Seismic migration processing ofP-SVconverted phases for mantle discontinuity structure beneath the Snake River Plain, western United States

2000 ◽  
Vol 105 (B8) ◽  
pp. 19055-19065 ◽  
Author(s):  
Anne F. Sheehan ◽  
Peter M. Shearer ◽  
Hersh J. Gilbert ◽  
Kenneth G. Dueker
Keyword(s):  
United States ◽  
Snake River Plain ◽  
Western United States ◽  
Snake River ◽  
Seismic Migration ◽  
Discontinuity Structure ◽  
River Plain ◽  
Mantle Discontinuity

Downward remagnetization of a ∼74-m-thick zone in lake sediments from palaeo-Lake Idaho (NW United States)—Locating the Gauss/Matuyama geomagnetic boundary within a dual-polarity zone

2020 ◽  
Vol 222 (2) ◽  
pp. 754-768
Author(s):  
Frederik J Allstädt ◽  
Erwin Appel ◽  
Wolfgang Rösler ◽  
Alexander A Prokopenko ◽  
Udo Neumann ◽  
...  
Keyword(s):  
United States ◽  
Transition Zone ◽  
Remanent Magnetization ◽  
Snake River Plain ◽  
Snake River ◽  
Depth Range ◽  
Magnetic Minerals ◽  
Reversed Polarity ◽  
Dual Polarity ◽  
River Plain

SUMMARY Remagnetization is an important issue in palaeomagnetism. Here, we discuss an extraordinarily thick (∼74 m) dual-polarity transition zone between the Gauss and Matuyama Chrons. The studied succession is from a drill core through lacustrine sediments of palaeo-Lake Idaho (Snake River Plain, NW United States of America) that are intercalated with basalt units. We identified detrital Ti-rich titanomagnetite and magnetite in lamellar exsolutions as the main carriers of a primary remanence, likely derived from the basalts that erupted in the Snake River Plain. Stepwise thermal demagnetization revealed a single-component remanent magnetization with reversed and normal polarities above and below the transition zone, respectively. Based on rock-magnetic results, microscopic observations, and previously known events in the evolution of palaeo-Lake Idaho, the reversed-polarity component in the transition zone represents a secondary chemical remanent magnetization caused by magnetic mineral alteration or partial neo-formation of magnetite, in association with strong depletion of the primary detrital magnetic minerals that affected a wide depth range below the level where the remagnetization event occurred. This remagnetization event was most likely related to lake-level lowering and partial desiccation of palaeo-Lake Idaho. Understanding the nature and origin of the remagnetization allows to identify the polarity boundary in the unusual case of a secondary magnetization with reversed polarity produced downward in a sequence to an extraordinary large depth. Based on available age information, the observed reversal represents the Gauss/Matuyama boundary, which provides an important age constraint for palaeoclimatic interpretation of the succession.

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