Transantarctic Mountain Front Research Articles

Thermal evolution of the lower crust beneath the Transantarctic Mountains

Models of uplift, rifting, and heat transfer in the Transantarctic Mountains (TAM) rely on knowledge of the thermal evolution of the lower crust, but such information has remained elusive. Granulite xenoliths entrained in <2 Ma rift basalts at the TAM front chronicle the long-term thermal history of the deep crust and yield insight into the Cenozoic evolution of the TAM. Major-element thermobarometry of the xenoliths record elevated temperatures of 860–920 °C at ∼0.8 GPa. In situ coupled U-Pb and trace element zircon and titanite data reveal that the deep crust experienced intense heating at least twice: once at 850–900 °C ca. 500 Ma and again at 740–900 °C starting ca. 37 Ma. The Cenozoic temperature-time path indicates a high geothermal gradient beneath the TAM front—with 800–900 °C at 20–30 km depth and 900–1000 °C temperatures at the Moho (30–35 km depth)—and is interpreted to reflect heating by the adjacent West Antarctic Rift System. Despite elevated temperatures in the lower crust, minimal melting beneath the present-day TAM is inferred given that the crust is refractory, having been previously depleted during Ordovician magmatism. The identification of hot crust beneath the TAM forces revisions to estimates of the elastic thickness of the lithosphere, which underpin models invoking a flexural origin for the high elevations of the TAM. The observed Cenozoic heating trend supports models that suggest uplift was driven by a buoyant thermal anomaly beneath the TAM front. The finding of dry and strong deep crust is also in line with models wherein the TAM have maintained high elevations since the Mesozoic.

Heterogeneous upper mantle structure beneath the Ross Sea Embayment and Marie Byrd Land, West Antarctica, revealed by P-wave tomography

We present an upper mantle P-wave velocity model for the Ross Sea Embayment (RSE) region of West Antarctica, constructed by inverting relative P-wave travel-times from 1881 teleseismic earthquakes recorded by two temporary broadband seismograph deployments on the Ross Ice Shelf, as well as by regional ice- and rock-sited seismic stations surrounding the RSE. Faster upper mantle P-wave velocities (∼+1%) characterize the eastern part of the RSE, indicating that the lithosphere in this part of the RSE may not have been reheated by mid-to-late Cenozoic rifting that affected other parts of the Late Cretaceous West Antarctic Rift System. Slower upper mantle velocities (∼−1%) characterize the western part of the RSE over a ∼500 km-wide region, extending from the central RSE to the Transantarctic Mountains (TAM). Within this region, the model shows two areas of even slower velocities (∼−1.5%) centered beneath Mt. Erebus and Mt. Melbourne along the TAM front. We attribute the broader region of slow velocities mainly to reheating of the lithospheric mantle by Paleogene rifting, while the slower velocities beneath the areas of recent volcanism may reflect a Neogene-present phase of rifting and/or plume activity associated with the formation of the Terror Rift. Beneath the Ford Ranges and King Edward VII Peninsula in western Marie Byrd Land, the P-wave model shows lateral variability in upper mantle velocities of ±0.5% over distances of a few hundred km. The heterogeneity in upper mantle velocities imaged beneath the RSE and western Marie Byrd Land, assuming no significant variation in mantle composition, indicates variations in upper mantle temperatures of at least 100°C. These temperature variations could lead to differences in surface heat flow of ∼±10 mW/m2 and mantle viscosity of 102 Pas regionally across the study area, possibly influencing the stability of the West Antarctic Ice Sheet by affecting basal ice conditions and glacial isostatic adjustment.

Variability in uplift, exhumation and crustal deformation along the Transantarctic Mountains front in southern Victoria Land, Antarctica

The Transantarctic Mountains (TAM) are an imposing topographic feature, forming the western shoulder of the Meso-Cenozoic West Antarctic Rift System. Although the TAM topography is similar to other continental rifts, some aspects such as the high topography and the transition in the mode of crustal extension from orthogonal to oblique rifting during the Cenozoic makes the TAM an anomalous rift margin. Here, we present a topography analysis of a 600 km long transect along the TAM front in southern Victoria Land combined with a large available thermochronological data-set to decode the tectonic signals hidden in the topography. An along-strike variability in tectonic, erosional and geomorphic characteristics is detected. We then focus our analysis on the Royal Society Range, where structural investigations were integrated with new fission track thermochronology in order to assess the morphotectonic evolution of the region. Fission-track data and topography of the Royal Society Range reveal remarkable differences with respect to the neighboring areas. Topography characteristics and thermal modeling suggest an increase in tectonic activity during late Eocene-early Oligocene times and structural analysis suggests that the Cenozoic rifting has been controlled by dextral transtension, as proposed for others sector of the TAM front. The detected along-strike variability in tectonic, erosional and geomorphic characteristics may reflect geodynamic complexities that should be taken into account in any further model of the TAM evolution.

