Mapping the northern limit of subduction in Cascadia using POLARIS broadband seismic stations
The Explorer region is a rapidly evolving oceanic micro-plate fragment that accommodates relative motion between the Pacific, Juan de Fuca, and North America plates in the vicinity of northern Vancouver Island, Canada. The northern limit of Explorer/Juan de Fuca subduction along the margin and the fate of the slab in northern Cascadia are poorly known. We use passive teleseismic recordings from a dense cross-shaped array of portable broadband seismic stations as part of the POLARIS-BC experiment to image upper mantle structure underneath northern Vancouver Island and the interior of British Columbia. One arm of the array trends NW-SE in the direction parallel to strike and straddles the assumed northern end of the subduction zone (LINE 1), and the second arm trends SW-NE in the direction perpendicular to strike, just north of the extension of the Nootka fault beneath Vancouver Island where subduction is observed (LINE 2). The array has been in operation since June 2005 and we use an average of 50 recorded events with high signal-to-noise ratio to compute receiver functions. A P-wave tomographic model of northern Cascadia is also derived to image deep (50-300 km) slab structure. The NVI array is complemented with data from a few permanent stations operated by the GSC and data from a previous experiment. Station spacing is approximately 5 km along both lines across northern Vancouver Island. We find a clear signature of subducted material extending northeast from Brooks Peninsula at crustal levels, into Georgia Strait and beyond deep into the mantle down to 300 km depth. The location of the sharp slab edge and complexities in slab topology result from Juan de Fuca ridge subduction and opening of a slab window, in agreement with heat flow and gravity modelling, geochemical data, and fault patterns in northern Vancouver Island. We propose a model of plate evolution for the Explorer region in which its separation from the Juan de Fuca plate is caused by the thermal erosion of the slab edge and slab thinning at shallow levels, which act to decrease convergence with North America and lead eventually to plate capture. Our model postulates that Juan de Fuca subduction is still active north and east of the detached Explorer slab, implying a revision of earthquake hazard potential along the northern Cascadia margin.
Surface Wave Tomography of the Nechako Basin, British Columbia, Canada, Using Ambient Seismic Noise
The Nechako Basin in British Columbia, Canada has been a difficult basin to explore due to the presence of Tertiary volcanic outcrop. The volcanic outcrop makes the use of conventional seismic methods difficult due to a strong velocity inversion at its base. An alternative to active source methods is the passive source method known as ambient noise surface wave tomography. The method, which examines the high-frequency surface wave field that is obtained from noise analysis, is sensitive to large-scale crustal structure and has been successfully applied to measuring the depths of sedimentary basins. Ambient noise surface wave tomography will thus help to unravel the structural composition of the Nechako Basin. We estimated station-to station Green's functions within the basin, by cross-correlating the vertical components of the seismic noise data recorded by 12 POLARIS and CNSN seismic stations between September 2006 and November 2007, using a two-station method. The cross-correlation produced estimates of the Rayleigh-wave Green's function for waves travelling between pairs of stations. The dispersion characteristics of the resulting Rayleigh waveforms were measured within the micro-seismic band. Inversion of the dispersion curves produced 2-D group velocity maps between 0.55 Hz and 0.03Hz, and 1-D S velocity models for Nechako Basin and its surrounding region. The average 1-D model within the basin suggests a six layered medium; surface/near surface sediment (~1.8 km), volcanics (~0.6 km), the sedimentary basin (~2.0 km), the Precambrian basement (~9.1 km), the lower crust (~17.0 km), and the upper mantle. Outside the basin, the model suggested a four layered medium, because of the absence of the volcanic and sedimentary layers. The 2-D group velocity maps show the lateral variations of rock composition within the Nechako region. High lateral variations are observed at frequencies between 0.55Hz and 0.3Hz, which is believed to correlate with the depth of the sedimentary basin within the region. Pockets of low group velocity structures within the area suggest that the region consists of a major deep and laterally extensive sedimentary package, and a shallow sedimentary package at the southern part of the Nechako Basin.
Crustal Thickness versus Elevation: Hot Backarc Cordillera versus Stable North America
With expanding data sets on continental crustal thickness we are now able to better define the systematics between elevation and crustal thickness, and the controls on isostasy. We first note that the average crustal thickness for the Cordillera (about 35 km) is less than that for stable North America (about 40 km) in spite of the ~1500 m higher Cordillera elevation; the high elevation is not supported by a crustal root. We find two well- defined linear relations between crustal thickness and elevation, one for the Cordillera and one for stable North America, with the Cordillera being about 1500 m higher for the same crustal thickness. The difference is interpreted to result from the systematically higher Cordillera lithosphere temperatures, 800-900C at the Moho compared to 400-450C for cratons, as previously concluded for almost all current or recent backarcs from a number of thermal indicators. Although there may be significant mantle and crustal composition differences, their effect on elevation appears to be second-order. The areas globally with very thick crust, including Tibet and the central South America Cordillera (Altiplano/Puna), fit well on the same hot backarc crustal thickness vs elevation relation as the North America Cordillera. We conclude that the elevation of most mountain belts is supported thermally by about 1500 m. Areas with unusually thick crusts have additional elevation support by crustal roots. It has been previously concluded that continents have a strongly bimodal division of thermal regimes; most backarcs are uniformly hot and systematically much hotter than stable areas. We now conclude that there is a similar bimodal division of elevation between high backarcs and low stable continental regions, after correction for differences in crustal thickness.
