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The Transantarctic Mountains (TAM) delineate the boundary between fundamentally different lithosphere in East and West Antarctica. Figure 3 shows the distribution of crustal age provinces across the entire continent.
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| Figure 3. Distribution of crustal age provinces in Antarctica. Borg et al., 1990. | ||||
Archean and Proterozoic crust comprises the East Antarctic craton, and compressive orogenesis last affected the margin of East Antarctica during the Triassic. In contrast, West Antarctica is composed of at least 5 dominantly Paleozoic-Mesozoic lithospheric blocks. Some of these blocks experienced differential rotation with respect to East Antarctica as late as the Cretaceous (Grunow et al., 1991). Compressive orogenesis affected parts of West Antarctica into the Cenozoic, and subduction continues beneath the Antarctic Peninsula today. Data from lower crustal xenoliths confirm a change in the character of the crust across the TAM (Kalamarides et al., 1987). Plots of isotopic and elemental composition produce groupings of East and West Antarctic xenoliths that are distinct from one another (Figure 4).
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| Figure 4. Results from isotopic and major element analyses on lower crustal xenoliths from across the TAM. Note that TAM and Ross Embayment (RE) xenoliths are distinct. Symbols: closed diamonds, RE xenoliths (900 m.y.); open circles, TAM xenoltihs (900 m.y.); closed circles, Kirkpatric basalts (Jurassic host). Kalamarides et al., 1987. | ||||
Figure 5 shows major tectonic events that affected the central TAM since the Proterozoic. A major point to take from this figure is that the western margin of East Antarctica has been the locus of numerous compressional and extensional tectonic episodes since the middle Proterozoic. The Ross and Beardmore Orogenies may be related to the Lachlan-Tasman Orogeny in Eastern Australia (this orogeny is treated on another Orogenic Systems website).
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| Figure 5. Chronology of tectonic events affecting the central TAM and East Antarctica. | ||||
The Kukri peneplain merits discussion because it is commonly used as a datum in the TAM. The Kukri separates rocks deformed in the Ross and earlier orogenies from mostly undeformed rocks of the Beacon Supergroup. The Kukri is an erosional unconformity of tens of millions of years duration that makes a useful planar marker in the TAM. For example, if the Kukri is offset across a narrow region, a fault is inferred. The inferred faults are usually covered by glaciers. The Kukri is exposed at elevations up to 3000 meters.