Preface.

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Sound change and coarticulatory variability involving English /ɹ/

English /ɹ/ is known to exhibit covert variability, with tongue postures ranging from bunched to retroflex, as well as various degrees of lip protrusion and compression. Because of its articulatory variability, /ɹ/ is often a focal point for investigating the role of individual variation in change. In the studies reported here, we examine the coarticulatory effects of alveolar obstruents with /ɹ/, presenting data from a collection of sociolinguistic interviews involving 162 English speakers from Raleigh, North Carolina, and a pilot corpus of ultrasound and lip video from 29 additional talkers. These studies reveal a mixture of assimilatory and coarticulatory patterns. For the sound changes in progress (/tɹ/ and /dɹ/ affrication, and /stɹ/ retraction), we find increases over apparent time, but no effect of covert variability in our laboratory data, consisting mostly of younger talkers. When a sound change has already become phonologized to a new phonemic target with a correspondingly different articulatory target, the original variability is obscured. In comparison, post-lexical coarticulation of word-final /s z/ before a word-initial /ɹ/ more closely resembles /s z/ in tongue posture, with an effect of anticipatory lip-rounding that introduces a low-mid frequency spectral peak during the sibilant interval, and greater reduction in the frequency of this peak for talkers who transition more rapidly to the /ɹ/. In order to uncover the role of covert variability in a sound change, we must look to sounds that exhibit synchronically stable articulatory variability

Discussion of 'Classification of fault breccias and related fault rocks', by Woodcock & Mort: the particular problem of pseudotachylyte

J. F. Magloughlin comments: I would like to compliment Woodcock & Mort on an important attempt to bring more order to the complex and under-attended world of fault rock classification. The authors aptly point out several of the persistent difficulties in fault rock terminology, such as the original cohesive-versus-incohesive dichotomy in Sibson's (1977) classification, and they appropriately attempt to create a more nongenetic classification scheme

A laser-probe 40Ar/39Ar study of pseudotachylite from the Tambach Fault Zone, Kenya: direct isotopic dating of brittle faults

Understanding the tectonic evolution of orogenic belts and intracratonic areas depends on our ability to determine the age of tectonic features on a variety of scales. This study demonstrates the value of the laser-probe 40Ar/39Ar dating technique, which, if applied to fault-derived pseudotachylites, may be used to directly determine the age of brittle faults. The laser-probe technique affords high spatial resolution, enabling a greater opportunity for discriminating between pseudotachylite matrix, host-rock clasts and alteration products that are often present in varying proportions within pseudotachylites. The laser-probe 40Ar/39Ar technique has been applied to pseudotachylite samples from the Tambach Fault Zone (TFZ), a major NW–SE trending strike-slip fault within the Kenyan part of the Late Proterozoic/Early Palaeozoic Mozambique Belt. The pseudotachylites of the TFZ were previously thought to have formed either (i) at about 530–430 Ma, or (ii) during the Cenozoic evolution of the Kenya Rift. In the latter case, seismic slip on the rift-bounding normal fault would have generated the pseudotachylites, due to the reactivation of old NW–SE trending structures in the basement. Based on our new data, we interpret the pseudotachylite formation age to be 400 Ma. This rules out the possibility that the pseudotachylites are related to the formation of the Kenya Rift. Although the inherited basement faults may have been locally reactivated as transfer faults, reactivation of these structures during rifting did not occur beyond the margins of the Kenya Rift

A geological fingerprint of low-viscosity fault fluids mobilized during an earthquake

The absolute value of stress on a fault during slip is a critical unknown quantity in earthquake physics. One of the reasons for the uncertainty is a lack of geological constraints in real faults. Here we calculate the slip rate and stress on an ancient fault in a new way based on rocks preserved in an unusual exposure. The study area consists of a fault core on Kodiak Island that has a series of asymmetrical intrusions of ultrafinegrained fault rock into the surrounding cataclasite. The intrusive structures have ductile textures and emanate upward from a low-density layer. We interpret the intrusions as products of a gravitational (Rayleigh-Taylor) instability where the spacing between intrusions reflects the preferred wavelength of the flow. The spacing between intrusions is 1.4 ± 0.5 times the thickness of the layer. This low spacing-to-thickness ratio cannot be explained by a low Reynolds number flow but can be generated by one with moderate Reynolds numbers. Using a range of density contrasts and the geometry of the outcrop as constraints, we find that the distance between intrusions is best explained by moderately inertial flow with fluid velocities on the order of 10 cm/s. The angle that the intrusions are bent over implies that the horizontal slip velocity was comparable to the vertical rise velocity, and therefore, the fault was slipping at a speed of order 10 cm/s during emplacement. These slip velocities are typical of an earthquake or its immediate afterslip and thus require a coseismic origin. The Reynolds number of the buoyant flow requires a low viscous stress of at most 20 Pa during an earthquake