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

Abstract

A B S T R A C TThe Heart Mountain allochthon is among the largest landslide masses in the rock record. The basal fault, the Heart Mountain detachment, is an archetype for the mechanical enigma of brittle fracture and subsequent frictional slip on low-angle faults, both of which appear to occur at ratios of shear stress to normal stress far below those predicted by laboratory experiments. The location of the detachment near the base of thick cratonic carbonates, rather than within subjacent shales, is particularly enigmatic for frictional slip. A broad array of potential mechanisms for failure on this rootless fault have been proposed, the majority of which invoke single-event, catastrophic emplacement of the allochthon. Here, we present field, petrographic, and geochemical evidence for multiple slip events, including crosscutting clastic dikes and multiple brecciation and veining events. Cataclasites along the fault show abundant evidence of pressure solution creep. Banded grains, which have been cited as evidence for catastrophic emplacement, are associated with stylolitic surfaces and alteration textures that suggest formation through the relatively slow processes of dissolution and chemical alteration rather than dynamic suspension in a fluid. Temperatures of formation of faultrelated rocks, as revealed by clumped isotope thermometry, are low and incompatible with models of catastrophic emplacement. We propose that displacement along the gently dipping detachment was initiated near the base of the carbonates as localized patches of viscous yielding, engendered by pressure solution. This yielding, which occurred at very low ratios of shear stress to normal stress, induced local subhorizontal tractions along the base of the allochthon, raising shear stress levels (i.e., locally rotating the stress field) to the point where brittle failure and subsequent slip occurred along the detachment. Iteration of this process over geological time produced the observed multikilometer displacements. This concept does not require conditions and materials that are commonly invoked to resolve the stress paradox for low-angle faults, such as near-lithostatic fluid pressures or relative weakness of phyllosilicates in the brittle regime. Cyclic interaction of viscous creep (here by pressure solution) and brittle failure may occur under any fluid pressure conditions and within any rock type, and as such it may be an attractive mechanism for slip on "misoriented" fault planes in general.

Fluid Flow, Brecciation, and Shear Heating on Faults: Insights From Carbonate Clumped‐Isotope Thermometry

“…An attempt was made to avoid large clasts when sampling, to aim for a more homogenous mixture of matrix material. The breccia varied widely from many meters thick and highly visible from up to 100 m away (e.g., Figure a) to under 1‐cm thick and barely detectable from 100 m away (Figure b; see also Swanson et al, ). Marble breccia (DET‐m) : These samples are exclusively from the unique basal cataclasite at White Mountain (Heart Mountain allochthon), where all of the hanging wall host is metamorphosed by local intrusions. These breccias were sampled from within 3 m of the detachment, but their distinctive appearance and proximity to metamorphic and magmatic features lead us to consider them separately from other detachment breccias (Figure c). Gouge (DET‐g for Mormon Mountains and HW‐g for Heart Mountain allochthon) : These samples are very fine‐grained cataclasites with no clasts visible, even with an optical microscope.…”

“…Only veins of at least 2‐mm width could supply enough material to analyze, limiting the number of these analyses. Detachment breccia (DET‐b) : These samples were all collected from within 1 m of the detachment surfaces (except ES12‐18, at 12 m below the detachment) and predominantly within 10 cm. They consist of a fine‐grained matrix and angular clasts (Figures a, c, and d), some of which appear to be clasts of an earlier brecciation episode Figure in Swanson et al, ). An attempt was made to avoid large clasts when sampling, to aim for a more homogenous mixture of matrix material.…”

“…We summarize these features below and show typical examples in Figures and . For more comprehensive structural characterization of these damage zones, including extensive documentation of features directly related to fluid flow such as veins and stylolites, we refer the reader to Anders et al (), Anderson et al (), Diehl et al (), and Swanson et al () for the Mormon Peak detachment and Anders et al () and Swanson et al () for the Heart Mountain detachment.…”

Geologic map of the east-central Meadow Valley Mountains, and implications for reconstruction of the Mormon Peak detachment, Nevada

“…Our approach is to present here a kinematic model based on reconstruction of the dismembered Mesozoic thrust system and does not depend on structural reconstruction of individual fault blocks in the Mormon Peak allochthon. Thus, although we acknowledge the central importance of fluid-assisted deformation in the development of the Mormon Peak and other carbonate allochthons (e.g., Swanson et al, 2012Swanson et al, , 2016, it is beyond the scope of this paper to address this important issue.…”

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

“…Generalized geologic map of the Heart Mountain slide area. Modified from Hauge (1993) and Malone et al (2014a). incremental movement potentially lasting millions of years (Hauge 1985(Hauge , 1990Templeton et al 1995;Swanson et al 2016), and catastrophic emplacement of Paleozoic carbonate and Eocene volcanic rocks over a period of minutes to hours (Craddock et al 2009). The mechanism of upper-plate movement that allows for catastrophic emplacement along a lowangle surface with only minor associated deformation preserved above, along, or below the detachment is poorly understood.…”