CENTRAL AND SOUTHERN TRANSANTARCTIC MOUNTAINS

Proceedings of a Workshop

September 1997

The Ohio State University

Columbus, Ohio 43210-1002 U.S.A.

Convenors

David H. Elliot and James W. Collinson

Department of Geological Sciences and Byrd Polar Research Center

The Ohio State University, Columbus, Ohio

William R. Hammer

Department of Geology, Augustana College, Rock Island, Illinois

1998

Acknowledgments

The workshop was supported by the Antarctic Geology and Geophysics program of the Office of Polar Programs, National Science Foundation (OPP 9710357). The conveners wish to acknowledge the participation of many scientists and the efforts of the group leaders in preparing the group reports. Lynn Tipton-Everett helped in the preparation of this report and Lynn Lay checked references. Illustrations are by Jeanne Jaros.

Executive Summary

The workshop on the Central and Southern Transantarctic Mountains was convened to assess the important geological objectives that might be addressed by concentrating logistic support at a limited number of sites. The general goal of the workshop was to identify the primary science objectives in the various fields of geology and geophysics, and establish a possible scenario of fieldwork so as to meet those objectives. In part, this workshop can be viewed as an implementation plan for one component of the report stemming from the workshop on Geodynamic Evolution of the Transantarctic Mountains and West Antarctic Rift System.

Thirty two geologists and geophysicists attended the workshop. Four general areas of research provided the initial focus for discussion of compelling problems and where to address them. Group Reports and Abstracts of presentations are included as appendices. Groups addressed Tectonics and Structure; Gondwana stratigraphy and paleontology; Cenozoic history; Basement Geology. As a result of workshop discussions, participants indentified six issues of global significance that can be addressed in the central and southern Transantarctic Mountains:

* Laurentia-Gondwana rift history and early Paleozoic intercontinental correlations

* Biotic evolution and paleoclimates at high latitudes

* Magmatism and continental break-up processes

* Geodynamic evolution of continental rifts

* Landscape evolution in polar regions

* Evolution of Cenozoic glaciation

Workshop participants concluded that compelling science objectives justify seeking major logistic support. Therefore, individual investigators and groups of investigators will submit Letters of Intent, which will include brief summaries of proposed science, scale of logistics, preferred field years, and associated or related science projects. Following on from these expressed interests, the recommendations are:

* The Office of Polar Programs announce that opportunities exist for helicopter-supported field research at remote locations.

* The Office of Polar Programs encourage individual investigators and groups of investigators to submit proposals utilizing such support and other appropriate logisitics for remote field locations.

Figure 1. Generalized setting of the Transantarctic Mountains and the West Antarctic Rift System. Diagonal pattern indicates the region that would lie below sealevel following deglaciation and isostatic rebound. Solid black represents areas of rock exposure. TM - Thiel Mountains; EM - Ellsworth Moutnains. Elongate box marks the geological-geophysical transect recommended by previous reports.

I.INTRODUCTION

Research in the central and southern Transantarctic Mountains has yielded many significant discoveries and results (Table 1) over the 40 years since the International Geophysical Year when the U.S. Antarctic Program was established. These results have been obtained by a combination of numerous individual remote field projects and five major field camps (between 1969-70 and 1995-96) with varying numbers of projects and scientists supported by helicopters. These results laid the foundation for on-going and anticipated research efforts directed at addressing specific questions.

Table 1. Major advances in geology in CTM from helicopter-supported field camps

1969-1971 helicopter-supported camps near Beardmore, Shackleton and Amundsen glaciers

1985-1986 helicopter-supported camp in Beardmore Glacier region

1990-1991 helicopter-supported camp in Beardmore Glacier region

1995-1996 helicopter-supported camp in Shackleton Glacier region (preliminary)

Thus the time seemed appropriate for a workshop to explore both the compelling scientific questions that can be addressed by further research in the central and southern Transantarctic Mountains, and how this research can best be achieved given the new possibilities for logistic support. This workshop builds on an earlier evaluation of the geodynamic evolution of the Transantarctic Mountains and associated West Antarctic Rift System as a whole (Wilson and Finn, 1996). This workshop also dealt with two aspects of the geology not specifically considered in that earlier report: the pre-Devonian basement rocks and the Beacon Supergroup.

II. Workshop Organization

The first day of this two-day workshop was devoted to science presentations and discussions, and the second to: science summaries; map, imagery and photographic needs; logistics questions; and preliminary planning of possible logistic requirements.

Established research scientists in each of four areas gave overviews of the science and important problems. Each overview was followed by short presentations by individual participants. At the start of the second day, the four groups met to refine their goals and objectives, and these were then presented to the workshop as a whole. Jean Claude Thomas, U.S.G.S., followed these presentations with a discussion of the potential for new imagery of the central and southern Transantarctic Mountains. The Antarctic Geologyand Geophysics Program Manager, Scott Borg, then discussed current thinking in OPP about logistic opportunities and capabilities; this was followed by a general discussion of this topic. The workshop then proceeded to identify possible areas of concentrated research interest, and concluded with a tentative framework for science and a summary of the logistics required to address these goals.

III. Workshop Results/Conclusions

Research was initially discussed under four general topics:

a. Basement geology

b. Gondwana stratigraphy and paleontology

c. Mesozoic-Cenozoic tectonics, including magmatism, structure, geophysics (hereafter separated into: magmatism and continental break-up; geodynamic evolution of rifts)

d. Cenozoic glacial history (hereafter separated into: Cenozoic glaciation and climate history; landscape evolution)

Discussions of these general topic areas were broad-ranging and directed toward identifying the high priority science that could be addressed by research in the Transantarctic Mountains south of the Byrd Glacier. Details of the science are given in Appendix A and Abstracts of presentations in Appendix B.

A. High priority science objectives

The highest priority can be attached to those aspects of the geology that make a unique contribution to earth sciences. These objectives provide the rationale for further research in the central and southern Transantarctic Mountains, and are listed below (NOTE that the topic areas have NOT been evaluated relative to each other and that these topics are NOT rank ordered).

1. Pre-Gondwana tectonic history. The SWEAT hypothesis is an example of application of the principles of plate tectonics to the Precambrian. The Neoproterozoic to Early Paleozoic record in Antarctica provides critical information for testing that hypothesis and understanding Rodinia- and Gondwana-wide tectonic events.

2. Biotic evolution and paleoclimates at high latitudes. Antarctica provides a unique opportunity to study a sedimentary sequence, deposited in a polar and near-polar position, that contains a nearly complete record of changes from "icehouse" to "greenhouse" conditions. This succession of upper Paleozoic to Mesozoic clastic sedimentary rocks contains exceptional vertebrate, invertebrate, plant and trace fossils. Fundamental questions concerning biotic evolution, the physical changes that occur during an "icehouse" to "greenhouse" transition, paleoclimate, paleoclimatic models, and tectonic evolution of southern Pangea can be addressed by studies of these rocks.

3. Magmatism and continental break-up processes. Assembly and break-up of supercontinents is a first order event in crustal history. The Ferrar tholeiites are an integral part of the flood basalt magmatism associated with the initial fragmentation of Gondwanaland. They contain unique information on source regions and evolution of mantle-derived magmas and their plumbing systems. They bear on the role of plumes, whether as heat sources alone or as heat and magma sources, plate-scale mantle processes, and the evolution of large igneous provinces (LIPs) in general. The tectonic setting of the linear belt of Ferrar tholeiites is critical in evaluating competing models for break-up processes.

4. Geodynamic evolution of continental rifts. The linked Transantarctic Mountains and West Antarctic Rift System, one of earth's major rift systems, have unique attributes that relate to intraplate deformation during Mesozoic break-up and subsequent rifting and uplift. These attributes include aspects such as the consistent asymmetry of the rift shoulder, the long duration of the crustal thickness and thermal boundaries between the two provinces, and the apparent aseismicity yet active volcanism, uplift and faulting. Furthermore, uplift is related to the glacial history and climate, which themselves have feedback into the denudation history.

