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SCAR Report No 16

Appendix 7

THE LATE MESOZOIC AND CENOZOIC PLATE TECTONIC AND CRUSTAL HISTORY OF ANTARCTICA: IMPLICATIONS FOR MARINE AND TERRESTRIAL SEQUENCES
D.H. Elliot, Byrd Polar Research Center and Department of Geological Sciences,
Ohio State University

Much of the Late Mesozoic and Cenozoic geologic history of Antarctica is contained in the sedimentary sequences on the continental shelves. That history, however, in large measure reflects the evolution of the Antarctic Plate, both its plate tectonic and its crustal development.
The Antarctic Plate was created by the break-up of Gondwanaland and today, except for the Scotia Sea region, is bounded by spreading ridges. The Antarctic continental shelves rimming the East Antarctic craton are passive margins resulting from the separation of southeastern Africa, India and Australia. The remaining part of the continent has a complex history which includes fragmentation into microcontinents or blocks, rotations and displacements of those blocks, and subduction and passive margin evolution.
In general, the post-breakup history can be divided into: the mid Jurassic to mid Cretaceous, by which time the Antarctic continent had attained its present configuration and polar location (Grunow et al., 1991); and the mid Cretaceous (100 Ma) to Recent. During the latter time interval break-up between Antarctica and Australia/New Zealand occurred, the link between South America and the Antarctic Peninsula was broken, and events leading up to the present day rift system through West Antarctica took place.

The mid Cretaceous (100 Ma.) setting

During the 75 m.y. between Mid Jurassic break-up and the mid Cretaceous, Africa and India separated from Antarctica, and South America from Africa; in addition, rifting had begun between Australia and Antarctica but no oceanic crust had formed. The Antarctic rim of the Gondwana Plate was an active convergent margin throughout most of this interval, although in the backarc region there was extension associated first with break-up and emplacement of the Ferrar tholeiites and second following New Zealand/Pacific-Phoenix Ridge collision in mid Cretaceous time.
Regions rifted during Middle Jurassic break-up were of longstanding by 100 Ma, and included the Weddell Embayment, Pensacola Basin, Lambert Graben, and probably the Jutulstraumen embayment and Lützow-Holm Bay region; other rifts may well be present. These rifts were likely the sites of epicontinental seaways into the interior of Antarctica. Marine incursions may well have occurred in other locations as well.
Initial disruption of the Gondwana Plate margin was probably accompanied by local crustal thinning, and hence seaways may have been established through the Ross-Weddell embayment region and connected to the Panthallasan Ocean. Fission track data, however, suggest significant Early Cretaceous (ca. 141-117 Ma) uplift of the Ellsworth Mountains (Fitzgerald and Stump, 1991), possibly as a result of "docking" against southern Palmer Land, thus blocking any major shallow passages in this region. Uplift of the southern Transantarctic Mountains started at about 125 Ma (Fitzgerald and Stump, 1997), but must have had a different cause. Initial extension in the Ross Embayment-Campbell Plateau region, with accompanying development of the Ross Sea sedimentary basins, and uplift were doubtless linked (Lawver et al., 1994). The underlying driving force for all the Ross Embayment events was probably related to extension in the backarc region of the active plate margin.

Plate tectonic events of the last 100 m.y.

Plate tectonic events of the last 100 m.y. brought about the physical isolation of the Antarctic continent. Arguably this is one of earth's most important events in that it precipitated conditions leading to the inception and evolution of Cenozoic glaciation.
The fragmentation of East Gondwanaland was initiated with the development of the Pacific-Antarctic Ridge between Campbell Plateau and West Antarctica, at about 85-90 Ma. (Cristoffel and Falconer, 1972; Cande et al., 1982) and the SE Indian Ocean-Antarctic Ridge between Australia and East Antarctica, at about 90 Ma (Cande and Mutter, 1982). The boundary connecting those two is complex and also involves the opening of the Tasman Sea at the same time (Weissel and Hayes, 1977). Pacific-Antarctic spreading involved a dextral component of motion whereas the initial opening between Australia and Antarctica was more orthogonal but very slow until the mid Eocene (C20 time) when there was a significant increase in rate (Cande and Mutter, 1982). Transform fractures linked the two principal ridges but complete crustal separation did not occur until the Late Oligocene when the South Tasman Rise finally cleared north Victoria Land.
The other Gondwanaland link, that with South America, was finally broken at about 29 Ma with the formation of ocean floor in Drake Passage (anomaly C10). Shallow water connections may have existed earlier although deepwater circulation was not possible till about 23 Ma when the South American continental crust cleared Peninsula crust at the Shackleton FZ (Barker and Burrell, 1977).

