Lori A. Kennedy

Low-temperature recrystallization in calcite: Mechanisms and consequences

Vein-calcite–dominated fault rocks collected from several locations show evidence for intense intracrystalline plasticity and interface (twin and grain boundary) mobility, leading to dynamic recrystallization of calcite at temperatures (150–250 °C) significantly below those at which these features are commonly anticipated. These observations require a reappraisal of calcite deformation at low temperature, particularly the capability for dynamic recrystallization in the apparent absence of significant, thermally activated recovery processes. The cyclic introduction of coarse-grained calcite veins is observed to be essential for the initiation of intracrystalline deformation and associated dynamic recrystallization. The introduction of veins generates an essentially monomineralic rock of a grain size larger than the protolith. As a result, the mylonitization does not occur within a given protolith, but rather in the introduced secondary calcite. Through Hall-Petch–type grain- size–dependent dislocation interactions, stress is locally increased, and the resulting increase in dislocation densities promotes grain-boundary migration. The recognition that nominal high-temperature creep processes and associated microstructures can occur outside their expected temperature range has implications for fault rheology (strength) and fault permeability and porosity.

Mantle shear zones revisited: The connection between the cratons and mantle dynamics

New geodynamic models, based on teleseismic data sets, propose that the westward motion of the North and South American plates is driven by asthenospheric flow coupled to the overlying lithosphere. Implicit in these models is the existence of a zone of partial coupling at the boundary between the lithosphere and asthenosphere. We propose that porphyroclastic peridotite xenoliths from kimberlite are samples of deep-mantle shear zones. These samples are a direct manifestation of the mechanical coupling that arises from the drag applied to the lithosphere by the underlying asthenospheric flow. We postulate that the formation of shear zones at the lithosphere-asthenosphere boundary is a quasisteady-state process attending plate movement beneath all cratons.

Origins of Mount St. Helens cataclasites: Experimental insights

Abstract The 2004-2006 eruption of Mount St. Helens produced a sequence of lava domes characterized by a 1-3 m thick outer carapace of highly brecciated and comminuted dacite fault rocks. This outer layer of fault rocks is proposed to be a physical manifestation of the “drumbeat” microseismicity, such that magma extrusion occurred via integration of rapid, co-seismic slip along small displacement (<5 mm) faults. A suite of deformation experiments performed on samples of Mount St. Helens (MSH) dacite under confining pressures of 0, 25, 50, and 75 MPa, at room temperature, and an average displacement rate of ~10-4/s were run to reproduce fault textures found in nature. The MSH dacite starting material has low porosity (7-8%), and a microcrystalline groundmass with little glass. A subsidiary set of deformation experiments on dacite samples from the 2006 Augustine eruption was run under identical experimental conditions to evaluate the effect of increased porosity (φ ~ 20-24%) on failure mechanisms. The MSH dacite samples show a progressive increase in peak strength with increasing confining pressure, are strong (peak stress at 75 MPa is 700 MPa) and failed by localized, brittle behavior, characterized by macroscopic fractures and rapid stress drops. In contrast, Augustine dacite deformed by distributed cataclastic flow and is much weaker (peak stress at 75 MPa is 220 MPa). We propose that the generation of low-porosity dacite was an important variable in promoting wholesale localized faulting and the attendant drumbeat microseismicity at Mount St. Helens. Microstructures of gouge developed experimentally at room temperature are remarkably similar to those developed at ~730 °C at MSH. Mode I microcracking, shear fracture of grains, and grain size comminution occurred in both natural and experimental fault rocks. Laser grain size particle analyses show peaks at 1.5 μm for experimental run products vs. peaks at 1.9 and 4 μm for natural MSH gouge. We conclude that because the MSH lava had solidified prior to faulting, temperature is secondary in importance in the formation of the gouge material. Based on the amount of fault displacement per microseismic event, the number of “drumbeats,” and the aggregate radial thickness (1-3 m) of the gouge, we calculate that the 3-6 m/day eruption rate allows for gouge-filled slip surfaces having thicknesses of 0.8-5 mm and strike lengths of 98-190 m per seismic event.

Microstructures of cataclasites in a limestone-on-shale thrust fault: implications for low-temperature recrystallization of calcite

Abstract The Hunter Valley thrust (HVT), a low-temperature foreland thrust fault in the Valley and Ridge province (Southern Appalachians), produced two distinct cataclasites: a limestone cataclasite, derived predominantly from the hanging wall limestone, and a shale cataclasite, derived predominantly from the footwall shale. The limestone cataclasite consists of fragments of calcite, limestone, pre-existing limestone cataclasite, quartz, and quartz aggregates in a fine-grained (

A low-load, high-temperature deformation apparatus for volcanological studies

Abstract We describe a new experimental apparatus designed to perform high-temperature, low-load (<1136 kg) deformation experiments relevant to volcanology. The apparatus accommodates samples that are up to 7.5 cm in diameter and 10 cm long, and can be used to run constant displacement rate and constant load experiments. The rig is ideal for volcanological studies because it uses experimental conditions that closely match those found in volcanic processes: temperature (25 to 1100°C), stress (0 to >50 MPa), strain rates (10-6 to 10-2s), and total strains of 0 to >100%. We present experimental data that show how total strain (εT) is distributed in pyroclastic material during welding. Our experiments use cores of analogue (glass beads) and natural (ash and pumice) materials. Coaxial deformation of the glass beads involves equal amounts of axial (εa; volume strain) and radial (εr; pure shear strain) strain until 40% strain where porosity is reduced to less than 10%. Radial strain dominates at this point. Natural materials show a different pattern because both the matrix and clasts are porous. High ratios of εa to εr are maintained until all porosity is lost (εT ≈ 80%). The implication is that welding in pumiceous pyroclastic deposits proceeds mainly by volume strain; in natural materials, pure shear strain is minimal except in special circumstances

Strain partitioning in accretionary orogens, and its effects on orogenic collapse: Insights from western North America

Changes in relative plate motions during the construction of accretionary orogens generally result in varying structural styles along the length of the orogen. These disparate structural styles can be interpreted as having been formed by different tectonic regimes along the orogenic axis that formed at the same time. If the orogen is considered at the large scale, the differences in the way in which the crust responds during accretion can be explained by large-scale strain partitioning within the same overall tectonic environment. The westernmost Canadian Cordillera records the transition from Late Cretaceous dextral strike-slip faulting to near-orthogonal compression along the orogenic axis. We postulate that the transition between strike-slip–dominated to compression-dominated tectonics represents a Late Cretaceous partitioning of strain that resulted in a significant difference in crustal rheology along strike of the orogeny. This had a dramatic effect on subsequent Tertiary orogen-scale extension. We propose that plate readjustments in the Tertiary led to orogen-perpendicular collapse in portions of the orogen, facilitated by decoupling between the middle and lower crusts along thermally weakened layers. In contrast, localized, orogen-parallel extension occurred in other portions of the orogen, along kinematically linked, large dextral strike-slip faults where the upper crust remained coupled to the middle and lower crust. New data indicate that partitioning of strain occurs across very large regions within any orogenic system, and that the way in which strain is partitioned can lead to dramatic differences in future orogenic processes. It becomes apparent from these data that orogens must be examined as a whole and that differing structural styles of similar ages are likely responses to the same overall tectonic regime.