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| Titel |
Constraints on lateral variations in upper mantle viscosity from Lake Bonneville shorelines |
| VerfasserIn |
Jacqueline Austermann, Christine Chen, Harriet C. P. Lau |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250149135
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| Publikation (Nr.) |
 EGU/EGU2017-13459.pdf |
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| Zusammenfassung |
| Lake Bonneville is an extinct pluvial lake that formed and catastrophically drained at the
onset of the last deglaciation (∼ 20 – 18ka). With a volume of just over 10 000 km3 this lake
was comparable in size to present-day Lake Michigan. During its existence the
excess load of water stored in Lake Bonneville depressed the crust and upper mantle.
After the drainage of the lake this area rebounded by up to 75 m, which is recorded
in the paleoshorelines around the lake periphery and on islands within the lake.
The rebound pattern has been used to infer the lithospheric thickness and upper
mantle viscosity structure of the area (e.g. Bill et al., 1994). In agreement with
the tectonic history of the Basin and Range area, the deformed shorelines point to
a thin lithosphere (< 30km) and low upper mantle viscosity (∼ 1019 Pa s). This
differs from the upper mantle viscosity inferred from post-glacial data in cratonic
regions (e.g., Hudson Bay, Fennoscandia), which is one to two orders of magnitude
larger (∼ 5 × 1020 Pa s). Direct constraints on the lateral variability of mantle
viscosity are invaluable but in order to utilize such constraints it is important to
consider the sensitivity range of different observations before comparing the inferred
viscosities.
In this study we revisit the earlier inversions of shoreline elevations for mantle and
lithospheric structure with an updated dataset of paleoshoreline elevations by Chen and
Maloof (2017). We construct depth-dependent sensitivity kernels for the lake rebound and
compare them to kernels associated with the rebound from glacial ice sheets over Canada and
Scandinavia. This comparison along with the inferred viscosities allows us to evaluate the
degree to which lateral viscosity variations are required. We additionally compare our results
to estimates of lateral viscosity variations based on perturbations in seismic shear wave speed
in the respective areas in order to assess the consistency of our results with independent
data.
The paleoshorelines of Lake Bonneville have been deflected by not only rebound
post-drainage, but also the longer-term subsidence of the Laurentide peripheral bulge. The
lake was located on the distal flank of the peripheral bulge of the Laurentide Ice Sheet and
after its collapse the peripheral bulge subsided leading to an additional northeast trending tilt
in shoreline elevations. We show that the degree of tilt is not only sensitive to shallow
mantle structure but has also sensitivity in the upper half of the lower mantle, in
contrast to the lake rebound pattern. We independently invert the degree of tilt for
mantle viscosity and examine its trade-off with uncertainties in the ice history. |
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