An integration of geological and geophysical techniques has been used to characterize the internal structure of the Little Valley Landslide in Draper, Utah, USA. The Little Valley Landslide is a pre-historic landslide as old as 13ka B.P. It is found to consist of chaotic and disturbed weathered volcanic units derived from Tertiary age volcanics that comprise a great portion of the Wasatch Range. Geotechnical investigations that were integrated with the geophysical results included excavation of trenches and drilling of boreholes. Geophysical methods, in particular high-resolution seismic data, were used to provide a framework for interpreting the geotechnical observations. High-resolution seismic reflection data, seldom used in landslide investigations, were acquired and processed in order to image the basal or glide surface of the landslide and the structure underlying the landslide. The integration of the geotechnical and geophysical investigations provided a better understanding of the geometry of a portion of the Little Valley Landslide. Trenching and drilling identified landslide material in the subsurface. The high-resolution seismic reflection data imaged the glide surface with the onset of coherent reflectivity. A decollement or glide surface underlies the landslide indicating a large mass movement. The glide surface is observed on the seismic reflection profiles to be deepest in the center portion of the landslide. It is observed in the seismic reflection images to shallow up slope and creating a trough-like shape feature. A contour map modeling the middle of the Little Valley Landslide is derived from the seismic data. This study shows that seismic reflection techniques can be successfully used in complex alpine landslide regions. They are also efficient and cost-effective tools when compared to trenching and drilling investigations. The seismic data can (1) provide a framework to link geological data and (2) take the place of an extensive trenching and drilling program.

The East Traverse Mountains are an E-W trending mountain range dividing Utah and Salt Lake valleys in northern Utah. Geologically perplexing, the nature of the East Traverse Mountains has been under investigation for 140 years. Previously, the mountain range was proposed to be a dismembered but still coherent down-faulted block that experienced 4 km of post-thrusting extension within the Charleston-Nebo thrust sheet. However, new insight on the origin of the East Traverse Mountains indicate that it is a mega landslide, roughly ~100 km3 in size, which catastrophically slid from the upper reaches of the Little-Cottonwood stock to its present-day location. The primary evidence for this landslide includes two unusual dike swarms whose roots are in the Wasatch Range and whose upper reaches are now in the East Traverse Mountains, 16 km to the SW. A swarm of pebble dikes, indicative of porphyry mineralization is found at the center of the East Traverse Mountains and contain pebbles of Little-Cottonwood stock as well as two other intrusions found at the center of a mineralized zone. These granitic clasts have phyllic alteration, contain molybdenite grains and are sourced from a subeconomic molybdenum-copper porphyry deposit located 16 km to the NE. The other dike swarm occurs on the SE corner of the range near Alpine, Utah, which contains various andesitic and phaneritic dikes of intermediate-felsic compositions (56-69 wt.% SiO2) with localized marble on their southern margin. These dikes range in U-Pb ages from 36-29 Ma. Moreover, other evidence includes brecciation of the entire mountain range as well as along the slide path of this landslide. Breccia, as well as pseudotachylyte and cataclasite have been discovered that formed in the rapid transportation of the 1-2 km thick detached block. Devitrified pseudotachylyte veins range in thickness from 1 cm to 1 m and are present in the roof zone of the pluton. Sixteen kilometers of sliding caused 70-80% of the Oquirrh Group rocks of the East Traverse Mountains to be fractured to less than 1-inch diameter clasts in breccias and broken formations, as documented by 16 years of mining. U-bearing opal replaced significant areas of brecciated volcanic rocks when hot water seeped into highly-fractured, argillically altered rock. U-Pb ages of 6.1 ± 0.9 Ma from these opalite areas could provide a minimum age for the emplacement of the mountain block. Underlying the East Traverse Mountains slide block is a layer of fallout tuff deposited in the Jordan River Narrows member with 40Ar/39Ar ages of 6.62 ± 0.07 Ma which provides a maximum age of emplacement. Therefore, we propose that the East Traverse Mountains mega-landslide occurred between 6.1 ± 0.9 Ma and 6.62 ± 0.07 Ma. Our interpretation for the East Traverse Mountains mega-landslide model builds upon previous research and data, with the addition of these recent findings. This new interpretation is crucial for understanding the potential for large normal fault systems to create significant landslide hazards.

