Dynamics of the emplacement of the Heart Mountain allochthon at White Mountain: Constraints from calcite twinning strains, anisotropy of magnetic susceptibility, and thermodynamic calculations

Abstract

White Mountain is centrally located in the bedding-plane portion of the Eocene HeartMountain detachment and contains the only upper plate Mississippian Madison Group rocks that have been metamorphosed into marble. The marble rests upon the thickest (1 m) part of a carbonate ultracataclasite that marks the detachment. Thermodynamic and mechanical calculations based on possible frictional melting of calcite and other minerals, geochemical data, the characteristics of the carbonate ultracataclasite, and the geometrical characteristics of White Mountain suggest a possible initial upper plate emplacement rate of 126-340 m/sec and that the duration of the emplacement event was less than 4 min, too brief a time to develop an emplacement-related calcite twinning strain overprint in upper or lower plate carbonates. While the detachment-related carbonate ultracataclasite did not form by melting, it does preserve a magnetic fabric where K max is parallel to the detachment slip direction and records a westward and down paleopole (287° and 27°), where magnetite is the carrier mineral. The Eocene (49.6 Ma) paleopole for this latitude in North America was southerly and upward (0° and 45°). This brief and catastrophic detachment event produced a signifi cant amount of CO 2 by fl ash heating. This report is the fi rst to quantify the emplacement rate of the upper plate of the Heart Mountain detachment based on physical and geochemical parameters.

Episodic Dissolution, Precipitation, and Slip along the Heart Mountain Detachment, Wyoming

“…4). However, these authors all dismiss this brecciation as either deformation that occurred earlier during the catastrophic event (Anders et al 2000) or "pre-Heart Mountain detachment deformation" (Craddock et al 2009). They do not explain their reasons for interpreting the deformation that was observed only along the Heart Mountain detachment as being unrelated to slip along that fault.…”

“…Previous studies discussing the presence of banded grains similar to those in figures 7 and 8 interpret them as having formed during suspension within a pressurized fluid at the base of the detachment, constituting evidence of catastrophic slip (Hughes 1970;Beutner and Craven 1996;Beutner and Gerbi 2005;Craddock et al 2009Craddock et al , 2012Anders et al 2010). In the discussion of the processes that might have created the banded grains, the analogues of volcanic accretionary lapilli and impact ejecta were invoked, with an interpreted fluid of volcanic gas preferred for Heart Mountain (e.g., Beutner and Gerbi 2005).…”

“…Given these facts, any mechanical conception of the Heart Mountain detachment as forming catastrophically has at its disposal a broad palette of extraordinary circumstances affecting an area of thousands of square kilometers (e.g., fluid overpressure, magmatic devolatilization, seismic accelerations, bolide impact) that need to operate only for as little as a few seconds prior to emplacement rather than a million years or more (e.g., Hughes 1970;Pierce 1973Pierce , 1980Voight 1973;Melosh 1983;Sales 1983;Beutner and Gerbi 2005;Aharonov and Anders 2006;Craddock et al 2009Craddock et al , 2012Anders et al 2010Anders et al , 2013. The gradual emplacement hypothesis, on the other hand, Figure 2.…”

Volcanic Initiation of the Eocene Heart Mountain Slide, Wyoming, USA

“…The base of allochthonous rocks at White Mountain is defined by a horizontal, 2-m-thick carbonate ultracataclasite (CUC; Craddock et al 2009) that fed eight injectites of the same material, which are as much as 120 m above the slide base within marble and Madison Group limestones (Craddock et al 2012). Existing structural, mineralogical, and geochemical constraints indicate that the Heart Mountain slide moved at ∼100 m/s (Craddock et al 2009).…”

“…Existing structural, mineralogical, and geochemical constraints indicate that the Heart Mountain slide moved at ∼100 m/s (Craddock et al 2009). …”