EROSIONAL HISTORY OF EAST ANTARCTICA FROM DOUBLE AND TRIPLE-DATING OF SINGLE GRAINS IN GLACIALLY- DERIVED SEDIMENTS FROM PRYDZ BAY
by
Clare J. Tochilin
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{J Dr. Stuart N. Thomson Lf{., I 1 s/:r/1z.. . (signature) (dat/i 1 Erosional history of East Antarctica from double and triple-dating of single grains in glacially-derived sediments from Prydz Bay
Clare J. Tochilin1, Peter W. Reiners1, Stuart N. Thomson1, George E. Gehrels1, Sidney R. Hemming1 1Department of Geosciences, University of Arizona, Tucson, AZ 2Lamont-Doherty Earth Observatory and Department of Earth and Environmental Sciences, Columbia University, Palisades, NY
Abstract
Since its onset in the early Oligocene, the East Antarctic Ice Sheet has covered up to 98% of the surface of East Antarctica. Because bedrock exposure is limited, little is known about its tectonic and erosional history. To better constrain the subglacial erosional record, we analyzed
Oligocene through Quaternary sediments from Prydz Bay, as these sediments contain geo- and thermochronologic information about the bedrock from which they are derived. A multi-dating approach was applied in the analysis of these sediments, involving U-Pb, fission track, and (U-
Th)/He dating of single apatite and zircon grains to measure a range of crystallization and cooling ages. Apatite and zircon U-Pb dates of grains spanning the entire sampled stratigraphic section are characterized by a dominant ~500 Ma age peak, recording widespread metamorphism and magmatism related to Pan-African orogenesis at this time. Zircon fission track dates record two periods of cooling at ~250-300 Ma and ~120 Ma due to rifting in the Lambert Graben during the Permian-Triassic and magmatic resetting during the Cretaceous. Mean apatite fission track dates show a decrease from ~280 Ma in early Oligocene samples to ~180 Ma in late Miocene samples, but show little change from ~180 Ma to ~150 Ma in Pliocene through Quaternary samples. These ages demonstrate a rapid decrease in lag time from ~250-180 My throughout the early Oligocene, indicating increased erosion rates during this time. The youngest measured apatite He ages also decrease from ~100 Ma in the Oligocene sediments to ~20 Ma in the
Miocene sediments. Taken together, these results indicate a significant increase in erosion rates
! 1! in the catchments draining to Prydz Bay between the early Oligocene and late Miocene, with relatively steady rates of erosion since that time. This erosion was primarily achieved by glacial incision into preexisting river valleys, based on glacial erosional modeling and observed valley shapes. Glaciers must have incised to depths of ~2.8-3.0 km by the late Miocene. This is consistent with models of the EAIS that show that it transitioned to less erosive cold-based conditions following the middle Miocene climatic optimum.
1. Introduction
The tectonic and erosional history of East Antarctica is poorly understood due to coverage of about 98% of the bedrock by the East Antarctic Ice Sheet (EAIS) (Barron et al.,
1991; Webb, 1990). The EAIS is the largest and longest-lived ice sheet on Earth and emcompasses ~90% of the ice in Antarctica (Bamber et al., 2000). One nucleation point for the onset of glaciation in East Antarctica in the early Oligocene is though to be the subglacial
Gamburtsev Mountains in the interior of the continent, which reach elevations of up to 4 km above sea level, but are entirely covered by ice (Bo et al., 2009; Cox et al., 2007; Cox et al.,
2010; DeConto and Pollard, 2003a, 2003b; Siegert, 2008; Siegert et al., 2008; Taylor et al., 2004; van de Flierdt et al., 2008). Rapid onset of the EAIS began at ~34 Ma as a result of global cooling related to declining atmospheric CO2 levels and the development of the Antarctic
Circumpolar Current and consequent thermal isolation of Antarctica. Glaciation remained dynamic and warm-based until ~14 Ma, after which time it underwent a shift to more stable, cold-based, and potentially less erosive glaciation as a result of further cooling (Bo et al., 2009;
DeConto and Pollard, 2003a, 2003b; Jamieson and Sugden, 2008; Jamieson et al., 2010; Naish et al., 2001; Sugden, 1996; Sugden et al., 1993; Young et al., 2011; Zachos et al., 1992).
! 2! Approximately 10-20% of the EAIS is drained by the largest ice stream in Antarctica, the
Lambert Glacier, which flows through the north-south trending Lambert Graben and into Prydz
Bay on the northeastern margin of the continent (Fig. 1) (Bamber et al., 2000; Barrett et al.,
1996; Cox et al., 2010, 2007; DeConto and Pollard, 2003a; Hambrey and McKelvey, 2000;
Jamieson et al., 2005). The Lambert Graben was formed primarily by Permo-Triassic rifting
(Cox et al., 2010; Kurinin and Grikurov, 1982; Lisker, 2002; Lisker et al., 2007b) and extends
~500 km to the south of Prydz Bay into the interior of the continent (O’Brien et al., 2007).
