2.1. Satellite imagery

The background for the Mount Murchison map is a Landsat 4 Thematic Mapper (TM) satellite image mosaic. The mosaic consists of five TM images, acquired between November 1989 and January 1990 (062-112, 17 January 1990; 062-112, 14 November 1989; 066-113, 17 January 1990; 066-111, 29 January 1990; 066-112, 29 January 1990). The TM images were digitally integrated using ERDAS Imagine mosaicking tools to produce a map with the dimensions of the USGS Mount Murchison quadrangle (1967). Landsat 4 TM data have a ground resolution of 28.5m and are collected simultaneously in seven spectral bands: three in the visible spectrum (bands 1–3), three in the near-infrared region (bands 4, 5 and 7) and one in the thermal infrared region (band 6). Here we use a false-colour composite (assigning red to band 4, green to band 3, and blue to band 2) at a 1 : 250 000 scale. The albedo of snow, firn and ice generally decreases from visible to near-infrared wavelengths (Reference FrezzottiFrezzotti, 1993; Reference Zibordi, Meloni and FrezzottiZibordi and others, 1996). This difference in spectral response is enhanced in the false-colour mosaic, allowing differentiation of distinct glacier facies (e.g. snow-covered accumulation regions appear white in the images, whereas blue-ice covered ablation areas appear blue). Aerial photography and additional satellite imagery (Landsat TM; Landsat multispectral scanner (MSS); Système Probatoire pour l’Observation de Terre (SPOT); European Remote-sensing Satellite (ERS)) were also used to estimate glacier velocities and ice-front fluctuations from 1964 to 1999.

2.2. Field survey and data analysis

The main landscapes of the region were surveyed through helicopter flights and detailed geomorphological surveys in key areas in the period 1996–2001. Landforms and deposits of glacial and periglacial environments, as well as forms related to mass wasting, wind action, weathering and geological structures, were identified and mapped. Features originating from different geomorphological processes are represented with appropriate symbols and colour-coded. Morpho-chronological and morpho-dynamical data are supplied where available. Different colour tones differentiate active and relict landforms. Particular attention was devoted to the relationship between relict erosional landforms and volcanic activity. For example, we measured and mapped the elevation of glacial erosional trimlines by means of ground surveys and detailed helicopter traverses using topographic maps and altimeter measurements (corrected by cross-checking points of known elevation). The estimated trimline elevation points are indicated on the map and have an uncertainty of about 50 m.

The main features related to present-day glacier morphology and dynamics are depicted on the map. The methods used to survey glaciological features (ice velocity, ice-front position, grounding line, aeolian morphology, etc.) are described in more detail by Reference FrezzottiFrezzotti (1997) and Reference Frezzotti, Capra and VittuariFrezzotti and others, (1998 Reference Frezzotti, Gandolfi, La Marca and Urbini, 2002). Maps, georeferenced satellite images and aerial photographs were analyzed, compared and input into a Geographic Information System (GIS) using ERDAS and Arc/Info software. Data from all other sources were also formatted so they can form attribute layers of a GIS.

2.3. Additional data sources

Additional information on the thickness of glaciers derives from airborne radar surveys conducted in 1995 (Reference Tabacco, Passerini, Corbelli and GormanTabacco and others, 1998). These radar profiles are inserted at the base of the map. The data were acquired by means of a radar system operating at 60MHz and linked to a GPS. Some themes (e.g. lithology, after Reference Capponi, Meccheri and PertusatiCapponi and others, 1997) are shown at reduced scales. Extensive geomorphological features, such as the main glacial troughs, relict alpine ridges and spurs, scoured bedrock and volcanic cones, are also depicted in the same insert.

Snow-accumulation measurements and mean temperatures are also reported. They derive from direct measurements and from studies on the isotopic and chemical composition of the snow as a function of elevation and distance from the ocean (Reference Allen, Mayewski, Lyons and SpencerAllen and others, 1985; Reference Piccardi, Udisti and CasellaPiccardi and others, 1994a Reference Piccardi, Barbolani, Bellandi, Casella and Udistib Reference Piccardi, Casella and Udisti, 1996; Reference Gragnani, Smiraglia, Stenni and TorciniGragnani and others, 1998; Reference MaggiMaggi and others, 1998; Reference Udisti, Becagli, Traversi, Vermigli and PiccardiUdisti and others, 1998a Reference Udisti, Traversi, Becagli and Piccardi, b Reference Udisti, Becagli, Castellano, Traversi, Vermigli and Piccardi1999; Reference StenniStenni and others, 1999 Reference Stenni, 2000). The firn cores were drilled between 1981 and 1993, and the annual accumulation rate was determined for the period 1971–92.

