Characterizing azimuthal anisotropy at the mid-lithospheric discontinuity in the Superior and Wyoming Cratons

Abstract:

The recent observation of a ubiquitous mid-lithospheric discontinuity (MLD) within tectonically stable Proterozoic- to Archean-aged lithosphere has led to questions about the formation of the MLD and the continental lithosphere. The MLD has been typically imaged as a drop in velocity from Sp receiver functions, but establishing a single mechanism for a global negative velocity gradient within stable continental lithosphere has proven difficult. One proposed mechanism, azimuthal anisotropy, has been suggested however, it is unclear how such anisotropy could produce a consistently negative phase in Sp receiver functions. More fundamentally, it is unclear how lithospheres with different tectonic histories would all have a boundary in anisotropy located at similar depths.

To better understand the relationship between the MLD, anisotropy and tectonic history we have calculated radial and transverse component Ps receiver functions for seven permanent US Array stations within the Superior and Wyoming Cratons. The two cratons are similar in age and collided during the Trans-Hudson orogen at ~1.8 Ga, forming the core of present day Laurentia. The receiver functions were generated using a multi-taper cross correlation technique and events were limited to distances of 30 to 100°, and magnitudes greater than 5.8. Seismograms were inspected for data quality; receiver function results were binned as functions of back azimuth and epicentral distance. At stations DGMT, EGMT, LAO, and LKWY, a large amplitude negative phase is observed 8-10 seconds after the direct P arrival on the radial component. This arrival corresponds to a depth of approximately 80 to 100 km and is interpreted to be the MLD. Preliminary analysis of the transverse component finds significant energy within the same time window as the radial component MLD with changes in polarity observed in some instances. Further work is needed before the character of these polarity changes, and their associated anisotropy, can be quantified. At stations AGMN, ECSD and RSSD, negative phase energy is found at 8-10 seconds on the radial component, however the energy is more diffuse, with multiple small phases present. Transverse component receiver functions also show evidence of anisotropy at MLD depths, but further work must be done before this can be verified.