Small-scale mafic magmatism associated with the break-up of Gondwana occurred in the
Lambert Graben/Prydz Bay area during the early to mid Cretaceous (Arne, 1994; Collerson and
Sheraton, 1986; Hambrey and McKelvey, 2000; Jamieson et al., 2005; Kurinin and Grikurov,
1982; Lisker et al., 2007a, 2007b, 2003).
In addition to hosting an ice stream currently draining a large portion of the EAIS, the
Lambert Graben is proposed to have been an important drainage route for fluvial systems prior to the onset of glaciation in East Antarctica. Jamieson and Sugden (2008) modeled the preglacial fluvial drainage patterns of this area (Fig. 1), showing that the Lambert Graben served as the primary path for rivers carrying sediment out of the Gamburtsev Mountains. This drainage system was also fed by sediments from the Prince Charles Mountains and Grove Mountains along the flanks of the Lambert Graben near Prydz Bay (Cox et al., 2010; Jamieson et al., 2005;
O’Brien et al., 2007). The current ice streams draining this part of East Antarctica likely follow these preexisting pre-glacial drainages within the Lambert Graben, Gamburtsev Mountains, and surrounding areas, ultimately delivering sediment from the East Antarctic interior to Prydz Bay.
To address the erosional history beneath the EAIS, we applied a multi-dating approach to sediments recovered from Prydz Bay by the Ocean Drilling Project (ODP). These sediments are
! 3! ultimately derived from the inaccessible subglacial bedrock and can provide geo- and thermochronologic records of their source areas as well as valuable information about the phases of East Antarctic glaciation since its onset at 34 Ma (Barron et al., 2001; Whitehead et al., 2006).
The three dating methods applied to detrital minerals in this study are U-Pb (apatite and zircon),
Fission Track (apatite and zircon), and (U-Th)/He (apatite; due to the limited number of zircon grains in our samples, zircon (U-Th)/He analysis was not conducted). This combination of multiple high and low temperature chronometers is extremely powerful, as it allows for the determination of both crystallization ages as well as a range of cooling ages on single grains, providing a more complete provenance and cooling history than any one method alone. We use this multi-dating method here to better constrain the subglacial exhumational history of East
Antarctica since the onset of the EAIS.
2. Geologic Setting
The East Antarctic Shield is composed primarily of an amalgamation of Archean-early
Cambrian crustal fragments, deformed and intruded during multiple episodes of tectonism
(Dalziel, 1992; Fitzsimons, 1997, 2000a, 2000b). The presence of Grenville-age and Pan-
African-age metamorphic belts and intrusive rocks indicates the continent’s integral involvement in the assembly of supercontinents Rodinia and Gondwana, respectively (Boger et al., 2002;
Fitzsimons, 2003, 2000b; Veevers, 2003; Veevers et al., 2008). In some areas within the Lambert
Graben, Precambrian bedrock is overlain by mostly undeformed Devonian-Pliocene sediments and sedimentary rocks (Fitzsimons, 2000a; McKelvey et al., 2001; O’Brien et al., 2006; Veevers and Saeed, 2007; Whitehead et al., 2006).
! 4! Several studies have measured U-Pb zircon dates of grains from the limited ice-free bedrock exposures in East Antarctica, including samples from the Prydz Bay coast, the margins of the Lambert Graben, the Prince Charles Mountains, and the Grove Mountains, in order to characterize portions of the exposed material in this region by age and extrapolate these ages to what may be beneath the ice (Fig. 2). These studies showed a wide range of crustal formation ages in East Antarctica ranging from ~3.5 Ga-0.5 Ga, with dominant peaks at 0.4-0.6 Ga, 0.9-1.2
Ga, 1.6 Ga, 2.2 Ga, 2.6-2.7 Ga, 3.0-3.1 Ga, and 3.4-3.5 Ga. Peaks at 0.5-0.6 Ga and 0.9-1.2 Ga from syntectonic intrusive rocks as well as metamorphic rims on some crystals correlate well with large-scale Pan-African and Grenville orogenic events known to have affected the region
(Boger et al., 2001; Carson et al., 2000, 1996; Fitzsimons, 2000b; Fitzsimons et al., 1997;
Hemming et al., 2007; Liu et al., 2007; Mikhalsky et al., 2006; van de Flierdt et al., 2008;
Veevers and Saeed, 2007; Veevers et al., 2008; Williams et al., 2007).
Additionally, a number of studies have measured 40Ar/39Ar ages on hornblende and mica from diamictite in Prydz Bay. In contrast with the variation observed in the U-Pb ages, 40Ar/39Ar dates from this region of East Antarctica are almost exclusively ~500 Ma, providing further evidence for a widespread tectonothermal event affecting the area during Pan-African time, resulting in a strong metamorphic overprint. 40Ar/39Ar studies also show a minor peak of
Grenville age (1.1-1.3 Ma) (Hemming et al., 2007; Phillips et al., 2007; Roy et al., 2007).
