RETIMING THE NEPHI SEGMENT OF THE WASATCH FAULT ZONE, UTAH

The Nephi segment is one of the most distinct, yet least understood parts of the Wasatch fault zone (WFZ) in Utah. Early work on this part of the WFZ by Woodward-Clyde geologists showed evidence for multiple Holocene surface ruptures at North Creek. Machette (USGS) mapped the surficial geology along the segment in 1983-84, and Michael Jackson (Univ. of Colorado) trenched the southern part of the segment and reconfirmed three Holocene earthquakes. With the creation of the Mt. Nebo Wilderness area, it seemed that all the good sites on the segment had been trenched. However, in 2003 Bill Lund (Utah Geological Survey) reassessed the segment's paleoseismic record for seismic hazards and Chris Duross (UGS) deciphered new slip rates from the scarp's morphology. As a result, the UGS identified the Nephi segment as Utah's highest priority for further paleoseismic research—the hunt was on for a new trenching site.

Remaining questions about the paleoseismic history of the segment include: 1) the most recent surface-rupturing earthquake (P1) is known only to be <1,110 yrs from charcoal in an unfaulted debris flow; 2) the penultimate (P2) earthquake is either >1,650 or >3,640 yrs based on four C14 dates from the A horizon on fault-scarp colluvium 2; 3) the oldest (P3, undated) recorded earthquake is inferred from a terrace inset into the uplifted block at North Creek and is known to be <4,580 yrs based on the age of charcoal in fan-head deposits of North Creek. (All dates are in ¹⁴C yrs B.P.). Jackson's results at Red Creek are equally problematic and do not refine the existing two-decade old chronology.

In 2004 we identified a small parcel of U.S. Forest Service land that was excluded from the Mount Nebo Wilderness area at Willow Creek near the middle of the Nephi segment. Here the WFZ has displaced fan and side-stream deposits about 8 m and formed steep multiple-event scarps. We dug two 4-5 m deep trenches at Willow Creek, identified 3 colluvial wedges, sampled charcoal and A-horizon organics for AMS radiocarbon dating, and sampled fine-grained eolian and colluvial sediments for luminescence dating. About 50% of the net vertical displacement is accommodated by gentle tilting of fan deposits on the hanging-wall block, perhaps due to massive landslide deposits beneath Holocene fan alluvium. Results from this site should answer most of the questions posed above.