Return to the Table of Contents for this chapter.

Structural and geomorphological observations at Cape Surprise, Shackleton Glacier area

Scott R. Miller, Paul G. Fitzgerald*, and Suzanne L. Baldwin, Department of Geosciences, University of Arizona, Tucson, Arizona 85721

Graeme Dingle, Mail Centre, Leigh, R.D. Warkworth, New Zealand

*Present address: Antarctica New Zealand, Christchurch, New Zealand.

Nearly 3 weeks were spent in the Cape Surprise (84°30.27'S 176°23.3'W) region of the central Transantarctic Mountains to extend its mapped coverage farther south, to review the previously mapped geology, and to sample granitoids in a transect perpendicular to the range front for apatite fission-track thermochronology (see Fitzgerald et al., Antarctic Journal, in this issue). Cape Surprise (figure) was named by the Southern Party of the New Zealand Geological Survey Antarctic Expedition (1963-1964) because the strata of the Beacon Supergroup and sills of Ferrar Dolerite they found there were completely unexpected. Along the 3,500-kilometer (km) length of the Transantarctic Mountains, this is the only occurrence of Beacon units exposed at the coast. The closest exposure of similar Beacon strata to those at Cape Surprise is in the range's steep frontal escarpment, 2,500 meters (m) higher in elevation and approximately 35 km to the south.

In the continuing study of the geological evolution of the Transantarctic Mountains, Cape Surprise is important because it provides constraints on the structural geometry of the range and the boundary of the west antarctic rift system. Models for the formation of the present-day Transantarctic Mountains describe a relatively simple rift-flank architecture with essentially range-parallel normal faults along the coast (e.g., Fitzgerald et al. 1986; Stern and ten Brink 1989). More recent work (Fitzgerald 1992; Sutherland 1995; Wilson 1995; Fitzgerald and Stump in press), however, suggests a more complex, dextral oblique Cretaceous and Cenozoic rift margin. Because Cape Surprise is one of only two places along the Transantarctic Mountains Front (Fitzgerald 1992) where post-Jurassic normal faulting has been observed directly--the other is in southern Victoria Land--and the only place where all associated range-front offset has been proposed along a single fault (Barrett 1965), it is an important locality for models of mountain uplift that require a large range-front normal fault (e.g., McGregor 1965; Stern and ten Brink 1989).

Cape Surprise comprises Permian strata of the Beacon Supergroup (Fairchild and Buckley Formations), Jurassic sills of Ferrar Dolerite, and Cambro-Ordovician granitoid of the Queen Maud Batholith (Barrett 1965; Stump 1995; Barrett personal communication). The contact between Beacon strata and granitoid on the south side of Cape Surprise was previously interpreted as a nonconformity, the Ordovician-Devonian Kukri Peneplain, by Barrett (1965) and subsequently by La Prade (1969). By extending the gently (approximately 5°) southwest-dipping Beacon beds atop Mount Wade north to the position of the cape, Barrett (1965) calculated a throw of 3,100 to 5,200 m that must have occurred to displace the units down to their present position at Cape Surprise. Without any structural data between Mount Wade and the cape, save for the presence of granitoid on Garden Spur, Barrett inferred that this offset took place across a single fault between Cape Surprise and Garden Spur, later named the "North Boundary fault" by La Prade (1969).

Our observations in the region around Cape Surprise included the following:

• The contact between Beacon strata and the granitoid southwest of Facet Peak (informal name, 84°31.2'S 174°26.8'W) is a normal fault, indicating that the Kukri Peneplain is not exposed near Cape Surprise. The well-exposed fault surface (N55°W 60°NE) (this study) is at an angle to bedding (N20°W 40°NE) (Barrett 1965). Slickenlines (raking 75°E) are present on the planar face (few square meters) of the granitoid that marks the contact, adjacent to which the sedimentary rock is brittley deformed into an approximately 20-m-wide crush zone of poorly calcite-cemented fault breccia.
• A number of other faults dissect the Cape Surprise area. Barrett (1965) inferred three: the North Boundary fault, trending west-northwest and two trending roughly north-south, cutting the western spur of the cape. In this study, west-northwest-to-northwest striking (dominantly normal dip slip) and north-to-northeast striking (including components of strike slip and rotational slip) sets of faults were observed north of Facet Peak, below spot-height 950, in the col between Facet Peak and spot-height 950, and near spot-height 700. Joints were measured in granitoid on Garden Spur, and these are divisible into similarly oriented sets.
• Fracture zones with evidence for intense hydrothermal alteration were observed, including those that cut a low spur northeast of spot-height 950 and the col between Facet Peak and spot-height 950. These zones, several centimeters to 40 m wide, often contain faults as shown by the presence of slickensides, fault breccia, gouge, offset strata, and drag folds. The trends of these zones (north-northeast and west-northwest) parallel those of other known faults in the area. Similar alteration zones have been documented by Wilson (1995) in southern Victoria Land. Craw and Findlay (1984) conclude that the hydrothermal alteration is probably the result of Jurassic Ferrar magmatism. These fracture zones have, therefore, existed with or without fault movement at least since that time.
• Ferrar Dolerite sills (approximately 80 m thick) and thinner dikes were found intruding the granitoid on Garden Spur and spot-height 950, both south of the inferred North Boundary fault. The presence of dolerite sills at these locations indicates that minimal displacement occurs between the two and that the major displacement across the North Boundary fault occurred between spot-height 950 and Facet Peak, contrary to the mapped interpretation of La Prade (1969). The basement sill must be at least 760 m below the Kukri Peneplain because granite above the sill at Garden Spur has 760 m relief. Assuming that the basement sill is no more than approximately 1,200 m below the Kukri Peneplain, as in southern Victoria Land (Hamilton et al. 1965), a revised estimate of throw across the North Boundary fault can be made: 2.4-3.1 km at its western end and 1.4-1.8 km at its eastern end. These estimates suggest that 1.3-3.8 km of throw is accommodated by normal faulting between Garden Spur and Mount Wade. To elucidate the structure of the Transantarctic Mountains on a transect between Cape Surprise and Mount Munson, we plan to apply apatite fission-track thermochronology to samples collected in systematic, vertical profiles (see Fitzgerald et al., Antarctic Journal, this issue, figure 2).
• Asymmetrical drainage patterns in the Cape Surprise area may be the result of westward fault-block tilting (see figure and Fitzgerald et al., Antarctic Journal, this issue, figure 1). Such drainage patterns are common indications of this style of deformation (Cox 1994; Keller and Pinter 1996). An angle between the strike of Beacon bedding and the inferred strike of the North Boundary fault (estimated by fault surface data and valley morphology) agrees with westward tilting, which may partially explain the east-west variance in offsets on the North Boundary fault. Block tilting at Cape Surprise and elsewhere between the Shackleton and Liv Glaciers may have been accommodated by north-south to north-east striking, east dipping normal faults underlying glaciers such as the Massam, Barrett, Gough, Le Couteur, and Morris. This west-directed drainage pattern is less distinct inland toward the Transantarctic Mountains frontal escarpment, and faults do not appear to offset Beacon strata in that face. We propose that activity on range-forming normal faults, such as the North Boundary fault, preceded block tiling along north-south to north-east striking faults and that the two episodes were separated by significant erosion and scarp retreat. Given the angles of both sets of faults to the general orientation of the range, both episodes appear related to dextral transtension in the west antarctic rift system.