Studies measuring apatite fission track and apatite and zircon (U-Th)/He dates of
Precambrian basement rocks, Paleozoic and Mesozoic sedimentary rocks, and sediment cores from the Prydz Bay region have also been conducted. Cox et al. (2010, 2007) analyzed grains in
Eocene fluvial sediment from cores in Prydz Bay, reporting apatite fission track dates of 207-612
Ma and (U-Th)/He dates of 97-377 Ma and 197-397 Ma for apatite and zircon, respectively.
! 5! They interpret these ages to reflect very slow erosion rates in the Gamburtsev Mountains and very little tectonic activity in the interior of the continent since Pan-African time. Other studies, however, have measured apatite fission track dates in Precambrian basement rocks and
Paleozoic-Mesozoic sedimentary rocks in the Prince Charles Mountains and the Lambert
Graben, showing age distributions primarily from ~100-300 Ma with an older late Paleozoic- early Mesozoic peak and a younger Cretaceous-Paleocene peak (Arne, 1994; Lisker et al., 2003,
2007a, 2007b). These data suggest two phases of cooling: an older peak associated with Permo-
Triassic rifting in the Lambert Graben area, and a younger peak that these authors propose to correspond to Cretaceous rifting and exhumation during the breakup of Gondwana. Lisker et al.
(2003, 2007a, 2007b) interpret their data to be consistent with 2-4 km of erosion associated with a Cretaceous rifting event, although independent geologic evidence for such an event is lacking.
In other areas of East Antarctica such as Dronning Maud Land (55°-15°E) and Wilkes
Land (155°-90°E) several of the same zircon U-Pb age peaks seen in the Prydz Bay/Lambert
Graben region are observed, especially the peaks of Grenville and Pan-African ages
(Fitzsimmons, 2000b; Roy et al., 2007). In Wilkes Land there is also a dominant zircon U-Pb age peak from 1.6-1.8 Ga not seen in the Prydz Bay catchment (Goodge, 2007). Hornblende
40Ar/39Ar studies show that all regions of East Antarctica are dominated by 500 Ma dates, with the exception of Wilkes Land, which shows primarily Mesoproterozoic and Paleoproterozoic dates. This implies that this area did not experience the Pan-African orogenic event as strongly as the rest of East Antarctica (Roy et al., 2007).
3. Samples and Stratigraphy
! 6! The samples analyzed in this study are early Oligocene-Quaternary sediments from ODP core 739C (67°16.57’S, 75°04.91’) recovered from Prydz Bay in January, 1988 (Shipboard
Scientific Party, 1999). The presence of a thick section of diamictites down to the early
Oligocene recovered in this core and other cores in the vicinity provides evidence for a fully developed EAIS by that time (Barron et al., 2001; Solheim et al., 1991). Volpi et al. (2009) reanalyzed the stratigraphy of core 739C (Fig. 3), slightly redefining the stratigraphic and age divisions previously determined by the Shipboard Scientific Party on the basis of new seismic profiles, which they integrate with existing lithologic, biostratigraphic, and magnetostratigraphic data. The stratigraphic ages in Figure 3 and any depositional ages of samples mentioned throughout this study are those reported by Volpi et al. (2009). Absolute ages were determined primarily through the dating of diatom fossils (Whitehead et al., 2006).
Core 739C consists of six main units of diamictites (Fig. 3). Unit A, the uppermost segment of the core, consists of soft Quaternary diatomaceous sand, clay, and poorly consolidated diamicton. Unit B, 24-80 meters below seafloor (mbsf), is a homogeneous, poorly sorted diamictite that is late Pliocene in age. Unit C (80-110 mbsf) is a thick, stratified diamictite, also late Pliocene in age. Unit D (110-140 mbsf) is a highly compacted diamictite of early Pliocene age. Unit E (140-174 mbsf) is a very highly compacted diamictite divided into two sections. E1 is diatomaceous and early Pliocene in age, while E2 is is late Miocene with few to no diatoms. Core 739C is the only core drilled in this area to contain Miocene sediments.
Below the Miocene unit is an unconformity, underlain by a very thick section of early Oligocene diamictites (174-486.8 mbsf) (Volpi et al., 2009).
We carried out geo/thermochronological analyes on twenty samples from this core, including one Quaternary sample (001R55: 0-9.5 mbsf), three late Pliocene samples (004R21:
! 7! 24.1-28.7 mbsf; 005R115: 28.7-38.3 mbsf; 013R10: 105.9-115.5 mbsf), three early Pliocene samples (014R29: 115.5-125.2 mbsf; 017R20: 135.0-140.0; 019R85: 144.7-149.7 mbsf), three late Miocene samples (021R70: 154.3-159.3 mbsf; 022R74: 159.3-164.0 mbsf; 023R110: 164.0-
169.0 mbsf), and ten early Oligocene samples (025R10: 173.6-183.2 mbsf; 030R91: 221.8-231.4 mbsf; 031R85: 231.4-241.4 mbsf; 034R80: 260.4-270.0 mbsf; 038R60: 298.9-308.6 mbsf;
041R110: 327.8-337.5 mbsf; 047R55: 385.9-395.6; 052R20: 434.2-439.2; 056R35: 453.5-458.5;
062R0: 482.6-486.8 mbsf).