5. Landscape evolution. Development of the landscape reflects the interplay of tectonics, climate and denudation. In the Transantarctic Mountains it links adjacent sedimentary basins to uplift of the range and to Cenozoic climate trends. The Transantarctic Mountains, and other high elevation ranges in East Antarctica, may be among the planet's least modified ancient mountain landscapes, parts of which may date from the Early Miocene or earlier. These mountains provide a unique window into Cenozoic history.

6. Cenozoic glaciation and climate history. Terrestrial sequences are an essential counterpart to marine sequences in the study of glaciation and climate history. The Sirius Group, with the most diverse geomorphic settings of any glacial unit in the Transantarctic Mountains, has been interpreted in terms of a dynamic East Antarctic ice sheet which attained its present state during mid-Pliocene cooling; this contrasts with the conventional view of long-term stability with the present polar conditions and East Antarctic ice sheet existing essentially unmodified since the early Miocene. These contrasting hypotheses have profoundly different implications for the course of Cenozoic climate.

B. Field and laboratory objectives

Research objectives for the central and southern Transantarctic Mountains are presented in full in Appendix A. Considerable overlap exists between the various areas. The goals are not exclusive; in due course other research objectives and endeavors will be formulated and will augment or supersede those listed here. Field and laboratory objectives for the six general areas, listed in the same order as above, are summarized here.

1. Basement geology (Pre-Gondwana tectonic history). Isotope mapping of igneous, metamorphic and sedimentary rocks. Neoproterozoic rift-margin history and relations to Laurentia. Crustal architecture of basement terrains. Early Paleozoic orogenic history and intercontinental correlations. Post-orogenic, pre-Devonian, uplift and denudation.

2. Beacon rocks (Biotic evolution and paleoclimate at high latitudes). Late Paleozoic to Early Mesozoic biotic and environmental changes, from a polar perspective, with respect to an icehouse to greenhouse climate shift. Data acquisition for testing general circulation models developed for modern climates.

3. Ferrar tholeiites (Mesozoic magmatism and continental break-up). Spatial diversity of Ferrar Dolerite geochemistry. Ferrar magma origin and source region. Ferrar plumbing system. Ferrar tectonic setting.

4. Mesozoic-Cenozoic tectonics (Geodynamic evolution of continental rifts). Structural architecture of the Transantarctic Mountains and adjacent regions. Post-Jurassic tectonic history and relationships to adjacent sedimentary basins to understand the evolution of a unique rift margin. Lithospheric structure in transects from the Ross embayment to the Wilkes-Pensacola basin. Relationships between uplift history and glaciation.

5. Landscape evolution. Extensive mapping of landscape elements. Systematic sample collection for cosmogenic nuclide, radiogenic isotope and fission-track dating, to provide a framework for landscape analysis.

6. Cenozoic glacial history (Cenozoic glaciation and climate history). Mapping and analysis of glacial deposits to develop a chronology of glaciation from inception, through growth and fluctuations, to present day conditions. Collection of fossil flora and fauna for paleoenvironmental studies.

C. Logistics to accomplish the science objectives

1. Available mix. Potentially, the basic logistic support consists of the heavy lift, ski-equipped LC-130 aircraft, the limited load-capacity and range performance Twin Otter, and helicopters. The LC-130 aircraft have placed many small parties in the field for self-contained operations with surface transport (snowmobiles). These aircraft have also serviced various helicopter-supported field camps, providing the ability to build temporary housing and other facilities at remote field sites and supply the large amounts of fuel needed for sustained helicopter operations. Twin Otter aircraft are a more recent addition to the logistic capabilities and have provided invaluable support for small parties needing transport to distant localities for short durations as well as for camp moves. Helicopter support was formerly provided exclusively by the US Navy UH1N Bell helicopters, but recently small Squirrel (A Star) helicopters have proven to be most effective in support of fieldwork.

2. Research support scenarios. Three basic scenarios can be envisaged:

i) A traditional large field camp. The Shackleton 1995-96 field camp is an example. It accommodated 12 individual projects with 56 scientists who were supported by 7 ASA camp personnel; two helicopters were deployed for eight weeks (three UH-1N helicopters provided equivalent support for previous remote field camps of this type); two separate weeks of Twin Otter time provided additional logistic resources.

ii) A small helicopter-supported field camp. The Beardmore 1990-91 field camp is an example. It accommodated eight projects with 29 scientists who were supported by 5 ASA personnel. Field projects also operated, prior to the 4 weeks of helicopter support, with surface transport. Twin Otter-type support (Ganovex Dornier aircraft) was also provided for a limited time.

iii) Individual field projects. Perhaps two to four projects located in the same region, placed in the field by LC-130 or Twin Otter, and requiring limited helicopter or Twin Otter support. Such support might be one to two weeks duration and perhaps split into two periods during the field season.

The principal science objectives to emerge from the workshop require a mix of the different modes of operation outlined above. The groups considered priorities within their subdisciplines and identified regions where objectives could best be met. Given the need for helicopter support to attain the objectives, the workshop as a whole then considered whether priorities in different groups had sufficient geographic overlap to justify seeking helicopter-supported field camps. The conclusion was that there are a number of localities where concentrated interest from one or more groups justifies deployment of helicopters to remote field camps for extended time periods. Rationales for possible camps are described below.

Table 2. Priorities for each interest group with respect to potential camp sites. Note that only the three highest priorities are listed (with 3 as the highest). Note that there is no ranking between subdisciplines; Table 2 does not reflect an absolute ranking. Note that potential camp sites are ordered geographically.

3. Field objectives for each possible camp site (Fig. 2). NOTE: the order is geographic from north to south.

Figure 2. Map of the Ross Sea sector of the Transantarctic Mountains. The region of interest lies between the Byrd Glacier and the Ohio Range and beyond to the Thiel Mountains (see Figure 1). Sites for possible Beardmore "South"and Shackleton camps are those used previously. Sites for other possible camps are approximate and would require detailed evaluation.

i) Nimrod Glacier. The Nimrod Glacier region has the most extensive exposures of

basement rocks between south Victoria Land and the Scott Glacier region. It is a prime target for study because of the thick siliciclastic and carbonate sequences, the rich biota in the carbonate beds, the post-orogenic coarse clastic sequences, the cross-strike extent of the sequences, and the structural relations between them. Furthermore, high-grade metamorphic assemblages cropping out at the head of the Nimrod Glacier represent deeper crustal levels in the Ross Orogen. Ongoing studies may answer many of the questions raised.

ii) Beardmore Glacier region (Beardmore South camp). The Queen Alexandra Range has the most complete Gondwana sequence of any region in the Transantarctic Mountains. It is of prime importance paleontologically; every season of fieldwork, starting in 1969-70, has turned up new vertebrate material and new paleobotanical discoveries. It is a key area for establishing Gondwana paleoenvironments, from glaciation, through wet-temperate forested conditions, to drier alluvial plains. The recent discovery of fossil crayfish and fossil-bearing lacustrine beds provides an additional focus, as does the presence of sequences crossing the Permian/Triassic boundary.

It is also a key area for addressing the sequence of magmatic events following cessation of Gondwana sedimentation. It is the only region where silicic pyroclastic rocks preceding basaltic volcanism occur in situ, and thus the stratigraphic relations and tectonic settings can be clarified. The basaltic pyroclastic rocks have greater variety here than anywhere else in the Transantarctic Mountains; paleovolcanological studies on the extensive exposures will clarify and elaborate on the processes and environments preceding eruption of the flood basalts. In this region the extensive exposure of dolerite sills will facilitate studies on geochemical variability, magma transport and the magma plumbing system.

Landforms have been developed primarily on the Beacon. There are indications of relict pre-glacial surface events preserved in toreva blocks resting on mesa and butte topography. There are also numerous surfaces suitable for exposure age dating. An array of glacial deposits occurs in this region in different geomorphic settings and at different elevations.

Basement rocks exposed to the north of the Queen Alexandra Range provide a transect across the Transantarctic Mountains. This is a key area for understanding relationships between various crustal terranes and between various lithostratigraphic units. Much of this would be more easily accomplished from a camp adjacent to the Nimrod Glacier (see: 1. Nimrod Glacier).

