Terra Antartica 2001, 8(1), 11-24 © Terra Antartica Publication Structure and Kinematics of the Central Transantarctic Mountains: Constraints from Structural Geology and Geomorphology near Cape Surprise

S.R. MILLER1*, P.G. FITZGERALD2 & S.L. BALDWIN2

1Department of Geosciences, Pennsylvania State University, University Park, PA 16802 - U.S.A. 2Department of Earth Sciences, Syracuse University, Syracuse, NY 13244 - U.S.A.

Received 7 February 2000; accepted in revised form 24 April 2001

Abstract - The transition zone between the Transantarctic Mountains (TAM) and the West Antarctic rift system is defined as the Transantarctic Mountain Front (TMF). In the vicinity of Cape Surprise (84°30’S) near the Shackleton Glacier in the central TAM, two fault sets have been mapped. Mesoscopic faults and geomorphic trends indicate that one set of normal faults within the TMF is generally dip-slip in nature and strikes parallel to the mountain range. The second fault set is oriented approximately perpendicular to this. Kinematic analysis of lineated fault surfaces reveals an extension axis oriented 020°-040°, orthogonal to the trend of the central TAM. However, asymmetric drainage patterns and a small number of lineated fault surfaces support a kinematic model of Cenozoic dextral transtension in the TMF. While age constraints on these two episodes of deformation are poor, it is likely that dextral transtension followed orthogonal extension, as suggested from other regions of the TAM. Apparent orthogonal extension in the TMF is thus consistent with early Cenozoic rift-flank uplift driven by isostatic forces. A middle to late Cenozoic period of transtension could be contemporaneous with strike-slip faulting in the Ross Sea, and could also indicate less strain partitioning between the rift system and the TMF after the uplift rate decreased.

INTRODUCTION for the episodes of Cretaceous exhumation recorded in the TAM (Fitzgerald and Baldwin, 1997; Fitzgerald, 2001). Mountain ranges and sedimentary basins are some Another proposed model for the Cenozoic uplift of the of the primary, large-scale geologic effects of tectonic TAM invokes isostatic uplift as a result of the Vening- processes. In Antarctica, the Transantarctic Mountains Meinesz effect due to crustally penetrative normal faulting (TAM) define the uplifted flank of the intracontinental along the margin of strong East Antarctic lithosphere, West Antarctic rift system (WARS), as indicated by their lateral heating of the thick East Antarctic lithosphere by topography, architecture, and outcrop pattern (Fig. 1A; the hotter lower lithosphere of the attenuated WARS, and Fitzgerald et al., 1986; Stern & ten Brink, 1989). The erosion (Stern & ten Brink, 1989; Bott & Stern, 1992, ten Transantarctic Mountains Front (TMF) (Barrett, 1979) is Brink et al., 1997). More recently, models that invoke the onland zone of evident and inferred normal faults that plastic necking (Chéry et al., 1992) and deep lithospheric accommodated rift-flank uplift. Although significantly necking aided by strong erosion-induced isostatic rebound lowered by erosion (e.g. Gleadow & Fitzgerald, 1987), the (van der Beek et al., 1994; Busetti et al, 1999) have been high elevations (~4000 m) that are common along the crest suggested. Recent geophysical work has, however, revealed of the TAM (e.g. Fig. 2) are the result of rock uplift, that there is no significant basin in the WARS adjacent to predominantly in the Cenozoic, that has occurred within the central TAM off-shore the Robb-Lowery Glaciers, as and adjacent to the TMF. there is off-shore of Victoria Land. This suggests a lack of The high elevations and long length (~3500 km) of the mechanical coupling between the rift system and the TAM places it amongst the largest non-contractional presumed isostatic response of the mountain range (ten mountain ranges in the world. How a range of such Brink et al., 1993). Combined with data showing that the dimensions formed is of fundamental importance to the main phase of TAM uplift was not coeval with the main understanding of continental tectonics in general and period of extension in the WARS (e.g. Fitzgerald & Antarctic tectonics in particular. Several mechanisms for Baldwin, 1997; Fitzgerald, 2001) plus evidence for the main Cenozoic uplift of this rift margin have been Cenozoic transtension in the TMF (e.g. Wilson, 1995), the proposed. These include simple shear linking different origin of the mountain range remains an outstanding lithospheric levels under the Ross Embayment and TAM tectonic problem. with strain partitioning accommodated on low-angle A mechanistic understanding of the tectonic history of detachment faults (Fitzgerald et al., 1986). Although this the TAM is by default linked to an understanding of the model as an explanation for Cenozoic uplift is now regarded kinematics of its mountain front, as well as to its degree of as unlikely, Cretaceous extension in the WARS was coupling with the rift system. Constraints on the kinematic probably accommodated, at least in part, along low-angle histories of the TMF and WARS are therefore critical. To detachment faults and this mechanism may be responsible date, the Cenozoic kinematics of the TMF has only been

*Corresponding author ([email protected]) 12 S.R. Miller et al.