Variations of the effective elastic thickness over the Ross Sea and Transantarctic Mountains and implications for their structure and tectonics

The effective elastic thickness (Te) is a proxy for lithospheric strength, and it depends primarily on the thermal gradient and composition of the lithosphere. Accordingly, spatial variations in Te reflect changes in lithospheric properties and can be used to better understand the structure and tectonics of particular regions. In this paper, we investigate the Ross Sea and Transantarctic Mountains in terms of Te using gravity and topographic data and the fan wavelet transform technique. The results reveal that relatively high Te values dominate in the extensional basins of the Ross Sea and the hinterland of Transantarctic Mountains, whereas very low Te values occur along the Transantarctic Mountain Front and in the deep ocean basin, with the lowest Te values are found the vicinity of Ross Island and onshore in northern Victoria Land. In addition, the spatial variations in Te correlate well with lithospheric structure at the regional scale. By combining these findings with published seismic and heat flow data, we conclude that the presence of a zone of anomalously lowTe values parallel to the coast indicates that the lithosphere beneath the Transantarctic Mountain Front is extremely weak due to Cenozoic volcanism and extension. The Te values increase from the Transantarctic Mountain Front (7km) toward the center of the continent (~80km), which indicates that the continental lithosphere underlying East Antarctica belongs to the classic Gondwanan craton. The increase in Te indicates that the Transantarctic Mountain Front marks the continent-continent boundary between East Antarctica and West Antarctica. The Te values in the other extensional basins of the Ross Sea exhibit little variation and average approximately 35km. The relatively high Te values are interpreted to indicate that the lithosphere cooled and became mechanically stronger between late Cretaceous extension and Eocene-Neogene deposition.

Variable thermal loading and flexural uplift along the Transantarctic Mountains, Antarctica

The high elevations of the Transantarctic Mountains (TAMs) have been suggested to be flexural in origin, but to date, the thermal contribution to uplift has yet to be quantified. Here, we present new P- and S-wave tomography images of the upper-mantle seismic structure beneath the central to northern TAMs, which reveal two, focused low-velocity anomalies beneath Ross Island and Terra Nova Bay that laterally extend beneath the TAMs front and that are connected by a low-velocity region constrained offshore within the Victoria Land Basin. The focused low velocities are interpreted as shallow regions of partial melt, connected by a broad region of slow (warm) upper mantle associated with Cenozoic extension along the Terror Rift. Thermal loading constraints based on our tomographic results are used to update flexural uplift models for the TAMs. Our findings confirm that thermal buoyancy is a principal component leading to the uplift of the TAMs but suggest that the thermal loading is variable along the TAMs front.

Widespread persistence of expanded East Antarctic glaciers in the southwest Ross Sea during the last deglaciation

It has been suggested that the grounding line of the Last Glacial Maximum (LGM) ice sheet in the Ross Sea, Antarctica, receded in an approximately north-to-south pattern during the Holocene. An implication of this hypothesis is that geological evidence from the southwestern Ross Sea has been used widely to interpret retreat histories of the West Antarctic Ice Sheet (WAIS) across the wider Ross Sea embayment. Accurately constraining the timing and pattern of marine-based ice sheet retreat in this embayment is critical to understanding the drivers that may have triggered this event, and its contribution to rapid sea-level rise events. Here, we present new multibeam swath bathymetry data that identifies well-preserved glacial features indicating that thick (>700 m) marine-based ice derived from the East Antarctic Ice Sheet coastal outlet glaciers dominated the ice sheet input into the southwestern Ross Sea during the last phases of glaciation. Subglacial geomorphic features indicate that ice derived from present outlet glacier valleys in South Victoria Land flowed southeastward. This is more consistent with flowlines from model-based interpretations of an earlier retreat of the WAIS in the central Ross Sea than with previous land-based geological reconstructions. This implies that coastal records of deglaciation along the Transantarctic Mountains front record only the final phases of glacial retreat in the Ross Sea. Therefore, chronological data from the central embayment are required to accurately constrain the timing of large-scale glacial retreat in the Ross Sea and to identify the mechanisms that drove it.

