Palaeoshoreline records of glacial isostatic adjustment in the Dry Valleys region, Antarctica

We present a new record of crustal deformation for the Dry Valleys and surrounding region of Antarctica. Values of crustal tilt resulting from the differential uplift of lacustrine strandlines are derived and linked with age data to provide a history of solid earth deformation since deglaciation. We present tilt directions and gradients for 13 strandlines formed c. 18 100– 2100 cal yr BP. Derived gradient magnitudes increase exponentially with age and indicate an ongoing response to deglaciation since the Last Glacial Maximum. Azimuths of crustal tilting are consistently down to the SE towards West Antarctica. This tilt pattern is opposite to that predicted by models of glacial isostatic adjustment for Antarctica. Tilt magnitudes are significantly larger than tilted strandlines documented elsewhere in the world, suggesting an influence from thin crust and weak mantle underlying the region. This study presents the first use of lacustrine strandline tilts to document crustal deformation due to glacial unloading in Antarctica and provides an important new datum for constraining glacial isostatic adjustment models. For more than a century, observations of uplifted and tilted marine and lacustrine strandlines have been used to understand glacial isostatic adjustment (GIA) from retreating ice sheets (e.g. Spencer 1891; Goldthwait 1908). As the Earth’s crust rebounds in response to ice mass loss, ice-proximal shorelines are raised and tilted away from ice load centres. These geological shoreline records can be used in different ways depending on whether original elevation and/or age of the features are known. When both the original elevation and age are known, uplift through time can be reconstructed. The most common application of this case is analysis of marine raised beaches and shorelines where past elevation is determined through comparison to present-day sea-level. Radiocarbon dates from organic material extracted from shoreline deposits are correlated with raised beach elevations to produce a relative sea-level curve (e.g. Hall & Denton 1999; Lambeck et al. 2002b; Baroni & Hall 2004). When corrected for eustatic sea-level change, relative sea-level curves reflect the influence of solid earth and geoid variations resulting from deglaciation, providing essential constraints for models of glacial isostatic adjustment that solve the sea-level equation (Farrell & Clark 1976; Peltier et al. 1978; Lambeck & Chappell 2001; Mitrovica & Milne 2003; Peltier 2004). For lake palaeoshorelines, the original shoreline elevation is unknown, but differential uplift since the time of deposition results in shoreline tilt. Analysis of strandline tilt through time provides a record of solid earth deformation since deglaciation. For the case of glacial isostatic adjustment, the total amount of uplift is a function of shoreline position relative to former ice mass centres. Greatest uplift occurs closer to the centre of ice mass loss and decreases away from the load; subsidence occurs in the forebulge region (e.g. Clark et al. 1978). Shorelines located between the ice load and forebulge tilt away from the centre of ice loss, whereas shorelines outboard of the forebulge tilt towards the load centre. In addition to tilt direction, both temporal and spatial variances in tilt magnitude manifest ice history and geometry. Older deglacial shorelines have a greater magnitude of tilt than younger shorelines from the same location, and shorelines closer to the centre of ice mass loss have a greater magnitude of tilt than shorelines of the same age located farther away from the load. Given these relationships, analysis of the direction, magnitude and rate of tilting provides valuable information regarding ice history. Intersections of projected strandline tilt directions have been used to interpret the location of past ice mass centres, improving reconstructions of former ice geometry (Barnett & Peterson 1964; Andrews & Barnett 1972). Tilted palaeoshorelines linked with age data provide a relaxation curve for the Earth’s crust since deglaciation, enabling studies of mantle rheology and lithospheric thickness, parameters crucial for understanding the geodynamic processes involved in From: Hambrey, M. J., Barker, P. F., Barrett, P. J., Bowman, V., Davies, B., Smellie, J. L. & Tranter, M. (eds) Antarctic Palaeoenvironments and Earth-Surface Processes. Geological Society, London, Special Publications, 381, http://dx.doi.org/10.1144/SP381.26 # The Geological Society of London 2013. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics by guest on September 16, 2013 http://sp.lyellcollection.org/ Downloaded from