This dataset is complemented by that of Thomson et al. (2011), in which apatite grains from Eocene fluvial sediments from ODP Leg 188, Hole 1166A and Oligocene-Quaternary diamictites from Holocene Jumbo Piston Core 34 (JPC34) were analyzed using the same triple- dating method (U-Pb, fission track, and (U-Th)/He dating) described in this study. However, core 739C provides a more complete syn-glacial stratigraphic section of Oligocene through
Quaternary sediments, as cores 1166A and JPC34 do not include any Miocene sediments or as extensive a section of early Oligocene sediments.
4. Methods
4.1. Fission Track Analysis
All samples were processed using standard mineral separation techniques for zircon and apatite (Gehrels, 2000). The zircon yield for all samples was very low, and many of the zircon grains were metamict, making them unsuitable for fission track and U-Pb analysis. Zircon and apatite grains from each of the twenty samples were mounted in teflon and epoxy, respectively, polished, and etched using the methods outlined in Hurford et al. (1991). The samples were analyzed by the external detector method (Tagami and O’Sullivan, 2005) and irradiated at the
! 8! Oregon State University Triga Reactor in Corvallis, OR, along with European Institute for
Reference Materials and Measurements uranium-dosed glasses IRMM 540R for apatite and
IRMM 541 for zircon to monitor neutron fluence. Spontaneous and induced tracks were counted using an Olympus BX51 microscope at 1250x magnification at the University of Arizona Fission
Track Laboratory. Central ages were calculated using the zeta calibration approach of Hurford and Green (1983). The zeta values used for zircon and apatite were 351.99 ± 8.94 and 113.21 ±
1.81, respectively. Analytical data for 1019 analyzed apatite grains and 218 analyzed zircon grains can be found in Appendix A, Tables A1 and A2.
4.2. U-Pb Analysis
Fission track mounts for zircon and apatite were attached to glass slides adjacent to standards mounted separately in epoxy or teflon for U-Pb analysis. Primary standards included a
Sri Lanka zircon (Gehrels et al., 2008), and Madagascar and McClure Mountain apatites
(Thomson et al., 2012). All grains dated by fission track analysis were located and analyzed at the Arizona LaserChron Center by laser ablation-multicollector-inductively coupled plasma mass spectrometry (LA-MC-ICPMS). Detailed procedures for zircon analysis are described by
Gehrels et al. (2008); procedures for apatite analysis are described by Thomson et al. (2012).
Three analyses were conducted in each apatite grain, depending on the size of the grain, to account for compositional variation. Concordia ages were later calculated to determine one age for each grain. In cases where very few zircons could be dated by fission track analysis, a small number of additional suitable zircon grains were analyzed to obtain a greater number of U-Pb analyses for those samples. The measured isotopic ratios for zircons are corrected for common
Pb using the measured 204Pb, assuming an initial Pb composition according to Stacey and
! 9! Kramers (1975). Measured isotopic ratios for apatites are also corrected for common Pb according to Stacey and Kramers (1975). However, due to the high common to radiogenic Pb ratio in most apatite, a five-step iterative process is also applied to correct for this (Thomson et al, 2012). Full analytical data for 982 apatite grains (multiple analyses per grain) and 262 zircon grains yielding data of acceptable precision (apatite: <10% 206Pb/238U age and <10% 206Pb/207Pb age; zircon: <15% 206Pb/238U age and <10% 206Pb/207Pb age) and discordance (<30% discordant) are reported in Appendix A, Tables A3 and A4.
4.3. (U-Th)/He Analysis
Individual apatite grains from five representative samples from different stratigraphic age divisions were analyzed by the (U-Th)/He method. Grains from one late Pliocene sample
(004R21), one early Pliocene sample (017R20), two late Miocene samples (022R74 and
023R110), and one early Oligocene sample (047R55) were analyzed, with a focus on the
Miocene samples due to their rarity in all other cores from this region. Double-dated grains were handpicked from the fission track mounts and measured lengthwise and widthwise for alpha ejection correction parameters. Each grain was then loaded into a Nb tube and analyzed using the methodology described in Reiners et al. (2004) at the Arizona Radiogenic Helium Dating
Laboratory. Because the grains had previously been polished for fission track and U-Pb analysis, a polished grain alpha ejection correction was applied to the raw age for each grain, assuming that 30 µm of the grain had been removed (Farley, 2002; Reiners et al., 2007). However, because all analyzed grains were anhedral and in many cases broken, it is unclear when the rims of the grains were removed and whether the alpha ejection correction should actually be applied. For this reason, a plot age was calculated for each grain by taking the average of the raw age and the
! 10! fully corrected age. The uncertainty assumed for each plot age is the difference between the plot age and the minimum (raw age minus analytical uncertainty) and maximum (fully corrected age plus analytical uncertainty) ages for that grain. All ages plotted in the apatite (U-Th)/He histograms are raw ages. Probability density function (PDF) curves (Ludwig, 2008) are shown for raw ages as well as plot ages. Analytical data for 103 apatite grains can be found in Appendix
A, Table A5.