Fig. 1 - (A) Location map, indicating the Transantarctic Mountains (TAM), the West Antarctic rift system (WARS), and the area of study. (B) Geological map of the Shackleton Glacier and Liv Glacier region. Note that the bar-and-ball on the faults signifies separation, not slip. The Spillway Fault actually varies along strike between normal and reverse, for instance; its movement is attributed to the Ross orogeny (e.g. Stump, 1995). Modified after La Prade (1969), McGregor & Wade (1969), and Stump (1995).

described at a few locations. Mapping and kinematic brittle fault kinematic analyses from southern Victoria analyses have suggested a significant component of strike- Land suggest that the TMF is dextrally transtensional slip deformation across the TMF in Victoria Land, where (Wilson, 1995), probably in response to transtension and normal faults strike obliquely to the range’s trend (Findlay strike-slip faulting in the greater WARS (Grindley & & Field, 1983; Fitzgerald, 1992; Wilson 1992; Olliver, 1983; Lawver & Gahagan, 1991; DiVenere et al., Stackebrandt, 1994). These observations and mesoscale 1994; Salvini et al., 1997). Whether such strain is present

Fig. 2 - Oblique aerial photograph of the Transantarctic Mountains Front (TMF) in the vicinity of Cape Surprise and Shackleton Glacier, looking south-southwest. The relief of the TAM in this region is just under 4000 m. The surface of the Ross Ice Shelf is ~200 m above sea level. Mt. Wade (4084 m above sea level) is the highest mountain. (U.S. Navy photograph, TMA 938, #75, F-33.) Structure and Kinematics of the Central Transantarctic Mountains 13 along the entire range and whether it was contemporaneous with a total of ~400 km of extension (e.g. Davey & with uplift has implications for the mechanism(s) of uplift. Brancolini, 1995) by the Late Cretaceous. In contrast, Wilson (1995), for example, pointed out that flexural whereas there were periods of exhumation along the TAM uplift of the TAM due to mechanical unloading would not in the Early and Late Cretaceous (e.g. Stump & Fitzgerald, be likely if the TMF were dominantly transtensional. 1992; Fitzgerald, 1994; Balestrieri et al., 1997; Fitzgerald In this paper we describe the general structural geology & Stump, 1997; Schäfer et al., 1998) significant rock uplift of the mountain front in the vicinity of Cape Surprise. An in the TAM did not begin until the early Cenozoic (Gleadow analysis of large-scale geomorphologic trends over a & Fitzgerald, 1987). Since about 55 Ma, ~5 km of rock broad region is shown to elucidate the regional fracture uplift, but approaching 10 km in some places, have occurred pattern. We present results of fault kinematic analyses and along the inland margin of the TMF (e.g. Gleadow & quantitative descriptions of drainage basin asymmetry Fitzgerald, 1987; Fitzgerald & Gleadow, 1988). The and from those we interpret the kinematic history of this amount of rock uplift decreases inland of the TMF, portion of the TAM. reflecting the mountain range’s overall tilt-block or flexure architecture (e.g. Gleadow & Fitzgerald, 1987; Fitzgerald & Stump, 1997). At least in some sectors of the TAM, GEOLOGIC AND TECTONIC SETTINGS Cenozoic uplift appears related to coeval extension localized along the western side of the rift system (e.g. The TAM mark the edge of the East Antarctic craton, Cooper et al., 1991). a fundamental lithospheric boundary since the Proterozoic. In the Prince Olav Mountains, the Kukri Erosion The region’s history of repeated mountain building and Surface is exposed ~2620 m above sea level (Mt. Munson), basin subsidence, as well as evidence for Cenozoic dipping gently (<5°) SSW (Barrett, 1965; La Prade, 1969). transtension along the rift margin of the TAM, suggests Farther inland, towards the south, the Kukri Erosion that the TMF marks the zone of lithospheric weakness and Surface and overlying units become subhorizontal (La pre-existing anisotropies (e.g. Fitzgerald et al., 1986; Prade, 1969), eventually disappearing beneath the ice cap. Wilson, 1995). In sharp contrast, Beacon sediments and Ferrar sills at The basement in the Cape Surprise region (Fig. 1B) is Cape Surprise occur at an elevation of ~200 m with dips composed primarily of late Proterozoic to Cambrian ranging from 30°NE to 35°SW (Barrett, 1965; La Prade, metamorphic rocks of the Ross Supergroup (McGregor, 1969). The presence of these units so near the coast makes 1965; Wade & Cathey, 1986) and Cambrian-Ordovician this location unique within the TAM. These strata and sills granitoids of the Queen Maud Batholith (Borg, 1983). have been down-thrown 3.1-5.2 km into their present During the Cambrian-Ordovician Ross orogeny (Stump, position across one or more approximately NE-dipping 1995), volcanic and marine sedimentary rocks of the Ross normal faults (Barrett, 1965; Miller et al., 1996). The Supergroup were metamorphosed and deformed and also inferred range-parallel fault immediately south of Cape intruded by granitoids (McGregor, 1965; Stump, 1981). Surprise, which was suspected of accommodating most or Exhumation of the basement following the Ross orogeny all throw, was named the North Boundary fault by La produced the low-relief Kukri Erosion Surface, Prade (1969). A number of approximately NE-striking unconformably overlain in the Shackleton Glacier region faults were also mapped by Barrett (1965) at Cape Surprise, by Carboniferous to Triassic glacial, alluvial, and shallow however the only other normal faults mapped in the marine sedimentary units of the Beacon Supergroup, Shackleton Glacier region (La Prade, 1969) were roughly which were deposited in elongate basins oriented parallel parallel to the range and south of the Prince Olav Mountains. to the modern mountain range (Barrett, 1991; Isbell, 1999). During the Jurassic, sills of the Ferrar Dolerite, dated at 177 ± 2 Ma (Fleming et al., 1997), intruded STRUCTURAL GEOLOGY basement and sedimentary cover (Barrett, 1965; Miller et al., 1996). At the same time, Kirkpatrick flood basalts STRUCTURES were erupted at the paleo-landsurface. This period of tholeiitic magmatism and its associated extensional Field work consisted of mapping the smaller-scale structures are often associated with the break-up of structures within the general architecture previously framed Gondwana (e.g. Kyle et al., 1981; Wilson, 1993). In the by Barrett (1965) and La Prade (1969). In addition, the Shackleton Glacier region, remnants of the Kirkpatrick authors mapped south of Cape Surprise where basement Basalt are limited to the southern, inland flank of the TAM rocks crop out (Fig. 3). Structures are herein simply (La Prade, 1969), reflecting the generally lesser amounts categorized into those associated with the Ross orogeny of erosion that have occurred with increased distance from (D1), those that predate or are contemporaneous with the the West Antarctic rift system. Ferrar Dolerite (D2), and those that postdate the Ferrar Since the Jurassic, the tectonics of the TAM-WARS Dolerite (D3). has been of general, overall extension, which was ultimately D1 structures include folds, metamorphic foliation, responsible for the formation of basins within the Ross reverse faults, and boudinage in the plutonic and Embayment and the rift-flank uplift. The majority of metasedimentary basement (McGregor, 1965; Stump, extension within the Ross Embayment occurred between 1981; Paulsen et al., 1999). Structures indicate contraction 105-85 Ma (Lawver & Gahagan, 1994, 1995), probably in a roughly NE-SW to E-W direction (Paulsen et al., via low-angle normal faults (Fitzgerald & Baldwin, 1997), 1999). For example, the Spillway fault, at the mouth of 14 S.R. Miller et al.