3.1. Glacier and sea-ice features

The Mount Murchison map covers an area extending from Priestley Glacier to the upper Rennick Glacier basin, Half Ration Névé, Hercules Névé and Lady Newnes Bay. The EAIS borders the area to the northwest, and feeds the Rennick and Priestley outlet glaciers. The former flows northward toward the Pacific Ocean, while the latter enters Terra Nova Bay, south of the study area. Aviator, Campbell, Tinker, Icebreaker, Fitzgerald and Meander glaciers are the main valley glaciers present in the map and drain the névé of the mountainous Victoria Land coastal zone. The catchments of most of these glaciers extend outside the Mount Murchison quadrangle.

The ice-thickness data reveal that ice caps are up to 900m thick (Hercules Névé). Radar profiles of Hercules Névé (radar profile G–H and K–L in the map) show strong reflectors at intermediate depths (200–400 m). The geomorphological position and continuous extensions of these reflectors suggest they are tephra layers ejected by local volcanoes. Indeed, tephra attributed to The Pleiades and Mount Rittmann volcanoes was identified 180 km west of Hercules Névé, in a blue-ice area of the Outback Nunataks (72˚57' S, 160˚28' E; ∼2040ma.s.l.; Reference Perchiazzi, Folco and MelliniPerchiazzi and others, 1999) and in an ice core drilled at Talos Dome (72˚48' S, 159˚06' E; 2316ma.s.l.; Reference Narcisi, Proposito and FrezzottiNarcisi and others, 2001). The Pleiades volcano (72˚45' S, 165˚29' E), located close to the northeast border of Hercules Névé, is known to have produced widespread pumice deposits. On the basis of snow-accumulation rates measured at Hercules Névé (Reference StenniStenni and others, 2000) and of reflector depths, we suggest that this eruption is of Holocene age.

The catchment areas of glaciers in the region vary in size from a few tens of km² to several thousand km². These catchment regions mainly develop above 1000 m elevation (80% of the Campbell Glacier basin area lies above 1000 m) and some exceed 3000 m. The glacier tongues have linear snow-filled depressions, which have been interpreted as the surface expression of bottom crevasses (Reference FrezzottiFrezzotti, 1997). The calving period for these glacier tongues is typically 7– 15 years (Reference FrezzottiFrezzotti, 1997). The presence of fast ice for most of the year, and of multi-year sea ice in Lady Newnes and Wood Bays, protects the ice fronts from storms and wave motion (Reference FrezzottiFrezzotti, 1997).

Aviator Glacier occupies a deep trough that crosses the Transantarctic Mountains. Its grounding zone is located about 90 km inland of the present ice front. Glacier ice is about 1500m thick at the grounding zone (Fig. 2). Ice velocity at the ice front is about 180 ma^–1 (Reference FrezzottiFrezzotti, 1993), and ice thickness is reduced to <200m due to basal melting and ice dynamics. The grounding zone of Icebreaker Glacier is about 30 km inland of the present ice front, with an ice thickness of about 900 m. The thickness of the Icebreaker ice tongue is also reduced to <200m at the ice front, which moves at a velocity of about 70 ma^–1.

Fig. 2. Present and former longitudinal profiles of Aviator Glacier derived from ice surface and thickness, trimlines and drift limit elevations.

3.2. Climatic data

Ten-metre core temperatures (Reference StenniStenni and others, 2000) indicate a sub-adiabatic lapse rate of 0.5˚C (100 m)^–1, in agreement with the results of Reference Vitale, Tomasi, Giovannelli and StefanuttiVitale and Tomasi (1994) which indicate values of 0.55 –0.62˚C (100 m)^–1 at Terra Nova Bay Station on the basis of meteorological radio sounding collected from 1987 to 1991. The distribution of accumulation rates shows an overall negative correlation with altitude and distance from the coast. Maximum values were measured at Aviator Glacier station (260 kg m^–2 a^–1), the lowest site (150ma.s.l.) closest to the coast. Among the less disturbed windy sites, the lowest accumulation rates (130–145 kg m^–2 a^–1) were measured on Hercules Névé, located at high altitudes (about 2900ma.s.l.) and far from the sea. The geographical distribution of accumulation rates indicates that depositional regimes may depend on orography and the direction of storms and are induced by wind-driven ablation. Reference StenniStenni and others (2000) calculated an average snow accumulation rate of about 170 kg m^–2 a^–1, and a linear relationship between mean δ¹⁸O and firn temperature with a gradient of 0.81‰˚C ^–1.