5. Results
5.1. Zircon U-Pb
All samples display a dominant age peak between ~450-550 Ma, and a smaller peak from
~800-1100 Ma (Fig. 4). Miocene through Quaternary samples contain a handful of grains ranging from 1.5-3.2 Ga. The Oligocene samples contain far fewer zircons than the younger samples overall, but have a much higher abundance of older Proterozoic and Archean grains.
5.2. Zircon Fission Track
Like the zircon U-Pb ages, zircon fission track ages are very consistent throughout the stratigraphic section (Fig. 4). Each set of samples grouped by stratigraphic age contains two main age populations – an older population at ~250-300 Ma, and a small younger population at ~120
Ma.
5.3. Apatite U-Pb
Apatite U-Pb ages are also consistent throughout the entire stratigraphic section (Fig. 5).
Like with the zircon U-Pb data, all samples display a dominant peak at ~450-550 Ma. Less than
! 11! 10% of grains in Miocene through Quaternary samples are older than 600 Ma. However, ~35% of grains in the early Oligocene samples have ages ranging from 600-1300 Ma.
5.4. Apatite Fission Track and (U-Th)/He
Apatite fission track cooling ages display a clearly decreasing trend moving from the early Oligocene through the Quaternary (Fig. 6). There are no distinct age populations within the age spectra of any of the samples, but rather a central cooling age population whose probability decreases fairly symetrically to both older and younger ages. Throughout the 5 myr period of the early Oligocene represented in core 739C, there is a clear decrease in the mean cooling age of each sample. The oldest Oligocene sample (062R0) has a mean age of ~280 Ma, and the youngest Oligocene sample (025R10) has a mean age of ~210 Ma. Above the unconformity in the 10 myr period from the late Miocene to the Quaternary, there is only a slight shift in mean fission track age from ~180 Ma to ~150 Ma.
Apatite (U-Th)/He ages show a similar trend of decreasing low-temperature cooling ages upsection (Fig. 7). Although the sample sizes are limited for all stratigraphic ages with the exception of the late Miocene, early Oligocene minimum ages are clearly older than those in the late Miocene. There are no grains with cooling ages younger than ~100 Ma in the early
Oligocene samples, and a large number of grains with cooling ages as young as ~20 Ma in the
Miocene-Quaternary samples.
6. Discussion
6.1. Zircon U-Pb
! 12! The dominant peak at 500 Ma in samples of every stratigraphic age records the Pan-
African orogenic event that produced widespread magmatism and metamorphism throughout
East Antarctica, as noted in previous U-Pb geo/thermochronologic studies. Similarly, the peak centered around 900-1000 Ma records an orogenic event of Grenville age, although this event does not appear to have produced the same scale of magmatism as the Pan-Afican event, at least as recorded in detrital zircon and apatite grains (Boger et al., 2001; Carson et al., 2000, 1996;
Fitzsimons, 2000b; Fitzsimons et al., 1997; Hemming et al., 2007; Liu et al., 2007; Mikhalsky et al., 2006; van de Flierdt et al., 2008; Veevers and Saeed, 2007; Veevers et al., 2008; Williams et al., 2007). All ages seen in the zircon age spectra are within the range of ages found in the exposed bedrock outcrops in the vicinity of Prydz Bay and the Lambert Graben (Fig. 2) and most likely also found in and around the Gamburtsev Mountains further into the interior of the shield.
The greater proportion of older grains in the Oligocene sediments as well as the sudden decrease in zircons found in these sediments is indicative of a possible shift in source area between the early Oligocene to the late Miocene. Additionally, the lack of zircons younger than ~400 Ma in any of the samples suggests that the Gamburtsev Mountains, where a large portion of these sediments are most likely sourced from based on known subglacial drainage patterns into Prydz
Bay, supports recent studies concluding that they are most likely an old feature (Cox et al., 2010;
Van de Flierdt et al., 2008) and not a young volcanic mountain range, as has been previously hypothesized (Sleep, 2006).
