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The 2004-2008 eruption of Mount St. Helens produced seven dacite spines mantled by cataclastic fault rocks, comprising an outer fault core and an inner damage zone. These fault rocks provide remarkable insights into the mechanical processes that accompany extrusion of degassed magma, insights that are useful in forecasting dome-forming eruptions. The outermost part of the fault core consists of finely comminuted fault gouge that is host to 1- to 3-mm-thick layers of extremely fine-grained slickenside-bearing ultracataclasite. Interior to the fault core, there is an ~2-m-thick damage zone composed of cataclastic breccia and sheared dacite, and interior to the damage zone, there is massive to flow-banded dacite lava of the spine interior. Structures and microtextures indicate entirely brittle deformation, including rock breakage, tensional dilation, shearing, grain flow, and micro faulting, as well as gas and fluid migration through intergranular pores and fractures in the damage zone. Slickenside lineations and consistent orientations of Riedel shears indicate upward shear of the extruding spines against adjacent conduit wall rocks. Paleomagnetic directions, demagnetization paths, oxide mineralogy, and petrology indicate that cataclasis took place within dacite in a solidified steeply dipping volcanic conduit at temperatures above 500 °C. Low water content of matrix glass is consistent with brittle behavior at these relatively high temperatures, and the presence of tridymite indicates solidification depths of <1 km. Cataclasis was coincident with the eruption's seismogenic zone at <1.5 km. More than a million small and low-frequency "drumbeat" earthquakes with coda magnitudes (Md) <2.0 and frequencies <5 Hz occurred during the 2004-2008 eruption. Our field data provide a means with which to estimate slip-patch dimensions for shear planes and to compare these with estimates of slip patches based on seismic moments and shear moduli for dacite rock and granular fault gouge. Based on these comparisons, we find that aseismic creep is achieved by micron-scale displacements on Riedel shears and by granular flow, whereas the drumbeat earthquakes require millimeter to centimeter displacements on relatively large (e.g., ~1000 m²) slip patches, possibly along observed extensive principal shear zones within the fault core but probably not along the smaller Riedel shears. Although our field and structural data are compatible with stick-slip models, they do not rule out seismic and infrasound models that call on resonance of steam-filled fractures to generate the drumbeat earthquakes. We suggest that stick-slip and gas release processes may be coupled, and that regardless of the source mechanism, the distinctive drumbeat earthquakes are proving to be an effective precursor for dome-forming eruptions. Our data document a continuous cycle of deformation along the conduit margins beginning with episodes of fracture in the damage zone and followed by transfer of motion to the fault core. We illustrate the cycle of deformation using a hypothetical cross section of the Mount St. Helens conduit, extending from the surface to the depth of magmatic solidification. [ABSTRACT FROM AUTHOR]

Fault rocks can function as barriers to subsurface fluid flow and affect the storage of CO2 in geological structures. Even though flow across faults often involves more than one fluid phase, it is typically modeled using only single-phase functions due to a lack of fault rock relative permeability data and complexities in incorporating two-phase flow properties into flow simulations. Here we present two-phase fluid flow data for cataclastic fault rocks in porous sandstone from the 90-Fathom fault (northeast England). The study area represents a field analogue for North Sea saline aquifers of Permian-Triassic age that are currently being considered for CO2 storage. We use the experimental data to populate a synthetic model of a faulted saline aquifer to assess the impact of these fault rocks on CO2 injection. We show that even fault rocks with low clay contents and very limited quartz cementation can act as major baffles to the flow of a non-wetting phase if realistic two-phase properties are taken into account. Consequently, pressure may increase far more rapidly in the storage compartment during CO2 injection than anticipated based on models that only incorporate absolute fault rock permeabilities. To avoid high pressures, which may lead to hydrofracturing and CO2 leakage, either more complex injection strategies need to be adopted or seismic data acquired to ensure the absence of faults in aquifers selected for CO2 storage. [ABSTRACT FROM AUTHOR]

Large hinterland thrust sheets are the dominant structures in retroarc fold-thrust belt (FTB) wedges. These internal sheets remain active throughout the history of an FTB and strongly influence the kinematics of the external portions of the FTB and the overall behavior of the entire tectonic wedge. Their emplacement and deformation account for a significant portion of the total mechanical energy budget in FTB evolution based on estimates using deformation features preserved within such sheets. Moreover, the deformation within dominant internal sheets can be used as a means to evaluate the tectonic history of the wedge as a whole. The principal deformation mechanisms in different parts of an FTB wedge are strongly controlled by variations in the geothermal gradient. Retroarc FTBs typically show a steeper geothermal gradient in the hinterland than in the foreland, resulting in a forelandward slope of the "brittle-ductile transition." Therefore, a change from plastic to elasticofrictional (EF) mechanisms is expected not only with decreasing depth, but also across strike as a dominant internal thrust sheet is transported toward the foreland and undergoes exhumation by synorogenic erosion. However, evidence for cataclasis overprinting early plastic deformation features is often not preserved in ancient FTBs because of extensive erosion of the wedge top. Even where evidence for late-stage cataclastic behavior exists in either ancient or modern (i.e., active) orogens, it is often not recognized or studied. Our work in the Canyon Range, central Utah segment of the Sevier FTB shows that unraveling the EF mechanisms, such as late-stage cataclastic flow, is critical to understanding the total deformation history of an internal thrust sheet. After initial emplacement under plastic deformation conditions, the CR thrust sheet was continuously reactivated under EF conditions, thereby helping to maintain critical taper of the FTB wedge during its development. Structures preserved in the CR thrust sheet, therefore, record a significant portion of the deformation history of the FTB as a whole. The Canyon Range (CR) syncline, part of the CR sheet, underwent fold tightening within the EF regime and the cataclasized rocks are well preserved. The geometry and progressive deformation patterns of the CR syncline reveal four distinct to partly overlapping deformation stages. The history of the rocks in the CR thrust sheet has been tracked by incrementally retro-deforming the FTB and by unraveling details of the deformation within the CR syncline at various scales for each stage. We use the CR syncline as a type example to show that unraveling EF mechanisms is critical to (1) understanding the state of a FTB wedge during its evolution, (2) calculating the energy involved in FTB emplacement, and (3) developing accurate restorations. Placing constraints on the depth of the rocks for successive stages of deformation is essential for developing retrodeformable balanced cross sections, and in turn, geometric and kinematic models for wedge evolution. [ABSTRACT FROM AUTHOR]

Comments on the issue about the study of pseudotachylite-bearing friction marks on andesitic blocks. Types of structures on block surfaces; Formation of pseudotachylites; Decrease of grain size on cataclastic layers.