ABSTRACT

The widespread (2 x 105 km2) infravolcanic, Eocene, allegedly planar “surface” in the Basin-Range (B/R) province is universally considered to indicate long-lasting erosion (“peneplanation”?) on vertically immobile crust deformed during the Laramide orogeny. This allegedly manifests secular tectonic stability between Laramide and B/R orogenies. Faults cutting this “surface” and the volcanic rocks overlying it are widely believed to demonstrate the onset of B/R faulting. I doubt this because: (1) the “surface” is not everywhere planar; its intrarange topographic relief exceeds 0.1 km; (2) its observed remnants lie on modern B/R range blocks; planar remnants of this surface may be parts of pediments formed on adjacent Eocene B/R ranges as regionally temporal but not necessarily widespread interrange hypsometric correlatives; (3) its unobserved alleged remnants in buried basin blocks may be depositional; (4) the pediments formed in warm, humid climates which possibly produced pedimentation rates high with respect to Eocene B/R range uplift rates, and few pediment gravels because of intense chemical weathering of mostly Paleozoic limestones; (5) in at least four B/R basins, the deeply buried (depositional) surfaces lie on drilled Eocene continental deposits over 2 km thick, but no Eocene deposits occur on adjacent B/R ranges between the Oligocene ignimbrites and the deeply eroded Paleozoic strata, thus indicating contemporaneity of B/R range uplift and adjacent B/R basin sinking in Eocene; and (6) the “surface” had been and was being formed up to the time of Oligocene volcanism, I conclude that the alleged Eocene “surface” was not one widespread static surface, but many local dynamic surfaces, formed in equilibrium with major Eocene B/R tectonism; throws of this “surface” are not necessarily manifestations of the onset time of B/R faulting.

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