3.3. Aeolian landforms

The snow deposition process is complicated in the Victoria Land mountains, characterized by strong, persistent katabatic winds and severe snowstorms. Converging katabatic winds in Terra Nova Bay greatly influence the environmental conditions in Victoria Land. The primary route for katabatic winds into Terra Nova Bay is the Reeves Glacier valley (south of Mount Murchison), but an important secondary source is provided by airflow down Rennick and Priestley Glaciers (Reference Bromwich, Parish and ZormanBromwich and others, 1990; Reference FrezzottiFrezzotti, 1998). Wind-field data (Fig. 3) derived from surface aeolian morphologies (snowdrift, sastrugi, wind scoops and drift plumes) show that katabatic winds from the west and southwest cross Eisenhower Range and Priestley Valley, and affect the Deep Freeze Range and the leeward area (hydrographic right) of Campbell Glacier (Reference Zibordi and FrezzottiZibordi and Frezzotti, 1996; Reference FrezzottiFrezzotti, 1998), where they are responsible for the extensive formation of blue-ice fields. Extensive blue-ice areas are also present in the Mesa Range along Rennick Glacier. In the eastern part of the Campbell Glacier valley (hydrographic left) the effect of winds strongly diminishes. Automatic weather station 7356 on Tourmaline Plateau (74˚11' S, 163˚29' E; 1700ma.s.l.; Reference BaroniBaroni and others, 1996) shows that prevailing winds mainly blow from the west and southwest. Winds can reach speeds of > 40 kts (20ms^–1), and speeds of > 7 kts (3.5ms^–1) persist for more than 50% of the year. The mountain area east of Campbell Glacier is characterized by aeolian features related to surface air blowing radially and uniformly away from the highest parts of the mountains (Hercules Névé, Southern Cross Mountains) to the coast along the main valleys. In the coastal area, aeolian morphologies are aligned southwest–northeast according to the direction of barrier winds (Reference FrezzottiFrezzotti, 1998).

Fig. 3. Wind-field directions derived from surface aeolian morphologies (snowdrift, sastrugi, wind scoops and drift plumes).

3.4. Volcanic and structural morphology

The geomorphology of the area is strongly controlled by the geological structure. The Mount Murchison quadrangle encompasses an Early Palaeozoic metamorphic basement (Reference Capponi, Meccheri and PertusatiCapponi and others, 1997 and references therein). It consists of two main terranes, the Wilson Terrane (greenschist facies metasediments, migmatites, gneiss) and the Bowers Terrane (low-grade sedimentary and volcanic rocks). Granites of the Granite Harbour Igneous Complex (Ordovician) intruded the Wilson Terrane. A north-northwest– south-southeast-trending shear zone marks the contact between the Wilson and Bowers Terranes. Devonian Admiralty intrusives and Gallipoli volcanic rocks were emplaced in both terranes. After this event, the basement was eroded and uplifted. An erosion surface (known in southern Victoria Land as Kukri Peneplain) was sculptured in the basement. The Triassic continental Beacon Sandstone deposited on this surface. The Kirkpatrick Basalt (Jurassic) covered the Beacon Sandstone that was in turn intruded by the Ferrar Dolerite, which built horizontal sills along the basal Beacon horizon. Columnar jointing is a common feature along the cliffs where dolerites and basalts crop out (i.e. Mesa Range, Lichen Hills, Illusion Hills, slope southwest of Deception Plateau). Flat surfaces related to the exhumation of the Kukri Peneplain are visible on the Southern Cross Mountains (Archambault Ridge; Fig. 4).

Fig. 4. Mesa structure (> 3000 m) to the west of Archambault Ridge. Ferrar Dolerite and Beacon Sandstone (in the background) cover the paleozoic Kukri Peneplain, exhumed in the foreground.

The last geological event was the emplacement of the Cenozoic McMurdo igneous rocks, consisting of the Meander intrusives suite (22–48 Myr) and the Melbourne alkali– volcanic suite (>14.5Myr to the present; Reference Kyle, LeMasurier and ThomsonKyle, 1990; Reference Armienti and BaroniArmienti and Baroni, 1999 and references therein). Effusive activity generated numerous volcanoes, the most evident being Mount Overlord, Mount Rittman, Mount Noice and Parasite Cone. Cinder cones are also widespread, particularly in the Southern Cross Mountains. Tephra accumulation on ice-free areas is evident on Mount Noice and in the Meander Glacier basin. Well-defined lava flows are present on the western slopes of Mount Overlord. Small lava flows related to minor volcanoes were found along the slopes of the Aviator Glacier valley and in the vicinity of Mount Kinet.

