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

Abstract

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.

Hydrologic variation during the last 170,000 years in the southern hemisphere tropics of South America

“…The two uppermost major mud units have been identified with paleolakes Tauca (26,000 -15,000 cal yr B.P.) and Minchin (46,000 -36,000 yr) (Baker et al, 2001a), which are associated with dated paleoshorelines (Bills et al, 1994;Servant et al, 1995;Sylvestre et al, 1999). Our new extended record indicates that these were also the two longest lacustrine intervals in the entire dated sequence.…”

“…5, 6a) suggests that paleolake Tauca was the deepest of the paleolakes, as indicated by intervals with the highest percentages of planktic diatoms and the dominance of Cyclotella choctawatcheeana, a planktic diatom characteristic of deep mesosaline (>10 g L À1 ) lakes and brackish marine systems (Fritz et al, 1993;Prasad, 1990). The highest outcropping paleoshorelines in the basin demonstrate that paleolake Tauca reached 140 m above the modern Salar (Bills et al, 1994). Stable-oxygen-isotopic values of carbonates in the Tauca unit (Fig.…”

Crustal strength in central Tibet determined from Holocene shoreline deflection around Siling Co

“…Moreover, shorelines developed during highstand levels represent a paleo-horizontal datum from which to measure deflections, as well as provide an estimate of the volume of the load. This methodology is a proven means of estimating the bulk constitutive behavior of the lithosphere and has provided key constraints in the Basin and Range (Bills et al, 1994a;Gilbert, 1890), the Andes (Bills et al, 1994b), and in the Mediterranean (Govers et al, 2009). It has also been recently used to place bounds on the rheology of the crust in west-central Tibet (England et al, 2013).…”

Time-dependent deformation of Uturuncu volcano, Bolivia, constrained by GPS and InSAR measurements and implications for source models

“…A weak felsic crust predicts a shallow brittle-ductile transition in agreement with seismicity (Jay et al, 2012), but a more intermediate middle and lower crustal composition may be justified given the andesitic composition of the APMB (Comeau et al, 2015). Independent constraints on the viscosity of the lower crust in the Central Andes are limited, but one study of lake loading indicates viscosities less than 10 20 Pa s below 40 km depth (Bills et al, 1994). Given an estimated shear modulus of 10-100 GPa for the lower crust, these viscosity values imply relaxation times on the order of 10-1000 years, which is similar to the timescale of ground deformation measured by geodesy and geomorphology (e.g., Perkins et al, 2016).…”