Fig. 3 - (A) Geological map of Cape Surprise and vicinity. Beacon stratigraphy modified after Barrett (1965, 1991, pers. comm.) and La Prade (1969). (B) Cross-sections through the Cape Surprise area. Section X- X’ crosses through the western end of Cape Surprise and Garden Spur, showing the large inferred normal fault that down-drops Beacon strata to the north. Note that the Buckley Formation is intruded by multiple sills of Ferrar Dolerite. Section Y-Y’ crosses through the eastern end of Cape Surprise, showing the normal fault that brings Beacon strata into contact with the Queen Maud Batholith.

the Liv Glacier, strikes ~315º and foliation measured near zones (n = 14) include a dominant set that strikes 111° and Cape Surprise strikes approximately N-S and dips steeply a secondary set that strikes 035° (Fig. 4A). Some of the west. large alteration zones of the primary set near the Cape, which were over 50 m long, dip approximately 50°N D2 structures include tholeiitic Ferrar sills and dikes. Fracture and/or fault zones with halos of mineralogic (Fig. 3). alteration, hereafter simply called alteration zones, are in We have adopted the statistical method outlined by some places spatially associated with these intrusions. Bull & Brandon (1998, p. 67) of discriminating significant Elsewhere the alteration zones are found alone. peaks in a histogram for our analysis of oriented, circular Sills found within the basement south of Cape Surprise data (e.g. Fig. 4A). This consists of defining the expected on Garden Spur appear oriented parallel to the Kukri distribution of data if they were selected randomly from a Erosion Surface and Beacon bedding: a 80 m-thick dolerite uniform parent population with no preferred orientations. sill (154°/30°SW) intruded into basement granites below This uniform parent distribution is shown graphically as spot-height 700 is approximately parallel to the SE-dipping a thick black circle on the rose diagram. The expected Beacon strata across the glacier at Cape Surprise. A single limits of variation due to random sampling of this Ferrar dike was found on Mt. Munson striking 107°. This population are expressed as the relative standard error. dike is surrounded by a 5 m-wide yellow to red zone of Two standard errors are depicted as a gray circular envelope intense mineralogical alteration. This alteration is likely around the uniform distribution. Peaks in the sample data hydrothermal and related to Jurassic Ferrar magmatism, that extend beyond this envelope are considered to have similar to alteration zones in southern Victoria Land significantly greater probabilities than that of a uniform basement granites (Craw & Findlay, 1984). Many alteration distribution to the ~95% confidence level. Both the zones were found close to Cape Surprise. These alteration dominant and secondary set of alteration zones are, thus, Structure and Kinematics of the Central Transantarctic Mountains 15

secondary set strikes ~041° (n = 6) with dips from 13° to 82° to both the SE and NW. Calculated against all sampled faults, the dominant set is a statistically significant cluster at the 95% confidence level following the criteria of Jowett & Robin (1988) whereas the number of faults in the secondary set is too small to determine significance. In a general geometric sense, these faults have attitudes similar to those of range-bounding normal faults and transfer faults in other rift margins (e.g. Gibbs, 1984; McClay & Khalil, 1998).

The NW-striking set of D3 faults, which includes the North Boundary fault, was responsible for the large magnitude offset across the TMF associated with uplift of the mountain range. An exposed example is the normal fault that marks the contact between Beacon sediments and the basement at Cape Surprise, previously mapped as the Kukri Erosion Surface (Barrett, 1965; La Prade, 1969), just southwest of Facet Peak (informal name, 84°31.2’S, 174°26.8’W). Granite in the footwall is juxtaposed against the Fairchild Formation (J. Isbell, personal communication, 1995) in the hangingwall. This fault is oriented 125°/ 60°NE, has steeply raking (70°-80°E) slickenlines on the footwall surface, a ~20 m-wide crush zone of poorly calcite-cemented breccia in the hangingwall, and truncates bedding with an attitude of 160°/40° NE.