Upper mantle seismic anisotropy beneath the Northern Transantarctic Mountains, Antarctica from PKS, SKS, and SKKS splitting analysis

AbstractUsing data from the new Transantarctic Mountains Northern Network, this study aims to constrain azimuthal anisotropy beneath a previously unexplored portion of the Transantarctic Mountains (TAMs) to assess both past and present deformational processes occurring in this region. Shear‐wave splitting parameters have been measured for PKS, SKS, and SKKS phases using the eigenvalue method within the SplitLab software package. Results show two distinct geographic regions of anisotropy within our study area: one behind the TAMs front, with an average fast axis direction of 42 ± 3° and an average delay time of 0.9 ± 0.04 s, and the other within the TAMs near the Ross Sea coastline, with an average fast axis oriented at 51 ± 5° and an average delay time of 1.5 ± 0.08 s. Behind the TAMs front, our results are best explained by a single anisotropic layer that is estimated to be 81–135 km thick, thereby constraining the anisotropic signature within the East Antarctic lithosphere. We interpret the anisotropy behind the TAMs front as relict fabric associated with tectonic episodes occurring early in Antarctica's geologic history. For the coastal stations, our results are best explained by a single anisotropic layer estimated to be 135–225 km thick. This places the anisotropic source within the viscous asthenosphere, which correlates with low seismic velocities along the edge of the West Antarctic Rift System. We interpret the coastal anisotropic signature as resulting from active mantle flow associated with rift‐related decompression melting and Cenozoic extension.

Crustal Vp-Vs ratios and thickness for Ross Island and the Transantarctic Mountain front, Antarctica

SUMMARY We investigate crustal Vp–Vs ratios and thickness along the Transantarctic Mountain (TAM) front and on Ross Island, Antarctica to determine if the TAM crust has been modified by the Neogene magmatism associated with Ross Island. A seismic low velocity zone (LVZ) in the upper mantle beneath Ross Island extends laterally ∼80 km under the TAM front, and mantle temperatures within the LVZ may be sufficiently elevated for partial melting to have occurred and modified the crust. Data for the study come from 16 temporary seismic stations that were part of the TAM Seismic Experiment and three permanent stations. Estimates of Vp/Vs (κ) and crustal thickness (H) have been obtained from receiver functions analysed using the H–κ stacking method for 10 of the stations, and for the remaining stations, crustal thickness has been calculated by using the Moho Ps arrival time with an assumed Vp/Vs value. A Vp/Vs value of 1.88 is obtained for Ross Island, consistent with the mafic composition of the volcanic rocks from Mt. Erebus. Vp/Vs values for stations in the TAM situated away from the LVZ range from 1.63 to 1.78, with a mean of 1.73, while values for stations in the TAM lying above the LVZ range from 1.67 to 1.78, with a mean of 1.72. This result indicates that there is little difference in bulk crustal composition for areas above and away from the LVZ, and together with a Vp/Vs value (1.73) that is typical for felsic to intermediate composition crust, suggests that the crust along the TAM front has not been altered significantly by mafic magmatism. Crustal thickness estimates along the coast are quite variable, ranging from 18 to 33 km, and increase to 39 km inland beneath the crest of the TAM. On Ross Island, crustal thickness estimates range between 19 and 27 km. In this study, we investigate crustal structure along the Transantarctic Mountain (TAM) front in the vicinity of Ross Island, Antarctica, to determine the extent to which the crust has been modified by the Neogene volcanism associated with Ross Island and the Terror Rift. The TAM represent Earth’s largest non-collisional mountain

Cenozoic range-front faulting and development of the Transantarctic Mountains near Cape Surprise, Antarctica: Thermochronologic and geomorphologic constraints

[1] The Transantarctic Mountains (TAM) define the western flank of the West Antarctic rift system. Cape Surprise, near the Shackleton Glacier in the central TAM, is the only location along the range's >3500 km length where upper Paleozoic Beacon Supergroup strata are down-faulted to near sea level. Previous studies have inferred a range front master normal fault accommodates extension and rock uplift across the TAM front in this region. The history of rock uplift is debated, suggested as early as Mesozoic, typically Cenozoic, or even Pliocene or younger. Structural observations, apatite fission track (AFT) thermochronology, and geomorphologic mapping undertaken within the TAM front east of the Shackleton Glacier indicate extension, faulting, and denudation was mostly late Eocene–late Oligocene and likely into the early Miocene. An exhumed AFT partial annealing zone is found at the coast and traced >50 km inland. The base of that exhumed partial annealing zone indicates denudation accelerated at ∼40 Ma near the coast and at 30–35 Ma on the inland side of the TAM front. Vertically offset AFT isochrones across the TAM front reveal a step-faulted architecture rather than a single master fault. The cumulative vertical offset of the 55 Ma isochrone is 2.3–2.7 km, compared to 2.4–2.6 km offset of Beacon strata, indicating that all significant normal faulting is Cenozoic, and not related to Mesozoic extension within the West Antarctic rift system. Denudation from <26 Ma to ∼14 Ma produced a locally preserved erosion surface within the TAM front. Erosion surface remnants indicate the TAM front has undergone minimal internal deformation and only 290–790 m of surface uplift along offshore faults since the middle Miocene.

