Fitzgerald, 2002, with permission from the Royal Society of New Zealand). Structural features along the TAM and in the West Antarctic Rift System are from Tessensohn and Worner (1991), Fitzgerald (1992), Fitzgerald and Baldwin (1997) and Salvini et al. (1997). DAZ: Discovery accommodation zone from Wilson (1999). Basin positions in the Ross Sea (light grey) are from Davey and Brancolini (1995) and include North Basin (NB), Eastern Basin (EB), Central Trough (CT) and the Terror Rift (TR) lying within the Victoria Land Basin (VLB). The approximate outline of the Wilkes subglacial basin is shown inboard of the TAM in a dot pattern. West Antarctic microplates are after Dalziel and Elliot (1982); see Fig. 7.3 for abbreviations. (B) Generalized crustal profile A-A (in A) across the Ross Sea is after Cooper et al. (1991). Early rift grabens lie beneath regional unconformity U6 (heavy line). Late rift-faults and intrusive rocks deform the VLB and some small basement grabens. Note intra-basement reflector (dashed line) on eastern edge of VLB, possibly a detachment fault extending under the entire basin. DSDP site 270 is projected onto this cross-
the Campbell Plateau against Marie Byrd Land and the remarkable match of the deduced ocean-continent boundary as well as the alignment of the eastern edge of the Eastern Ross Basin with the northern edge of the Campbell Basin on the Campbell Plateau are evidence that the entire Eastern Ross Basin should have experienced extension prior to initiation of the Pacific-Antarctic Ridge at -85 Ma.
The Pacific-Antarctic spreading centre played a major role in the region in late Cenozoic time. With its initiation, a triple junction developed with three extensional arms (Australian-Antarctic spreading centre, Australian-Pacific arm and the Pacific-Antarctic spreading). Geochronological constraints are provided by early sea-floor spreading anomalies identified between Tasmania and the South Tasman Rise. Key tectonic events include (i) the jumping of the Australia-Antarctica spreading centre about Anomaly 30 (-65 Ma) to between the South Tasman Rise and northern Victoria Land, (ii) the completion of sea-floor spreading in the Tasman Sea at Anomaly 24 time (~ 54 Ma) as a consequence of the reformation of the ridge-ridge-ridge triple junction as a new spreading centre that went between Campbell Plateau and the Tasman Sea anomalies and (iii) the opening of the — 40 km wide Adare Rift within ''older'' sea-floor off the western edge of the Ross Sea sometime after Anomaly 13 (-33 Ma) (Cande et al., 2000; Cande and Stock, 2006).
Satellite gravity data suggest that the Adare Rift steps east, and then is transformed westward along the southern margin of the Coulman High to the region of the Terror Rift, a comparable structure in width, located along the western edge of the VLB in the western Ross Sea.
Striking products of the large-scale Cenozoic geodynamic evolution in the Ross Sea area are four major sedimentary basins which developed by rifting of previously thinned continental crust, possibly along pre-existing crustal faults, and coincide with major localized crustal thinning (to less than 10 km), and, in the case of the VLB (the westernmost one), with high heat flow and alkaline volcanism (Blackman et al., 1987; White, 1989; Cooper et al., 1991; Behrendt et al., 1993; LeMasurier, 2007). Extensive late Cenozoic volcanism is also inferred from aeromagnetic data. These basins include from E to W: the Eastern Basin - underlying most of eastern Ross Sea; the Central Trough - running north-south discontinuously through central Ross Sea; the Northern Basin - underlying the north-eastern Ross Sea margin; and the VLB underlying south-western Ross Sea, adjacent to the Transantarctic Mountains.
The region has been extensively investigated through a number of geophysical surveys. But, due to the lack of appropriate age data for the main seismostratigraphical units, inferences on the timing of Cenozoic rifting in the entire Ross Sea remain largely speculative. Based on the results of the
Cape Roberts Drilling Project (Barrett et al., 2001), the VLB is now considered to be mostly late Eocene or younger in age. Recent drilling on the western margin of this basin has indeed indicated an onset of subsidence at about 34 Ma, significantly younger than the onset of uplift of the adjacent Transantarctic Mountains (55 Ma) (Fitzgerald, 1992) and thus identifying an issue in the relationship of the uplift of rift margin mountains to the subsidence of the adjacent rift basin. However, the effect of uplift episodes at this margin on the sedimentary sequences recorded may need to be considered. The fact that the basin extends only from the Ross Island region to Terra Nova Bay, whereas the Transantarctic Mountains continue further north and south, would indicate that the basin may have originated from other processes in addition to simple extension. A transtension or ''pull-apart'' process has been suggested, limited by transfer mechanisms associated with the igneous activity recorded in the north by the ''Polar Three'' anomaly off Coulman Island and in the south by the magnetic anomalies south of the Ross Sea Fault (Bosum et al., 1989).
Another significant geological component in the Ross Sea area tectonic evolution is represented by an extensive alkalic magmatic province, one of the largest in the world and including two active volcanoes (Mt. Erebus at Ross Island and Mt. Melbourne on the Ross Sea coast in northern Victoria Land; LeMasurier and Thomson, 1990). These rocks, either exposed or suggested by aeromagnetic studies in the area under the Ross Sea and West Antarctic Ice Sheet (Behrendt et al., 2002; Bell et al., 2006), occur on either side of the WARS in Marie Byrd Land (West Antarctica) and in the western Ross Sea. In the western Ross Sea, the oldest rocks are restricted to coastal areas in northern Victoria Land where they consist of Eocene to Oligocene alkali intrusive rocks (Meander Intrusive), interpreted as the eroded remnants of subvolcanic magmatic complexes (Muller et al., 1991; Rocchi et al., 1999, 2002). Elsewhere, the alkaline magmatism is predominantly volcanic and has been subdivided into the informal, but geographic and petrologic distinct, Hallett, Melbourne and Erebus volcanic provinces (Kyle, 1990b). Two small isolated occurrences of basalts dated at 16-20 Ma occur around the head of the Scott Glacier (Stump et al., 1980).
In the Erebus volcanic province (Kyle, 1990a), there is a continuous eruptive sequence from 19 Ma to the present day, with the Mt. Erebus crater showing a persistent anorthoclase phonolite lava lake. The oldest exposed rocks of this province occur on the northern slopes of Mt. Morning where 19 Ma trachyandesites lavas are intruded by 16-18 Ma trachyte dikes. Volcanic ash (tephra) layers in CRP drill cores demonstrate evidence for older Miocene trachytic eruptions. The tectonic setting suggests that the southern extent of the volcanism may be controlled along a major transfer fault, which coincides with the southern boundary of the Terror Rift (Wilson, 1999). Many of the larger volcanic centres have a radial distribution around Mt. Discovery or Mt. Erebus. The radial distribution is interpreted as a result of upwelling of a mantle plume (the Erebus plume), which was centred under Mt. Discovery prior to 4 Ma and then migrated to its present position under Mt. Erebus (Kyle, 1990a).
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