Continental crustal events of the last 100 m.y.

Except for the Antarctic Peninsula, the continent has been rimmed by passive margins for the last 100 m.y. Those from the head of the Weddell Sea round to about 90EE are progressively younger, from Middle Jurassic to Early Cretaceous; the Australia-Antarctica sector dates from mid Cretaceous time, and the rest from the Late Cretaceous. All presumably have been subsiding at normal rates. The Antarctic Peninsula remained a convergent margin, where the Phoenix Plate was subducted, until Miocene time (Larter and Barker, 1991); since then the principal crustal motions there have been vertical.
In the rest of the continent, uplift and subsidence, together with extension and associated volcanism, have dominated crustal events. Uplift in the Lambert Graben region is thought to have occurred in Miocene to Recent time (Wellman and Tingey, 1982; McKelvey et al., 1995). The present elevation of the East Antarctic craton in the Queen Maud Land-Enderby Land sector may reflect the same episode of uplift. The Lambert Graben contains a number of small highly alkaline intrusive and extrusive bodies (Sheraton, 1983), suggesting the extensional regime is of longstanding and is probably still active. The sub-glacial Gamburtsev Mountains are possibly a major alkaline volcanic field related to the Lambert Graben (Elliot, 1994).
Uplift in the Ellsworth Mountains in the late Cretaceous and Cenozoic was limited (Fitzgerald and Stump, 1991). Nevertheless, the magnitude of the rifts between those mountains and the base of the Antarctic Peninsula suggests more recent continental block readjustments.
Episodic uplift is inferred from fission track data for the Transantarctic Mountains (Fitzgerald, 1992, 1994; Fitzgerald and Gleadow, 1988; Fitzgerald and Stump, 1997). The two principal episodes are Late Cretaceous and Eocene, the latter being the best documented and most widespread. Fission track data provide only a date on the inception of more rapid denudation and do not yield information on its termination. Thus the fission track data do not constrain the denudation/uplift history during the Late Cenozoic, except that in the lower Beardmore Glacier region there appears to have been as much as 4 km of denudation in the last 30 m.y. (Fitzgerald, 1994).
Episodic uplift is also inferred for Marie Byrd Land (Richard et al., 1994), with initial denudation at 100-94 Ma and a later event at about 80-70 Ma. Subsequent tectonic quiescence allowed the formation of a regional erosion surface in Marie Byrd Land, interpreted to be correlative with a similar surface in New Zealand and offshore on the Campbell Plateau and given a Late Cretaceous (75 Ma) age (LeMasurier and Landis, 1996). In Marie Byrd Land the surface is found at elevations up to 2,700 m. The cause of the uplift is thought to be a Late Cenozoic (<30 Ma) mantle plume which lead to eruption of the alkaline volcanic rocks of Marie Byrd Land.
The principal region of active extension in the last 100 m.y. is West Antarctica. The regionally thin crust (20-30 km) of the Ross Embayment was most likely formed following the mid Cretaceous Pacific-Phoenix ridge-crest collision with the trench off New Zealand and the Campbell Plateau (Bradshaw, 1989). Initial development of the elongate basins located on the shelf and extending beneath the Ross Ice Shelf occurred at this time. An extensional regime is also suggested by 100-105 Ma anorogenic granites in Marie Byrd Land (Weaver at al., 1994). This episode of extension, basin formation, and basin filling was terminated by rifting between Marie Byrd Land and Campbell Plateau. The Eocene changes in plate motions and the major episode of denudation in the Transantarctic Mountains may correlate with formation of the Ross Sea regional unconformity U6 (Davey and Brancolini, 1995). Late Cenozoic extension appears to be confined to the Terror Rift in south Victoria Land but contemporaneous alkaline volcanism is widespread in the Transantarctic Mountains and West Antarctica.
The age of the inferred rift basins in central West Antarctica - Byrd Subglacial Basin and the Bentley Subglacial Trench - is unclear but their development may also have a significant Late Cenozoic component. Magnetic data for the whole region have been interpreted to suggest the presence of a major subglacial flood basalt province of Late Cenozoic age (Behrendt et al., 1994).
The remaining region of possible crustal extension lies in Wilkes Land (about 90E-160EE) and includes the Wilkes Sub-glacial Basin, with its continuation into the Pensacola Basin, the Aurora Basin, and the graben-like setting of Lake Vostok and other narrow troughs. The Wilkes and Aurora Sub-glacial Basins may be the counterpart of the Cretaceous depression in Australia (Veevers, 1982). The Wilkes Basin may have been reactivated during the Cenozoic on uplift of the Transantarctic Mountains. The Wilkes-Pensacola Basin contains at least scattered evidence for marine incursions, as indicated by the reworked floras and faunas in the Sirius Group deposits.

