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Relatively unmetamorphosed Paleozoic miogeoclinal carbonate rocks in the Basin and Range of E Nevada, SW Nevada and adjacent California, and W Utah yield low-inclination magnetizations that reflect pervasive, regional remagnetization around the close of the Paleozoic. The rocks range in age from mid-Cambrian through Pennsylvanian and lie generally in a broad belt between the mid-Paleozoic Roberts Mountain Thrust and the late Cretaceous Sevier thrusts. Most of the magnetizations reside in magnetite, but at one site the magnetization is evidently carried by pyrrhotite. Preliminary rock-magnetic data suggest samples with magnetite-borne remanences have wasp-waisted hysteresis curves typical of remagnetized carbonates. The origin of the remagnetization is problematic and probably polygenetic: both the Permo-Triassic Sonoma orogeny and deformation associated with the Ancestral Rockies seem too spatially limited, but magnetite from smectite destruction seems difficult to reconcile with the great stratigraphic extent of late Paleozoic remagnetization unless combined with thermal resetting of the lowermost units. A number of sites also appear to have undergone some vertical-axis rotation, and the sense and magnitude of these rotations are grossly consistent with independent geologic evidence. However, the probably large age range of the low-inclination components complicates their use for resolving tectonic rotations. Younger, intermediate-stability components of magnetization, probably of Cretaceous or Cenozoic age, are also found in many sites and also probably have multiple origins. At sites farther W, the late Paleozoic component is not found, which probably reflects its destruction by later Mesozoic or Cenozoic heating. At sites farther E, on and near the Colorado Plateau, gray carbonates yield only Cenozoic magnetizations. Some reddish, oxidized carbonates there locally contain a hematite-borne magnetization of late Paleozoic age. However, it is probably related to the development of thick continental redbed sequences in overlying strata on the plateau rather than to the remagnetization process(es) in the miogeocline. [Copyright &y& Elsevier]

Lake Washington, which forms the eastern boundary of Seattle, is located in a tectonically active area containing several strands of the Seattle fault. High-resolution seismic reflection profiling, sidescan swath imagery, and sediment coring were used to define deformation of Holocene lake sediments and the distribution, geometry, age, and causes of submarine landslides. Numerous large block slides, sediment slumps, and debris flows are present throughout the lake, and large landslides obscure the surficial structure of Seattle fault strands as they pass through the lake. In addition, most bays along the lake margin are the headwalls of large submarine slides. Submerged forests show evidence of deep-seated block failures that have exposed glacial sediments and Tertiary rocks. The massive submarine block slides, and retrogressive submarine slope failures were triggered most likely by large (m[sub b]>7) earthquakes on the Seattle fault and/or large to great (m[sub b]>8) temblors occurring elsewhere in Cascadia. Buried landslides suggest that submarine slope failures and mass wasting occurred more than once in the last 11,000 yr. Sediments in Lake Washington preserve a record of episodic deposition of turbidites possibly caused by seismically induced submarine landslides. Magnetic susceptibility profiles on 36 gravity cores show a characteristic series of magnetic peaks that can be traced throughout the lake. X-radiography and grain size analyses suggest that the magnetic peaks represent anomalous detrital layers that in some cases are turbidites. The areal extent and magnetic signatures of many of the deposits suggest multiple sources, which is consistent with numerous local landslides caused by large earthquakes. Radiocarbon dating and correlation of the downcore magnetic profiles establish a sediment record in which episodic sedimentary disturbances occurred seven times in the last 3500 yr. If these deposits are seismically induced turbidites (seismites), then large earthquakes have occurred about every 300-500 yr in the Puget Sound region. Keywords: paleoseismology; seismotectonics; Holocene; landslide; Seattle; Washington; Lake Washington. [ABSTRACT FROM AUTHOR]