Ralph A. Haugerud

High-resolution lidar topography of the Puget Lowland, Washington - A bonanza for earth science

More than 10,000 km2 of high-resolution, public-domain topography acquired by the Puget Sound Lidar Consortium is revolutionizing investigations of active faulting, continental glaciation, landslides, and surficial processes in the seismically active Puget Lowland. The Lowland—the population and economic center of the Pacific Northwest—presents special problems for hazards investigations, with its young glacial topography, dense forest cover, and urbanization. Lidar mapping during leaf-off conditions has led to a detailed digital model of the landscape beneath the forest canopy. The surface thus revealed contains a rich and diverse record of previously unknown surface-rupturing faults, deep-seated landslides, uplifted Holocene and Pleistocene beaches, and subglacial and periglacial features. More than half a dozen suspected postglacial fault scarps have been identified to date. Five scarps that have been trenched show evidence of large, Holocene, surfacerupturing earthquakes.

Holocene fault scarps near Tacoma, Washington, USA

Airborne laser mapping confirms that Holocene active faults traverse the Puget Sound metropolitan area, northwestern continental United States. The mapping, which detects forest-floor relief of as little as 15 cm, reveals scarps along geophysical lineaments that separate areas of Holocene uplift and subsidence. Along one such line of scarps, we found that a fault warped the ground surface between A.D. 770 and 1160. This reverse fault, which projects through Tacoma, Washington, bounds the southern and western sides of the Seattle uplift. The northern flank of the Seattle uplift is bounded by a reverse fault beneath Seattle that broke in A.D. 900–930. Observations of tectonic scarps along the Tacoma fault demonstrate that active faulting with associated surface rupture and ground motions pose a significant hazard in the Puget Sound region.

Late Cretaceous and early Tertiary plutonism and deformation in the Skagit Gneiss Complex, North Cascade Range, Washington and British Columbia

The Skagit Gneiss Complex forms a more-or-less continuous terrane within the northern, more deeply eroded part of the North Cascade Range. The complex comprises abundant plutons intruded at mid-crustal depths into a variety of metamorphosed supracrustal rocks of both oceanic and volcanic-arc origin. A plethora of syntectonic pegmatite, small plutons, and granitic dikes gives the complex a migmatitic aspect. U-Pb zircon ages from gneissic plutons within and near the Skagit Gneiss Complex indicate magmatic crystallization between 75 and 60 Ma. Deformation, recrystallization, and migmatization in part postdate intrusion of the 75-60 Ma plutons. This latest Cretaceous and earliest Tertiary plutonism and migmatization may reflect thermal relaxation following early Late Cretaceous orogeny documented else-where in the North Cascades. The complex was ductilely extended northwest-southeast shortly after intrusion of granite dikes at ∼45 Ma, but before emplacement of the earliest (∼34 Ma) plutons of the Cascade arc. Outcrops of Late Cretaceous and earliest Tertiary plutons, migmatites of the Skagit Gneiss Complex, and rocks with young ductile deformation are roughly coextensive, all apparently marking a region of greater middle Eocene unroofing. Unroofing was apparently contemporaneous with east-west extension in the Okanogan region to the east and north-south and northwest-southeast strike-slip faulting within the North Cascades.

On numerical modeling of one-dimensional geothermal histories

Abstract Numerical models of one-dimensional geothermal histories are one way of understanding the relations between tectonics and transient thermal structure in the crust. Such models can be powerful tools for interpreting geochronologic and thermobarometric data. A flexible program to calculate these models on a microcomputer is available and examples of its use are presented. Potential problems with this approach include the simplifying assumptions that are made, limitations of the numerical techniques, and the neglect of convective heat transfer.

Holocene tectonics and fault reactivation in the foothills of the north Cascade Mountains, Washington

We use LiDAR imagery to identify two fault scarps on latest Pleistocene glacial outwash deposits along the North Fork Nooksack River in Whatcom County, Washington (United States). Mapping and paleoseismic investigation of these previously unknown scarps provide constraints on the earthquake history and seismic hazard in the northern Puget Lowland. The Kendall scarp lies along the mapped trace of the Boulder Creek fault, a south-dipping Tertiary normal fault, and the Canyon Creek scarp lies in close proximity to the south-dipping Canyon Creek fault and the south-dipping Glacier Extensional fault. Both scarps are south-side-up, opposite the sense of displacement observed on the nearby bedrock faults. Trenches excavated across these scarps exposed folded and faulted late Quaternary glacial outwash, locally dated between ca. 12 and 13 ka, and Holocene buried soils and scarp colluvium. Reverse and oblique faulting of the soils and colluvial deposits indicates at least two late Holocene earthquakes, while folding of the glacial outwash prior to formation of the post-glacial soil suggests an earlier Holocene earthquake. Abrupt changes in bed thickness across faults in the Canyon Creek excavation suggest a lateral component of slip. Sediments in a wetland adjacent to the Kendall scarp record three pond-forming episodes during the Holocene—we infer that surface ruptures on the Boulder Creek fault during past earthquakes temporarily blocked the stream channel and created an ephemeral lake. The Boulder Creek and Canyon Creek faults formed in the early to mid-Tertiary as normal faults and likely lay dormant until reactivated as reverse faults in a new stress regime. The most recent earthquakes—each likely M w > 6.3 and dating to ca. 8050–7250 calendar years B.P. (cal yr B.P.), 3190–2980 cal. yr B.P., and 910–740 cal. yr B.P.—demonstrate that reverse faulting in the northern Puget Lowland poses a hazard to urban areas between Seattle (Washington) and Vancouver, British Columbia (Canada).