ROLE OF A RIGID BEDROCK SUBSTRATE ON EMPLACEMENT OF THE BLUE DIAMOND LANDSLIDE, BASIN AND RANGE PROVINCE, EASTERN SPRING MOUNTAINS, SOUTHERN NEVADA

Rock avalanches are high velocity mass movements of fragmented rock that experience unusually long runouts. Many explanations exist for their enhanced mobility. Whereas most of these hypotheses address intrinsic mechanisms, few have addressed the influence of runout path substrate material. Here, we investigate the role of a bedrock substrate in the emplacement of the Blue Diamond landslide deposit.

Sedimentary fabrics and internal morphologies were mapped and interpreted using standard facies models for large-rock avalanche deposits. The presence of basal fabrics and morphologies rule out a frictionless emplacement as these features indicate prolonged interactions at the avalanche-substrate interface. Observations are consistent with the rock avalanche enduring significant basal shear resistance during motion. However, height/length estimates indicate the rock avalanche experienced a low to moderate basal resistance.

Outcrop mapping and clast count data from 120 stations across the deposit revealed the intrinsic mechanisms that occurred during emplacement. Negative correlations between matrix and carbonate count frequencies indicate that areas with more matrix experienced greater levels of fragmentation and comminution. We hypothesized that basal shear stresses were transmitted upward through the rock mass during emplacement, resulting in the vertical transition from matrix-rich to matrix-poor breccia facies. Data on proportional breakage and enhanced mobility may best be explained by dynamic fragmentation.

Uncertainties about the genesis and distribution of the landslide deposit as originally interpreted led to an alternate hypothesis. The estimated deposit volume is about twice that of the proposed source area. Additionally, distal isolated landslide blocks are exposed on Blue Diamond Hill, ~200 m higher than the main deposit. We propose that the landslide breccia was derived from slightly north of the previously suggested source area, and that it slid into the Blue Diamond Hill site. Thereafter, the underlying formation underwent gravity gliding, to its current position, carrying the landslide breccia with it. This new hypothesis is consistent with the present distribution of the landslide breccia and estimated volume loss from the headwall.