Simple models for late Holocene and present-day Patagonian glacier fluctuations and predictions of a geodetically detectable isostatic response

The late Holocene glacial moraine chronology in the southernmost South American Andes includes four C-14 dated Neoglacial advances and retreats. These are used as proxy information to characterize mass fluctuation of the Patagonian icefields during the last 5000 years. Modelled ice loads force a phase-lagged viscoelastic gravitational deformation of the solid Earth. The ancient glacier fluctuations may, therefore, drive present-day crustal motion even in the absence of present-day ice mass imbalance. Numerical models show that such rates of present-day uplift and subsidence are larger than those driven by the viscous memory of late Pleistocene deglaciation. Both spherical and flat-earth models are employed, the latter being used to study exhaustively the effects of glacial load history on the predicted vertical crustal velocity. Recent assessment of net mass balance from 1944 to 1985 indicates that the Southern Patagonian icefield has significantly deteriorated due to snout retreat and thinning. Volume loss rates are estimated at about 3.4-9.3 km(3) yr(-1). The predicted vertical isostatic response to this recession and to the modelled Holocene Neoglaciations is at a marginally detectable level (approximate to 1 mm yr(-1)) if the mantle/asthenosphere beneath Patagonia has a viscosity of about 10(21) p, S. However, for reduced mantle viscosities, the younger Holocene glacial load histories predict larger signatures. In fact, if the viscosity is about 2 x 10(20) Pa s, or lower, then geodetically detectable vertical motion may be driven by a regional Little Ice Age (LIA) (1400-1750 AD) glacier advance and subsequent 20th century retreat. Although this value for mantle viscosity is lower than thought typical of continental shield mantle (approximate to 10(21) Pa s), it is consistent with inversions for post-seismic relaxation time constants in island are environments and in regions with significant Neogene continental tectonism. In the viscosity regime of 5 x 10(18)-2 X 10(19) Pa s, the predicted rates of vertical crustal motion are similar to those presently occurring in Fennoscandia and Hudson Bay (5-10 mm yr(-1)). Geodetic data may be sensitive to the time-integrated growth and subsequent retreat of Patagonian glacier ice mass over the last 600 years.