Tectonic Evolution of the Transantarctic Mountains

The Transantarctic Mountains, one of the major young mountain chains on Earth, separate East Antarctica from the West Antarctica Rift System and the grounded East Antarctic Ice Sheet from the marine-based West Antarctic Ice Sheet over a substantial portion of the Antarctic interior along the margin of the Ross embayment (Fig. 7.5).

The regional structural architecture of the Transantarctic Mountains remains poorly known in most regions because of the extensive ice cover. The East Antarctic Ice Sheet hides the structure of the mountains along the Polar Plateau, preventing the identification of the extent of mountain structures into East Antarctica. In the McMurdo Sound coastal area of the Ross embayment, thin but extensive piedmont glaciers obscure the structural boundary (''Transantarctic Mountains Front'') with the off-shore Victoria Land rift basin. The main drainage for the East Antarctic Ice Sheet into the Ross embayment of West Antarctica is through outlet glaciers carved through the mountains. It has long been inferred that these outlet glaciers developed where faults cut transverse to the mountain trend (e.g. Gould, 1935; Gunn and Warren, 1962; Grindley and Laird, 1969; Davey, 1981; Wrenn and Webb, 1982; Cooper et al., 1991; Tessensohn and Worner, 1991; Fitzgerald, 1992), and the occurrence of pseudotachylites has been proved a valuable tool to constrain the age of some faults (Di Vincenzo et al., 2004). In most cases, however, there is little direct evidence either for the existence of a fault or of its age. This is evidently a key issue to be addressed in order to determine if appropriate structures are present to allow differential uplift of discrete mountain blocks (e.g. van der Wateren et al., 1999), to understand the localization of valley incision and how erosion has contributed to mountain uplift, and to provide constraints on the development of the pathways for drainage of the East Antarctic Ice Sheet.

In contrast to other young mountain belts, such as the Alpine-Himalayan system and the North American and Andean Cordilleras, which formed at convergent plate boundaries, there is a tight genetic link between the uplift of the Transantarctic Mountains and intra-plate processes associated with the rifting within the Antarctic plate. These mountain ranges are generally interpreted as a high-relief rift flank uplift (van der Wateren et al., 1999), which occurred during the Mesozoic-Cenozoic break-up of the Gondwana supercontinent (Cooper et al., 1987, 1991; Tessensohn and Worner, 1991; Davey and Brancolini, 1995; Fitzgerald and Stump, 1997). The development of the Transantarctic Mountains in an extensional, rather than a contractional, tectonic regime was already recognized by pioneering Antarctic geologists, who interpreted the mountain chain as a fault-bounded horst block (David and Priestley, 1914; Gould, 1935). More recent structural investigations indicate that the Transantarctic Mountains consist of a linear to curvilinear chain of asymmetric tilt blocks bounded on the West Antarctic edge by a major normal fault zone and subdivided by transverse faults (Fitzgerald et al., 1986; Tessensohn and Worner, 1991; Fitzgerald, 1992; Tessensohn, 1994a,b; Fitzgerald and Baldwin, 1997). Active rift tectonism and mountain uplift have been inferred from the presence of active volcanism and Neogene-Quaternary age faulting in the western portion of the rift and the Transantarctic Mountains (Behrendt and Cooper, 1991; Davey and Brancolini, 1995; Jones, 1997). The Cenozoic-Cretaceous asymmetric uplift and subsequent erosion exposed basement rock and older sediments along the coastward side of the Transantarctic Mountains, leaving younger sediments only on the inland side. Apatite fission track analysis in the McMurdo Sound area indicate an uplift of ~6 km since c. 55 Ma, though other sectors of the Transantarctic Mountains record denudation events in the Late Cretaceous also (Fitzgerald, 1992, 1995; Studinger et al., 2004) (Fig. 7.6) and in the Early Cretaceous (e.g. Scott Glacier area: Stump and Fitzgerald, 1992; Fitzgerald and Stump, 1997).

