Abstract

The Ruby-East Humboldt Range consists of pre-Miocene igneous, metamorphic, and sedimentary rocks of complex structure. The adjoining basins contain deformed beds of the upper Miocene Humboldt formation. The boundary structure of the mountain block is well exposed because of dissection by the through-flowing Humboldt River.

This range is a westward-tilted horst bounded by normal faults which dip 60 to 70 degrees basinward. Displacements on the east boundary faults have been at least twice as great (5500 to 6000 feet) as on the west boundary faults (2000 feet). The northern termination of the mountains is due to intersection of the east and west boundary faults. The structure of the pre-Miocene rocks is discordant with the trend and shape of the range.

Five periods of Basin-Range faulting have been established: (1) Late middle or early upper Miocene, displacement small (open to question); (2) upper Miocene during deposition of the Humboldt formation, displacement larger: (3) later upper Miocene to Pliocene, younger than the Humboldt formation and older than the Pliocene (?) lava, amount of displacement unknown; (4) Pliocene to Pleistocene, later than the Pliocene (?) lava and extending to middle or late Pleistocene, the period of last major uplift of the range, displacement large; (5) late Pleistocene to Recent, later than the earliest Wisconsin, displacement small.

Lack of evidence of intense late Cenozoic compression indicates that the uplift is to be attributed to vertical forces operating while the crust was in a neutral or tensional state.

The scarps bounding the range are composite, in places rejuvenated by late Pleistocene or Recent fault movements.

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