|
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
The rift setting for the Transantarctic Mountains (TAM) system is well established. (Not convinced? See the pages on the crust and the mantle). But when did rifting and uplift occur in the TAM? How much rock uplift, surface uplift, and denudation occurred in the TAM? How is uplift partitioned in time and space? These issues are covered in this section.
Jurassic-Cretaceous rifting
The latest episode of rifting in the TAM appears to have begun in the Late Jurassic or Early Cretaceous. Abundant mafic magmatic rocks of Jurassic age are found in the TAM. These include the Ferrar Dolerite in Victoria Land, the Kirkpatrick Basalt in the central TAM, the Dufek gabbro in the Pensacola Mountains, and other flows, sills, and dikes in the TAM. Schmidt and Rowley (1986) suggested that these mafic rocks are evidence of Jurassic rifting of Antarctica and Asutralia from Africa. Rocks of similar age and composition are found in Tasmania, Australia, and Southeast Africa (including the Karoo Dolerite). This rift was probably built on the roots of the older Gondwanide Orogeny, termed the Weddell Orogeny in the TAM.
 |
||||
| Figure 24. Diagram showing the age of basalts associated with the breakup of Gondwana for different fragments of the super-continent. Note that rift basalts in Antarctica are Jurassic in age. Schmidt and Rowley, 1986. | ||||
The Jurassic rift system extended along the present day TAM from Northern Victoria Land , through the Ellsworth and Pensacola Mountains, to the Theron Mountains. Sense of extension was down to the east. Note that there are apparently no reversals of asymmetry polarity along the entire length of the rift! Dextral strike slip probably played a major role in the rift system, perhaps shifting all of West Antarctica to the right relative to East Antarctica. This shift alleviates problems of Jurassic plate reconstruction, in which West Antarctica overlaps with the tip of South America.
 |
||||
| Figure 25. Map of Antarctica showing the Jurassic rift system and inferred Jurassic dextral strike-slip faults. These faults are inferred to have moved West Antarctica into it's present position. Schmidt and Rowley, 1986. | ||||
But how much uplift occurred during the Mesozoic? Apatite fission track analyses consistently show episodes of denudation and uplift in the Cretaceous across the TAM. In the Ellsworth Mountains, denudation occurred between 140 and 117 Ma, with a maximum rock uplift of 3 km (Fitzgerald and Stump, 1991). Relief in the Ellsworth Mountains at this time was at least 1.8 km, and this relief has continued to the present. For definitions of surface uplift, rock uplift, and denudation (exhumation), see England and Molnar (1990).
In the Scott Glacier region of the TAM (near the South Pole), denudation episodes are recorded at 125 Ma and 95 Ma, with magnitudes of 0.5-2 km (Fitzgerald and Stump, 1997). These episodes are shown schematically in Figure 26.
 |
||||
| Figure 26. Episodes of exhumation in the Scott Glacier region of the TAM. Note that the earliest occurs in the Early Cretaceous, consistent with the volcanic evidence for Jurassic rifting (above). Fitzgerald and Stump, 1997. | ||||
In central Victoria Land, denudation began at about 115 Ma, with a maximum of 2 km of denudation (Fitzgerald, 1994). This is shown in Figure 27.
 |
||||
| Figure 27. Magnitude and timing of exhumation in central Victoria Land. Fitzgerald, 1994. | ||||
The Ford Ranges in Marie Byrd Land, West Antarctica, also show evidence of Late Cretaceous denudation and rock uplift (Richard et al., 1994).
The time delay between initiation of rifting and uplift of rift flanks is reasonable if uplift is caused by lateral conduction of heat from the center of the rift to the flanks.
Cenozoic rifting
Most of the exhumation and rock uplift in the TAM occurred during the Cenozoic (Figures 26 and 27). The magnitudes of Cenozoic rock uplift and denudation are quite large. In central Victoria Land, a point 40 km inland (The Cloudmaker) experienced about 7 km of rock uplift during the Cenozoic. About 10 km of rock uplift is estimated for ranges at the coast (Fitgerald, 1994). Southern Victoria Land experienced about 6 km of Cenozoic uplift (Fitzgerald and Stump, 1997; Fitzgerald, 1992; Gleadow and Fitzgerald, 1987). Note that up to 5 km of Cenozoic sediment fills the Victoria Land Basin in the Ross Sea, so the total magnitude of Cenozoic extensional offset could be up to 15 km.
The most rock uplift in the TAM occurred at the coast (Fitzgerald and Stump, 1997; Fitgerald, 1994; 1992), and the magnitude of rock uplift decreases landward. The orientation of normal faults is at an angle to the axis of maximum uplift (Figure 7). This observation implies a component of dextral strike-slip to the motion on the normal faults.