The Hudson Bay Lithospheric Experiment - University of Bristol

BASTOW ET AL.: HUBLE2: Receiver functions(a)S waveP wavePpPs PsPs Pp Ps+PpSsthree-componentseismic stationMohoReceiver functions are time series, computedfrom three-component seismograms, whichshow the relative response of Earth structurenear the receiver (Phinney 1964). Thewaveform is a composite of P-to-S convertedwaves that reverberate in the structurebeneath the seismometer (figure 5a). Theythus carry valuable information aboutvelocity discontinuities in the Earth suchas the crust–mantle boundary (the Moho).A radial receiver function (figure 5b) candirect P wave(b)PsPpPsPpSs+PsPs5: Receiver function analysis of velocity structure beneath a seismic station. (After Ammon 1991)PsSsbe computed by deconvolving the verticalcomponent seismogram from radial (SV)component (and the same procedure for thetangential component, SH). In the frequencydomain this can be written:H(w) = R(w) / Z(w) (1)where w is the angular frequency 2f.Z(w) and R(w) are the Fourier transformsof the vertical and radial seismograms.H(w) is the Fourier transform of the receiverfunction.3: Shear-wave splittingSeismic anisotropy – the directionaldependence of seismic wavespeed – canresult from the alignment of minerals suchas olivine in the crust and mantle, andthe preferential alignment of fluid. In ananisotropic medium, a shear-wave willsplit into two orthogonal components,one travelling faster than the other. Thisphenomenon is known as shear-wavesplitting. Analysis of shear-wave splittingis common amongst seismologists (e.g.Silver and Chan 1991) because the splittingparameters (the orientation of the fastshear wave) and t (the lag-time betweenthe fast and slow shear wave, see figure6) can be related readily, for example, tomantle flow (e.g. Fouch et al. 2000), thepreferential alignment of magma (e.g.Blackman and Kendall 1997, Bastow et al.2010), pre-existing fossil anisotropy frozendeep in plates (e.g. Helffrich 1995, Bastowet al. 2007), or any combination thereof.sources of cultural noise, has been extremelyhigh. This, in turn, has enabled us to place fundamentalnew constraints on the deep structureof the Earth beneath this region.A receiver function (see box 2) study of crustalstructure (Thompson et al. 2010) providedfresh insight into the processes that shaped andformed Earth’s oldest crust. Formation of theCanadian Shield is likely to have evolved fromprocesses characterized by a hot ductile regimeduring the Paleoarchean, to those more closelyresembling modern day-style plate tectonicsby the Paleoproterozoic. An SKS shear-wavesplitting (see box 3) study of seismic anisotropy(Bastow et al. 2011) provided further evidencein support of this view, with strong anisotropicfabrics preserved deep in the North Americanplate. These recorded the 1.8 billion-year-oldmountain-building event called the Trans-Hudson Orogen that geologists have speculatedwas of similar scale and nature to the currentTibetan–Karakoram–Himalayan orogen ofAsia (e.g. Hoffman 1988, St-Onge et al. 2006).As well as shedding light on the ancient lithosphericprocesses that shaped northern Canada,our seismic data are capable of addressing thegeodynamic puzzle of why Hudson Bay (thelargest intracratonic basin in the world) existsat all. One hypothesis for the existence of theBay is that a down-welling in the underlyingmantle is dragging the Hudson Bay basin downbelow sea level. Using receiver function (box 2)analysis to measure the thickness of the mantletransition zone, however, we have shown thatthere is no evidence for a thermal anomaly –hot or cold – beneath the region. This impliesthat whatever the reason for the existence of theBay, it is almost certainly confined to the uppermantle (Thompson et al. 2011 in press). Ouranalysis of shallow crustal structure beneaththe region also indicates that the Hudson Baybasin may owe its existence, at least in part, tocrustal thinning (Pawlak et al. 2010).The innovative seismograph station designsemployed during the HuBLE experiment showclearly that successful seismic experiments canbe conducted in even the harshest and remotestof environments. The challenge now is thus toexplore seismically Earth’s more remote regions,as well as the sunnier climes where our databaseof constraints is increasingly well established. ●I D Bastow (ian.bastow@bristol.ac.uk), J-MKendall, G R Helffrich, D A Thompson, J Wookey,A Horleston, School of Earth Sciences, Universityof Bristol, UK; A M Brisbourne, D Hawthorn,SEIS-UK, Geology Department, Universityof Leicester, Leicester; D Eaton, Departmentof Geoscience, University of Calgary, Alberta,Canada; D B Snyder, Geological Survey ofCanada, Ottawa, Ontario, Canada.Acknowledgments. HuBLE-UK was supportedby NERC grant NE/F007337/1, with logisticalsupport from the GSC, CNGO, SEIS-UK and FirstNations communities of Nunavut. J Beauchesneand J Kendall provided invaluable assistance in thefield. IB is funded by the Leverhulme Trust.ReferencesAmmon CBull. Seis. Soc. Am. 81(6)Bastow I et al.Geophys. Res. Lett. 34(5)Bastow I et al.Geology 39(1)6: Shear-wave splitting in an anisotropicmedia. When the two pulses reach aseismometer, seismologists can measurethe differential time and polarization of theenergy. These parameters can then be usedto characterize the anisotropy.Bastow I et al.Geochem. Geophys. Geosyst.Q0AB10Blackman D and J KendallPhil. Trans. R. Soc.Lond. 355Cawood P et al. GSA Today 16(7)Eaton D and F DarbyshireTectonophysicsEaton D J et al. EOS Transactions 86Fouch M A et al. J. Geophys Res. 105Furnes H et al. Science 315(5819)Hamilton WGSA Today 13(11)Helffrich G J. Geophys. Res. 100(B9)Hoffman PAnnual Review of Earth and PlanetarySciences 16Hopkins M et al. Nature 456Pawlak A et al. Geophys. J. Int. 184Phinney R A J. Geophys. Res. 69Silver P and G Chan J. Geophys. Res. 96St-Onge M et al. Tectonics 25 (4)Stern R Geology 33(7)Thompson D A Earth Planet. Sci. Lett. 297Thompson D A et al.Earth Planet. Sci. Lett. 3126.24 A&G