Lithospheric Thickness and Thermal Regime in the Canadian Shield: Constraints from Heat Flux and S wave Travel Time Delays
Available heat flux data cover the Southern part of the Canadian Shield and extend northward to the regions
east and west of James Bay. The northernmost data are located near the core of the North American craton
where one expects the
lithosphere to be thickest. All the heat flux values in the James Bay area are less
than 34 mW~m-2 and are amongst the lowest values recorded so
far in the Shield.
In the Canadian Shield, heat flux variations occur at wavelengths
<100 km and are mostly of crustal origin. Local averages in two
200×200 km windows located on Archean areas at high
latitudes (north of the 51°N parallel) on either side of
James Bay are 29 mW~m-2 and 31 mW~m-2, the lowest values
found so far at this scale in the Canadian Shield. In order to
elucidate the deep thermal structure of the Canadian Shield
lithosphere, we have used S-wave travel-time delays from the tomographic
model of Shapiro and Ritzwoller . Travel time delays vary between
-0.5 and -3s but show no significant correlation
with averaged surface heat flux,
confirming that these surface heat flux variations are
mostly of crustal origin. Travel-time
delays require only small variations in the heat flux at the base of the lithosphere.
These lateral variations in basal flux beneath the Canadian
Shield are not
larger than ±3 mW~m-2. Such small variations of heat supply at the base of
Precambrian lithosphere are associated with changes of lithospheric
thickness that may be as large as 100 km.
[0.25cm] Shapiro, N.M., and M.H. Ritzwoller (2002), Monte-Carlo inversion for a global shear velocity model of the crust and upper mantle, Geophys. J. Int., 151, 88-105.
The EarthScope USArray Observatories: Status and Results
The EarthScope USArray program includes three seismic and two magnetotelluric components. The USArray
seismic components consist of the Transportable Array (TA), the Flexible Array (FA), and the Reference
Network. The TA component of USArray has now occupied over 700 sites in the western United States, from
the Pacific coast through the Rocky Mountains. The three component broadband TA stations are deployed in a
grid-like arrangement, with 70 km separation between stations. At any given time there are approximately 400
station sites, occupying a ~2000 km by 800 km "footprint." Each station is operated for two years. The FA
component of USArray provides a pool of instruments, ranging from high frequency geophones to three-
component broadband sensors, and these instruments are typically deployed for focused geological targets
for time periods ranging from days to years. Finally, the Reference Network provides a fixed, permanent
reference frame for the TA and FA, with approximately 100 broadband stations deployed across the contiguous
US, at roughly 300 km spacing. The magnetotelluric (MT) component of USArray consists of both a fixed
reference network as well as a transportable array of instruments that are deployed campaign style, using a 70
km by 70 km grid.
The geographical extent of USArray allows unprecedented observation of geophysical targets. Instruments
have been deployed across the west and mid-west of the US, with TA stations presently moving into the states
spanning a north-south line from North Dakota to Texas. MT observations in Cascadia have been augmented
by corresponding observations in Canada. Similarly, as the seismic TA moves east, plans are being
developed to collaborate on TA seismic observations on both sides of the US-Canada border in the region of
the Great Lakes.
We will present the current status of USArray activities and progress to-date, with a special emphasis on
standardized data products that are produced from USArray data, including phase picks, wave-field
animations, observations of the ambient noise field, and MT transfer functions. We will also provide an
overview of USArray deployment plans, to facilitate collaborative experiments and investigations, and discuss
opportunities for the seismological education and research communities to participate in and leverage the FA
and TA efforts.
Ambient noise and earthquake tomography: the structure of the crust and uppermost mantle beneath the western US
The western US is undergoing broad and diverse deformation caused by strike-slip motion (California plate margin), subduction (Juan de Fuca and Gorda plates beneath the Pacific northwest), extension (Basin and Range province), and large-scale uplift (Colorado Plateau). These active tectonic processes are manifest in a variety of crustal and lithospheric structures. An integrated 3D seismological model of the crust and upper mantle across the entire western US on the spatial scale of these features is needed to identify the principal structural features across the western US, determine the relations between the features themselves and their surface expressions, and provide clues about their nature, origin and evolution. The ongoing deployment of the Earthscope/USArray Transportable Array (TA) coupled with advancements in seismic methodology allows for the development of a high-resolution 3D seismic model of the crust and upper mantle beneath the western US. In this study, we apply two methods together, ambient noise tomography and multiple-plane-wave tomography, to seismic data observed at over 700 TA broad-band seismic stations. We produce broad-band surface wave dispersion maps from 8 to 100 sec across the entire western US with unprecedented resolution and use the local Rayleigh wave phase speed curves to construct a unified isotropic 3D Vs model to a depth of 150 km. Crustal and uppermost mantle features that underlie the western US are revealed in striking relief. High velocities are seen associated with various tectonic processes, including the subducting slab of Juan de Fuca and Gorda plates, the downwelling lithosphere beneath the southern Central Valley and the Transverse Ranges, and the thick lithosphere of the Rocky Mountains and Colorado Plateau. Low velocities are imaged beneath the High Lava Plains, the Great Basin, and the Snake River Plain in the upper mantle associated with elevated temperature and/or possibly the presence of partial melt.
Utilizing USArray to image the structure and infer the evolution of the North-American Plate