Abstract. Field observations of two overlapping normal faults and associated deformation document features common to many normal-fault relay zones: a topographic ramp between the fault segments, tapering slip on the faults as they enter the overlap zone, and associated fracturing, especially at the top of the ramp. These observations motivate numerical modeling of the development of a relay zone. A three-dimensional boundary element method numerical model, using simple fault-plane geometries, material properties, and boundary conditions, reproduces the principal characteristics of the observed fault scarps. The model, with overlapping, semicircular fault segments under orthogonal extension, produces a region of high Coulomb shear stress in the relay zone that would favor fault linkage at the center to upper relay ramp. If the fault height is increased, the magnitude of the stresses in the relay zone increases, but the position of the anticipated linkage does not change. The amount of fault overlap changes the magnitude of the Coulomb stress in the relay zone: the greatest potential for fault linkage occurs with the closest underlapping fault tips. Ultimately, the mechanical interaction between segments of a developing normal-fault system promote the development of connected, zigzagging fault scarps.

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Interactions among fluids, deformation structures, and chemical changes in sediments impact deformation of the shallow crust, influencing the preservation and extraction of the economic resources it contains. These interactions have been studied along the Moab Fault, in the Paradox Basin, Utah, where diagenetic cements, joints, cataclastic deformation bands and slip surfaces developed during faulting are thought to control fault permeability. Previous fluid inclusion micro-thermometry and stable isotopic data from calcite cements collected along segments of the Moab Fault suggest cements precipitated from hot basin fluids that migrated up the fault and interacted with a shallower meteoric groundwater source. In this study, we investigate the interactions of these fluids with deformation structures using clumped isotope thermometry of calcite cements along the Moab Fault. Guided by prior high-resolution mapping of deformation structures and calcite cements, we measured the growth temperature of calcite cements collected at varying distance from fault segments and fault intersections. Cement temperatures from individual segments vary greatly; cements along a relatively simple fault segment indicate temperatures ranging from 67 to 128°C, similar to previously published fluid inclusion homogenization temperatures from a cement sample collected in the same locality, while a nearby fault intersection hosts cements with temperatures of 13 to 88°C. The spatial pattern of cement temperatures revealed by clumped isotope thermometry suggests that intensely jointed zones associated with fault intersections enable rapid down-fault migration of cool surface waters and that deformation-band faults with their associated slip surfaces may further compartmentalize fluid flow, restricting fluid sources to warm waters thermally equilibrated with the country rock outside the jointed zone. Our data confirm that the relationship between faults and fluid flow can vary greatly over short length scales, and suggest that some fracture zones can be highly conductive to depths as great as 2 km.

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We describe a low-cost application of digital photogrammetry using commercially available photogrammetric software and oblique photographs taken with an offthe-shelf digital camera to create sequential digital elevation models (DEMs) of a lava dome that grew during the 2004-2008 eruption of Mount St. Helens (MSH) volcano. Renewed activity at MSH provided an opportunity to devise and test this method, because it could be validated against other observations of this well-monitored volcano. The datasets consist of oblique aerial photographs (snapshots) taken from a helicopter using a digital single-lens reflex camera. Twelve sets of overlapping digital images of the dome taken during 2004-2007 were used to produce DEMs and to calculate lava dome volumes and extrusion rates. Analyses of the digital images were carried out using photogrammetric software to produce three-dimensional coordinates of points identified in multiple photos. The evolving morphology of the dome was modeled by comparing successive DEMs. Results were validated by comparison to volume measurements derived from traditional vertical photogrammetric surveys by the US Geological Survey Cascades Volcano Observatory. Our technique was significantly less expensive and required less time than traditional vertical photogrammetric techniques; yet, it consistently yielded volume estimates within 5% of the traditional method. This technique provides an inexpensive, rapid assessment tool for tracking lava dome growth or other topographic changes at restless volcanoes.

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