We present results from a carbonate clumped-isotope thermometric study of 42 carbonate samples collected within ~1 m or less of the Mormon Peak detachment, a large-slip Miocene normal fault in the Basin and Range province of southern Nevada. Samples include cataclastic rocks, narrow vein fillings and larger void-filling carbonates. Our results are consistent with earlier measurements of O and C isotopic ratios and fluid inclusion temperatures, and provide independent constraints on the isotopic composition and temperature of both syntectonic and post-tectonic pore waters. The results reveal a wide range of precipitation temperatures (24 to 137 °C) associated with deformation, and indicate that the pore waters were meteoric, with δ 18 O as low as-11.6‰ (VSMOW) and δ 13 C as low as-8.0‰ (VPDB). The results do not provide any direct evidence for hightemperature thermal decarbonation reactions (ca. 500 to 800°C) that are widely expected to result from flash heating along upper crustal faults, although they do not rule them out so long as recarbonation occurs at very low temperature, or the products of these reactions are volumetrically minor. The results are difficult to reconcile with recent suggestions that the detachment is the base of one or more catastrophically emplaced, surficial landslides. In concert with other lines of evidence, the data are most simply interpreted as recording deformation and precipitation events through a long history of slip, accompanied by relatively deep (>3 km) circulation of meteoric pore waters along the detachment plane.

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.

Abstract-Efficient detection and high-fidelity quantification of surface changes resulting from underground activities are important national and global security efforts. In this investigation, a team performed field-based topographic characterization by gathering high-quality photographs at very low altitudes from an unmanned aerial system (UAS)-borne camera platform. The data collection occurred shortly before and after a controlled underground chemical explosion as part of the United States Department of Energy's Source Physics Experiments (SPE-5) series. The highresolution overlapping photographs were used to create 3D photogrammetric models of the site, which then served to map changes in the landscape down to 1-cm-scale. Separate models were created for two areas, herein referred to as the test table grid region and the nearfield grid region. The test table grid includes the region within *40 m from surface ground zero, with photographs collected at a flight altitude of 8.5 m above ground level (AGL). The near-field grid area covered a broader area, 90-130 m from surface ground zero, and collected at a flight altitude of 22 m AGL. The photographs, processed using Agisoft Photoscan Ò in conjunction with 125 surveyed ground control point targets, yielded a 6-mm pixelsize digital elevation model (DEM) for the test table grid region. This provided the B3 cm resolution in the topographic data to map in fine detail a suite of features related to the underground explosion: uplift, subsidence, surface fractures, and morphological change detection. The near-field grid region data collection resulted in a 2-cm pixel-size DEM, enabling mapping of a broader range of features related to the explosion, including: uplift and subsidence, rock fall, and slope sloughing. This study represents one of the first works to constrain, both temporally and spatially, explosion-related surface damage using a UAS photogrammetric platform; these data will help to advance the science of underground explosion detection.

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