At least 105% nearly east-west extension of late Cenozoic age has been accommodated by down-to-the-east normal and oblique-slip faults in the central Wassuk Range. One northwest striking, right-lateral strike-slip fault probably contributes an additional component of WNW-ESE slip. Movement on the normal and oblique-slip faults in this area tilt the Ternary and Mesozoic section and older faults westward; faults were initiated at high-angles (45-75°) and rotated to shallower dips during movement and by movement on more recent faults. Late Cenozoic faults can be divided into four age groups: 1) the earliest in the area was a down-to-the-east, spoon-shaped, normal fault, presently dipping gently to the east. This fault dipped moderately to steeply eastward when active and displaces Tertiary and Mesozoic rocks a minimum of 3000 ft. Movement on this fault post-dates eruption of silicic ash-flow tuffs (22-28 Ma), which are tilted moderately to steeply westward (40° to overturned), and pre-dates deposition of Wassuk Group sedimentary rocks and flow rocks ([approximately]9 Ma). The Wassuk Group is tilted shallowly to moderately westward (14-64°). 2) Several parallel, northwest sulking, moderately dipping, down-to-the-east normal faults were active during deposition of the base of the Wassuk Group ([approximately]9 Ma). The sum of movement on these faults is 6150± 250 ft. 3) One vertical right-lateral strike-slip fault and two younger, moderately dipping, coeval oblique-slip faults began movement during deposition of the Wassuk Group, and pre-date eruption of basalt flows (7 Ma). The strike-slip fault has components of vertical and horizontal movement with a maximum horizontal offset of about 11,000 ft. The two oblique-slip faults have an offset of about 13,750 ft. One oblique-slip fault strikes northwest and has components of right- and dipslip; the second oblique-slip fault strikes northeast to east and has components of dip- and left-slip. One or all of these faults have drag folded parts of the Wassuk Group into a syncline. 4) North-northwest striking, down-to-the-east range-front normal faults were active after eruption of the basalts (7 Ma), which are tilted 5-12° westward. There is evidence of Holocene movement on some of these faults. The orientation of extension was S85-89°E during the earliest phase of faulting ([approximately]22- 9 Ma) and underwent a small change in orientation to S69-77°E, before or during movement on the oblique- and strike-slip faults at [approximately] 9 Ma. It is uncertain if this extension orientation persisted during movement on the range-front fault system. Pebble count data suggest that the Cottonwood Springs, Penrod Canyon, and Reese River faults were active during deposition of the Wassuk Group. This, taken in conjunction with the sedimentological evidence (angular clasts, coarse clast size, and poor sorting), suggests that clasts were mostly locally derived and were deposited on alluvial fans adjacent to fault scarps.
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