The cause of this uplift and denudation is the subject of continuing debate, the reconstruction of Cenozoic tectonic processes being complicated by the complex interplay between a number of factors including the regional plate geodynamics, rifting style, erosion rates, subsidence and formation of thick sedimentary layers, the volcanic activity and the glacial processes. The possible mechanisms for the uplift include thermal buoyancy due to conductive or advective heating from the extended upper mantle of the hotter West Antarctic lithosphere (Stern and ten Brink, 1989; ten Brink and Stern, 1992), simple shear extension (Fitzgerald et al., 1986), isostatic rebound due to stretching of the lithosphere through normal faulting (Bott and Stern, 1992), plastic necking (Chery et al., 1992), elastic necking (van der Beek et al., 1994) and rebound response to erosion (Stern and ten Brink, 1989). In the McMurdo Sound area, some of these mechanisms are based on specific assumptions about the crustal and upper mantle structure beneath the ''Transantarctic Mountains Front'', as well as about the timing of the

Tectonic Evolution

Figure 7.6: Schematic diagram showing the variation of exhumation events along the TAM at different localities (after Fitzgerald, 2002, with permission from the Royal Society of New Zealand). SCG, Scott Glacier area; BDM, Beardmore Glacier Area; SHG, Shackleton Glacier area; SVL, Southern Victoria Land; TNB, Terra Nova Bay; NVL, Northern Victoria Land. A relative scale only is shown for exhumation as the amount of exhumation at any one locality will vary across the range. Early or late Cretaceous exhumation is not always present throughout an area (e.g. Scott Glacier region; see Fitzgerald and Stump, 1997).

Figure 7.6: Schematic diagram showing the variation of exhumation events along the TAM at different localities (after Fitzgerald, 2002, with permission from the Royal Society of New Zealand). SCG, Scott Glacier area; BDM, Beardmore Glacier Area; SHG, Shackleton Glacier area; SVL, Southern Victoria Land; TNB, Terra Nova Bay; NVL, Northern Victoria Land. A relative scale only is shown for exhumation as the amount of exhumation at any one locality will vary across the range. Early or late Cretaceous exhumation is not always present throughout an area (e.g. Scott Glacier region; see Fitzgerald and Stump, 1997).

rift-related processes in the nearby VLB. Some constraints have already been provided by gravity studies (e.g. Davey and Cooper, 1987; Reitmayr, 1997), and by seismic reflection data (O'Connell and Stepp, 1993; Della Vedova et al., 1997). Data from the ACRUP seismic experiment indicate thickening crust beneath the Transantarctic Mountains to a depth of 38 km, and quite low P-wave velocities (7.6-7.7 km/s) in the mantle beneath the VLB (Della Vedova et al., 1997), while slow S-wave velocities are inferred at 60-160 km depth from surface wave analysis (Bannister et al., 1999), suggesting that the upper mantle is anomalously warm at that depth.

More recently, the findings of drilling projects in the McMurdo Sound area (CIROS, Cape Roberts Project) have resolutely constrained the age of the onset of subsidence at the westernmost margin of the VLB. The first direct geological evidence of a major pre-Oligocene uplift phase of the Transantarctic Mountains comes from the oldest strata cored in the CIROS-1 and CRP-3 drill-holes (Barrett et al., 1989, 2001). These include granitic clasts eroded from exposed basement to the west, implying that the Transantarctic Mountains were at least half of their present height by then, for erosion had cut through the more than 2,000 m of Devonian-Jurassic Gondwana cover beds to basement (Barrett et al., 1989, 2001). In the Cape Roberts drill core, the presence of the Devonian Arena Formation (Beacon Supergroup) as bedrock beneath the Cenozoic sediments indicates that significant uplift and unroofing of the Transantarctic Mountains must have occurred prior to the Oligocene (Barrett et al., 2001).

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