Joint 118th Annual Cordilleran/72nd Annual Rocky Mountain Section Meeting - 2022

Paper No. 16-2
Presentation Time: 8:30 AM


OSBORN, Gerald, Geoscience, University of Calgary, 2500 University Dr NW, Calgary, AB T2N1N4, Canada and GRIFFITH, Martin, Reno, NV 89434

Acknowledgement of the Basin and Range Province (BRP) as a premier example of extensional tectonics did not come easily. The popular 19th-century notion that mountain building was wrinkling of the crust as the earth cooled and contracted influenced the first interpretations in the BRP. The earliest field work (1867-72) was by Clarence King for the Survey of the 40th Parallel; he thought the ranges were mainly eroded remnants of “anticlinals” and the basins “synclinals”, but he also recognized displacement along faults which cut the folds. In 1872-73 G.K. Gilbert worked in the Great Basin for the Wheeler Survey and concluded that modern range relief was produced by faulting, not folding; he thus conceived of a whole new way to make mountains. Crustal extension was not yet a concept and Gilbert invoked “vertical tectonics”.

Scottish naturalist John Muir made three trips across parts of the Great Basin in 1876, 77, and 78 as a guide for the USCGS Survey of the 39th Parallel. He was unaware of Gilbert’s 1874 and 1875 reports and misinterpreted tectonic geomorphology to be glacial geomorphology, but made a startling observation recorded in his 1876 journal: "It is interesting to note that in this land of volcanic energy corrugated with successive mtns & deep sandy sinks as if the crust of the globe stretched weakly & uncertainly over a shifting fluid foundation...”. The grammar is shaky and Muir provides no elaboration or context, but this was the first time anyone had even imagined what we now call extension, in the BRP or elsewhere in the world.

In a 1901 revisit to the “Basin Ranges” Gilbert recognized that range-base faults were tilted and faulting necessitated lateral extension, but this conclusion was not explicitly published. Dissent emerged from J. Spurr, who in 1901 argued that alleged fault scarps were mostly erosional features, and C. Keyes, who in 1909 argued that the ranges were residual mountains left after wind erosion. But meanwhile G.D. Louderback in 1904 corroborated the fault-block theory using repeated stratigraphy. Fault orientations led W.M. Davis to conclude in 1928 that the region “has suffered a pronounced extension of its former east-west breadth”. By 1966 W. Hamilton and W. Myers argued for 100% extension in the BRP, but according to Hamilton it was not till almost 1980 that large-scale BRP extension was widely accepted.

  • gsa22powerpoint.pptx (42.2 MB)