A camp in this region could provide a convenient staging post in support of the meteorite program.

iii) Mill Glacier. The upper Beardmore Glacier region has the most extensive Cenozoic glacial deposits south of McMurdo. The Oliver Bluffs region has been particularly important and still has much to offer. In the Mill Glacier region as a whole, there are large expanses of relatively flat terrain where other glacial deposits undoubtedly occur. These deposits will augment and build on the current database and yield much information on landscape evolution as well as glacial history. The possibility exists for developing a substantially larger database for interpretation of the landscape and glacial history and for comparison with other regions in the Transantarctic Mountains. Furthermore, inferred neotectonic displacements have been mapped on the Dominion Range and, given the significance of these features for uplift scenarios, need to be examined more closely.

iv) Shackleton Glacier. The frontal fault system, expressed by a series of tilted fault blocks from Cape Surprise southward for about 100 km, represents a prime target for investigating the structural evolution of the Transantarctic Mountains. Wide expanses of exposed rock in this region provide a prime target for landscape analysis. Inferred neotectonic faults reported from the head of the Shackleton Glacier are a significant target for structural studies related to the Late Cenozoic history of uplift and glaciation.

v) Scott Glacier region. This region provides the most extensive transect of the Transantarctic Mountains basement rocks apart from north Victoria Land. Although high-grade metamorphic basement (craton) is not exposed, all the components of the Ross Orogen are present and exposed over a wide area. It is the best place to conduct a geological transect of the range, with primary emphasis on the structural and tectonic history. Relatively short distance projection of the geological boundaries of basement rock units along strike will intersect corridors where oversnow and aerogeophysical traverses can be conducted parallel to Scott Glacier. The geophysical traverses can be linked to ongoing surveys in West Antarctica, and extended onto the polar plateau, thus contributing to the major transect corridors identified in previous reports (e.g., Polar Research Board, 1986; Wilson and Finn, 1996).

This region is key to distinguishing the geometry and kinematics of fault arrays related to Cretaceous and Cenozoic uplift episodes documented by fission-track data. The exposures of Cenozoic volcanic rocks may offer the opportunity to develop constraints on timing of faulting. This area, and the mountains to the east, are pivotal in establishing structural-kinematic links between development of the Transantarctic Mountains and motions of West Antarctic blocks, particularly the Ellsworth-Whitmore block.

This general region is also of prime importance for studies of Cenozoic glacial history. Sirius Group deposits are known (e.g., along the flanks of the Reedy Glacier), but require detailed studies. To date, geomorphological studies have been cursory; this region has extensive areas of outcrop and will provide a contrast and complement to the Beardmore-Mill Glacier region and to south Victoria Land where most landscape evolution studies have been conducted so far.

Knowledge of the Ferrar Dolerite in this part of the range is extremely limited; it is important for assessing geographic trends in geochemistry and time of emplacement, as well as the magma plumbing system. This region provides the link to the Dufek Intrusion and the Weddell Sea end of the Transantarctic Mountains.

Although only Permian Beacon strata (glacials to coal measures) are exposed along the plateau edge, those outcrops will yield critical information on the along-axis basin trends identified in the Beardmore-Shackleton region.

Because of the distance from McMurdo Station, a camp could facilitate support of the meteorite program, both in providing helo-supported reconnaissance of icefields as well as Twin Otter access to a variety of locations.

D. Community action

The Workshop agreed that Letters of Intent from individual investigators and groups of investigators will be submitted directly to OPP. For major field camps, it is the intent that one scientist will co-ordinate Letters of Intent and provide an overview of the science that might be conducted and the logistics required. Letters of Intent will include, as appropriate: brief summary of the proposed science, the scale of logistics, preferred field year(s), and associated or related field projects.

IV. Recommendations

A. Major field operations

Based on the expressed interests of participants in the workshop, and recognizing the logistics constraints imposed by rebuilding of South Pole Station, it is recommended that OPP announce that opportunities exist for helicopter-supported remote field camps.

Locations for possible Beardmore "South" and Shackleton Glacier camp sites are known quantities. The specific sites for possible camps in the other areas considered will require evaluation based on scientific need and logistics considerations. Suggested locations (see Table 2 and Figure 2) are not exclusive and specific scientific needs may well determine different sites.

B. Community participation

It is recommended that OPP encourage individual investigators and groups of investigators to submit proposals that take advantage of the new and more cost-effective ways of conducting research in remote field areas.

References

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Appendices

A. Group Reports

B. Abstracts

C. Participant list

Basement Rocks of the Central Transantarctic Mountains (TAM)

Compiled by: Bert Rowell

Late Precambrian - Early Paleozoic

Introduction and Background

Recent studies of basement rocks near the Weddell Sea terminus of the Transantarctic Mountains challenge traditional interpretations for the origin and subsequent tectonic history of the east Antarctic cratonic margin (Rowell et al., 1994, 1997; Millar and Storey, 1995, Gose et al., 1997). Although it is probable that the SWEAT hypothesis (Moores, 1991; Dalziel, 1991, 1992; Hoffman, 1991) correctly describes first-order relationships among Proterozoic cratons, it is no longer clear whether the entire margin of the TAM was the product of Neoproterozoic rifting. Investigations in several sectors of the TAM also question the existence of a distinct Beardmore orogeny (Goodge, 1997, Rowell et al., 1997) and associated late Neoproterozoic magmatism (Encarnaci--n and Grunow, 1996). Perhaps even more significantly, a clearer understanding of the timing of the Ross orogeny, the oldest of the Phanerozoic orogenies, is emerging (Rowell et al., 1992, 1997; Goodge et al., 1993).

Rifting and passive-margin sedimentation are characteristic features of the late Neoproterozoic to Cambrian period globally. During the Neoproterozoic, many thousands of kilometers of new continental margin were formed (Bond et al., 1984) by the breakup of the supercontinent Rodinia (Moores, 1991; Dalziel, 1991). Most of these margins continued in a passive state during the early Paleozoic and accumulated thick lower Paleozoic sedimentary successions as a consequence of thermal subsidence and lithospheric thinning that occurred during the rift phase. The Transantarctic margin of East Antarctica was unusual because, contrary to widespread opinion (Stump 1995), its passive-margin history was brief at best. By middle Early Cambrian time, subduction-related magmatism was widespread in the Queen Maud sector of the TAM (Rowell et al., 1995, in press; Encarnaci--n and Grunow, 1995, 1996; Van Schmus et al., 1997), and although periods of relative tranquillity occurred at various places along the margin of the East Antarctic craton, it was an active margin for much of the Cambrian. Tectonic activity appears synchronous with Gondwana-wide consolidation. Folding and magmatism continued into the Early Ordovician as an expression of the Ross orogeny (Laird, 1981; Stump, 1995). In the Pensacola Mountains, however, it is now apparent that by the close of the Cambrian, orogenic effects were modest and that maximum deformation occurred much earlier during the mid-Early and early Middle Cambrian interval (Rowell et al., 1997). Data are permissive, but not conclusive, that this time period also witnessed maximum deformation in the Beardmore - Byrd sector of the range (Rowell et al., 1992).

Understanding the nature and timing of orogenic events along the Neoproterozoic to early Paleozoic active margin is critical to our global understanding of Cambrian sea-level changes and major episodes of biotic diversification and extinction (e.g., Kirschvink et al., 1997). To do this adequately, it is important to ensure that isotopic dates are tied to series and stage boundaries using the best possible correlationÕs. Most major events in the lower Paleozoic are still expressed in terms of their position relative to stage and series boundaries (thus, the upper Toyanian extinctionÕs and the early Middle Cambrian Hawke Bay event). Commonly a disconnect exists between these biostratigraphic events and magmatic or deformation episodes, which are normally measured isotopically. Can episodes of increased tectonic activity be correlated to abrupt changes in sea-level and accommodation known from many parts of the world? Is it possible to recognize a common driving force for abrupt changes in sea level, magmatism, and deformation? Were relative plate movements superimposed on true polar wander?

