ALBERT

Description: Inference of the mantle viscosity from observations for glacial isostatic adjustment (GIA) process has usually been conducted through the analyses based on the simple three-layer viscosity model characterized by lithospheric thickness, upper- and lower-mantle viscosities. Here, we examine the viscosity structures for the simple three-layer viscosity model and also for the two-layer lower-mantle viscosity model defined by viscosities of 670, D (670- D km depth) and D ,2891 ( D -2891 km depth) with D -values of 1191, 1691 and 2191 km. The upper-mantle rheological parameters for the two-layer lower-mantle viscosity model are the same as those for the simple three-layer one. For the simple three-layer viscosity model, rate of change of degree-two zonal harmonics of geopotential due to GIA process (GIA-induced 2 ) of –(6.0–6.5)  x  10 –11 yr –1 provides two permissible viscosity solutions for the lower mantle, (7–20)  x  10 21 and (5–9)  x  10 22  Pa s, and the analyses with observational constraints of the 2 and Last Glacial Maximum (LGM) sea levels at Barbados and Bonaparte Gulf indicate (5–9)  x  10 22  Pa s for the lower mantle. However, the analyses for the 2 based on the two-layer lower-mantle viscosity model only require a viscosity layer higher than (5–10)  x  10 21  Pa s for a depth above the core–mantle boundary (CMB), in which the value of (5–10)  x  10 21  Pa s corresponds to the solution of (7–20)  x  10 21  Pa s for the simple three-layer one. Moreover, the analyses with the 2 and LGM sea level constraints for the two-layer lower-mantle viscosity model indicate two viscosity solutions: 670,1191  〉 3  x  10 21 and 1191,2891  ~ (5–10)  x  10 22  Pa s, and 670,1691  〉 10 22 and 1691,2891  ~ (5–10)  x  10 22  Pa s. The inferred upper-mantle viscosity for such solutions is (1–4)  x  10 20  Pa s similar to the estimate for the simple three-layer viscosity model. That is, these analyses require a high viscosity layer of (5–10)  x  10 22  Pa s at least in the deep mantle, and suggest that the GIA-based lower-mantle viscosity structure should be treated carefully in discussing the mantle dynamics related to the viscosity jump at ~670 km depth. We also preliminarily put additional constraints on these viscosity solutions by examining typical relative sea level (RSL) changes used to infer the lower-mantle viscosity. The viscosity solution inferred from the far-field RSL changes in the Australian region is consistent with those for the 2 and LGM sea levels, and the analyses for RSL changes at Southport and Bermuda in the intermediate region for the North American ice sheets suggest the solution of 670, D  〉 10 22 , D ,2891  ~ (5–10)  x  10 22  Pa s ( D  = 1191 or 1691 km) and upper-mantle viscosity higher than 6  x  10 20  Pa s.