The NE-striking set of D3 faults is exemplified by two faults mapped by Barrett (1965) west of Facet Peak. Smaller examples were also found southeast of Facet Peak (Fig. 3), of which one has a counterclockwise sense of scissor or rotational slip around a horizontal axis—the same sense as determined on another, larger fault by Barrett (1965). Across other small NE-striking fault zones, Beacon strata are slightly drag-folded in a way implying a component of dextral slip. These faults are surrounded by halos of alteration up to ~10 m-wide. Given this Fig. 4 - Stereonets of the major brittle structures in the Cape Surpise region of the TMF. (A) Rose diagram showing the trends of hydrothermal evidence, these faults are either older than Cenozoic and alteration zones. The thick circle denotes the uniform distribution and are part of the D2 deformation episode, or the alteration the gray envelope shows the expected extent of random variability to 2 zones were reactivated as faults in the Cenozoic. Even standard errors. Measured alteration zones are from the Cape Surprise though their exact origin is not well constrained, these vicinity. (B) Poles to planes of bedding in the Beacon Supergroup at Cape Surprise. Note there are two distinct dip domains: bedding that dips faults’ reactivated nature is a characteristic of most faults north in the eastern half of Cape Surprise and that which dips south in the that strike at high angles to rift trends, such as transfer west. (C) Poles to planes of the south-dipping contact between the Ferrar faults (e.g. Davison, 1994). Dolerite sill and the Queen Maud Batholith (QMB) on Garden Spur. (D) The best evidence for N- to NE-striking faulting in the Fault planes and slickenlines measured at Cape Surprise, Olliver Peak, Sage Nunataks, and Olds Peak. The upper left stereonet is a plot of the basement south of Cape Surprise, with a similar sense of poles of fault planes. To the lower right, the faults (great circles) and their slip, is in the variation in the dip of the Ferrar sill on spot- slip lines (points) are depicted. The gray circle highlights the normal height 700 (30°SW) and spot-height 950 (~5°SW). In fault that juxtaposes granite and Beacon strata on the southern flank of contrast to the structure of the coastal area, no significantly Facet Peak. All contouring was by Kamb’s (1959) method on equal-area projections. The contour interval is 2σ. vertical offsets of the Kukri Erosion Surface along the Prince Olav escarpment were observed, indicating that these small, approximately NE-striking faults, as observed considered significant. at Cape Surprise, do not extend that far inland. Following Jurassic magmatism, D3 deformation tilted the sills and sedimentary beds into their present attitudes KINEMATIC ANALYSIS OF FAULTS (Figs. 4B & 4C) and faults cross-cut these sills (Fig. 3). The mean strike of Beacon strata at Cape Surprise is 131°, Kinematic analysis of fault slip yields information but can be separated into two discrete dip domains with about progressive strain (Marrett & Allmendinger, 1990). mean attitudes of 134°/32°S and 123°/22°N (n = 25). This Slip measurements comprised the orientations of basement differential tilting was accommodated by scissor-slip fault planes and the trends and plunges of striae on these faulting. D3 faults, both large and small (Fig. 4D), are planes. Slip magnitudes were generally not determined divisible into two sets. The dominant set strikes 123° (n = due to ice, snow, or scree cover, and a lack of identifiable 21) with dips ranging from 37° to 61°NE whereas the offset markers. We assume scale invariance of kinematics 16 S.R. Miller et al.

Fig. 5 - Stereonets of fault planes and slickenlines from the Cape Surprise area, Olds Peak, Olliver Peak, and Sage Nunataks and their calculated kinematic solutions. (A) A stereonet of all measured faults. The fault planes (n = 24) are plotted as great circles and the slip lineations (n = 27) are plotted as points on these circles. Arrows denote the movement of the hangingwall. Incremental extension, S1, axis poles for each fault plane-lineation σ pair are contoured by the Kamb (1959) method, for which the contour interval is 2 . S1 axes define a statistically non-random distribution at the 95% confidence level (Jowett & Robin, 1988). (B) A similar stereonet showing the kinematic solutions for faults (n = 3) with multiple lineation sets (n

= 6). Solid black S1 axis squares correspond with black lineation circles, and gray squares with gray circles. It is evident that the two sets of lineations produce two separate sets of S1 axes. (C) Stereonet of faults, lineations, and their associated contoured S1 axes (n = 21), excluding those more NW- trending S1 axes. The moment tensor eigenvectors, which approximate the mean principal incremental strain axes, are located with white squares and ° ° ° ° ° ° trend and plunge: S1 = 200 /5 ; S2 = 291 /1 ; S3 = 036 /85 . These results differ insignificantly with those in (A).