These are fundamental problems that need to be addressed and the Antarctic is seemingly one of the best places to address them. The active Antarctic margin was continuous with that of eastern Australia (Flottmann et al., 1993), yet despite extensive cover by ice and snow, outcrops in the Transantarctic Mountains are generally more revealing than those in Australia. No place in Australia, for example, affords documentation of Early and Middle Cambrian volcanic activity as well as the Queen Maud Mountains. Modern logistics available to USAP are adequate to overcome the problems inherent in remote field work and we believe the following objectives are attainable.

Critical Scientific Problems

The group focused its efforts on problems associated with initiation of sedimentation along the margin of the East Antarctic craton, together with its subsequent history prior to Devonian deposition of the unconformably overlying Beacon Supergroup. We recognize two general classes of problems of wide significance because they relate to processes that operate on a global scale. These processes are associated with large-scale plate motions and commonly initiate eustatic sea-level changes and associated biotic reactions that are potentially detectable over all the Earth. The third group of problems is of regional significance within Antarctica.

Problems of Global Significance

A. Inferred rift-margin phase

i) What was the Neoproterozoic geographic position of the TAM margin of East Antarctica relative to the other cratons (Laurentia in particular)? Given that the TAM contain one of the most extensive geologic records from the Neoproterozoic to early Paleozoic period in Antarctica, can we obtain reliable paleopole positions for the craton?

ii) When was rifting initiated? What lithostratigraphic units were deposited during this interval, what was their provenance, and what are their ages?

iii) What was the structural geometry of the rift margin? Has this inherited basement geometry controlled subsequent events?

iv) Can this rifting phase be correlated with well-known rift and passive-margin deposits of the Windermere Group or other units along the margin of Laurentia? Were the margins formerly contiguous?

B. Orogenic history

i) How many phases of deformation occurred and what was their timing?

ii) What was the style and geometry of deformation?

iii) For any given episode of deformation, how do geometry and timing vary along the length of the orogen? Is there evidence for postulated translation or partitioned oblique motion? What about syn-tectonic sedimentation?

iv) How did magmatism vary in composition, space and time? What was the role of magmatism in deformation and strain history?

v) What was(were) the inferred plate-tectonic setting(s) of orogenesis?

Problems of regional significance

C. Post-orogenic history

i) What were the magnitudes and/or rates of post-orogenic exhumation and denudation? By what mechanism did post-orogenic exhumation occur? How much variation is recorded along the orogen?

References

Bond, G.C., P.A. Nickeson, and M.A. Kominz. 1984. Breakup of a supercontinent between 625 Ma and 555 Ma: new evidence and implications for continental histories. Earth Planet. Sci. Lett., 70, 325-345.

Dalziel, I.W.D., 1991. Pacific margins of Laurentia and East Antarctica as a conjugate rift pair: evidence and implications for an Eocambrian supercontinent. Geology, 19, 598-601.

Dalziel, I.W.D., 1992. Antarctica- a tale of two supercontinents? Ann. Rev. Earth Planet. Sci., 20, 501-526.

Encarnaci--n, J.P. and A.M. Grunow. 1995. New U-Pb ages from the Transantarctic Basement and the timing of Ross Magmatism, VII Int. Sym. Antarct. Earth Sci., Siena, Abstr., p. 120.

Encarnaci--n, J. P. and A.M. Grunow. 1996. Changing magmatic and tectonic styles along the paleo-Pacific margin of Gondwana and the onset of early Paleozoic magmatism in Antarctica. Tectonics, 15, 1325-1341.

Flšttmann, T., T.N. Gibson, G. Kleinschmidt. 1993. Structural continuity of the Ross and Delamerian orogens of Antarctica and Australia along the margin of the paleo-Pacific. Geology, 21, 319-322.

Goodge, J.W., 1997. Latest Neoproterozoic basin inversion of the Beardmore Group, central Transantarctic Mountains, Antarctica. Tectonics, 16, 682-701.

Goodge, J.W., V.L. Hansen, and N.W. Walker. 1993. Neoproterozoic-Cambrian basement - involved orogenesis within the Antarctic margin of Gondwana. Geology, 21, 37-40.

Gose, W.A., M.A. Helper, J.N. Connelly, F.E. Hutson, and I.W.D. Dalziel. 1997. Paleomagnetic data and U-Pb isotopic age determinations from Coats Land, Antarctica: Implications for late Proterozoic plate reconstructions. J. Geophys. Res., 102, 7887-7902.

Hoffman, P.E., 1991. Did the breakout of Laurentia turn Gondwana inside out? Science, 252, 1409 -1412.

Kirschvink, J.L., R.L. Ripperdan, and D.A. Evans. 1997. Evidence for large-scale reorganization of Early Cambrian Continental Masses by Inertial Interchange True Polar Wander. Science, 277, 541-545.

Laird, M.G., 1981. Lower Palaeozoic rocks of Antarctica. In Holland, C.H., Lower Palaeozoic of the Middle East, Eastern and Southern Africa, and Antarctica, Wiley and Sons, New York, 257-314.

Millar, I.L., and B.C. Storey. 1995. Early Palaeozoic rather than Neoproterozoic volcanism and rifting within the Transantarctic Mountains. J. Geol. Soc. Lond., 152, London, 417-420.

Moores, E.M., 1991. Southwest U.S.- East Antarctic (SWEAT) connection: A hypothesis. Geology, 19, 425- 428.

Rowell, A.J., D.A. Gonzales, L.W. McKenna, K.R. Evans, E. Stump, and W.R. Van Schmus. 1995. Lower Paleozoic rocks in the Queen Maud Mountains: Revised ages and significance. VII Int. Sym. Antarct. Earth Sci., Abstr., Siena , Italy, p. 329.

Rowell, A.J., D.A. Gonzales, L.W. McKenna, K.R. Evans, E. Stump, and W.R. Van Schmus. 1997. Lower Paleozoic rocks in the Queen Maud Mountains: Revised ages and significance. VII International Symposium on Antarctic Earth Sciences. In Ricci, C.A. (ed.), The Antarctic Region: Geological Evolution and Processes, Terra Antarctica Publication, Siena, Italy, 201-207.

Rowell, A.J., M.N. Rees, and K.R. Evans. 1992. Evidence of major Middle Cambrian deformation in the Ross orogen, Antarctica. Geology, 20, 31-34.

Rowell, A.J., W.R. Van Schmus, A.H. Fetter, L.W. McKenna, and K.R. Evans. 1997. Cambrian deep-water sedimentary rocks of the Patuxent Formation and the main pre-latest Middle Cambrian phase of the Ross Orogeny in the Pensacola Mountains, Antarctica. Geol. Soc. Amer., Abstracts with Programs, 29.

Rowell, A.J., W.R. Van Schmus, L.W. McKenna, and K.R. Evans. 1994. Early Paleozoic continental-rise deposition off East Antarctica: The Patuxent Formation of the Pensacola. Antarct. J. U.S., 29(5), 42-44.

Stump, E, 1995. The Ross Orogen of the Transantarctic Mountains. Cambridge Univ. Press, Cambridge, 284 pp.

Van Schmus, W.R., L.W. McKenna, D.A. Gonzales, and A.J. Rowell. 1995. U/Pb geochronology of Parts of the Pensacola, Harold Byrd, and Queen Maud Mountains, Antarctica. VII Int. Sym. Antarct. Earth Sci., Siena, Abstr., p. 390.

Van Schmus, W.R., L.W. McKenna, D.A. Gonzales, and A.J. Rowell. 1997. U-Pb geochronology of Parts of the Pensacola, Thiel, and Queen Maud Mountains, Antarctica. In Ricci, C.A. (ed.), The Antarctic Region: Geological Evolution and Processes, Terra Antarctica Publication, Siena, Italy, 187-200.

