Continuity Between Surface and Deep Lithospheric Deformational Grain for the Northern Cordillera Using Transportable Array Receiver Functions

Abstract:

We present results from azimuthal receiver function analysis imaging structural contrasts and anisotropic fabrics that together compose tectonic grain of the northern Cordillera crust and lithosphere. We compare the imaged tectonic grain at depth to surface geology as well as other geophysical observables such as shear wave splitting, surface wave anisotropy, and stress-strain indicators from geodesy and seismicity.

The best-resolved quantities from azimuthally varying receiver function conversions are the strike and depth of contrasts in dipping fabric or of dipping interfaces between isotropic bodies. We find a strong geographic gradient in such anomalies, with large amplitudes extending inboard from the present-day subduction margin, the Aleutian arc, and an influence of flat-slab subduction of the Yakutat microplate north of the Denali fault. Anomaly amplitudes correlate with structural intensity (spatial abundance of aligned geological elements) but are highest in areas of strong Cenozoic deformation. Imaged subsurface strikes show alignment with a variety of surface geological features such as major faults and shear zones including Denali and Tintina-Kaltag. However, intra-fault blocks show strikes with deep alignments of fabric parallel to major block-bounding faults, but also sets of imaged strikes with oblique orientations. The latter include alignments with ancient metamorphic fabrics as well as neotectonic deformation features, belts of seismicity, fault lineaments, and prominent metallogenic belts, all at high angles to block-bounding faults.

Of particular interest are concentric strikes seen around arc volcanoes that imply dipping magmatic structures or fabric well into the middle crust. We observe general coherence of the signal across the brittle-plastic transition and suggest that the underlying tectonic grain in the northern Cordillera is broadly distributed rather than strongly localized. Our results show variations at scales corresponding to regional geology and interstation scales. They resolve features finer than those seen in more smoothed results from ambient noise anisotropic surface wave tomography from the crust and SKS splitting presumably originating mostly in the mantle, which both show broad-scale alignment with major block-bounding fault orientations.