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Hydro‐isostatic deflection and tectonic tilting in the central Andes: Initial results of a GPS survey of Lake Minchin shorelines

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  • Sufficiently large lake loads provide a means of probing rheological stratification of the crust and upper mantle. Lake Minchin was the largest of the late Pleistocene pluvial lakes in the central Andes. Prominent shorelines, which formed during temporary still‐stands in the climatically driven lake level history, preserve records of lateral variations in subsequent net vertical motions. At its maximum extent the lake was 140 m deep and spanned 400 km N‐S and 200 km E‐W. The load of surficial water contained in Lake Minchin was sufficient to depress the crust and underlying mantle by 20–40 m, depending on the subjacent rheology. Any other differential vertical motions will also be recorded as departures from horizontality of the shorelines. We recently conducted a survey of shoreline elevations of Lake Minchin with the express intent of monitoring the hydro‐isostatic deflection and tectonic tilting. Using real‐time differential GPS, we measured topographic profiles across 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 highest shoreline was occupied only briefly (probably less than 200 years) and radiocarbon dates on gastropod shells found in association with the shore deposits constrain the age to roughly 17 kyr. The basin‐wide pattern of elevations of the highest shoreline is composed of two distinct 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 rate of tilting is too high to be plausibly attributed to steady tectonism, and presumably reflects some unresolved combination of tectonism plus the effects of oceanic and lacustrine loads on a laterally heterogeneous substrate. The history of lake level fluctuations is still inadequately known to allow detailed inferences of crust and mantle rheology. However, it is already clear that the effective elastic plate thickness is closer to 40 km than the 60–70 km crustal thickness in the central Andes and the effective viscosity is less than 5 10²⁰ Pa s.
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  • Bills, B. G., S. L. de Silva, D. R. Currey, R. S. Emenger, K. D. Lillquist, A. Donnellan, and B. Worden (1994), Hydro‐isostatic deflection and tectonic tilting in the central Andes: Initial results of a GPS survey of Lake Minchin shorelines, Geophysical Research Letters, 21(4), 293–296, doi:10.1029/93GL03544.
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  • 21
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  • 4
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  • This research was supported by the NASA Geodynamics Program through RTOP 465-13-04 at GSFC, grant NAG5-1734 at ISU, and grant NAG5-1822 at UU.
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