We thank the National Science Foundation, Antarctic Support Associates, the Antarctic Development Squadron 6 (VXE-6), the Air National Guard, Ken Borek Air, Helicopters New Zealand, and the staff at the Shackleton Glacier camp for support during the season. This research was supported by National Science Foundation Grant OPP 93-16720. Scott R. Miller is also grateful for discussions with Robert Casavant.

References

Barrett, P.J. 1965. Geology of the area between the Axel Heiberg and Shackleton Glaciers, Queen Maud Mountains, Antarctica. New Zealand Journal of Geology and Geophysics, 8(2), 344-370.

Barrett, P.J. 1996. Personal communication.

Cox, R.T. 1994. Analysis of drainage-basin symmetry as a rapid technique to identify areas of possible Quaternary tilt-block tectonics: An example from the Mississippi Embayment. Geological Society of America Bulletin, 106, 571-581.

Craw, D., and R.H. Findlay. 1984. Hydrothermal alteration of Lower Ordovician granitoids and Devonian Beacon Sandstone at Taylor Glacier, McMurdo Sound, Antarctica. New Zealand Journal of Geology and Geophysics, 27(4), 465-476.

Fitzgerald, P.G. 1992. The Transantarctic Mountains of southern Victoria Land: The application of apatite fission track analysis to a rift shoulder uplift. Tectonics, 11(3), 634-662.

Fitzgerald, P.G., S.L. Baldwin, S.R. Miller, and G. Dingle. 1996. Geologic and thermochronologic studies along the front of the Transantarctic Mountains near the Shackleton and Liv Glaciers. Antarctic Journal of the U.S., 31(2).

Fitzgerald, P.G., M. Sandiford, P.J. Barrett, and A.J.W. Gleadow. 1986. Asymmetric extension associated with uplift and subsidence in the Transantarctic Mountains and Ross Embayment. Earth and Planetary Science Letters, 81, 67-78.

Fitzgerald, P.G., and E. Stump. In press. Cretaceous and Cenozoic episodic denudation of the Transantarctic Mountains, Antarctica: New Constraints from fission track thermochronology in the Scott Glacier region. Journal of Geophysical Research.

Hamilton, W., P.T. Hayes, R. Calvert, V.C. Smith, P.S.D. Elmore, P.R. Barnett, and N. Conklin. 1965. Diabase sheets of the Taylor Glacier region, Victoria Land, Antarctica (U.S. Geological Survey Professional Paper, 456B). Washington, D.C.: U.S. Government Printing Office.

Keller, E.A., and N. Pinter. 1996. Active tectonics: Earthquakes, uplift, and landscape. Upper Saddle River, New Jersey: Prentice Hall.

La Prade, K.E. 1969. Geology of the Shackleton Glacier area, Queen Maud Range, Transantarctic Mountains, Antarctica. (Ph.D. thesis, Texas Technological College, Lubbock, Texas.)

McGregor, V.R. 1965. Geology of the area between the Axel Heiberg and Shackleton Glaciers, Queen Maud Range, Antarctica. Part 1--Basement complex, structure, and glacial geology. New Zealand Journal of Geology and Geophysics, 8(2), 314-343.

Stern, T., and U.S. ten Brink. 1989. Flexural uplift of the Transantarctic Mountains. Journal of Geophysical Research, 94, 10315-10330.

Stump, E. 1995. The Ross Orogen of the Transantarctic Mountains. New York: Cambridge University Press.

Sutherland, R. 1995. The Australia-Pacific boundary and Cenozoic plate motions in the SW Pacific: Some constraints from Geosat data. Tectonics, 14(4), 819-831.

Wilson, T.J. 1995. Cenozoic transtension along the Transantarctic Mountains-west antarctic rift boundary, southern Victoria Land, Antarctica. Tectonics, 14(4), 531-545.