on visible small faults in ridge outcrops and invisible large If every fault plane-slickenline pair formed faults covered by valley glaciers (following Wilson, 1995). synchronously, then the kinematic solution would be That is, the faults we analyzed should reflect regional tenable, but because some fault planes host multiple sets kinematics. We derived kinematic solutions using the of lineations this solution is not. The polykinematic faults software Fault Kinematics 3.5 (Allmendinger et al., 1993) each have two kinematic solutions and two derivable in order to maintain consistency with previous workers in mean S1 axes (Fig. 5B), trending NNE and NW. A plot of the TAM (Wilson, 1995). The graphical method of Marrett moment tensor eigenvectors of a Bingham axial distribution & Allmendinger (1990) solves for the axes of incremental of S1 axes (Fig. 5C) calculated for NE-trending set (n = 21) extension (S ) and shortening (S ) based on the fault plane ° ° 1 3 yields the mean principal strain axes: S1 = 200 /5 , S2 = attitude, the lineation’s plunge and trend, and the sense of ° ° ° ° 291 /1 , and S3 = 036 /85 , statistically indistinguishable fault slip for individual faults. Following the Monte Carlo from the results in figure 5A. The remaining NW-trending approach of Jowett & Robin (1988), these axes cluster into set (n = 3) of extension axes produce eigenvectors with statistically significant sets. A Bingham distribution of ° ° mean trends and plunges as follows: S1 = 281 /6 ; S2 = incremental strain axes (Fisher et al., 1987) was assumed ° ° ° ° 012 /13 ; and S3 = 167 /76 . Given the lack of clear cross- only to objectively estimate their mean. If the individual cutting relationships, the relative ages of the NW- and incremental strain axes represent a plausibly discrete set, NNE-trending S1 axis sets are unknown. for example one that approximates a Bingham distribution, The fault kinematic data presented above are, of course, then their mean orientation is a tenable approximation of for faults with unknown, but probably small, amounts of the bulk incremental strain axis of that set. It should be displacement. It is possible that they record slightly different noted that these axes, individual and mean, do not extension directions than faults with greater displacement. necessarily coincide with the finite strain axes and the A sufficient number of large faults would, of course, better magnitudes of these axes are not known. characterize the kinematics of the area. Our data set Because of the lack of identifiable markers in the includes only one unarguably large fault: that which granite, the slip senses on most faults were inferred from juxtaposes granite and Fairchild Formation on the southern mineral steps in slip-fiber lineations (Davis & Reynolds, flank of Facet Peak (Figs. 3 & 4). Considering the thickness 1996). These showed a consistent normal sense of slip of missing strata (Barrett, 1965; La Prade, 1969), separation where present. For purposes of the kinematic analysis on the fault must be at least 320 m. This normal fault and here, those lineated fault surfaces with ambiguous slip the slickenlines measured on it yield an incremental senses were considered to be normal, consistent with the extension direction that trends 042°-049°. Although this majority of faults measured within the TMF (e.g. Wilson, value is ~20°-30° off of the mean extension axis calculated

1995). The S1 axes (n = 24) document a NNE-SSW trend for all of the faults, we note that it is consistent with with a mean trend and plunge of 206°/4° (Fig. 5A) in a extension that is very close to orthogonal with the coastline significant cluster at the 95% confidence level (Jowett & and mountain range. With no proper knowledge about Robin, 1988). This compares closely to the kinematic how to weight the value of this incremental extension axis solution for fault planes with unambiguous slip-fiber against those of the rest of the faults, it is probably safe to lineations (n = 11), for which the mean trend and plunge conclude that the true mean extension axis in the Cape ° ° of S1 axes is 021º/12º. Surprise area trends 020 -040 . Structure and Kinematics of the Central Transantarctic Mountains 17

Fig. 6 - Maps of glacier and ridge networks in the region of Shackleton and Liv Glaciers with rose diagrams that shows trends of landforms in the vicinity of the study area. Glaciers are marked by medium weight black lines whereas ridges are depicted as thin gray dashed lines. These were digitized from the “Shackleton Glacier” and “Liv Glacier” 1:250,000-scale Antarctic series topographic maps by the United States Geological Survey. The trace of the Kukri Erosion Surface is shown as a thick gray line. The rose diagrams represent individual analyses subdivided into (A) the basement west of Shackleton Glacier, (B) the basement between Shackleton and Liv Glaciers, (C) the Beacon and Ferrar cover west of Shackleton Glacier, (D) the cover between Shackleton and Liv Glaciers, and (E) the basement in the Cape Surprise vicinity. A gray box surrounds the sampled Cape Surprise vicinity. Joint, fault, and alteration zone orientations measured in outcrops in this region are compared to valley orientations from the same small area in (E). A heavy black circle denotes the expected probability of a hypothetical uniform distribution of landform orientations. Each rose diagram has a gray envelope that represents ±2 standard errors of plausible random variation in data density about what would be the expected uniform distribution if there were no preferred valley or ridge orientations. Frequencies that exceed the envelope are ~95% likely to be non- random and thus reflect a preferred geomorphic orientation. Class interval = 10°.

GEOMORPHOLOGY subregions, each lying either within basement or Paleozoic- Mesozoic cover, west or east of the Shackleton Glacier. GEOMORPHIC TREND ANALYSIS Preferred landform orientations were recognized as those that are more common than random orientations, Geomorphic trend analysis was used to determine which is considered here the scatter (±2 standard errors) whether landforms have a preferred orientation. Preferred about the uniform distribution of azimuthal orientations trends not parallel to the regional surface gradient are most (Bull & Brandon, 1998). Preferred landform orientations likely to reflect underlying structure (Morisawa, 1963; theoretically correlate to an underlying structural fabric Scheidegger, 1983). Geomorphic trend analysis (Scheidegger, 1983), which finds some support in complements regional lineament mapping in the central comparisons with mesoscale structures in the Cape Surprise TAM (Paulsen & Wilson, 1998), providing less information area (Fig. 6). A WNW trend, parallel to the coastline and on where large structural features are but more statistical significant at the 95% confidence level is present in most characterization of the structural sets within that region. southern sub-regions. Northern sub-regions in the basement Following previous studies in glaciated and non- all have strong N to NNE trends and weaker WNW trends. glaciated terrain (e.g. Scheidegger, 1983; Ai et al., 1982; Eyles et al., 1997) we analyzed the glacial network, DRAINAGE BASIN ASYMMETRY digitized from 1:250 000-scale United States Geological Survey (USGS) Antarctic series topographic maps. The Based on field observations, drainage networks in the center-lines of glaciers composing the drainage network TMF near Cape Surprise appear asymmetric (Miller et al., were transformed into dashed lines with segments with 1996). For example, tributary glaciers are often present to lengths equivalent to 750 m on the ground. Each segment the SE of the NNE-trending trunk glaciers (Fig. 7). (The was separated by a gap equivalent to 250 m on the ground distinction between trunk and tributary glaciers is somewhat so that image analysis software (namely, NIH Image) arbitrary. Here, we principally distinguish them based on could discriminate and measure the azimuths of the their mapped width, length, and geometry: trunk glaciers individual line segments. These azimuths were measured are often wider, longer, and more linear than their with respect to a local standard meridian (174°W), because tributaries. For purposes of objectivity in our analysis, we at such high latitudes the meridians are significantly non- present a topological distinction below.) As a corollary to parallel at regional scales. The Liv and Shackleton Glaciers the above observation of confluences, spurs that diverge were excluded because of the bias their self-evident trends off of NNE-trending ridges more often point to the NW would introduce. The region was divided into four than the SE (Figs. 2 & 7). Miller et al. (1996) attributed 18 S.R. Miller et al.