Seismic facies and stratigraphy of the Cenozoic succession in McMurdo Sound, Antarctica: Implications for tectonic, climatic and glacial history

Integration of data from fully cored stratigraphic holes with an extensive grid of seismic reflection lines in McMurdo Sound, Antarctica, has allowed the formulation of a new model for the evolution of the Cenozoic Victoria Land Basin of the West Antarctic Rift. The Early Rift phase (Eocene to Early Oligocene) is recorded by wedges of strata confined by early extensional faults, and which contain seismic facies consistent with drainage via coarse-grained fans and deltas into discrete, actively subsiding grabens and half-grabens. The Main Rift phase (Early Oligocene to Early Miocene) is represented by a lens of strata that thickens symmetrically from the basin margins into a central depocenter, and in which stratal events pass continuously over the top of the Early Rift extensional topography. Internal seismic facies and lithofacies indicate a more organized, cyclical shallow marine succession, influenced increasingly upward by cycles of glacial advance and retreat into the basin. The Passive Thermal Subsidence phase (Early Middle Miocene) is recorded by an evenly distributed sheet of strata that thickens somewhat into the depocentre but is continuous across and over the earlier rift strata to the margins of the basin. Internally, it contains similar facies to the underlying Main Rift, but preserves more evidence for clinoform sets and large channels, and in core comprises many short, condensed and strongly top-truncated stratal cycles with continued, periodic glacial influence. These patterns are interpreted to record accumulation under similar environmental conditions but in a regime of slower subsidence. The Renewed Rifting phase (Middle Miocene to Recent, largely unsampled by coring thus far) is represented by intervals that thicken significantly into the basin depocentre and that are complicated by evidence of magmatic activity (McMurdo Volcanic Group). This succession is further divided into lower and upper intervals, separated by a major unconformity that displays increasing angular discordance towards the western basin margin and Transantarctic Mountain Front. The youngest part of the stratigraphy was accumulated under the influence of flexural loading imposed by the construction of large volcanic edifices, and was formed in an environment in which little sediment was supplied from the western basin margin, suggesting a change in environmental (glacial) conditions at possibly c. 2 Ma. The Cenozoic stratigraphy of the southern Victoria Land Basin preserves archives of both climate change and the complex rift history of the basin, and coincidences between key stratal surfaces in seismic data and evidence for environmental change in drillcores suggest that tectonic and climatic drivers may be causally linked.

The geological evolution of southern McMurdo Sound - new evidence from a high-resolution aeromagnetic survey

Magnetic anomaly data are presented from a new helicopter-borne high-resolution aeromagnetic survey in southern McMurdo Sound. Anomaly data have been acquired at a common 305 m elevation above the McMurdo and southern McMurdo ice shelves and draped over the volcanic islands that pin them. The resulting anomaly patterns provide a significant advance in the understanding of the rift related geology beneath the floating ice shelves. More extensive Erebus Volcanic Province (McMurdo Volcanic Group) rocks are indicated along with a significant blanket of glaci-volcaniclastic sediment on the seafloor between the volcanic islands in southern McMurdo Sound. These glaci-volcaniclastic sediments are inferred to originate from former grounding of the southern McMurdo Ice Shelf as a marine ice sheet. A strong N-S fabric is also observed in the anomaly data suggesting that the rift structure observed in the Victoria Land basin persists to the south beneath the McMurdo and southern McMurdo ice shelves. W-N-W transfer faults identified within the Transantarctic Mountain rift flank to the west are not obvious in the aeromagnetic data set, implying that the 'Discovery Accommodation Zone' may be restricted to the region between a southward extension of the range bounding fault that marks the limit of the Victoria Land Basin and the right lateral offset in the Transantarctic Mountain front in southern Victoria Land.

Cenozoic tectonics of the Cape Roberts Rift Basin and Transantarctic Mountains Front, Southwestern Ross Sea, Antarctica

We conducted a multichannel seismic reflection survey offshore Cape Roberts, Antarctica, and combined our findings with the results of the Cape Roberts International Drilling Project (CRP). This allows us to interpret Cenozoic tectonics in the southwest sector of the Ross Sea including the history of uplift of the Transantarctic Mountains (TAM) and subsidence of the Victoria Land Basin (VLB). Seismic stratigraphic sequences mapped offshore Cape Roberts arc tilted eastward and thicken into the VLB where they comprise more than half the fill seen on seismic records. Normal faults a few kilometers offshore cut these sequences and define a north trending rift graben. Drilling results from the CRP show that these strata are latest Eocene (?), Oligocene, and younger in age; much younger than previously inferred. We interpret this pattern to be due to an episode of E‐W extension and related subsidence that occurred across the major basins in the western Ross Sea during the early Cenozoic. The rift graben offshore and adjacent to Cape Roberts is bounded on the west by a major north trending fault zone. At Cape Roberts this fault system may have from 6 to 9 km of vertical separation. This fault system is part of a larger zone along the coastline in southern Victoria Land that accommodated uplift of the TAM in Oligocene time. We name it here the McMurdo Sound Fault Zone. A late Oligocene angular unconformity that is seen in seismic data and sampled by CRP drilling marks the end of east tilting of the stratigraphic sequences. We interpret this as the end of the main uplift of the TAM coinciding with a change from E‐W extension to NW‐SE oblique rifting at that time. Uplift of the TAM and subsidence in the VLB may be linked with seafloor spreading on the Adare Trough to the northwest of the Ross Sea between 43 and 26 Ma. This would imply a plate boundary between East and West Antarctica crossing through the western Ross Sea in Eocene and Oligocene time.