The Role of Antarctica in Changing Global Systems as Shown by the Late Paleozoic and Early Mesozoic Succession:

"Icehouse" to "Greenhouse" Transition and Biotic Response

Compiled by : John L. Isbell

Discussants: Rosemary A. Askin, Loren E. Babcock, James W. Collinson, William R. Hammer, Molly F. Miller, Edith L. Taylor, and Thomas N. Taylor

Purpose

Studies of Pangean rocks, primarily in Europe and North America, have shown that dramatic biotic changes and a climatic change from an "icehouse" to a "greenhouse" state occurred during the late Paleozoic and early Mesozoic. Because of the equatorial location of the Euramerican rocks, our understanding of the biotic and climatic record of Pangea is biased toward events that occurred at low paleolatitudes. Antarctica, however, provides a unique opportunity to study upper Paleozoic and lower Mesozoic rocks deposited in a polar and near-polar position. The thick succession of siliciclastic sedimentary rocks in the central and southern Transantarctic Mountains contains vertebrate, invertebrate, plant, and trace fossils that provide a nearly complete record of changing high-latitude conditions within southernmost Pangea. Fundamental questions concerning biotic evolution, the physical changes that occur during an "icehouse" to a "greenhouse" transition, paleoclimate, paleoclimatic models, and the tectonic evolution of southern Pangea can be addressed by studies of these rocks.

Introduction

Pangea formed by the collision of Gondwanaland and Euramerica in the Carboniferous and by a collision with Siberia in the Early Permian (Ross and Ross, 1985; Veevers, 1988; Scotese and McKerrow, 1990). Surrounded by the Panthalassan Ocean, this supercontinent stretched from pole to pole. Pangea drifted northward throughout the Permian and Triassic, resulting in changing paleolatitudes for many sectors within the landmass (Scotese and McKerrow, 1990; Powell and Li, 1994; Ziegler et al., 1997). Antarctica, however, was unique within Pangea as it was located in a near-south-polar position throughout the supercontinent's northward drift.

Major changes in dominant fauna and flora occurred globally during the late Paleozoic to early Mesozoic, resulting in tremendous taxonomic diversification. Diversification occurred in spite of a major extinction at the end of the Permian. In the marine realm, Paleozoic faunas, dominated by complexly tiered, epifaunal suspension feeders, were replaced by a more modern-type fauna dominated by predators and a mobile, highly tiered infauna (Sepkoski, 1990). A parallel change in dominant elements in the terrestrial realm occurred in vascular plants and in tetrapods. In the latest Paleozoic, floras dominated by seed ferns and cordaites were replaced in the Mesozoic by conifers in the northern hemisphere and corystosperms in the southern hemisphere (Niklas et al., 1983). Also during this time, a late Paleozoic labyrinthodont, anapsid, and synapsid-dominated tetrapod fauna was superseded by a Mesozoic diapsid, dinosaur, and pterosaur-dominated fauna (Benton, 1985). Pangean floras display a change from low provinciality and broad latitudinal distributions during the Early Carboniferous (Raymond, 1985), to high provinciality with apparent latitudinal gradients during the Late Carboniferous and Permian (Ziegler, 1990; Ziegler et al., 1993; Hallam, 1994). Not enough studies of late Paleozoic terrestrial tetrapods have been conducted to define latitudinal zonations; however, early Mesozoic faunas appear to be dominated by cosmopolitan taxa (Shubin and Sues, 1991). Much of the work done on the biota of Pangea is from low paleolatitudes (e.g., Embry et al., 1994; Hallam, 1994). The role that high latitude communities played within Pangea is unclear at the present time. The study of high latitude late Paleozoic and early Mesozoic terrestrial fossils in Antarctica, which include decapods, tetrapods, and vascular plants (e.g., Babcock et al., 1996; Taylor and Taylor, 1990; Hammer, 1990; Hammer and Hickerson, 1994; Isbell et al., in press) may provide answers to this problem.

Within Pangea, Late Carboniferous and Early Permian ice sheets extended across much of the southern Gondwanan portion of the continent. Their waxing and waning resulted in cyclic sedimentation and peat accumulation in equatorial Euramerica (Veevers and Powell, 1987; Scotese and McKerrow, 1990). Extensive glacial deposits and cyclothems within Pangea suggest high latitudinal climatic gradients with mean annual temperatures colder than at present (Frakes et al., 1992; Ziegler et al., 1990, 1997). An absence of glacial deposits and associated ice rafted debris, coupled with widespread evaporites, red beds, and the expansion of carbonates (bioherms/reefs) during the Late Permian and Triassic suggest a warmer-than-present world climate punctuated with arid intervals (Parrish, 1990; Frakes et al., 1992). Although Late Carboniferous to Triassic climatic amelioration occurred within northward-drifting sectors of Pangea, a progressive change from glacial, to coal-bearing, to tetrapod-bearing fluvial deposits in Antarctica indicates that the polar regions also experienced a change from cold-glacial to warm-temperate conditions during the same interval (Collinson et al., 1994). The transition from glacial to post-glacial deposits represents a change from an "icehouse" to a "greenhouse" world (e.g., Fischer, 1981, 1984a, 1984b). However, present numerical simulations based on general circulation patterns and on energy balance models cannot explain the apparent latest Paleozoic and early Mesozoic climatic conditions in the south polar sectors of Pangea. The models suggest seasonal extremes in climate (Crowley et al., 1989; Kutzbach and Gallimore, 1989; Kutzbach and Ziegler, 1993), whereas the Antarctic rocks suggest temperate conditions (Collinson et al., 1994).

A complete picture of changing conditions within Pangea requires documentation of the rocks and fossils from low and high paleolatitudes. Because of late Paleozoic and early Mesozoic plate motion, most sectors of Pangea moved northward from one latitudinal and climatic zone to another. Antarctica's constant near-polar position was unique, which therefore makes it the best place in Pangea to investigate high latitude depositional, biotic and climatic conditions. Fundamental questions that can be addressed using such a "stable high latitudinal platform" include: 1) How did individual faunal and floral elements adapt to polar latitudes? 2) What role did high latitude communities play in terms of evolutionary innovations within the Earth's biota? 3) What changes were occurring within the polar landmass during the change from an "icehouse" to a "greenhouse" world? 4) What was the paleogeography of Antarctica like? 5) How can the physical and biological events within Antarctica be dated? and 6) How can geological and biological evidence be used to calibrate physical climate models? These questions are addressed below.

High Latitude Faunas and Floras as Sources of Evolutionary Innovation

Documentation of late Mesozoic and Cenozoic biota of the polar regions over the last century has shown that these areas have been home to thriving communities of organisms. In several examples (e.g., Hickey et al., 1983; Zinsmeister and Feldmann, 1984), groups of organisms described from polar regions predate their descendants in low latitudes by tens of millions of years. This concept (heterochroneity) seems to apply to a wide variety of terrestrial and marine organisms. Recognition of heterochroneity within various groups of organisms suggests that the high latitude regions have played an important role in the development and diversity of late Mesozoic and Cenozoic biotas (Zinsmeister and Feldmann, 1984). Rather than acting merely as conduits for dispersal (or, alternatively, as refugia from originally much larger geographic distributions), high latitude regions may have acted as major areas of biological innovation (Hickey et al., 1983; Zinsmeister and Feldmann, 1984).

The possibility exists that high-latitude-to-low-latitude dispersal patterns documented for the late Mesozoic and Cenozoic may have a parallel in late Paleozoic and early Mesozoic biotas. This would contrast with more conventional interpretations that low latitude areas acted as centers of most biological innovation (e.g., Schram, 1977; Olson, 1979). Conditions that facilitated heterochroneity during the late Mesozoic and Cenozoic (namely, land and sea masses near at least one pole, and access through biogeographic dispersal pathways to low latitude areas) were also present during intervals of the late Paleozoic and early Mesozoic. In particular, assembly of Pangea would have been favorable for the distribution of terrestrial organisms that originated in southern Pangea (present-day Antarctica). Oceanic circulation along the Pangean shelf may have facilitated dispersal of marine organisms from high to low latitudes.

One example of possible heterochroneity is exhibited in freshwater crayfish (Babcock et al., 1996). Early crayfish or their burrows are present in high latitudes of the Antarctic sector of Pangea, but the next-youngest occurrences are in low-latitude areas of Laurasia (present-day western USA). Antarctic specimens predate their North American descendants by about 65 million years. Crayfish, one of the most important animals in modern freshwater ecosystems, probably evolved from marine lobsters in southern Pangea and then dispersed through freshwater systems of Pangea prior to its breakup. It is our impression that, with further study, other such late Paleozoic-early Mesozoic examples of high-latitude-to-low-latitude dispersal patterns will be documented.