Fig. 7 - (A) Map of asymmetric drainage in the TMF, between the Shackleton and Liv Glaciers. North is towards the top. Drainages are numbered as follows: (1) Massam Glacier, (2) lower Barrett Glacier, (3) unnamed glacier in the Gabbro Hills, (4) lower Gough Glacier, (5) Le Couteur Glacier, (6) Morris Glacier, and (7) unnamed glacier northeast of Mt. Daniel. Note the NW-pointing ridgelines in basins 1, 2, 5, 6, & 7. Shaded-relief base maps are the “Shackleton Glacier” and “Liv Glacier” from the 1:250,000-scale Antarctic series by the United States Geological Survey. (B) Analytical results of the topographic symmetry factor (T), a normalized measure of drainage asymmetry, are presented in a polar scattergram. The magnitudes of the analytical data are shown graphically as the distances from the center of the circle, where T = 0 at the center and T = 1 at the edge. The azimuth is measured clockwise from the top of the circle. A square marks the mean T value of the entire dataset. (C) Schematic model illustrating the relationship between asymmetrical stream patterns and active or relatively recent block-tilting, for instance associated with normal faulting. If applicable to the TMF between Shackleton and Liv Glaciers, this would be analogous to a view towards the north or northeast.

this pattern to possible down-to-the-northwest block tilting. In a more general way, asymmetry in drainage basin geometry is commonly thought to indicate neotectonic block tilting in fluvial landscapes (e.g. Hare & Gardner, 1985; Cox, 1994; Keller & Pinter, 1996). Evidence for Cenozoic pediplanation in the Cape Surprise area (Miller et al., in press) and elsewhere in the TAM (Sugden et al., 1995), which is indicative of a subaerial, nonglacial environment of formation, is grounds for us to assume the glacial drainage networks near Cape Surprise occupy formerly fluvial networks. To determine whether the basins in the Cape Surprise area are asymmetric, we used the technique of Cox (1994). The cross-sectional symmetry or asymmetry of a drainage basin is quantified using the transverse topographic symmetry factor (T), which is defined as

D T = a . (1) Dd Fig. 8 - A diagram of a drainage basin showing the measurements used

in calculating the topographic symmetry factor (T). Da is the distance Da is the distance from the mid-line of the basin to the mid- from the basin mid-line to the trunk stream and Dd is the distance from line of the trunk glacier (measured perpendicular to the the basin mid-line to the basin divide. Structure and Kinematics of the Central Transantarctic Mountains 19

basin mid-line) and Dd is the distance from the basin mid- Although the measure of basin asymmetry as a tectonic line to the basin divide (see Fig. 8). In this glacierized indicator has not been applied before to glaciated regions, terrain, we arbitrarily designate the longest, widest glacier we suggest that asymmetric drainage in the Cape Surprise the trunk glacier. All glaciers of first order, topologically region: (1) predates glaciation, (2) is fluvial in origin, and analogous to Strahler’s (1952) first order streams, were (3) resulted from tectonic tilting down to the NW. A case excluded or considered tributaries of the trunk glacier. As for the importance of fluvial erosion in the landscape

Da and Dd are vectors, measured perpendicular to the trunk development of the TAM, and the persistence of such glacier, T is a vector of normalized length that represents landforms to this day, has been made, for example, by the mean azimuth and intensity of basin asymmetry. For Sugden et al. (1995). example, the measured cross-section of the basin is The exact origin of the asymmetry in the Cape Surprise symmetric if T = 0. Its asymmetry is a maximum where region, however, and therefore the age of deformation T = 1 (i.e. the trunk stream flows directly adjacent to one remain open to interpretation. Two possible hypotheses of the basin’s divides). emerge. Hypothesis (1): The deformation could have been T was measured for 7 drainage basins, or coastal contemporaneous with development of the extant landscape portions of basins, covering 1460 km2 (Fig. 7A). Areas of in the TMF, in which case the asymmetric pattern likely basins measured were those that do not directly drain the developed in direct response to down-to-the-WNW tilting frontal escarpment of the Prince Olav Mountains. This due to range parallel transtension (Fig. 7C). Small was done in order to prevent any bias in the shape of the displacement normal faulting and the formation of half valley network that might result directly from scarp retreat. graben in a range-front, low relief pediplain, postulated by The basins’ boundaries and the trunk glaciers were extracted Miller et al. (in press), might be responsible. The existence from 1:250 000-scale USGS topographic maps of the of this pediplain, its present position only several hundred region. Seventy-seven measurements of T were made at meters above sea level, and the fact that it is not evidently a frequency of one measurement per 2 km along each trunk faulted by range-parallel faults, suggests that fluvial glacier (Fig. 7B). For the 7 measured basins, mean T has processes contributed to landscape development in the a magnitude of 0.22 and an azimuth of 293°, which Cape Surprise area after most rift-flank uplift had occurred. indicates that the basins tend to be centered around NNE- Hypothesis (2): The existing drainage network pattern in directed trunk glaciers and portions of the basins ESE of granite could be a feature inherited from antecedent streams the these trunk glaciers tend to be larger than those on the that evolved an asymmetry in once-overlying Beacon WNW sides. strata that had some component of NW dip. Although it is To test whether the measured samples of T are difficult to falsify either hypothesis, and thus better statistically significant, the Rayleigh test of significance constrain the age of the drainage network, both invoke for two-dimensional vector data was used to determine the fault blocks with similar kinematics. probability (p) that a combination of random vectors would produce a mean T that was greater in magnitude (Curray, 1956). This probability is defined as DISCUSSION

