Return to the Table of Contents for this chapter.
The Ellsworth Mountains are part of a geologically and geophysically defined terrane that lies between the Transantarctic Mountains and West Antarctica. The approximately 13,000-meter-thick succession of Cambrian through Permian strata exposed in the Heritage Range has paleogeographic and paleobiogeographic affinities to the margin of Gondwanaland (Webers, Craddock, and Splettstoesser 1992), but the Early Paleozoic depositional and tectonic history of the terrane is equivocal (Thorstenson, Duebendorfer, and Rees 1994; Curtis 1995; Rees et al. 1995). Establishing the geological history of the range is critical for assessing plate tectonic models of the paleo-Pacific-facing margin of Gondwanaland.
Our fieldwork during the 1995-1996 season was concentrated in the northern Heritage Range of the Ellsworth Mountains ( figure). Our field party consisted of Ernest M. Duebendorfer (structural geologist), Margaret N. Rees (stratigrapher), Eugene Smith (volcanologist), and Lucylle J. Smith (mountaineer). On 24 November 1995, an LC-130 aircraft with a VXE-6 crew airdropped four 55-gallon drums of motor-gas on the Balish Glacier during our reconnaissance flight, and on 29 November, they put our party into the field near the drop (79°32'14"S 84°25'11"W). Using four snowmobiles and four Nansen sledges, we traversed the area and established temporary camps. Our party, fuel drums, gear, and rock samples were pulled out of the field on 3 January 1996.
Our geological mapping documents at least three deformational events in the northern Heritage Range, not just the single Mesozoic contractional event recognized by other workers. Furthermore, our recognition of structures not previously mapped calls into question the stratigraphic order, ages, and depositional relationships of several map units.
The earliest documented deformational event (D1) is manifested by refolded folds and crosscutting cleavages in the Middle Cambrian Springer Peak Formation. D1 structures are nearly completely overprinted by the dominant fabric of the range. A minimum age for D1 may be constrained by the angular unconformity between the Late Cambrian Minaret Formation and the overlying Crashsite Group in the Webers Peaks area. The contact previously was described as conformable by Webers et al. (1992) and disconformable by Goldstrand et al. (1994). The unconformity is bracketed between post-early Late Cambrian and pre-Devonian based on fauna that we collected immediately above and below the contact (Rees et al. 1995). Thus, deformation of Cambrian succession may predate or have been coeval with the development of this unconformity. The D1 event and the unconformity may be related to either the Ross Orogeny, activity along the Mozambique suture, or other movements along the paleo-Pacific margin of Gondwanaland.
The dominant structures in the Heritage Range are north-northwest-striking folds associated with the Triassic Ellsworth/Gondwanide Orogeny (D2). Our work and that of Curtis (1995) suggest that structures attributed to this event record not only northeast-southwest shortening but also involve a significant component of dextral shear deformation.
In the Soholt Peak-Edson Hills region of the northern Heritage Range (figure), we documented a series of east-vergent thrust sheets associated with D2. Several contacts between formations that were originally interpreted as depositional are major thrust-sense, cataclastic shear zones that dip 40-60° west. Kinematic indicators record dextral, top-to-the-east tectonic transport. The structurally lowest of these thrust sheets places the lower Middle Cambrian Drake Icefall Formation over the undated Union Glacier and Hyde Glacier Formations along the Drake Icefall shear zone. The structurally intermediate-level Conglomerate Ridge shear zone places the Conglomerate Ridge Formation in the hanging wall over the Drake Icefall thrust plate. The structurally highest shear zone places rocks originally mapped as Springer Peak Formation against the Conglomerate Ridge Formation.
An additional structure in the Soholt Peaks area is an inferred cross fault that strikes northeast from Mount Bursik (figure). This structure is inferred on the basis of right-lateral separation of formation contacts and of right-lateral displacement of a regional anticline. If this interpretation is correct, then the cross fault has a minimum of 5 kilometers of right-slip displacement.
A third, possibly late- or post-Gondwanide deformational event is suggested by the orientation of F2 folds. F2 fold hinges show a strong bimodal distribution suggesting that they have been refolded about subhorizontal, northeast-trending fold axes (F3). In addition, a set of northeast-striking, subvertical joints is developed throughout the northern Heritage Range. The mean orientation for these joints is similar to the orientations of the inferred F3 fold axes. This joint set may therefore represent axial planar tension joints associated with the northeast-trending F3 fold axes.
This research was supported by National Science Foundation grant OPP 93-12040.
References
Curtis, M.L. 1995. Gondwanian age dextral transpression within the Ellsworth Mountains, West Antarctica. In VII International Symposium on Antarctic Earth Sciences, Siena, Italy, 10-15 September 1995. Siena: Dipartimento de Scienze della Terra, University di Siena, Italy.
Goldstrand, P.M., P.G. Fitzgerald, T.F. Redfield, E. Stump, and C. Hobbs. 1994. Stratigraphic evidence for the Ross Orogeny in the Ellsworth Mountains, West Antarctica: Implications for the evolution of the paleo-Pacific margin of Gondwana. Geology, 22, 427-430.
Rees, M.N., E.M. Duebendorfer, E.T. Wallin, and D.J. Thorstenson. 1995. Ellsworth Whitmore Mountains terrane of Antarctica: Record of a Neoproterozoic-Cambrian active tectonic margin. In VII International Symposium on Antarctic Earth Sciences, Siena, Italy, 10-15 September 1995. Siena: Dipartimento de Scienze della Terra, University di Siena, Italy.
Thorstenson, D.J., E.M. Duebendorfer, and M.N. Rees. 1994. Evidence for a Late Cambrian-Devonian deformational event in the Heritage Range, Ellsworth Mountains, Antarctica--The Ross Orogeny? Geological Society of America Program and Abstracts, 26, A-505.
Webers, G.F., R.L. Bauer, J.A. Anderson, W. Buggisch, R.W. Ojakangas, and K.B. Sporli. 1992. The Heritage Group of the Ellsworth Mountains, West Antarctica. In G.F. Webers, C. Craddock, and J.F. Splettstoesser (Eds.), Geology and paleontology of the Ellsworth Mountains, West Antarctica (Geologic Society of America Memoir 170). Boulder, Colorado: Geological Society of America.
Webers, G.F., C. Craddock, and J.F. Splettstoesser. 1992. Geological History of the Ellsworth Mountains, West Antarctica. In G.F. Webers, C. Craddock, and J.F. Splettstoesser (Eds.), Geology and paleontology of the Ellsworth Mountains, West Antarctica (Geologic Society of America Memoir 170). Boulder, Colorado: Geological Society of America.
Webers, G.F., and K.B. Sporli. 1983. Palaeontological and stratigraphic investigations in the Ellsworth Mountains, West Antarctica. In R.L. Oliver, P.R. James, and J.B. Jago (Eds.), Antarctic earth science. New York: Cambridge University Press.