More important than simply documenting disjunct times of evolution or migration in high and low latitudes, however, is developing an understanding of the mechanisms or pathways of biological innovation in high latitudes. Climatic forcing, oceanic circulation, availability of sunlight, and other factors must be considered in the search for answers to this interesting problem.

High Latitudes as a Source of Information Concerning the Change

From an "Icehouse" to a "Greenhouse" World

Data summarized by Fischer (1981, 1984a) suggest that eustatic sea level, world volcanism, continental aggregation/dispersal, and global climate fluctuate in a 300-million-year-long supercycle. The supercycles may extend back 2 Ga, and depending on the criteria used to identify the cycles, they may range from 300 to 500 million years in duration (Fischer, 1981, 1984a; Worsely et al., 1984; Veevers, 1990). Fischer (1981, 1984a) divided the climatic cycle into an "icehouse" and a "greenhouse" stage. Icehouse conditions are characterized by continental glaciation, low stands of eustatic sea level, cold oxygenated deep waters, strong oceanic circulation, distinct latitudinal climatic gradients, little volcanism, and the existence of a supercontinent. Greenhouse conditions are characterized by high stands of eustatic sea level, high rates of volcanism, warm sluggish oceans, oxygen-depleted deep waters, indistinct latitudinal climatic gradients, and times of continental dispersion. Fischer (1981, 1984a, 1984b) attempted to link plate motion, biotic evolution, mass extinction and biotic innovations to these cycles, and to the transition points separating icehouse and greenhouse states. Possible causes of the supercycle include: 1) changing concentrations of CO₂ in the atmosphere, 2) venting of mantle CO₂ during increased plate activity, 3) plate motion and changing continental configurations, 4) variations in the amount of solar energy received by the Earth due to orbital perturbations, 5) changes in the frequency and intensity of solar radiation, and 6) variations due to galactic rotation (Fischer, 1984a; Veevers, 1990; Hallam, 1994; Wopfner and Casshyap, 1997).

The last complete icehouse-to-greenhouse cycle began 320 Ma with the formation of Pangea in the Carboniferous. Depending on how the icehouse-to-greenhouse cycle is defined, the cycle either ended 35 Ma at the Eocene-Oligocene boundary (Fischer, 1984a), or has continued on to the present (Veevers, 1990) . Within the last cycle, Fischer (1984a) and Veevers (1990) noted that the icehouse-to-greenhouse transition occurred during the mid-Triassic, just prior to the initial breakup of Pangea. Although icehouse and greenhouse intervals, and the crossover points between the two conditions, are thought to reflect the Earth's prevailing climate at the time, the stages and their boundaries are often more closely linked to plate motion and predicted climate than to the actual climatic record (Hallam, 1994). Major anomalies within the last cycle include: 1) the termination of Gondwanan glaciation during the mid-Permian, which occurred near the predicted glacial peak within the icehouse state; 2) the apparent warm/ice-free conditions from the Late Permian to the Early Jurassic, which occurred during the last half of the icehouse state and the beginning of the greenhouse state; and 3) the glacial activity (indicated by the presence of ice rafted debris) extending from the Middle Jurassic to the Early Cretaceous, which occurred within the greenhouse state (cf., Frakes et al., 1992). Although the supercycle model is appealing, the examples cited above suggest that the causes and timing of the icehouse-to-greenhouse transition are more complicated than the model suggests.

Antarctica's unique position in high latitudes during the late Paleozoic and early Mesozoic makes it the ideal platform from which to collect a database from the sedimentary record documenting the change from an icehouse to a greenhouse world. Establishment of such a database is critical in developing an understanding of the mechanisms driving Earth's large-scale physical cycles; in interpreting the biota's response to such changes; and in predicting future changes to the Earth's physical, chemical, and biological systems. Questions that should be addressed include: 1) How extensive were glacial and interglacial events? 2) How quickly did the glacial-postglacial transition occur? 4) What caused the final collapse of the Gondwanan ice sheet? 4) Were uplands still covered by ice following the collapse of the Gondwanan ice sheet? 5) Were the post-glacial rocks deposited in a lacustrine or marine environment? 6) How extensive were the water bodies and what effect did they have on climate amelioration? 7) Do the Permian coal measures and Triassic fluvial rocks display cyclicity that can be linked to Milankovitch cycles? 8) What was the climate during coal measure time? and 9) Was the Triassic as warm as the fossils and rocks suggest?

Late Paleozoic and Early Mesozoic Paleogeography of Southernmost Pangea

Upper Paleozoic and lower Mesozoic sedimentary rocks of the central and southern Transantarctic Mountains contain the biotic, climatic, and tectonic records of southern Pangea. These rocks form a 2.5- to 3-km-thick (composite section) succession exposed from the Byrd Glacier to the Ohio Range. The rocks consist of: 1) 0- to 710-m-thick Devonian to Carboniferous(?) shallow marine and nonmarine rocks; 2) 0- to 440-m-thick Upper Carboniferous to Lower Permian glacial-marine and glacial-terrestrial rocks; 3) 350-m-thick Lower Permian marine, deltaic, fluvial, and lacustrine rocks; 4) 750-m-thick Upper Permian fluvial, lacustrine, and deltaic coal measures; and 5) 0- to 1100-m-thick Triassic fluvial rocks. Although the Byrd to Shackleton area consists of terrestrial rocks, and the Ohio Range consists predominantly of marine rocks (e.g., Elliot, 1975; Aitchison et al., 1988; Barrett, 1991; Barrett et al., 1986; Collinson et al., 1994; Isbell et al., 1997), fundamental questions concerning the nature and the distribution of these rocks remain because large areas exist where sedimentologic and stratigraphic investigations were last conducted prior to the advent of modern sedimentologic, stratigraphic, and plate tectonic concepts (e.g., 1958-62, Ohio Range, Long, 1964; 1962-63, Mt. Weaver, Minshew, 1967; 1963-64, Axel Heiberg Glacier, Barrett, 1965; 1964-65 Wisconsin Range, Minshew, 1967; 1970-71 Nilsen Plateau; Coates, 1985).

Upper Paleozoic and lower Mesozoic rocks are similar throughout the central and southern Transantarctic Mountains, which has led to the conclusion that these strata accumulated within a single, elongate, depositional basin (e.g., Collinson et al., 1994). This basin may have formed the central portion of a larger depositional basin that stretched from South America to Australia (Veevers et al., 1994). Hypotheses on the origin and evolution of the Antarctic basin include development as: 1) a passive-margin basin, 2) a rift basin, 3) a back-arc basin, 4) a cratonic basin, 5) an ice-loaded basin, and/or 6) a foreland basin (Barrett et al., 1986; Bradshaw and Webers, 1988; Collinson et al., 1994; Woolfe and Barrett, 1995; Isbell et al., 1997). Investigators apply different conditions to their paleogeographic interpretations depending on which basin model they adopt (cf. Collinson et al., 1994; Woolfe and Barrett, 1995; Isbell et al., 1997a, 1997b). These assumed conditions and the resultant paleogeographic models then serve as a foundation for constraining interpretations of tectonic activity, biotic ecology, and evolutionary pathways, as well as modeling climate in southern Pangea. Every effort should be made to provide a database for accurate basin interpretations, thus strengthening our understanding of Antarctic paleogeography.

Isbell et al. (1997a) recently questioned the single basin hypothesis for the Devonian to Early Permian rocks, suggesting that a complex paleogeography with deposition in multiple basins occurred. If their assumptions are correct, then the similarity of rocks within the central and southern Transantarctic Mountains may in part be the result of a high latitude position rather than accumulation within a single basin. Collinson et al. (1994) and Isbell et al. (1997b) summarized the data that suggest the occurrence of a latest Paleozoic and early Mesozoic foreland basin. This hypothesis is largely accepted by the scientific community; however, it has not been tested on rocks outside the area located between the Byrd and Shackleton Glaciers.

