BASEMENT ROCK CHARACTERISTICS

An understanding of the bedrock geology beneath the ice sheets is important to help determine how and when subglacial aquatic environments formed and whether these

GEOLOGICAL AND GEOPHySICAL SETTING

environments are connected. Like much information about the Antarctic continent, the basement rocks of the East Antarctic Shield are still poorly understood compared to other continents. This lack of knowledge is a result of the fact that 98 percent of the area is ice covered, and the 2 percent of it that is exposed, generally in coastal regions, is challenging to access. For this reason, current understanding of the basement geology of East Antarctica is constrained largely by seismic data, modeling of gravity and aeromagnetic data, and extrapolations of coastal rock exposures and deformational structures. However, recent examination of the assembly of the East Antarctic Craton suggests that Neoproterozoic to Cambrian rocks (900 million to 543 million years ago), similar to those in the Pinjarra Orogen of Western Australia (Figure 2.2), may exist in the vicinity of some subglacial lakes, including Lake Vostok (Fitzsimons 2003).

A similar age for the basement rocks beneath the ice sheet was indicated from geo-chemical (⁸⁷Sr/⁸⁶Sr versus ¹⁴³Nd/¹⁴⁴Nd) and mineralogical characterization as well as dating (neodymium model age) of a millimeter-size bedrock inclusion recovered from the accreted ice section of the Lake Vostok core (Delmonte et al. 2004). This study made the assumption that the inclusion samarium to neodymium ratio is representa-tive of the basement in spite of its small size. In agreement with the neodymium model ages, new uranium-thorium-lead ion microprobe determinations on detrital zircon and monazite, also from the Lake Vostok core, have yielded ages of between 1800 million and 600 million years (Rodionov et al. 2006). This range is similar to that observed at the surface in the Prince Charles Mountains, Prydz Bay, and Wilkes Land.

The fact that “young” ages in the range of 1800 million to 600 million years are being recognized from materials recovered in the Vostok core, suggests that faults and sutures exist that are juxtaposing basement rocks of a variety of ages. This is relevant to the study of subglacial aquatic environments because if the interior of Antarctica has fractured rocks as suggested by Figure 2.2, then it is not only likely but inevitable that these subglacial aquatic environments constitute a connected hydrologic system.

Further evidence for a potentially connected hydrologic system comes from the hydraulic conductivities¹ of bedrock beneath the ice sheet. The hydraulic conductivities of unfractured, igneous, and metamorphic rocks comparable to those observed in East Antarctic coastal exposures are low (3 × 10^–14 to 2 × 10^–10 m s^–1), which means that water does not flow through these rocks readily. However, if the interior of the East Antarctic Craton in the vicinity of Ridge B are dominated by fractured crystalline rocks, as suggested by models 2 and 3 of Fitzsimmons (2003) in Figure 2.2, then the local hydraulic conductivity is likely to be several orders of magnitude greater (8 × 10^–9 to 3 × 10^–4 m s^–1) than that of solid rocks (Domenico and Schwartz 1998).

From a number of studies, it is evident that the East Antarctic basement rocks are Precambrian in age (3800 million to 543 million years ago), but definitive ages of large sections of the basement rocks remain scarce. Palinspastic reconstructions¹ show that the East Antarctic basement rocks were formerly the nucleus around which the southern continents were amalgamated to form the supercontinent Gondwanaland. Therefore, this part of the continent is generally regarded as a tectonically stable cratonic region stabilized by ~800 million years when the supercontinent Rodinia fragmented. The Gamburtzev Mountains, which lie beneath the ice just west of Lake Vostok, have an estimated relief of 3500-4000 m. This relief, which is anomalously high for a cratonic

1A paleogeographic or paleotectonic map showing restoration of the features to their original geographic positions, before thrusting or folding of the crustal rocks.

EXPLORATION OF ANTARCTIC SUBGLACIAL AQUATIC ENVIRONMENTS

2.02

FIGURE 2.2 The bedrock geology beneath most of interior East Antarctica is not well known, and three possible interpretations for the architectural makeup of the crust beneath the East Antarctic lakes have been proposed and are labeled 1, 2 and 3. Interpretations 1 and 2 place Paleoproterozoic (2500 million to 1600 million years ago) to Mesoproterozoic (1600 million to 1900 million years ago) rocks of the Mawson Craton beneath the Lake Vostok region. Inter-pretation 3 on the other hand places Neoproterozoic to Cambrian (900 million to 543 million years ago) rocks similar to those in the Pinjarra Orogen in Western Australia beneath the area.

This figure illustrates that the interior of the East Antarctic Craton may not be homogeneous, but rather is faulted and may comprise terranes of different ages and origins. The structural relationships may be a boundary condition for how and where lakes form and the extent to which they might be connected. SOURCE: Fitzsimons 2003.

GEOLOGICAL AND GEOPHySICAL SETTING

interior, has led to speculation that volcanism may have erupted through the craton as recently as Cenozoic times (Dalziel 2006). This hypothesized volcanism would likely be in a zone of high heat flow with potential to accelerate melting of the ice sheet in certain areas. It should be noted that most of these ideas are based on very few con-crete data from the underlying rocks, and until samples of these rocks are obtained, speculation about the timing and processes by which these environments could have formed will remain.