The East Traverse Mountains are an E-W trending mountain range dividing Utah and Salt Lake valleys in northern Utah. Geologically perplexing, the nature of the East Traverse Mountains has been under investigation for 140 years. Previously, the mountain range was proposed to be a dismembered but still coherent down-faulted block that experienced 4 km of post-thrusting extension within the Charleston-Nebo thrust sheet. However, new insight on the origin of the East Traverse Mountains indicate that it is a mega landslide, roughly ~100 km3 in size, which catastrophically slid from the upper reaches of the Little-Cottonwood stock to its present-day location. The primary evidence for this landslide includes two unusual dike swarms whose roots are in the Wasatch Range and whose upper reaches are now in the East Traverse Mountains, 16 km to the SW. A swarm of pebble dikes, indicative of porphyry mineralization is found at the center of the East Traverse Mountains and contain pebbles of Little-Cottonwood stock as well as two other intrusions found at the center of a mineralized zone. These granitic clasts have phyllic alteration, contain molybdenite grains and are sourced from a subeconomic molybdenum-copper porphyry deposit located 16 km to the NE. The other dike swarm occurs on the SE corner of the range near Alpine, Utah, which contains various andesitic and phaneritic dikes of intermediate-felsic compositions (56-69 wt.% SiO2) with localized marble on their southern margin. These dikes range in U-Pb ages from 36-29 Ma. Moreover, other evidence includes brecciation of the entire mountain range as well as along the slide path of this landslide. Breccia, as well as pseudotachylyte and cataclasite have been discovered that formed in the rapid transportation of the 1-2 km thick detached block. Devitrified pseudotachylyte veins range in thickness from 1 cm to 1 m and are present in the roof zone of the pluton. Sixteen kilometers of sliding caused 70-80% of the Oquirrh Group rocks of the East Traverse Mountains to be fractured to less than 1-inch diameter clasts in breccias and broken formations, as documented by 16 years of mining. U-bearing opal replaced significant areas of brecciated volcanic rocks when hot water seeped into highly-fractured, argillically altered rock. U-Pb ages of 6.1 ± 0.9 Ma from these opalite areas could provide a minimum age for the emplacement of the mountain block. Underlying the East Traverse Mountains slide block is a layer of fallout tuff deposited in the Jordan River Narrows member with 40Ar/39Ar ages of 6.62 ± 0.07 Ma which provides a maximum age of emplacement. Therefore, we propose that the East Traverse Mountains mega-landslide occurred between 6.1 ± 0.9 Ma and 6.62 ± 0.07 Ma. Our interpretation for the East Traverse Mountains mega-landslide model builds upon previous research and data, with the addition of these recent findings. This new interpretation is crucial for understanding the potential for large normal fault systems to create significant landslide hazards.

Development of homes, roads, and commercial buildings in northern Utah has grown significantly during the last several decades. Construction has expanded from the valley floor to higher elevations of benches, foothills, and other elevated regions of the Wasatch Mountain Front. Construction in the higher elevation areas are a concern due to potential for landslides, both new and reactivated. Landslides have been identified in this region and are dated as Pleistocene to historical in age. A possible landslide of about 0.5 km2 on the south slope of Traverse Mountain has been mapped by the Utah Geological Survey in 2005. Its surface exhibits hummocky topography and is comprised of Oligocene-age volcanic ash, block and ash flow tuffs, and andesite lava. Landslides along the Wasatch Mountain Front are complex features usually characterized by dense vegetation and poor outcrop and require a combination geological and geophysical methods to study their thickness, slope, lateral extent, and style of emplacement. Our study incorporates trenching, boreholes, and LiDAR aerial imagery. Unique to the study of landslides is our use of seismic reflection with a vibroseis source over the mapped landslide deposit. The seismic parameters of source, station spacing, and processing method provide a coherent, albeit low-resolution, image of the upper 500 m of the subsurface beneath the landslide. A major reflector boundary in our seismic profiles has an apparent dip of 4° to the south, approximately parallel with the surface topography. Its elevation and seismic character are indicative of a contact between the Oligocene-age volcanic rocks on top of a portion of the Pennsylvanian-age Bingham Mine Formation, a mixed carbonate and siliciclastic sequence. The reflector defines an asymmetric graben-like structure bounded by a north-northwest-trending normal fault system. Analysis of trenches, boreholes and local geology reveals a faulted, chaotic body of block and ash flow tuffs, surrounded by andesite lavas. Using LiDAR and surface geological reconnaissance, a possible toe or margin of a landslide has been interpreted in the north-west portion of the study area. The combination weakened block and ash flow tuffs and abundant clay production from this unit contribute to the likelihood of a coalescence of landslides in this mapped landslide area. The integration of LiDAR, trenching, boreholes and reflection seismology provides the range and resolution of data needed to assess the complex geology of landslides.