Although the upper Paleozoic and lower Mesozoic rocks are known in a general way, many of the details have not been fully ascertained. Sedimentologic, stratigraphic, and basin analysis studies of these rocks are not only important in determining the depositional history, but are critical in establishing a foundation from which the tectonic, biotic, and climatic histories of southern Pangea can be interpreted.

Biostratigraphy

A sound biostratigraphic framework is the fundamental requirement for evaluation of the nature and rates of evolutionary and sedimentary processes, biotic distributional trends, tectonic and climatic changes, and subsequent biotic responses. Biostratigraphic control of the predominantly terrestrial Paleozoic-Mesozoic succession in the central to southern Transantarctic Mountains rests on the terrestrial fossil record, which for these rocks includes mainly plant and vertebrate fossils. Because they are abundantly and widely distributed through a variety of rock types, and include distinctive species with relatively rapid evolutionary change, plant microfossils (spores and pollen from land plants) make ideal biostratigraphic tools, and have provided the most precise means of correlation in the region (e.g., Kyle, 1977; Kyle and Schopf, 1982; Farabee et al., 1990). Key evolutionary or extinction events in the vertebrate and plant megafossil record also provide good biostratigraphic markers, although these fossils, in particular the vertebrates, are not preserved throughout the succession, and indeed may not have lived in the region during certain intervals. Plant microfossils are the only fossils that can potentially provide a continuous record.

The primary goal of palynological studies is to improve and refine the current biostratigraphic framework. An initial palynostratigraphic zonation was proposed for southern Victoria Land (Kyle, 1977) and extended to the central and southern Transantarctic Mountains (Kyle and Schopf, 1982). Subsequent additional assemblages (Farabee et al., 1989, 1990, 1991; Masood et al., 1994) have furnished data for parts of the succession in the Beardmore Glacier area, as has more recent ongoing work in the Shackleton Glacier area (e.g., Askin and Cully, in press; Askin, in prep.) and reconnaissance work on the Nimrod Glacier area (Askin and Isbell, in prep.). The palynostratigraphic framework still, however, includes gaps where palynological data are scarce or lacking, and there are some details on distributional trends throughout the Transantarctic Mountains that have yet to be obtained or clarified. Based on previous information, much of which is of a reconnaissance nature and lacking detailed biostratigraphic coverage, several areas and units have been identified that should fill these gaps in our current knowledge (see Table in H below).

The only major drawback with using spores and pollen for biostratigraphy is that they, like all organic matter, are progressively altered and eventually destroyed by oxidative and thermal effects. This has meant that syndepositional oxidation (associated with soil and redbed development in the lower Fremouw Formation, for example), and thermal metamorphism during Jurassic volcanism and emplacement of dolerite sills, has destroyed the fossil record in some areas or in some units, or made the microfossils very fragile and corroded, opaque black, and difficult to work with. Detailed sample collection from a wide geographic area, and in particular those areas outlined in the above-mentioned Table (and already proven to yield good to excellently preserved palynomorphs), should yield a complete palynological succession. Furthermore, well-preserved material from other unexpected sources (such as the recycled Middle and Upper Triassic palynomorphs from Sirius Group sediments in southern Victoria Land; Askin and Fleming, in prep.) can also provide an excellent record of species not previously encountered in in situ samples.

Use of Geologic and Biologic Evidence in the Calibration of

Physical Climate Models

The Central Transantarctic Mountains area has been in the southern polar region since the Late Carboniferous. The Lower Permian-to-Jurassic Gondwana sequence in the Central Transantarctic Mountains and the Mesozoic sedimentary sequences in the Antarctic Peninsula region suggest that climates were generally, but not always, much warmer than at present. These sequences document the end of a major Permo-Carboniferous glaciation and the transition in the Late Permian to relatively warm climates that appear to have persisted throughout the Mesozoic during the breakup of Pangea. Paleomagnetic pole models place the Central Transantarctic Mountains area at high latitudes throughout this long interval of time. However, paleontologic data appear to contradict global climate models that predict great annual extremes in the southern part of the Pangean supercontinent.

We suggest a cooperative scientific effort in compiling a comprehensive database that will be used to determine the climate history of Antarctica. Investigators should represent varied scientific disciplines and perspectives. Any data that might have some bearing on paleoclimates should be included. These data could be used to improve climate models and perhaps resolve the paradox of relatively warm climates in polar regions. In addition to filling gaps in the paleontological record, it is essential that existing and future data be compiled in a chronostratigraphic framework. Stratigraphic sequences have been correlated only to the series level, with a precision no better than several million years. Better time control will be required to document short term changes in paleoclimate. Questions include: Were polar paleoclimates really as mild as present data suggest? Were polar paleoclimates continuously warm or were they generally much cooler and punctuated by short warm periods? How did fauna and flora cope with darkness and cold during the polar winter? What were local conditions that may have ameliorated the climate?

Required Disciplines

A multidisciplinary approach is required to address the problems described above. Although individual projects must stand on their own merits, only collaboration among groups and a sense

of esprit de corps will result in the resolution of the larger-scale problems. The table below lists the individual disciplines and what each discipline contributes to resolving the problems outlined in this document. All disciplines listed contribute to the development of a climatic database.

Rationale for Studies in the Central and Southern Transantarctic Mountains

Antarctica's Carboniferous-to-Triassic polar and near-polar position makes it the most significant continent on which to search for high-latitude, biologic and environmental signatures within the late Paleozoic and early Mesozoic rock record. Within Antarctica, the thickest and most complete Devonian to Upper Triassic rocks are exposed in the central and southern Transantarctic Mountains. There, composite exposures consist of a nearly complete 2.5- to 3-km-thick succession of rocks that consist of: 1) Devonian rocks deposited during a greenhouse state; 2) Upper Carboniferous to Lower Permian glacial rocks deposited during an icehouse state; 3) Lower Permian to upper Permian marine and terrestrial rocks deposited during an icehouse-to-greenhouse transition; and 4) Upper Permian to Jurassic terrestrial rocks deposited during a greenhouse state. Significant fossil faunas and floras contained in these rocks include: 1) a diverse Devonian marine fauna, 2) the oldest freshwater decapod fossils, 3) evidence for interglacial and postglacial faunal recolonization and diversification, 4) one of the southernmost fossil floras ever described, including the highest latitude fossil forests and Permian and Triassic silicified peat localities, 5) the southernmost Triassic and Jurassic tetrapod faunas, which includes the southernmost Lystrosaurus and Cynognathus faunas, the southernmost dinosaur and pterosaur faunas, and the oldest known allosaurid dinosaur. Within the central and southern Transantarctic Mountains, the Amundsen and Scott Glacier area separates Devonian to Lower Permian marine rocks in the Ohio and Wisconsin Ranges from nonmarine rocks exposed in the area between the Byrd and Amundsen Glaciers. Due to this regional segregation of rocks, a comparison of the biotic and environmental polar signature contained within both marine and nonmarine deposits can also be addressed. Because of the completeness of these rocks and their contained fossil faunas and floras, the central and southern Transantarctic Mountains are the single most important sites on Earth for addressing late Paleozoic and early Mesozoic biotic and environmental changes from a polar perspective.

Priorities -- Central and Southern Transantarctic Mountains

The table below lists areas in the central and southern Transantarctic Mountains where the objectives of this multidisciplinary work can best be addressed. The list is arranged from highest to lowest priority. The list also provides rationale for each area's ranking.

References

Aitchison, J.C., M.A. Bradshaw, and L. Newmann. 1988. Lithofacies and origin of the Buckeye Formation: Late Paleozoic glacial and glaciomarine sediments, Ohio Range, Transantarctic Mountains, Antarctica. Palaeogeog. Palaeoclimat. Palaeoecol., 64, 93-104.

Askin, R.A. (In prep) Late Triassic palynology of the Schroeder Hill area, Cumulus Hills, Shackleton Glacier, Antarctica. Palynology.

