HYDRO-ISOSTATIC DEFLECTION AND TECTONIC TILTING IN THE CENTRAL ANDES - INITIAL RESULTS OF A GPS SURVEY OF LAKE MINCHIN SHORELINES

Sufficiently large lake loads provide a means of probing rheological s tratification of the crust and upper mantle. Lake Minchin was the larg est of the late Pleistocene pluvial lakes in the central Andes. Promin ent shorelines, which formed during temporary still-stands in the clim atically driven lake level history, preserve records of lateral variat ions in subsequent net vertical motions. At its maximum extent the lak e was 140 m deep and spanned 400 km N-S and 200 km E-W. The load of su rficial water contained in Lake Minchin was sufficient to depress the crust and underlying mantle by 2040 m, depending on the subjacent rheo logy. Any other differential vertical motions will also be recorded as departures from horizontality of the shorelines. We recently conducte d a survey of shoreline elevations of Lake Minchin with the express in tent of monitoring the hydro-isostatic deflection and tectonic tilting . Using real-time differential GPS, we measured topographic profiles a cross suites of shorelines at 15 widely separated locations throughout the basin. Horizontal and vertical accuracies attained are roughly 30 and 70 cm, respectively. Geomorphic evidence suggests that the highes t shoreline was occupied only briefly (probably less than 200 years) a nd radiocarbon dates on gastropod shells found in association with the shore deposits constrain the age to roughly 17 kyr. The basin-wide pa ttern of elevations of the highest shoreline is composed of two distin ct signals: (27 +/- 1) m of hydro-isostatic deflection due to the lake load, and a planar tilt with east and north components of (6.8 +/- 0. 4) 10(-5) and (-5.3 +/- 0.3) 10(-5). This Tate of tilting is too high to be plausibly attributed to steady tectonism, and presumably reflect s some unresolved combination of tectonism plus the effects of oceanic and lacustrine loads on a laterally heterogeneous substrate. The hist ory of lake level fluctuations is still inadequately known to allow de tailed inferences of crust and mantle rheology. However, it is already clear that the effective elastic plate thickness is closer to 40 km t han the 60-70 km crustal thickness in the central Andes and the effect ive viscosity is less than 5 10(20) Pa s.