The geologic basis for a reconstruction of a grounded ice sheet in mcmurdo sound, antarctica, at the last glacial maximum

A grounded ice sheet fed from the Ross Embayment filled McMurdo Sound at the last glacial maximum (LGM). This sheet deposited the little‐weathered Ross Sea drift sheet, with far‐traveled Transantarctic Mountains (TAM) erratics, on lower slopes of volcanic islands and peninsulas in the Sound, as well as on coastal forelands along the TAM front. The mapped upper limit of this drift, commonly marked by a distinctive moraine ridge, shows that the ice‐sheet surface sloped landward across McMurdo Sound from 710 m elevation at Cape Crozier to 250 m in the eastern foothills of the Royal Society Range. Ice from the Ross Embayment flowed westward into the sound from both north and south of Ross Island. The northern flowlines were dominant, deflecting the southern flowlines toward the foothills of the southern Royal Society Range. Ice of the northern flowlines distributed distinctive kenyte erratics, derived from western Ross Island, in Ross Sea drift along the TAM front between Taylor and Miers Valleys. Lobes from grounded ice in McMurdo Sound blocked the mouths of TAM ice‐free valleys, damming extensive proglacial lakes. A floating ice cover on each lake formed a conveyor that transported glacial debris from the grounded ice lobes deep into the valleys to deposit a unique glaciolacustrine facies of Ross Sea drift. The ice sheet in McMurdo Sound became grounded after 26,860 14C yr bp. It remained near its LGM position between 23,800 14C yr bp and 12,700 14C yr bp. Recession was then slow until sometime after 10,794 14C yr bp. Grounded ice lingered in New Harbor in the mouth of Taylor Valley until 8340 14C yr bp. The southward‐retreating ice‐sheet grounding line had penetrated deep into McMurdo Sound by 6500 14C yr bp. The existence of a thick ice sheet in McMurdo Sound is strong evidence for widespread grounding across the Ross Embayment at the LGM. Otherwise, the ice‐sheet surface would not have sloped landward, nor could TAM erratics have been glacially transported westward into McMurdo Sound from farther offshore in the Ross Embayment.

Cenozoic structural segmentation of the Transantarctic Mountains rift flank in southern Victoria Land

The Transantarctic Mountains (TAM) rift flank in southern Victoria Land is offset across the transverse Discovery accommodation zone. The Discovery accommodation zone is characterized by two discrete structural blocks bounded by west–northwest-trending lineaments interpreted to mark transfer faults. North–northeast-trending lineaments are interpreted to mark the main longitudinal normal fault zone of the TAM front, which steps progressively eastward from south to north across the transfer faults. Sinistral-shear displacement along the transfer faults accommodates changes in the magnitude of extensional transport in the offset rift-flank structural blocks. Correlation with offshore structures indicates that the Discovery accommodation zone is a regional feature of the Ross Embayment rift system as well as the TAM rift flank. Neogene volcanism in the Erebus Volcanic Province was focused regionally within the Discovery accommodation zone and was localized along transverse structures crossing the zone. Transverse structures of the accommodation zone channeled glacial drainage, and possibly older fluvial drainage, across the rift flank. There appears to have been a unique interplay between developing structural, volcanic, sedimentary and glacial features during the Cenozoic evolution of the TAM rift flank within the Discovery accommodation zone.

Cretaceous and Cenozoic episodic denudation of the Transantarctic Mountains, Antarctica: New constraints from apatite fission track thermochronology in the Scott Glacier region

Apatite fission track thermochronology utilizing vertical sampling profiles, with results interpreted using the concept of exhumed partial annealing zones, is applied in the Scott Glacier area (86°S) of the Transantarctic Mountains (TAM). Patterns in age profiles indicate that episodes of denudation in the Early Cretaceous, Late Cretaceous, and Cenozoic were separated by periods of relative tectonic stability. Thermal modeling of time‐temperature histories compared to observed data indicates that denudation episodes commenced at ∼125 Ma, ∼95 Ma, and 50–45 Ma. Magnitude of denudation is constrained only as &gt;700 m for the Early Cretaceous and from barely detectable to 1.5 km for the Late Cretaceous. Since the early Cenozoic, denudation within the TAM Front was similar in magnitude to other localities along the TAM (∼4–6 km), decreasing inland. Rock uplift was also a maximum at the coast, decreasing inland. Patterns of rock uplift and denudation are complicated by Cenozoic faulting, mostly by faults oriented ∼45° to the TAM Front. Along the length of the TAM there is an apparent systematic variation in the angle of these Cenozoic faults to the TAM Front, possibly reflecting greater components of dextral transtension southward along the TAM. The three denudation episodes correspond to regional tectonic events: Early Cretaceous southward translation of the Ellsworth‐Whitmore Mountains block of West Antarctica relative to East Antarctica; Late Cretaceous extension in the Ross Embayment between East and West Antarctica; and Cenozoic rejuvenated faulting, magmatism, and deformation within the Victoria Land Basin and its presumed southward extension under the Ross Ice Sheet.

