GSA Annual Meeting in Denver, Colorado, USA - 2016

Paper No. 89-11
Presentation Time: 11:10 AM


ANDERS, Mark H., Department of Atmospheric, Oceanic, and Earth Sciences, George Mason University, Fairfax, VA 22030,

Unlike rock falls and rock avalanches, slide blocks are masses of rock that are emplaced with minimum breakup of the upper plate. Here I will present a detailed examination of the base of large (>km3) slide blocks to assess their respective emplacement mechanism. Because the upper plate is not substantially broken up, it is often assumed the emplacement was due to incremental movement over long periods of time. To the contrary, most of the slide blocks I have studies exhibit evidence of a single catastrophic emplacement event. The evidence of rapid emplacement includes basal layers of disaggregated material formed either by overriding material or breakup of the upper plate. This material exhibits fluid flow pulsing and inverse grading typical of fluidized granular material. In the upper plate of slide blocks there are extensive clastic dikes that are linked to the basal fluidized layer. These fluidized clastic dikes are driven by basal pressure and extend 10s to 100s of meters into the upper plate. The pulses of fluidized material that are injected into the upper plate can be observed to have intruded into one another during the chaos of catastrophic breakup of the upper plate, however, the vast majority of dike networks are continuous. Slide blocks also exhibit asymmetric deformation where within a meter of the base there is intense deformation of the upper plate while deformation in the lower plate can be minimal to non-existent. Furthermore, the base of slide blocks lack evidence of stress cycling as demonstrated by the absence of a microfracture aureole typically produced by repeated rupturing of a tectonic fault. Large carbonate slide blocks also exhibit evidence of decarbonization that can facilitate catastrophic movement on low-angle surfaces by reducing basal friction. Many of the features observed in the basal layer have been reproduced experimentally by simulating catastrophic emplacement. Moreover, recent studies of clumped isotopes of carbonate rock surrounding the basal layer support catastrophic over incremental emplacement.