− ()()−Ln2410 pe= (2) The dominant existing structural kinematic model for the Transantarctic Mountains presents the TMF as a zone where e = base of the natural logarithm, L = mean magnitude of normal faulting with a significant component of dextral of T in percent, and n = number of basin segments translation (Findlay & Field, 1983; Lanzafame & Villari, measured. For our data, p = 0.024, indicating that the 1991; Fitzgerald, 1992; Wilson, 1995; Fitzgerald & Stump, measured values of T connote an asymmetry significant to 1997). This characterization, which has been gleaned the 95% confidence level. from the obliquity of Cenozoic fault strikes to the mountain The phenomenon of drainage basin asymmetry has front and, in a few locales, the oblique rakes of fault slip been studied in landscapes occupied by stream networks. lineations, has been assigned by some to the TMF for the In these, basin asymmetry has been interpreted as indicating entire period of Cenozoic uplift (e.g. Wilson, 1995). that processes external to the fluvial system (e.g. lithologic Indeed, oblique fault slip on NNE- to NE-striking faults in structure, tectonics, and climate) dominate over internal southern Victoria Land during the late Quaternary may processes (Cox, 1994). Internal processes include fluvial indicate that dextral transtension continues to this day dynamics. Being more stochastic, these processes tend to (Jones, 1996). These lines of evidence for oblique extension produce basin planforms that are more symmetrical in the TMF agree with evidence for dextral transtension (Morisawa, 1963). Whereas climate and prevailing winds and strike-slip faulting in the development of the Ross can produce asymmetric valleys (Wilson, 1968), structure Embayment after the Jurassic (Grindley & Oliver, 1983; and tectonics are thought to influence the asymmetry of Schmidt & Rowley, 1986; Lawver & Gahagan, 1991; drainage networks and basins. Asymmetry may reflect Storey, 1991; DiVenere et al., 1994; Sutherland, 1995; dipping planes of variable resistance to weathering and Salvini et al., 1997), lending support to the inference that erosion (e.g. joints, strata, or sills) or neotectonic tilting of the TMF is in coupled transtension with the West Antarctic the landsurface (e.g. Hare & Gardner, 1985; Cox, 1994; rift system. Based on the simplest assumption of Keller & Pinter, 1996). Both result in differential rates of homogeneous strain within the rift system, one would headward fluvial erosion. predict a larger strike-slip component to transtension in 20 S.R. Miller et al.

Fig. 9 - Comparison of existing constraints on the Cenozoic kinematics of the TAM and the WARS. Arrows depict the relative Cenozoic motion between West Antarctica (and/or the WARS, which includes the Ross Embayment as well as the TMF) and East Antarctica (Wilson, 1992, 1995; Bosum et al., 1989; Sutherland, 1995). Note the extension direction for the Beardmore Glacier region is based on only 16 faults (Wilson, 1993). The extension axis shown at Cape Surprise trends 030°. The motion the West Antarctic rift system relative to the central TAM is based on Australian, Pacific, and Antarctic relative plate motion reconstructions and corresponds to the period 45 Ma to present (Sutherland, 1995). Cretaceous-Cenozoic basins (from Davey & Brancolini, 1995) include North Basin (NB), Eastern Basin (EB), Central Trough (CT), and Victoria Land Basin (VLB), in which lies the Terror Rift (TR). Also shown are major normal faults in the Ross Sea, transverse faults in the TAM, and strike-slip faults (Tessensohn & Wörner, 1991; Fitzgerald, 1992; Salvini et al., 1997), oceanic fracture zones (Royer et al., 1990), and West Antarctic microplates (Dalziel & Elliot, 1982). West Antarctic microplates: Marie Byrd Land (MBL), Thurston Island (TI), Ellsworth Mountains (EWM), and Antarctic Peninsula (AP).

