Abstract

In the Basin and Range between Salina, Utah and Lake Mead, Nevada, several types of late Tertiary synextensional structures record contrasting styles of extension-normal contraction at or near the Basin and Range/Colorado Plateau (BR/CP) boundary. The structures include 1) conjugate northeast-striking sinistral and northwest-striking dextral faults, 2) zones of tectonic escape, 3) components of strike slip on northerly striking non-conjugate faults, 4) zones of steep-axis bending and associated faulting, and 5) east-west trending, extension-parallel folds, many of which appear to absorb strain associated with faults of categories 1 and 2. Following are large-scale examples of each type, but each ranges widely in scale. 1) The broadly contemporaneous northwest-striking right-lateral Las Vegas Valley shear zone and northeast-striking left-lateral Lake Mead fault system, each with tens of kilometers of displacement, displace the Great Basin sector of the Basin and Range southward. These faults intersect in the Lake Mead area where 20–50 km of Miocene north-south shortening has occurred. 2) Temporally and genetically associated with the fault intersection in 1) is a mobile zone of westerly tectonic escape that accommodated much of the displacement on the conjugate faults by at least 65 km of westerly tectonic transport of an east-narrowing structural wedge. Frenchman Mountain and the basin beneath Las Vegas to the west are part of the escaped wedge, which may be a unique boundary-condition feature in the cordillera. 3) Directly west of the BR/CP boundary at Salina and Gunlock, Utah and in the vicinity of Virgin Canyon, Arizona, subparallel right- and left-slip faults bound km-scale blocks that appear to have been displaced approximately parallel to BR/CP, as though by a component of basal traction related to boundary-parallel flow of substrate. 4) West of Cedar City, geologic and paleo-magnetic data define a crudely triangular zone of strong counterclockwise steep-axis rotation and associated faulting extending for as much as 120 km east-west and 40 km north-south—the Caliente-Enterprise zone. This large rotational strain has been interpreted as driven by basal traction and, depending on the traction model chosen, the strain could record 0- 20 km of north-south shortening. I favor traction caused by complexly deflected ductile flow subparallel to the province boundary, a choice that allows for the maximum amount of north-south shortening. A large area of clockwise steep-axis rotation in the vicinity of Lake Mead probably also reflects large north-south shortening associated with tractional forces. These interpretations are at odds with widely accepted interpretations of the rotations as shear-related bending strains. 5) Syn-extension folds with axes subparallel to the extension direction are widespread in the vicinity of the BR/CP boundary. As with the structures of category 4), there is no general agreement on their tectonic significance. Folds range from outcrop size to crustal scale with amplitudes as much as 10 km. Many are well developed and excellently exposed in the Lake Mead area where they reflect the late stages of extension-normal contraction associated with the conjugate faults of category 1) above. Some folds absorb components of strike slip on faults of category 3) above, an interpretation I apply to the deep basins along Interstate 15 directly west of Virgin Canyon. Others appear to reflect fault-parallel plan-view rock flowage commonly exhibiting displacement gradients that increase toward the faults, and still others may simply reflect variations in displacement gradients along the trace of normal faults with possible examples beneath the Rush Lake and Quichapa portions of Cedar Valley. At most, the latter would reflect small amounts of extension-normal contraction. Future progress in understanding the tectonic evolution of the BR/CP boundary will depend more on investigations of the extreme variation in structure along it than across it. Much of that extreme variation is associated with extension-normal contraction.