| Abstrakt: |
Prior to Cenozoic extension, the area now occupied by the northern Great Basin in Nevada and Utah was an elevated (exactly how much is debated) orogenic plateau inherited from earlier Mesozoic crustal shortening. Ash flow tuffs, which erupted during the Eocene--Oligocène "ignimbrite flare-up," were able to disperse across vast distances by exploiting paleovalleys carved into this landscape by late Mesozoic or early Tertiary erosion, but not yet blocked by major extensional fault scarps. Normal faulting during this time was locally significant--for example, in the Elko-Carlin region of northeastern Nevada--but appears to have been spatially restricted to areas of active magmatism and was of limited magnitude on a province scale. Major extension began in the middle Miocene throughout much of what is now the northern Great Basin as the Sierra Nevada began to move westward away from the stable continental interior. The total magnitude and distribution of extensional strain during this time is not fully understood, but it appears to have been strongly partitioned into highly-strained vs. unextended domains. For example, the Colorado River region of southern Nevada was subject to greater than 100 percent extensional strain in the middle Miocene and became a type locality for large-magnitude continental extension, while a large portion of the Oligocene caldera belt in central Nevada to the north remained relatively undeformed over the same time period. Continued faulting from the late Miocene to the present, along with the development of the Walker Lane dextral shear zone in western Nevada, overprinted mid-Miocene and older structures and created the modern basins and ranges that now dominate the region. Outside of the large metamorphic core complexes in eastern Nevada and northwestern Utah, Cenozoic extension was accommodated primarily by normal faults that initiated at steep angles in the upper crust and rotated to shallower dips as they moved, resulting in tilting of pre-extensional Tertiary rocks and deposition of syn-extensional sediments in adjacent basins. The effect of extension on mineral deposits depends on their location and whether they formed prior to major extension (e.g., in the Eocene and Oligocene) or during widespread Miocene and younger normal faulting. As a first-order approximation, the degree of tilting in Cenozoic volcanic and sedimentary rocks deposited on older basement can be used as a guide to how much extension a given region is likely to have undergone. Generally, deposits or potential host rocks overlain or surrounded by gently-dipping to flat Oligocène and older deposits are likely undisturbed by major extension, while those overlain by strongly tilted Tertiary rocks are likely to have been displaced and tilted along with their cover rocks. [ABSTRACT FROM AUTHOR] |