Geological structure also played an important role in the development of valley networks, operating both passive and active tectonic control. Block faulting affected the mesas in the Cretaceous and Cenozoic (Reference Petri, Salvini, Storti and RicciPetri and others, 1997), and a major northwest–southeast-trending dextral strike–slip fault system developed in northern Victoria Land (after 80 Myr; Reference Salvini, Brancolini, Busetti, Storti, Mazzarini and CorenSalvini and others, 1997; Reference Salvini and StortiSalvini and Storti, 1999). Cenozoic north–south to north-northwest–south-southeast transtensional faults developed in the Cenozoic (after 32 Myr). Major glacier valleys trend northwest–southeast, following the main regional strike–slip fault; however, a significant portion of the valley system trends north–south according to extensional faults (Reference Salvini, Brancolini, Busetti, Storti, Mazzarini and CorenSalvini and others, 1997). The resulting drainage system is complex, with dendritic, parallel and sub-rectangular patterns. The relationship between active tectonic control and the drainage network is marked by a number of selected lineaments. Some lineaments are interpreted as traces of faults or fault-line erosional scarps. Some of these scarps are associated with morphostructural features like saddles and triangular facets, suggesting that they actively determined the evolution of landscape and drainage pattern.

Flat-topped summit surfaces are visible at the Mesa Range, where summit mesas are interpreted as relict landforms scalloped by glacial erosion along their margins (Fig. 5). Differential weathering produced denudation terraces along the slopes of the mesas. A number of residual buttes are also associated with the mesas. A gentle regional tilting of about 2˚ is indicated in structural blocks of the Mesa Range and of the area in the vicinity (Reference Petri, Salvini, Storti and RicciPetri and others, 1997). Considering the low dip of the strata, almost imperceptible in the field, we indicate these landforms as mesas; those structures that show a more evident dipping of Mesozoic formations are indicated as cuestas. A sequence of esplanade terraces is found in the Deep Freeze Range, between Mount Hewson and Priestley Glacier.

Fig. 5. Mount Frustum (3100 m), Tobin Mesa. Flat-topped surfaces are interpreted as old relict landforms scalloped by glacial erosion on their margins. A diffluent lobe of Rennick Glacier enters from the left. Local debris-covered glaciers (to the right) rest in glacial cirques.

3.5. Glacial landforms and deposits

3.5.1. Erosional landforms

Valley troughs are occupied by outlet glaciers (Rennick and Priestley Glaciers) and local valley glaciers that form an articulated, complex drainage system, with main and tributary valleys producing dendritic, parallel and subrectangular patterns. Several hanging valleys are also present. Considering that well-developed relict alpine morphologies are perfectly preserved in deglaciated areas, the valley network is interpreted as the remnant of an old topography sculptured by temperate alpine glaciers. If this interpretation is correct, glaciers occupy the valley network and ridges mantling the relict landscape.

Relict alpine topographic features consist of sharp ridges with delicate serrated spires (Fig. 6), glacial cirques, horns and truncated spurs. These features continue across different bedrock lithologies. Well-preserved erosional trimlines are common features on the highest summits and on ridges (Reference Orombelli, Baroni and DentonOrombelli and others, 1991). They smoothly decrease in elevation along the valleys from the interior to the coastal margin (Fig. 2). No striations or erratics are found above the trimlines. On the contrary, the surfaces below the trimlines were sculptured by glacial abrasion into rounded summits, scoured areas and roches moutonnées. Grooves, crescentic marks and extremely well-preserved fine striations on polished surfaces are widespread and were used to reconstruct ice-flow directions. The bedrock above this erosional limit exhibits strong staining and deep cavernous weathering. Diffuse tafoni with enlarged visors, pseudokarstic rectilinear grooves and weathering pits characterize the exposed granitic rocks on horns and ridges.

Fig. 6. Southwest Chisholm Hills photographed from the northeast, with Mesa Range in the background. A sharp ridge with delicate and serrated spires (up to 30 m high) is sculptured in Ferrar Dolerite. A double trimline at about 2800 m limits the ridge that was never glacially eroded.

