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Erosional History of East Antarctica from Double and Triple-Dating of Single Grains in Glacially- Derived Sediments from Prydz Bay

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Erosional History of East Antarctica from Double and Triple-Dating of Single Grains in Glacially- Derived Sediments from Prydz Bay EROSIONAL HISTORY OF EAST ANTARCTICA FROM DOUBLE AND TRIPLE-DATING OF SINGLE GRAINS IN GLACIALLY- DERIVED SEDIMENTS FROM PRYDZ BAY by Clare J. Tochilin A Prepublication Manuscript Submitted to the Faculty of the DEPARTMENT OF GEOSCIENCES In Partial Fulfillment of the Requirements for the Degree of MASTER OF SCIENCE In the Graduate College THE UNIVERSITY OF ARIZONA th May 7 , 2012 STATEMENT BY THE AUTHOR This manuscript, prepared for publication in Geochemistry Geophysics Geosystems, has been submitted in partial fulfillment of requirements for the Master of Science degree at The University of Arizona and is deposited in the Antevs Reading Room to be made available to borrowers, as are copies of regular theses and dissertations. Brief quotations from this manuscript are allowable without special permission, provided that accurate acknowledgment of the source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the Department of Geosciences when the proposed use of the material is in the interests of scholarship. In all other instances, however, permission must be obtained from the author. (author 's signature) (date) APPROVAL BY RESEARCH COMMITTEE As members of the Research Committee, we recommend that this prepublication manuscript be accepted as fulfilling the research requirement for the degree of Master of Science. Dr. Peter W. Reiners 2. Major Advisor (signature) (date) Dr. George E. Gehrels 2otz_ (signature) (date) {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.
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