6.2. Zircon Fission Track
All zircon fission track ages are from zircon grains with U-Pb ages that are Pan-African and older, therefore representing later cooling and/or resetting events. The ~250-300 Ma
! 13! population of fission track cooling ages most likely reflects cooling associated with Permo-
Triassic rifting in East Antarctica related to initial formation of the Lambert Graben (Arne,
1994). Normal faulting associated with this rifting would have exhumed the basement rock underlying the Lambert Graben, with subsequent erosion of uplifted material producing the
Amery Group sediments of that age that now overlie the basement rock within the graben. If exhumation by normal faulting is the mechanism responsible for the widespread ~250-300 Ma zircon fission track ages, then this rifting would require tectonic exhumation to depths of ~8-12 km to reach basement temperatures of 200-300 °C, assuming a geothermal gradient of 25 °C/km.
If the geothermal gradient during rifting was elevated to ~30 °C/km in the area directly surrounding the rift (Lisker et al., 2003), the basement rocks would only need to have been exhumed through normal faulting to depths of ~6-10 km to cool through the range of fission track closure temperatures. Some of this cooling is also likely related to erosion on uplifted rift flanks.
A number of authors have proposed a second period of rifting in the Lambert Graben area during the Cretaceous associated with the final breakup of Gondwana (Arne, 1994; Ferraccioli et al., 2011; Lisker et al., 2003, 2007a, 2007b) that Lisker et al., (2003) use to explain their apatite fission track data. However, the zircon fission track cooling ages of ~120 Ma form a single well- defined peak more consistent with local resetting related to alkaline mafic magmatism of this age in the Beaver Lake area of the Northern Prince Charles Mountains (Coffin et al, 2002). This magmatic activity is associated with widespread magmatism of this age related to initial surface manifestation of more widespread Kerguelen hotspot magmatism (Coffin et al., 2002). A regional cooling related to exhumation would be expected to yield partially reset zircon fission track ages between 250-300 and 120 Ma, which we do not observe.
! 14!
6.3. Apatite U-Pb
The ~500 Ma apatite U-Pb age peak is dominant in all samples. However, the early
Oligocene samples have a greater proportion of Precambrian grains. In nine out of ten late
Miocene through Quaternary samples, ≥85% of analyzed grains are between 440-560 Ma. In sample 023R110 (late Miocene), 75% of grains were within this range. In all ten early Oligocene samples, however, only ≥65% of grains are within this age range. A Kolmogorov-Smirnov test
(Press et al., 1986) comparing the 440-560 Ma age population in each sample with the same population in every other sample produced P-values greater than 0.05 in 178/190 possible sample pairings. This indicates that there is a 95% level of confidence that this age population in each sample is not statistically different from the same population in any other sample.
The peak at ~500 Ma likely includes both apatite grains that were reset during widespread metamorphism associated with the Pan-African orogeny as well as grains that crystallized during magmatic events at that time. However, whether the majority of grains of this age are recording crystallization ages or ages of metamorphism cannot be elucidated from the detrital grains alone. The presence of a small number of zircon and apatite U-Pb ages older than
~500 Ma indicates that there are older rocks in the source area, suggesting that some of the apatite grains with ~500 Ma U-Pb ages represent Pan-African metamorphic resetting in line with overprinting indicated by widespread monazite U-Th-Pb data of this age in the southern Prince
Charles Mountains (Phillips et al., 2009). However, the majority of the zircon U-Pb crystallization ages in these samples are ~500 Ma, recording a magmatic event during which many of the ~500 Ma apatite grains may have crystallized as well.
! 15! The ~500 Ma age of metamorphism and crystallization is consistent with that seen in
40Ar/39Ar hornblende ages from other sediments recovered from Prydz Bay (Hemming et al.,
2007; Phillips et al., 2007; Roy et al., 2007). The older grains seen mainly in the Oligocene samples are most likely grains sourced from areas of East Antarctica that did not experience a high enough degree of metamorphism at this time to be reheated to 450-550 °C, the range of temperature at which the apatite U-Pb system is reset, depending on factors such as cooling rate and grain radius (Cherniak et al., 1991; Thomson et al., 2012). Evidence that such areas of East
Antarctica exist can be seen in 40Ar/39Ar dates of hornblende from Wilkes Land, the only region of East Antarctica dominated by 40Ar/39Ar dates older than Pan-African age (see Section 2) (Roy et al., 2007).
6.4. Apatite Fission Track and (U-Th)/He
If erosion rates increase through time, the cooling ages measured in the detrital record of this exhumation will become younger through time. We interpret the upsection-younging trends in apatite fission track and (U-Th)/He cooling ages to reflect increasing erosion rates and possibly localized glacial incision over time. The relationship between changes in cooling age and depositional age can be addressed through the determination of lag time, or the difference between the time of exhumation of a grain through its closure temperature for a given thermochronometric system and its time of deposition (Brandon and Vance, 1992; Cerveny et al.,
1988). The rapid decrease in mean apatite fission track cooling ages during the early Oligocene corresponds with a substantial decrease in lag time from ~240 My to ~180 My over a period of only ~5 My (Figs. 7 and 8). This implies slow but increasing erosion rates in the source areas of these sediments during this time. In contrast, fission track ages for the late Miocene-Quaternary
! 16! samples show only a slight decrease from ~170 My to ~150 My over ~10 My, implying slow, near steady-state erosion throughout this time. Likewise, apatite (U-Th)/He cooling ages are all greater than 100 Ma in the early Oligocene sample. Beginning in the late Miocene, cooling ages younger than 100 Ma become abundant and remain relatively constant throughout the Pliocene and Quaternary, supporting the slow, steady erosion implied by fission track lag times.
