Catastrophic emplacement of giant landslides aided by thermal decomposition: Heart Mountain, Wyoming

Thomas M Mitchell,Steven A.F Smith,Mark H Anders,Giulio Di Toro,Stefan Nielsen,Andrea Cavallo,Andrew D Beard

doi:10.1016/j.epsl.2014.10.051

Abstract

The Heart Mountain landslide of northwest Wyoming is the largest known sub-aerial landslide on Earth. During its emplacement more than 2000 km3 of Paleozoic sedimentary and Eocene volcanic rocks slid >45 km on a basal detachment surface dipping 2°, leading to 100 yr of debate regarding the emplacement mechanisms. Recently, emplacement by catastrophic sliding has been favored, but experimental evidence in support of this is lacking. Here we show in friction experiments on carbonate rocks taken from the landslide that at slip velocities of several meters per second CO2 starts to degas due to thermal decomposition induced by flash heating after only a few hundred microns of slip. This is associated with the formation of vesicular degassing rims in dolomite clasts and a crystalline calcite cement that closely resemble microstructures in the basal slip zone of the natural landslide. Our experimental results are consistent with an emplacement mechanism whereby catastrophic slip was aided by carbonate decomposition and release of CO2, allowing the huge upper plate rock mass to slide over a ‘cushion’ of pressurized material.

Highlights

In the early 1900s, a low-angle detachment was recognized at Heart Mountain and initially attributed to thrusting of Paleozoic rocks onto Eocene rocks (Dake, 1916)
Consensus amongst workers today is that emplacement was catastrophic, taking on the order of minutes to hours, and that initial movement of the landslide was triggered by Eocene volcanic activity (Aharonov and Anders, 2006; Anders et al, 2010; Beutner and Craven, 1996; Craddock et al, 2009; Goren et al, 2010a, 2010b; Hughes, 1970; Malone et al, 2014; Melosh, 1983)
In an attempt to reproduce some of the distinct microstructures seen in the basal slip zone of the landslide at White Mountain, we performed ten low-to-high velocity friction experiments on powdered carbonate rocks collected from the landslide

Summary

Introduction

In the early 1900s, a low-angle detachment was recognized at Heart Mountain and initially attributed to thrusting of Paleozoic rocks onto Eocene rocks (Dake, 1916). Of the Heart Mountain landslide include hydroplaning on cognate waters (Voight, 1973), injection of pressurized volcanic gases at the slide’s base (Beutner and Gerbi, 2005; Hughes, 1970), earthquake acoustic fluidization (Melosh, 1983), granular dynamic fluidization (Anders et al, 2000), hydrothermal overpressuring (Aharonov and Anders, 2006; Goren et al, 2010a, 2010b) and, more recently, overpressuring from generation of CO2 gas caused by thermal decomposition of the basal carbonates (Anders et al, 2010; Beutner and Gerbi, 2005). We show that hypotheses of landslide fluidization by CO2 gas generation, hitherto speculated at, are well supported by a combination of our experimental results and field observations

Natural observations

Experimental methods

Results

Microstructural observations

Estimates of temperature increase within experimental slip zones

Discussion