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Mesozoic-Cenozoic Tectonics of the

Central and Southern Transantarctic Mountains

Compiled by: David H. Elliot and Terry J. Wilson

Discussants: Robin Bell, Tom Fleming, Philip Kyle, B. Marsh, Ralph von Frese, and Philip Wannamaker

The Transantarctic Mountains (TAM) form one of Earth's major intraplate mountain belts (Dalziel and Elliot, 1982). The evolution of the TAM, stemming from the time of break-up of Gondwanaland, reflects regional- and global-scale processes. This starts with the initial fragmentation within the broader context of the Gondwana plate framework, and is recorded in the Ferrar tholeiites and their tectonic setting. The East-West Antarctic intraplate boundary was initiated at that time. Another aspect is development of that intraplate boundary and West Antarctic rifting, with accompanying inversion of the break-up rift to form the present mountain belt. Uplift of the range is also linked to the initiation, growth and fluctuations of the Antarctic icesheets. All these aspects have significance outside Antarctic earth sciences.

A comprehensive evaluation of the geodynamic evolution of the Transantarctic Mountains and associated West Antarctic Rift System, together with recommendations for research throughout that region, was presented in a recent workshop report (Wilson and Finn, 1996). This section deals with only part of that region, the central and southern Transantarctic Mountains, and identifies specific areas where particular problems might be addressed.

I. Ferrar Magmatic Province

A. Background

The Ferrar tholeiites constitute the magmatic rocks in Antarctica associated with the initial rifting of Gondwanaland (Elliot, 1992). Recent zircon and baddeleyite dating (Encarnación et al., 1996) shows that the Ferrar is contemporaneous with a major part of the Karoo (Duncan et al., 1997), and preliminary dating of the Vestfjella lavas suggests their contemporaneity with the Ferrar. Further, ⁴⁰Ar/³⁹Ar dating and geochemical studies (Fleming et al., 1997) suggest that the Ferrar was derived from a single source which was most likely related in some manner to the inferred break-up plume centered in the region of Queen Maud Land and the adjacent coast of Africa (White and McKenzie, 1989; Storey, 1995). The Jurassic tholeiitic magmatism along the margin of the East Antarctic craton is an integral part of the break-up process.

The principal questions concerning the Ferrar relate to the petrogenetic processes, geochemical variability, transport of magmas for thousands of km and emplacement either in supracrustal rocks or at the surface, and the tectonic setting. Petrogenetic studies have been hampered by the fact that even the most mafic Ferrar rocks, with about 9% MgO, carry a significant crustal imprint. Therefore, the prospect of unraveling the contributions of the asthenospheric mantle, lithospheric mantle, and crust, and the processes involved, has been problematic. Although a substantial body of data (Fleming et al., 1995 and references therein) exists for the extrusive rocks, the geochemical variability of Ferrar Dolerite sills and dikes (Fleming et al., 1997) are poorly documented outside Victoria Land. For instance, it is not known whether there are along-strike systematic changes, subtle or otherwise, in isotope and trace element compositions.

The source location is likely to have been in the Weddell Sea region (Fleming et al., 1997; Minor and Mukasa, 1997). Such a location implies magma transport for great distances, and in fact somewhat greater than those for the well-documented Mackenzie Dyke Swarm (Baragar et al., 1996). Magma transport was most likely at mid to upper crustal depths, with final dispersal and emplacement involving magma migration directly to the surface as well as supracrustal transport, as shown by the numerous sills within the Beacon sequence. The significance of the Dufek layered basic intrusion in the Ferrar magma system is unclear; it has been postulated to be the proximal source for all Ferrar magmas rather than a discrete and isolated plutonic body. The plumbing system for the Ferrar tholeiites is essentially unconstrained.

Structural studies on Ferrar dikes (Wilson, 1992, 1993) have demonstrated a stress field interpreted to indicate extension perpendicular to the trend of the Transantarctic Mountains. Petrologic, paleovolcanologic and other data suggest a rift setting into which the extrusive Ferrar rocks were erupted (Elliot, 1992, 1996; Elliot and Larsen, 1993; Hanson and Elliot, 1996). The regional extent of the surface expression of rifting is indicated by sparse and fragmentary supporting data from Victoria Land.

B. Key questions

Understanding of the Ferrar magmatic province requires information on:

1. Geochemical variability. What is the compositional variation of the Ferrar Dolerites on a regional scale? Nd model ages of the crust vary along the length of the Transantarctic Mountains (Borg et al., 1990); is there a geochemical signature related to the crustal ages? Are there patterns of compositions that suggest local centers of emplacement or distinct phases of emplacement? Do any such patterns indicate magma transport effects or source region evolution (as has been suggested for the Mackenzie Dyke Swarm; Baragar et al., 1996)?

2. Regional magma budgets. Generalized estimates of volumes have been presented for the Dufek intrusion, Ferrar Dolerite and Kirkpatrick Basalt (Ford and Himmelberg, 1991; Kyle et al., 1981; Fleming et al., 1997). Within the limitations of the rock exposure, what are the volumes in the various sectors of the Transantarctic Mountains? Is there any regional pattern? Do the patterns reveal information about the plumbing system?

3. Regional plumbing system. The Ferrar Magmatic Province is over 4,000 km long; if not derived from a whole series of local mantle sources, then it implies transport even farther than the Mackenzie Dyke Swarm. How were Ferrar magmas transported through the crust over such long distances? Was a feeder dike swarm responsible for lateral transport or could magma have been transported via massive sills? Can consistent regional patterns of flow from the Weddell Sea region be established? What is the role of the Dufek intrusion in the plumbing system?

4. Local sources for supracrustal dispersal. Detailed studies in the Dry Valleys (Marsh, 1996) have documented vertical magma transport and lateral dispersal in a series of sills, as exemplified by the Basement Sill. Are there regions, other than the Dry Valleys, where evidence exists for rise of large volumes of magma through the basement rocks and its emplacement supracrustally? Is there evidence for shallow depth magma chambers, such as suggested for the Butcher Ridge region (Behrendt et al., 1995) or might be inferred from layered dolerite intrusions such as the Warren Range? How do such centers link into a regional plumbing system?

5. The nature and extent of the rift topography and associated structural and tectonic features. The basaltic pyroclastic rocks underlying the flood basalts are probably the largest and most extensive basaltic phreatomagmatic field on earth. Are there features of this field of explosive volcanism that will further constrain the eruptive paleoenvironments? Are there any features indicating actual eruptive centers? Are there structural features, such as the monoclines observed in the Queen Alexandra Range, that will help constrain the location of rift valleys?

C. Study localities

Addressing these problems requires well-focused regional surveys and detailed studies in restricted areas. The outcrop pattern indicates that intensive studies will likely be most informative in the region from the Queen Alexandra Range to Nilsen Plateau, with those two areas as potentially the most rewarding.

1. The Queen Alexandra Range has the only complete and almost continuously exposed stratigraphic section from the basement to the Jurassic lavas. The estimated thickness of dolerite sills is more than 1,000 m and comprises at least six major sills. The section from the uppermost Beacon to the basalts is well exposed. The range in geochemistry of the sills and dikes in a single region can be addressed as well as lateral homogeneity and extent of individual sills. Magnetic anisotropy measurements and petrofabric studies may offer a way of establishing magma transport directions and thus possible locations of feeder systems. The relations between the Beacon Supergroup and the younger silicic and basaltic rocks is better exposed here than anywhere else in the whole of the Transantarctic Mountains. The inferred unconformities between the Beacon, the overlying Hanson Formation, and the Prebble Formation can be clarified and the possible geographic extent of Jurassic vertical tectonism addressed through sandstone petrology. Furthermore, more detailed studies of the paleovolcanology of the phreatomagmatic rocks of the Prebble Formation offer the possibility of developing the setting within which those rocks were erupted.

2. Nilsen Plateau has a section extending from basement granite to the Lower Triassic Fremouw Formation. A number of thick sills cut the Beacon rocks as well as the basement. These rocks include some of the few known Ferrar olivine dolerites and offer the possibility of extending the range of dolerite compositions to higher MgO contents.

3. The Shackleton Glacier region also has extensive exposures of sills. As in the Queen Alexandra Range, vertical and lateral variations in geochemistry