Hennigh, Quinton T., Evolving Gold Corp., 105 South Sunset St., Units K & L, Longmont, CO 80501
Consensus among geologists is rare, yet there is near unanimity of opinion that extensional tectonism has given rise to the Great Basin and its basin and range geomorphology. That, however, is where agreement ends. Views concerning the magnitude of extension vary from a mere 10% to an extreme, >100%. A growing body of evidence indicates that the latter of these numbers may prove closer to correct. Continental-scale seismic analysis indicates that the crust of the Great Basin is ~50% or less as thick as surrounding continental crust. This is readily evident in its anomalously high geothermal gradient. Voluminous bi-modal volcanism, indicative of lower crustal and upper mantle melting, has accompanied extension from Late Eocene to present, another testament to high heat flow resulting from significant crustal thinning. Seismic velocity profiles across parts of the Great Basin not only provide evidence that the crust has been appreciably attenuated, these data suggest that upper-crustal “blocks” are segregating, essentially pulling apart from one another. Seismic sections from the Great Basin closely resemble those from highly attenuated passive margins. Gravity data not only indicate crustal thinning, they provide clues concerning crustal block dismemberment. While it is acknowledged that sharp, ubiquitous gravity gradients reflect contrasting densities of subsurface lithologies, especially bedrock versus valley fill, it is the author’s opinion that these are actually providing us a view of something even more fundamental. Sharp gradients appear to define an expanded “jig-saw puzzle” of upper-crustal blocks across the region. Patterns of normal faulting serve as the best testament to pervasive hyper-extension. Cross sections from the Great Basin often illustrate ubiquitous “piano-key” normal faulting. Early extensional faults at times appear to lay flat and are cut by successively younger, steeper normal faults. In extreme cases, early faults are rotated into positions that give them an appearance of having a reverse sense of motion. Brittle rocks such as limestone where juxtaposed against ductile rocks such as shale sometimes generate boundinage-like deformation during extension. Structural patterns relating to extension can be observed at nearly every scale. Careful analogue modeling can provide insight into the processes of extension. These models replicate extensional fault patterns seen in the field and even illuminate subtle elements of deformation that, in some cases, have been misinterpreted as being related to compression. Analogue models sometimes reveal striking similarities to seismic sections from various parts of the Great Basin. Although the author acknowledges that the compilation of data presented in this paper is nowhere near complete, this hyper-extension model can serve as a worthy “working hypothesis” for exploration in the Great Basin by generating new ideas whether it be looking for fault offsets of a vein or the severed half of a mining district. Indeed, exploration is precisely what is needed to test the model's validity.