2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 6
Presentation Time: 9:50 AM


MCCALPIN, James P., GEO-HAZ Consulting, Inc, P. O. Box 837, Crestone, CO 81131, mccalpin@geohaz.com

The East Traverse Mountains (ETM) is a 10 km-long, 400 m-high bedrock ridge in the hanging wall of the Wasatch fault zone (WFZ), at the boundary between the Salt Lake City and Provo segments. The Wasatch fault makes a right-angle bend here, forming a southwest-plunging intersection line (or “nose”) in the subsurface fault plane with an estimated plunge angle of ca. 25 degrees. The ETM is perched on this nose and is sliding SW down it, and is thus being pulled apart in 2 directions, perpendicular to the nose (NW-SE) and parallel to it (NE-SW). This tensional stress field has created a ridge-wide set of NE- and NW-trending normal faults, and shorter sets of N-S and E-W faults. The NE-trending (ridge-parallel) set has the largest displacement and intersects the Alpine-Draper water tunnel in 2 places 250 m below the surface. Between Nov. 2003 and August 2005 over 80 trenches and 20 boreholes have been dug as part of a landslide investigation for a 4000-acre residential subdivision in the central one-third of the ETM. Numerous planes of displacement have been exposed in the trenches, but it is often unclear whether they are nontectonic (landsliding, sackung), tectonic but nonseismogenic (secondary faults to the Wasatch), or tectonic and seismogenic. We employ the method of multiple working hypotheses to separate these 3 classes of structures. In trench exposures, landslide-related faults are composed of a 0.3-1 m-thick zone of yellow-green, banded, striated clay with friction angles as low as 16 degrees (in tuff bedrock) or alternating bands of cataclastic andesite and clay (in andesite flows). In some cases steep (rejuvenated?) normal fault scarps lie above flattened (backtilted) topographic benches, but no discrete landslide margins can be identified. These areas are interpreted as deep-seated gravitational spreading (sackung) that employs the upper parts of Tertiary faults. Finally, some fault scarps face across-slope or upslope and are unrelated to gravity processes, and may indicate tectonic movement on secondary faults that sole into the WFZ at a depth of about 1500 m below the ridge crest. Because the city geologic hazard ordinance regulates active faults differently than landslides, these distinctions between faults, sackung, and landslides have practical importance.