Rock Deformation at the Base of Large Carbonate Block Slides: The Key to Assessing Emplacement Mode

We studied the bases of a number of large carbonate block slides in order to assess their modes of emplacement. There is a general assumption that if a large slide is not extensively broken up, it was emplaced by incremental movement over thousands to millions of years. However, our study of the basal surfaces of several large block slides indicates that many of them involve a single catastrophic emplacement event. In many of the block slides we examined, we found evidence of a basal layer we interpret as due to a single fluidization event. The basal layer is often inversely graded and can be traced into the upper plate as clastic dikes. This layer exhibits no crosscutting cataclastic flow or reincorporation of earlier-formed material. We suggest that the basal layer is due to calcining of carbonates by frictional heating during the early stages of a single catastrophic emplacement event. Examination of lower plate carbonates just below the basal layer by cathodoluminence reveals calcite filling the interstitial space of breccias representing a single fluid influx event. Moreover, there is a lack of healed microfracturing in arenites and of repeat brecciation of carbonates in the lower plate, which is also inconsistent with multiple movement events.

Where there is no clear evidence of gravity sliding – such as presence of toes, source areas, pressure ridges, or other supporting geophysical evidence -- differentiating between a block slide and a rooted detachment can be difficult. We use the criteria discussed above to assess the mode of emplacement in several debated examples. These include several allochthonous rock masses in the Shadow Valley of California, the Mormon Mountains of Nevada and the Beaver Dam Mountains of Utah that have been interpreted as due to significant crustal extension. We, in contrast, find them to be catastrophically emplaced block slides.