A marine and terrestrial Sirius Group succession, middle Beardmore Glacier-Queen Alexandra Range, Transantarctic Mountains, Antarctica

A succession of Sirius Group glacigene sediments which crop out along the western margins of the Beardmore valley between Cherry Icefall and Hewson Glacier, below The Cloudmaker, is designated as the stratotype of the Cloudmaker Formation. This new formation overlies a multiply glaciated pavement (Dominion Erosion Surface) cut into the Precambrian Goldie Formation, and is disconformably overlain by the Meyer Desert Formation (Sirius Group). The Cloudmaker Formation comprises bedded and massive diamictons, bedded sands and silts, and laminated clays. Assemblages of foraminifera occur throughout the Cloudmaker Formation and indicate that these basal Sirius Group sediments were deposited in brackish glacial marine environments. The general absence of diatoms suggest these marine waters were ice-covered. Similar marine assemblages are also present in basal Sirius Group sediments at Oliver Bluffs, Dominion Range. Recycled marine diatom assemblages in the Sirius Group at the latter locality indicate that the host sediments have an age of < 3.8 Ma (Pliocene). The Cloudmaker Formation is placed in the Pliocene, although a latest Miocene age for the basal sediments cannot be ruled out. Stratigraphie, sedimentologic, and paleontologic evidence suggests that Beardmore valley was occupied by a fjord and tidewater glacier system that extended at least 165 km through the Transantarctic Mountains from the southwestern Ross Sea. The stratigraphy of The Cloudmaker Formation consists of a succession of members separated by disconformities. It is hypothesised that these strata were deposited by a dynamic valley glacier system that underwent a history of glacier advance and grounding alternating with glacier retreat and flotation over a marine water column. A combination of fjord basin sediment filling and sea-level oscillations may also have influenced the pattern of glacier ice advance and retreat within Beardmore Paleofjord. The marine Cloudmaker Formation is overlain by the terrestrial diamicton dominated Meyer Desert Formation. At Oliver Bluffs, the Meyer Desert Formation diamictons are interbedded with fluvial, and lacustrine sediments; successions that contain in situ vascular plant fossils (principally the Southern Beech Nothofagus), mosses, and beetle remains. A Magellanic-type flora and fauna occupied the coastal margins of the Beardmore Paleofjord. The vertical transition from the basal marine Cloudmaker Formation to terrestrial Meyer Desert Formation provides a sea-level datum that can be used to assess the extent of post-Sirius Group tectonic uplift. Uplift rates at the Cloudmaker section, 90 km inland from the Transantarctic Mountain front or rift shoulder margin in the Queen Alexandra Mountain block are determined to be ~ 429 or ~ 350 m/Myr. This assumes a total uplift of 1331 m for the uppermost marine sediments of the Cloudmaker Formation, and maximum diatom-based ages for the Sirius Group of < 3.1 Ma or < 3.8 Ma. Gross similarities in stratigraphy and interpreted paleoenvironments are apparent between The Cloudmaker succession (Beardmore Paleofjord) and the upper Miocene-Pliocene successions at the mouth of Taylor Paleofjord, 800 km to the north. Contrasting present day elevational settings for these two widely separated marine successions indicates the post-Sirius rate of tectonic uplift for the Transantarctic Mountains has been significantly greater in the Queen Alexandra block.

Late Cenozoic Antarctic paleoclimate reconstructed from volcanic ashes in the Dry Valleys region of southern Victoria Land

