Seismic reflection profiling across exposed upper and middle crustal rocks of the Ruby-East Humboldt metamorphic core complex delineates important characteristics of the crustal structure developed during Tertiary extensional deformation. The goals of this study were to trace a Tertiary extensional shear zone from mylonitic surface outcrops into a seismic section and to characterize the deeper crustal fabric associated with the polyphase deformational history of the complex. Reflections in the shallow surface correlate with a plastic to brittle shear zone that formed during the tectonic unroofing of the middle crustal rocks. Constructive interference from strong planar layering in the mylonitic shear zone is considered chiefly responsible for generating the reflections. East and southwest dipping reflectors in the seismic section appear to correlate with exposed, opposing dipping mylonitic foliation domains. The opposing dips of the mylonitic layering may reflect warping of the normal-sense shear zone during tectonic exhumation coupled with an overall anastomosing character. Deeper in the section, a heterogeneous reflection character correlates with increases in velocity shown from wide-angle measurements in this region and is interpreted as penetrative fabric originating from extensional flow facilitated by broad-scale pure shear and localized simple shear. Lower crustal rocks apparently achieved granulite facies metamorphism during extension. A maximum of 6–8 km of mafic material could have been added to the crust during Cenozoic extension. The Moho reflection has such a high amplitude that it may be caused by partially molten rocks interleaved with peridotite. Reflections from the Moho show no significant upwarping of the base of the crust beneath the core complex. This suggests that the lower-crustal configuration is at least as young as the period of extensional activity that lasted from approximately 40 to 20 Ma which was responsible for the exhumation of the metamorphic core complex.
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