Implications Of The Plate Tectonic And Crustal History

Antarctica's role in biotic evolution and dispersal.

Paleobiogeography: Antarctica is central to the issue of migration of vertebrates to Australia. Should reptiles, including dinosaurs, of Triassic through Early Cretaceous age be found in Australia, land bridges must have been maintained across Antarctica. With the inferred rifting and marine incursions in the Ross Embayment, the Ellsworth Mountains would have been a critical stepping stone between the Antarctic Peninsula and the Transantarctic Mountains. Marsupial migration (Woodburne and Case, 1996) occurred after the start of Tasman Sea opening and prior to about 64 Ma by which time the South Tasman Rise was submerged, although the submerged Rise did not clear Antarctica till about 30 Ma; similarly, the land connections must have included the Ellsworth Mountains.
The physical isolation and polar position of Antarctica are crucial factors in evolution of the present marine biota. The presence or absence of shallow-water paths across the Antarctic continent would have played an important role in dispersal and endemism.
Significant questions include: What were the the timing, duration, and elevations of the stepping stones that constituted the land bridges? Where will records of vertebrate dispersal be found and in what sort of sequences? Are there remnants of pre-glacial sedimentary sequences along the flanks or on the shoulders of outlet valleys of the Transantarctic Mountains? How did the evolving shallow and deep seaways influence the evolution of marine biota?

Climate history

Oceanic circulation: The reorganization of oceanic circulation to a thermo-haline driven system occurred during the Cenozoic. Today it is governed by processes occurring around Antarctica. The evolution to a thermo-haline driven system is not well documented but presumably is related to the development of the circumantarctic currents and the upwelling that is a consequence of the ice sheets.

Significant questions include: when was present day thermo-haline circulation initiated? What is the relationship between the evolving shallow and deep-water passages and the development of that circulation?

Glacial history: Glacial onset occurred in the Paleogene. Eocene (>49.5 Ma) glaciation has been reported from King George Island in the South Shetland Islands (Birkenmajer, 1991), but such early glaciation is in conflict with most paleoclimatic data (see Elliot, 1997); based on Sr isotope data, that glaciation is possibly Late Oligocene (Dingle and Lavalle, 1998). Kerguelen Plateau and Maud Rise data have been interpreted to suggest glacial conditions at about 46 Ma (Ehrmann and Mackensen, 1992), however the reported Eocene IRD is now thought to be downhole contamination (S. Wise, this workshop report). In the Ross Embayment, striated clasts in drill core (Hambrey and Barrett, 1993) suggest nearby glaciation in early late Eocene time (Wilson et al., 1998), but there remains the question of whether it reflects mountain glaciation rather than continent-wide ice sheets. Evidence for initiation of substantial ice sheets comes from Prydz Bay and suggests ice down to sea level by Eocene-Oligocene time (Barron et al., 1991; Hambrey et al., 1991). By Late Oligocene time ice-sheet glaciation was widespread and in the Miocene the marine ice-sheets of West Antarctica were first formed. The marine record suggests there have been major fluctuations since then.

The older terrestrial record of glaciation comes from King George Island off the Antarctic Peninsula where the only well documented sequences indicate glaciation (Polonez and Melville glaciations) in Early Miocene time (Smellie et al, 1998). The record from West Antarctica is no older than about 10 Ma. In East Antarctica the antiquity and subsequent fluctuations of glaciation is a matter of on-going debate (see GSA Today, 1998).
Significant questions include: when did ice sheets evolve from wet-based to the present day cold-based condition? To what extent have there been fluctuations in that aspect of the ice-sheets? What role has Cenozoic uplift played in the initiation of ice sheets and in the change to cold-based conditions? Can the timing of uplift, and its duration and amount, be better constrained for the various now-uplifted crustal blocks in and around the margin of the continent?

Key Issues

The timing and kinematics of tectonic events is critical for understanding cause and effect relationships. Paleo-oceanographic history is dependent on knowing the onset and duration of shallow and deepwater circulation patterns, which themselves are dependent on the tectonic history. The uplift and subsidence of crustal blocks, together with crustal extension, control the nature, extent and infilling of sedimentary basins. These vertical movements likewise exerted a major control on the location of centers for glaciation which affected at least regional climate. The patterns of biotic evolution in the South Polar region are similarly dependent on tectonic, glacial and paleo-oceanographic events.

Summary

In broad outline, the significant mid Cretaceous to Recent tectonic events were:

 

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