Research Article| February 01, 1996 Late Cenozoic Antarctic paleoclimate reconstructed from volcanic ashes in the Dry Valleys region of southern Victoria Land David R. Marchant; David R. Marchant 1Institute for Quaternary Studies, University of Maine, Orono, Maine 04469 Search for other works by this author on: GSW Google Scholar George H. Denton; George H. Denton 2Department of Geological Sciences and Institute for Quaternary Studies, University of Maine, Orono, Maine 04469 Search for other works by this author on: GSW Google Scholar Carl C. Swisher, III; Carl C. Swisher, III 3Berkeley Geochronology Center, Berkeley, California 94709 Search for other works by this author on: GSW Google Scholar Noel Potter, Jr Noel Potter, Jr 4Department of Geology, Dickinson College, Carlisle, Pennsylvania 17013 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1996) 108 (2): 181–194. https://doi.org/10.1130/0016-7606(1996)108<0181:LCAPRF>2.3.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation David R. Marchant, George H. Denton, Carl C. Swisher, Noel Potter; Late Cenozoic Antarctic paleoclimate reconstructed from volcanic ashes in the Dry Valleys region of southern Victoria Land. GSA Bulletin 1996;; 108 (2): 181–194. doi: https://doi.org/10.1130/0016-7606(1996)108<0181:LCAPRF>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract We report the discovery of numerous in situ Miocene and Pliocene airfall volcanic ashes that occur within the hyperarid Dry Valleys region of the Transantarctic Mountains in southern Victoria Land, Antarctica. Ashes that occur above 1000 m elevation rest at the ground surface, covered only by a thin ventifact pavement 1 to 2 cm thick. The ash deposits are loose and unconsolidated and show no signs of chemical weathering. Laser-fusion 40Ar/39Ar analyses of volcanic crystals and glass shards indicate that the ashes range from 4.33 Ma to 15.15 Ma in age. The Arena Valley ash (4.33 ± 0.07 Ma) rests on the surface of a well-developed desert pavement and ultraxerous soil profile at 1410 m elevation. Lack of geomorphic evidence of liquid water on surficial sediments coeval and older than the Arena Valley ash, together with the pristine condition of volcanic crystals and lack of authigenic clay formation, indicates a cold desert at and since 4.33 Ma. The Beacon Valley ash (10.66 ± 0.29 Ma), the Koenig Valley ash (13.65 ± 0.06 Ma), and the Nibelungen Valley ash (15.15 ± 0.02 Ma) fill the upper half of relict sand-wedge troughs that form only in cold-desert conditions. The lack of authigenic clay-sized minerals in these ash deposits, along with preservation of sharp lateral contacts with surrounding sand-and-gravel deposits, suggests that frozen conditions (without rain or well-developed active layers during summer months) have persisted in Beacon, Koenig, and Nibelungen Valleys since ash deposition. Ash-avalanche deposits that rest on rectilinear slopes contain matrix ash dated to 7.42 ± 0.31 Ma in upper Arena Valley and 11.28 ± 0.05 Ma in lower Arena Valley. Little slope development has occurred since emplacement of these ash-avalanche deposits. Such slope stability is consistent with cold-desert conditions well below 0 °C. Taken together, these ash deposits point to persistent polar conditions similar to the present at elevations above 1000 m in the western Dry Valleys region during at least the last 15.0 m.y. This conclusion contradicts the view that, during part of the Pliocene epoch, East Antarctica was largely free of glacier ice and that scrub vegetation (Nothofagus, Southern Beech) survived along the Transantarctic Mountain front in the Dry Valleys region and to at least lat 86°S (Webb and Harwood, 1993). Instead, it supports marine and geomorphological evidence that calls for a stable Antarctic cryosphere, much the same as today, since middle Miocene time. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The Transantarctic Mountains of southern Victoria Land: The application of apatite fission track analysis to a rift shoulder uplift

A fission track study of the Transantarctic Mountains (TAM) in the Granite Harbour and Wilson Piedmont Glacier areas of southern Victoria Land reveals information on the timing of uplift, the amount of uplift and erosion, and the structure of the mountains, especially the onshore Transantarctic Mountain Front (TAM Front), which represents the boundary between East and West Antarctica. Apatite ages are &lt; 175 Ma and represent a thermal regime established after heating accompanying Jurassic magmatism. An apatite age profile from Mount England records a break in slope indicating uplift began at ∼55 Ma. Horizontal sampling traverses, plus fieldwork, delineate the structure of the TAM Front as a zone of north‐south striking, steeply dipping normal faults, with displacements, dominantly down to the east, of 40–1000 m. The overall structure of the mountains in the area studied can be envisaged as a large tilt block or flexure. Its westerly limb dips gently under the ice cap, compared to its faulted eastern edge, the TAM Front. The bounding structure to the south is the Ferrar fault and to the north is a graben through which the Mackay Glacier drains the polar plateau. The edge of the flexure, or axis of maximum uplift, lies at Mount Termination, ∼30 km west of the McMurdo Sound coast. There has been ∼6 km of uplift since the early Cenozoic and 4.5–5 km of erosion along this axis. The amount of uplift decreases to the west at the same rate as the decrease in dip of the Kukri Peneplain, but the amount of erosion decreases more quickly as indicated by the increasing height of the mountains to the west. The axis of maximum uplift is traced north to Granite Harbour. The axis does not parallel the coast but has a more northerly trend. North‐south striking longitudinal faults that delineate the structure of the TAM Front lie at an acute angle to the axis, indicating a dextral component to the dominantly east‐west extension in the Ross Embayment. Architecture of the TAM typifies the features of an upper plate passive mountain range, whereas the Ross Embayment has the characteristics of a lower plate. The TAM Front represents an upper plate breakaway zone. Transfer faults may exist up major outlet glaciers that cut the TAM. The inflection point in the coastline at the southern end of McMurdo Sound may be due to the presence of a major transfer fault up or near the Skelton Glacier.

