P. R. Stoddard

A kinematic model of ridge-transform geometry evolution

Spreading ridge-transform geometries will remain stable so long as accretion is symmetric. Asymmetric accretion, however, will cause lengthening or shortening of transforms and, in extreme cases, may result in zero-offset transforms (ZOTs) and very-long-offset transforms (VLOTs) such as the Ninetyeast and Chagos transforms. We use a simple kinematic model to examine the effects of various parameters on the evolution of zero-offset transforms and very-long-offset transforms. Starting with the transform length spectrum found along the Mid-Atlantic Ridge distributed in a randomly determined ridge-transform configuration, we allow for asymmetric accretion along ridge segments, assuming that individual ridge segments act independently. We analyze the effects of initial configuration, degree of asymmetry, and degree of bias in asymmetry on the generation of very-long-offset and zero-offset transforms. Finally, we examine the effect of these parameters on the possible steady-state nature of the transform length spectra. This model predicts that zero-offset transforms can be generated with a minimum of asymmetry, and that bias in asymmetry and initial ridge-transform-ridge configuration have no effect on generation of ZOTs. Similarly, random variations in spreading asymmetry have difficulty generating significant increases in transform length, so VLOTs may be manifestations of dynamic processes. Of the parameters tested, only lack of ‘memory’ of zero-offset transforms has any effect on transform length distribution, and therefore, the transform length spectrum remains steady-state if ZOTs have some degree of memory.

Age estimation of the Deccan Traps from the North American apparent polar wander path

It has recently been proposed that flood basalt events, such as the eruption of the Deccan Traps, have been responsible for mass extinctions. To test this hypothesis, accurate estimations of the ages and duration of these events are needed. In the case of the Deccan Traps, however, neither age nor duration of emplacement is well constrained; measured ages range from 40 to more than 80 Ma, and estimates of duration range from < 1 to 67 m.y. To make an independent age determination, the authors use paleomagnetic and sea-floor-spreading data and estimate the associated errors. The authors compare the Deccan paleomagnetic pole with the reference apparent polar wander path of North America by rotating the positions of the paleomagnetic pole for the Deccan Traps to the reference path for a range of assumed ages. Uncertainties in the apparent polar wander path, Deccan paleopole position, and errors resulting from the plate reconstruction are estimated. The authors suggest 83-70 Ma as the most likely time of extrusion of these volcanic rocks.

A comparison of brittle deformation models for the Gorda plate

Abstract The Gorda plate is an actively deforming subplate of the Juan de Fuca system, located off the west coast of North America. The site of intense seismicity and unusually curved magnetic lineations, the Gorda has defied rigorous explanation since it was originally mapped in 1961. A variety of models for its deformation mechanism have been proposed, two of which are analyzed in depth here. The right-lateral shear and flexural-slip models are compared using constraints imposed by geometry, seismicity, stress, and basement fabric. Also examined are the questions of convergent motion between the Pacific and Gorda plates at the Mendocino Transform Fault and shortening by means of pure shear of the magnetic lineations. While both deformation models fit most of the constraints, neither fits all, but the flexural-slip model best fits the most well-known of the constraints. If, in fact, flexural-slip is the preferred deformation mechanism, lineation shortening must be ruled out in favor of convergent motion.