the central TAM than that observed in southern Victoria TAM in Victoria Land (Salvini et al., 1997). Land (Fig. 9). Thus, two models for the general kinematics of the As pointed out by Wilson (1995), evidence for Cenozoic WARS and TMF exist against which to compare transtension implies that mechanical unloading is not our data. Model (1): Regional transtension has occurred likely to be an effective mechanism for uplift of the TAM. in the TMF since the inception of uplift in the early However, certain geodynamic models of this uplift favor Cenozoic. Model (2): The kinematic history of the TMF lithospheric heat transfer from the rift as the primary cause can be divided into two phases, in order to generally for isostatic uplift of the rift flank (Stern & ten Brink, conform to the Cenozoic history of the WARS. In model 1989). Still, the existing transtensional model holds that (2), orthogonal extension or dextral transtension with a such kinematics existed pretty much the length of the TMF relatively small oblique component occurred from the during the major phase of uplift, that strike-slip faulting initiation of the main phase of uplift of the TAM until ~30 along the range front may have even initiated the uplift Ma, followed by transtension with a much larger oblique, (ten Brink et al., 1997). or wrench, component. In contrast to this interpretation, recent seismic studies Whereas the strikes of the faults and trends of the suggest that dextral strike-slip faulting and transtension in extension axes are oblique to the trend of the range front the Ross Sea does not predate ~30 Ma (Salvini et al., in southern Victoria Land, the dominant post-Jurassic (D3, 1997). Prior to this, extension in the Ross Sea was probably Cenozoic) structures in the Shackleton Glacier approximately east-west, or roughly orthogonal to the region tend to parallel the trend of the TAM and the most Structure and Kinematics of the Central Transantarctic Mountains 21 significant extension direction is orthogonal to the range attributed to isostatic response or thermal buoyancy, front. This implies a deformation regime that differs from for example (Stern and ten Brink, 1989; van der Beek that in southern Victoria Land in two ways. Firstly, the et al., 1994; Busetti et al., 1999), and responsible for most significant direction of maximum mean incremental rift-flank uplift overwhelmed any strike-slip component extension at Cape Surprise does not indicate transtension. to faulting that was acting in the greater rift system, Secondly, this axis of extension is not parallel to that in outboard of the TMF. Thus, apparent early Cenozoic southern Victoria Land. These are in notable contrast with extension directions in different regions of the TAM any TMF kinematics thus far predicted from Cenozoic are possibly more a reflection of pre-existing large- strain within the Ross Sea (e.g. Salvini et al., 1997) or from scale structures than any far-field, plate tectonic stress plate motions (Sutherland, 1995). field. In this scenario, an increased component of Despite these significant differences, the second set of approximately range-parallel extension was the result fault lineations and the asymmetric drainage patterns in of increased strain coupling with the WARS as the rate the Cape Surprise region indicate the likelihood of a of rift-flank uplift subsided. Some of this deformation discrete episode of deformation characterized by a was accommodated by both NW- and NE-striking maximum extension axis subparallel to the trend of the faults in the Cape Surprise area. TAM. Although the constraints on the timing of this Although it is not possible to delineate between the two deformation episode are not well established, the kinematics scenarios with the data we have collected from the Cape of this deformation is consistent with existing models for Surprise region, considering other structural data sets dextral transtension in the TMF. In general, the kinematic within the TMF and in the WARS, the bulk of the evidence pattern at Cape Surprise may be similar to patterns of would appear, at this stage, to favor the first scenario. That dominantly dip-slip normal faulting elsewhere in the is, following the onset of significant rift-flank uplift in the TAM followed by more oblique slip (Wilson, 1992; 1995). early Cenozoic, extension in the TMF near Cape Surprise As such, transtensional deformation in the TMF, especially was nearly orthogonal to the trend of the range. At about with an extension axis oriented at as low an angle as 30 Ma, the stress regime in the TMF changed to include a appears in the Cape Surprise region, is not expected to be significant dextral strike-slip component, coincident with contemporaneous with the large magnitude of range uplift strike-slip faulting within the WARS. In this case, the observed. Although the data do not preclude the potential asymmetric drainage basins in the Cape Surprise area of a large strike-slip fault initiating uplift along the range were most likely formed following 30 Ma, as a result of front in the early Cenozoic (ten Brink et al., 1997), they are down-to-the-northwest block faulting during dextral not consistent with a model describing the entire TMF as transtension. transtensional throughout the Cenozoic. Inconsistent extension axes measured in the TMF at different locations along the TAM principally argue against CONCLUSIONS range-wide uniform and steady transtension during the Cenozoic. More importantly, they appear to question the Fault kinematic, geomorphic trend, and drainage presence of transtension everywhere along the TAM during basin asymmetry analyses from the Cape Surprise region the early phases of uplift. Although the ages of different have led to a number of conclusions regarding the structure faulting episodes need to be better constrained at Cape and kinematics of the TMF in the central TAM: Surprise and elsewhere along the TAM, we propose two 1. The architecture of the front of the range, which locally end-member explanations for the kinematics seen in our trends NW, is composed of NW-striking normal faults study area. and NNE-NE-striking normal or transfer faults. South 1. Following the onset of significant rift-flank uplift in of the TMF’s inland margin (i.e. the axis of maximum the early Cenozoic and prior to ~30 Ma, extension in uplift) the NE-striking set is not evident except possibly the TMF near Cape Surprise was nearly orthogonal to beneath the Shackleton Glacier. This is a different the trend of the range. Any transtension in the TMF pattern to that documented in southern Victoria Land coincided with strike-slip faulting within the WARS where range-front faults strike obliquely to the trend of and was initiated at ~30 Ma, well after the onset of rift- the TAM. flank uplift. 2. Faults in the Cape Surprise region record at least two 2. The two episodes of deformation in the TMF near Cape separate extension directions. The primary, statistically Surprise both occurred during transtension in the significant mean incremental extension axis is WARS. Strain partitioning (Teyssier et al., 1997) approximately perpendicular to the trend of the range, could explain the occurrence of dominantly range- or 020°-040°. The highly raking slip lineations, the parallel normal faults and orthogonal extension in the likelihood of fault reactivation, and the extension TAM. In such a scenario, the wrench component of direction are consistent with the hypothesis that uplift transtension could be taken up by a strike-slip fault (or of the TAM was driven by isostasy rather than a faults), most likely below the Ross Ice Shelf in the mechanical result of regional extension or transtension. WARS. The kinematics of the WARS would, therefore, A poorly defined secondary set of incremental extension be more effectively partitioned from kinematics in the axes trending roughly parallel to the range is TMF during the major phase of rift-flank uplift in the differentiated from the primary set on polykinematic early Cenozoic. Locally, the differential stresses faults. 22 S.R. Miller et al.

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