Assuming that sediments were derived from the same general source areas during the early Oligocene and the Miocene-Quaternary and that the depth-age structure in this region is like that of a very slowly eroding region, the younger fission track and (U-Th)/He cooling ages and near-constant lag times in the late Miocene-Quaternary samples indicate that the majority of the erosion producing these younger cooling ages had already been achieved by the late
Miocene, with little additional exhumation taking place since that time. This, as well as the change in erosion rates between the early Oligocene and late Miocene determined from fission track cooling ages, is consistent with climatic evidence seen in the marine isotope record as well as evidence from ice sheet erosional modeling (Bo et al., 2009) pointing to a transition from an initial phase of unstable, warm-based glaciation from 34-14 Ma to a phase of predominantly cold-based, less erosive glaciation from 14 Ma to the present (see Section 1). This suggests that the majority of erosion associated with the EAIS in the Prydz Bay/Lambert Graben region most likely occurred during the early Oligocene in the first 5 My of glaciation (based on the depositional age of the early Oligocene sediments), with slower, limited glacial erosion since that time.
Based on ice sheet erosional models and the characteristic U-shape of glacial valleys and troughs observed in East Antarctica (Bo et al., 2009; Jamieson et al., 2010), the majority of the erosion taking place during the early Oligocene in and around the Lambert Graben and
! 17! Gamburtsev Mountains was likely in the form of glacial incision rather than widespread erosion over the entire region. Glacial incision patterns generally follow the preglacial fluvial drainage patterns along the margins of East Antarctica, leading to overdeepening of these preexisting river valleys (Jamieson and Sugden, 2008; Jamieson et al., 2010).
6.5. Modeling Depths of Incision
The following discussion of modeled erosional depths assumes that erosion along the eastern margin of Antarctica was accomplished primarily by glacial incision into preexisting river valleys, as described previously in Section 6.4. To determine the depth of incision that must have been achieved by the late Miocene based on our measured apatite fission track and (U-
Th)/He ages, we have modeled the predicted cooling ages of grains derived from various depths beneath the surface of an area since the onset of glaciation at ~34 Ma using a time-temperature history likely to be a good estimate for the Prydz Bay/Lambert Graben region. To determine the predicted apatite fission track and (U-Th)/He cooling ages, we use a simple time-temperature model produced in the HeFTy program of Ketcham (2005). The model of Ketchem et al. (2007a,
2007b) is used to predict fission track cooling ages, and the Durango model of Farley (2000) is used to predict (U-Th)/He cooling ages. The parameters for this model include a period of rapid cooling at 280 Ma followed by slow cooling until 34 Ma, with accelerated cooling to surface temperatures between 34 Ma and the present day. During the period of slow cooling from 280-34
Ma, fission track and (U-Th)/He ages would have decreased steadily with depth, defining a thermochronologic stratigraphy. Assuming a normal geothermal gradient of 25 °C/km, this stratigraphy and the cooling ages predicted by the HeFTY model can be used to correlate our measured cooling ages with depths of incision.
! 18! The predicted cooling age curves generated by this model are shown in Figure 8, along with histograms of measured fission track and uncorrected (U-Th)/He ages from two combined late Miocene samples (022R74 and 023R110). By comparing the youngest measured cooling ages in each histogram with the cooling ages predicted by the HeFTy model and their corresponding incision depths, we can estimate the depth of incision that must have been achieved by the late Miocene to produce the cooling ages measured in the Miocene samples. The youngest measured fission track ages of ~80 Ma correspond to a predicted incision depth of ~2.8 km since the late Miocene. Measured (U-Th)/He ages indicate a slightly deeper predicted depth of incision, with the youngest ages of ~25 Ma corresponding to ~3 km of incision by the late
Miocene.
The presence of young ages of apatite grains in late Miocene sediments that must have come from ~2.8-3.0 km of depth supports other evidence indicating that the majority of glacial incision occurred during the early Oligocene in the first 5 My after the onset of glaciation in East
Antarctica. During this time, the EAIS was primarily warm-based, making it erosive and capable of incising valleys of these depths. After its transition to cold-based glaciation at ~14 Ma, however, the EAIS would have become far less erosive. Therefore, the depths of incision predicted by the HeFTY model must have been achieved before this transition to produce the corresponding cooling ages in the late Miocene apatite grains.