The mesa edges are also scalloped by the alpine topography. In this particular geomorphological context, where local ice caps descend from summit plateaus and feed the valley glaciers, an upper trimline is sculptured on the ridges. This trimline is irregular and was probably eroded by summit glaciers descending into the main valleys. K/Ar and Ar/Ar ages of volcanic rocks sculptured by glacial erosion show that alpine topography is older than 8.2 Myr. Landform features younger than 7.5 Myr are unaffected by wet glacial erosion (Reference Armienti and BaroniArmienti and Baroni, 1999).

At the confluence between Burns and Tinker Glaciers, at an elevation of about 300 m, volcanoclastic rocks buried a granitic bedrock intruded by basalt dykes (Reference Verbers, Smith and vanVerbers and others, 1995). Fine striations on granite and dykes were produced by a glacier moving eastward. The volcanoclastic rocks are presumably older than 12.5 Myr and predate the glacial abrasion of the granite bedrock.

3.5.2.

Glacial deposits and constructional landforms Since most of the Mount Murchison quadrangle, particularly in the entire area east of the Campbell Glacier–Mesa Range alignment, is an accumulation region, supraglacial drift is rapidly covered by snow and incorporated within glacier ice. Supraglacial debris re-emerges in blue-ice fields where katabatic winds prevent snow accumulation and induce strong ablation, forcing the ice to flow there continuously. Englacial debris in these areas is progressively accumulated on the ice surface. Debris traps are concentrated on the western portion of the map, and are almost absent along the central and eastern valley glaciers. Most of the debris-covered ice areas are located close to the slopes and inside pocket valleys where strong katabatic winds induce effective ablation. Supraglacial debris may be scattered on the ice or form a continuous cover some decimetres to a few metres in thickness. Supraglacial drift protects the ice from wind and radiation, thereby reducing ablation; as a result, debris-covered ice stands at a higher elevation with respect to the surrounding bare ice. Supraglacial till mainly consists of rocks from adjacent topography, suggesting that a glacial flow starts from the base of the same relief. Indeed, the debris-covered ice is a local glacier within a main glacier; a moraine of basal debris from the local glacier often marks its boundaries (Fig. 5). In some places the local glacier flow is dammed by the opposite inflow of the main glaciers. In this case, a double moraine delimits the two contrasting flows. Rennick Glacier enters with diffluent lobes into the Lichen Hills and insinuates up-valley (Fig. 7). The lobe west of Lemaster Bluff also deposited a small Holocene ice-cored moraine on the bedrock. Reference Meneghel, Bondesan, Salvatore and OrombelliMeneghel and others (1999) described a particular kind of banded floating moraine at Lichen Hills. Bands of different lithology (light-coloured granite and dark-coloured volcanic rocks) possibly formed in relation to the progressive lowering of the ice surface due to glacier retreat after the Last Glacial Maximum (LGM). If so, the marked difference in the lithological composition of the moraines is due to the different rocks exhumed by the lowering of the glacier surface during the Late Glacial and Holocene periods. Holocene moraines related to small local glaciers rest on ice-free areas of Gair Mesa, Lichen Hills and the Archambault Ridge.

Fig. 7. Lichen Hills photographed from northwest. A diffluent lobe of Rennick Glacier flows up-valley into a blue-ice area where ablation due to katabatic winds forces the ice to flow continuously. A continuous cover of supraglacial debris is visible on the lobe margin close to the slopes.

Late Wisconsin glacial drift occurs at several sites at different elevations. It consists of massive matrix-supported and clast-supported diamictons composed of granitic, metamorphic and volcanic rocks. Clasts are mainly angular and subangular, but some pebbles are bullet-shaped and show surface striations. Where present, the sandy to silty matrix is olive-grey. The debris shows a low degree of weathering and pale staining. On the basis of morphological position, degree of weathering and soil evolution, this deposit can be correlated with the ‘Terra Nova drift’ described by Reference Orombelli, Baroni and DentonOrombelli and others (1991). The Late Wisconsin drift is mapped in only a few localities because it is discontinuous, with a patchy distribution where present. Scattered patches of glacial drift and erratics are more common and were mapped at several sites. Drift distribution and elevations were used to reconstruct the maximum level reached by the glacial surface of the LGM (Fig. 2). On the basis of their position and morphology, some glacial drift can be interpreted as frontal deposits of small local glaciers (i.e. at Mount Frustum and north of Mount Hewson).

A number of moraine ridges related to recessional phases of the LGM are also indicated as Late Wisconsin drift. They were mapped at the Lichen, Illusion and Vantage hills.

Some erratics predating the LGM are present on the ridge west of Scarab Peak, in the Mesa Range.