Flexural uplift of the Transantarctic Mountains

The Transantarctic Mountains have formed at the continent‐continent boundary between East and West Antarctica. High heat flow, thin crust, normal faulting, and past and present volcanism indicate that this approximately 3000‐km‐long boundary is divergent in character. Three principal structures have developed at and adjacent to the boundary: the Transantarctic Mountains, the Wilkes Basin, and the Victoria Land Basin. The Transantarctic Mountains form the east edge of East Antarctica and consist of a block‐tilted mountain range up to 4500 m high. Running parallel but 400–500 km behind, or to the west of, the Transantarctic Mountains is the Wilkes Basin. This is a broad subglacial basin where the bedrock surface is now as much as 1 km below sea level. East of and immediately adjacent to the Transantarctic Mountain front is an area of extension called the Victoria Land Basin where at least 4–5 km of Cenozoic sediments have been interpreted from seismic reflection data. The wavelengths and amplitudes of these three structures can be accounted for by the elastic flexure of two cantilevered lithospheric plates if the boundary between East and West Antarctica is taken as a stress‐free edge. Specifically, the Wilkes Basin is modeled as a flexural “outer low” coupled to uplift of the Transantarctic Mountains. Similarly, subsidence within the Victoria Land Basin is also linked to uplift of the Transantarctic Mountains via the Vening Meinesz uplift‐subsidence mechanism and sediment loading. The maximum flexural rigidity for East Antarctica is estimated to be about 1025 N m (or effective elastic thickness, Te, of 115±10 km), one of the highest values for continental rigidity from long‐term loads. Flexural rigidity for the Ross Embayment in West Antarctica is, on the other hand, found to be more than 2 orders of magnitude less at 4×1022 N m (Te = 19 ± 5 km). This rigidity variation suggests a marked contrast in effective thermal age, and hence geotherms, between East Antarctica and the western Ross Embayment. Accordingly, one of the principal uplift mechanisms for the Transantarctic Mountains is considered to be a thermal uplift associated with lateral heat conduction from the extended and thinned West Antarctic lithosphere into the thicker lithosphere of East Antarctica. Augmenting thermal uplift of the Transantarctic Mountains are the effects of erosion and the Vening Meinesz uplift effect.

Metasomatised Xenoliths from Foster Crater, Antarctica: Implications for Lithospheric Structure and Processes beneath the Transantarctic Mountain Front

Xenoliths in basanite from Foster Crater in the foothills of the Transantarctic Mountains have been used to construct a model of the subcontinental lithosphere and lithospheric processes in this segment of Antarctica. The mafic and ultramafic xenoliths define two mineralogically, texturally and chemically distinct populations. One group is identical to Group II xenoliths, as defined by Frey & Prinz (1978). These have igneous cumulate textures and include wehrlites, rare websterites, dunite and amphibolite. In these xenoliths, the amphibole is always kaersutite and the (very rare) mica a Ti-rich phlogopite (TiO2 > 5%). The other xenoliths have complex metamorphic textures ranging from protogranular to porphyroclastic and are dominated by clinopyroxenites (cpx: Mg/(Mg + Fe) = 0·85–0·95; Ca/(Ca + Mg)>0·53) which show varying degrees of replacement by metasomatic phlogopite. Spinel and anorthite are also present. Collectively, these xenoliths are called the High Calcium Pyroxene Suite (HCPS) in deference to their unusual pyroxene compositions. Moreover, their spinel assemblages indicate equilibrium under variable fO2 conditions contrasting with the Group II spinels which equilibrated at relatively constant fO2. The mineralogy and chemistry of the HCPS xenoliths are unlike meta-igneous rocks and more closely resemble calc-silicate granulites. Thermobarometric calculations on Group II websterite assemblages and comparison with experimental equilibria on granulites and phlogopite stability suggest equilibration temperatures between 850 and 950° at 5 kb pressure, consistent with equilibration at mid to lower crustal depths. A number of processes have acted to modify Group II and HCPS xenoliths including K-metasomatism, dynamic recrystallization, melt generation, melt infiltration and oxidation. The first two are restricted to the HCPS xenoliths whereas the latter affect both groups. The interaction of these processes highlights the differences between Group II and HCPS protolith assemblages and is consistent with derivation from a complex section of lithosphere which has experienced major changes in tectonomagmatic activity since late PreCambrian times.