7. Conclusions
Multi-dating of single apatite and zircon grains from diamictites in Prydz Bay allows for the identification and interpretation of major tectonic and exhumational events throughout the history of East Antarctica. Zircon U-Pb crystallization ages and apatite U-Pb crystallization and
! 19! metamorphic ages record the widespread Pan-African orogeny at 0.5-0.6 Ga, which resulted in deformation and intrusive activity throughout much of East Antarctica. Zircon fission track ages record a rifting and exhumational event in the Permian-Triassic, as well as a magmatic heating event during the Cretaceous (Coffin et al., 2002; Ferraccioli et al., 2011; Lisker et al., 2003).
From low temperature cooling ages measured on apatite grains, erosional history can be constrained and estimated depths of glacial incision can be determined. Both the apatite fission track and (U-Th)/He cooling ages show a rapidly decreasing trend throughout the early
Oligocene, and a more slowly decreasing trend from the late Miocene through the Quaternary.
Lag times determined from mean apatite fission track cooling ages decrease from ~250-180 Ma during the early Oligocene, but remain relatively constant at ~160 Ma from the late Miocene to
Quaternary. This is indicative of increasing erosion rates throughout the early Oligocene, followed by near steady-state erosion from the Miocene to Quaternary. Although a greater number of apatite (U-Th)/He ages from Quaternary, Pliocene, and especially early Oligocene samples would be necessary to make this dataset more robust, these trends are indicative of accelerated glacial incision during the first 5 m.y. after the onset of the EAIS, followed by more steady slower incision from the late Miocene to the present. HeFTy time-temperature modeling compared with measured apatite fission track and (U-Th)/He cooling ages corresponds to ~2.8-
3.0 km of incision that must have occurred by the late Miocene. Most of this incision likely occurred during the early Oligocene before the transition from a warm-based, erosive EAIS to the cold-based, stable ice sheet that exists today.
! 20! Acknowledgements
This work was supported by NSF grants ANT-0838722 and EAR-1032156 (Arizona
LaserChron Center). Thanks to Mark Pecha, Nicky Giesler, Chelsi White, Intan Yokelson, and
Percival Gou in the Arizona LaserChron Center and Loren Hill and Uttam Chowdhuri in the
Arizona Radiogenic Helium Dating Laboratory for their assistance with sample preparation and analysis.
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Figure 1. Isostatically rebounded subglacial topography (80 times vertically exaggerated) of the Lambert Graben/Prydz Bay region of East Antarctica showing the location of core 739C used in this study and the modeled pre-glacial drainage into the Lambert Graben (Jamieson and Sugden, 2008; after Cox et al., 2010).
! 29!
Figure 2. Bedrock geology map of the area surrounding the Lambert Graben and Prydz Bay showing location of core 739C (after van de Flierdt et al., 2008; Fitzsimons, 2003).
! 30!
Figure 3. Stratigraphy of ODP core 739C. Analyzed segments of the core are indicated with arrows along the left side of the column (Volpi et al., 2009; Shipboard Scientific Party, 1989).
! 31!
Figure 4. (a) Zircon U-Pb and (b) zircon fission track probability density plots. Samples from each age division of core 739C are combined in each plot. Note the dominant ~500 Ma peak in all of the U-Pb spectra and the increasing occurrence of Precambrian grains in the Oligocene samples. Also note the two populations of FT ages (~250-300 Ma and ~120 Ma). (n=number of analyses)
! 32!
Figure 5. Apatite U-Pb probability density plots. Samples from each age division of core 739C are combined in each plot. Note the dominant ~500 Ma peak in every age of sediments. (n=number of analyses)
! 33!
Figure 6. Apatite fission track probability density plots. The data for each sample is shown separately and correlated with its location in core 739C. Note the decreasing trend in mean cooling age throughout the Early Oligocene, corresponding with a change in lag time from ~240- 180 My, compared with a change in lag time during the late Miocene through Quaternary from ~170 Ma-150 Ma. (n=number of analyses)
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Figure 7. Apatite (U-Th)/He probability density plots. All samples for each age distribution of core 739C are combined in each plot. Raw ages are plotted in the histograms. Red PDF curves represent raw ages. Green PDF curves represent plot ages, or the average between the raw age and the fully corrected age of a grain. Note the shift to younger ages (<100 Ma) after the Early Oligocene. (n=number of analyses)
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Figure 8. Plot of stratigraphic age versus central apatite fission track age for all samples. Black lines represent constant lag time. Note the decrease in lag time in the Early Oligocene samples from ~250 My to ~180 My, compared with the nearly constant lag time of ~160 My (nearly steady-state erosion) in the late Miocene through Quaternary samples. Error bars are ±1σ for central fission track ages and ±1 Ma uncertainty for depositional ages.
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Figure 9. HeFTy time-temperature models with histograms showing apatite fission track (blue) and raw apatite (U-Th)/He (green) age spectra for late Miocene samples. Youngest fission track age (~80 Ma) corresponds to a modeled incision depth of ~2.8 km by the late Miocene. Youngest (U-Th)/He age (~25 Ma) corresponds to an incision depth of ~3 km by the late Miocene.
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