Robert J. Carson

Lake Tapps tephra: An early Pleistocene stratigraphic marker in the Puget Lowland, Washington

Abstract The rhyolitic Lake Tapps tephra was deposited about 1.0 myr ago, shortly after culmination of the early phase of the Salmon Springs Glaciation in the Puget Lowland. It is contained within sediments that were deposited in ponds or lakes in front of the reteating glacier. An herb-dominated tundra existed in the southern Puget Lowland at that time. Lake Tapps tephra is most likely the product of an eruption that in part was phreatomagmatic. It forms an early Pleistocene stratigraphic marker across the southern sector of the Puget Lowland and provides a link between Puget lobe sediments of the Cordilleran Ice Sheet and sediments deposited by Olympic alpine glaciers.

Problems of interpreting radiocarbon dates from dead-ice terrain, with an example from the Puget Lowland of Washington

Abstract Radiocarbon dates of organic matter collected from ablation till or from the base of peat bogs in dead-ice deposits may postdate retreat of an active glacier terminus by hundreds or even thousands of years, and therefore provide only minimum estimates for the time of glacial maximum and the beginning of ice recession. Logs incorporated in Vashon till close to the drift border postdate recession of the Puget Lobe of the Cordilleran Ice Sheet by some 1400 years, and probably were buried when drift-mantled stagnant ice melted away, causing collapse of a superglacial forest.

Late Quaternary faults and their relationship to tectonism in the Olympic Peninsula, Washington

Late Quaternary movement on three reverse faults and the presence of one possible strike-slip fault have been recognized in the southeastern part of the Olympic Peninsula of Washington. Movement along all four faults occurred during or after a pre-Fraser glaciation, and the last movement on the Saddle Mountain East fault appears to have occurred about 1,240 to 1,235 yr B.P. Displacements along these faults correlate well with other workers9 recent geophysical analyses of tectonic events in the adjacent Puget Lowland. The Dow Mountain and Cushman Valley (?) faults each approximate an orthogonal plane derived by focal-mechanism solutions for two groups of recent shallow-focus earthquakes in the Puget Lowland. The Saddle Mountain faults do not correlate with recent earthquake data, but the Saddle Mountain East fault, when plotted with its orthogonal plane, is compatible with the modern regional stress field determined by other workers. The Saddle Mountain faults are believed to be Holocene features developed within an older northeast-trending zone of fracturing. This older zone possibly developed during a late stage of complex folding and doming of the Olympic Mountains, before the development of the northwest-southeast compressive stress system that has characterized this region during the Holocene.

The Egiin Davaa prehistoric rupture, central Mongolia: a large magnitude normal faulting earthquake on a reactivated fault with little cumulative slip located in a slowly deforming intraplate setting

Abstract The prehistoric Egiin Davaa earthquake rupture is well-preserved in late Quaternary deposits within the Hangay Mountains of central Mongolia. The rupture is expressed by a semi-continuous 80 km-long topographic scarp. Geomorphological reconstructions reveal a relatively constant scarp height of 4–4.5 m and a NW-directed slip vector. Previous researchers have suggested that the scarps exceptional geomorphological preservation indicates that it may correspond to an earthquake that occurred in the region c. 500 years ago. However, we constrain the last rupture to have been at least 4 ka ago from morphological dating and <7.4 ka ago based on radiocarbon dating from one of two palaeoseismic trenches. Our study shows that discrete earthquake ruptures, along with details such as the locations of partially infilled fissures, can be preserved for periods well in excess of 1000 years in the interior of Asia, providing an archive of fault movements that can be directly read from the Earths surface over a timescale appropriate for the study of slowly deforming continental interiors. The Egiin Davaa rupture involved c. 8 m of slip which, along with the observations that it is largely unsegmented along its length and that the ratio of cumulative slip (c. 250 m) to fault length (c. 80 km) is small, suggests relatively recent reactivation of a pre-existing geological structure. Supplementary material: All scarp profiles are available at http://www.geolsoc.org.uk/SUP18871