Laboratory and field studies of oblique rifting.

摘要

This thesis examines the manner in which fault population systematics vary as a function of rift obliquity and how fault populations evolve through time in oblique rift zones. Rift obliquity is related to the acute angle, α, between the rift trend and the displacement direction of the rift walls, so that the value of α is inverse to the degree of obliquity. I used scaled experimental models to simulate fault growth in oblique rift zones and analyzed photographs of the model surfaces to determine their fault population statistics. First I examined a single displacement increment for a whole suite of models where α is varied in 15° increments between 90° and 0°. Results show increases in the range of azimuths as rift obliquity increases; the length of the longest faults, the sum of fault lengths, and the width of the deformed zone all increase as rift obliquity decreases. Major changes in fault population statistics occur between α = 45° and 30°, when the stress state changes from both horizontal stresses being tensional to one becoming compressional. Next I used scaled clay models to study the temporal evolution of fault populations in experiments of moderately oblique (α = 60°) and highly oblique (α = 30°) distributed extension. The degree of rift obliquity places constraints on the style of fault growth from the earliest stages of fault population evolution. Faults nucleate and grow more rapidly, and their growth is less restricted in the moderately oblique model. Fault length is severely limited by rift geometry in the highly oblique model and two sets of faults link to form complex, branching structures. Finally, I have used air photos and field measurements to analyze fracture population systematics for the Reykjanes fissure swarm in order to determine the effect of rift obliquity on the evolution of fracture populations on the Reykjanes Peninsula, SW Iceland. Data show significant differences between the azimuths, lengths and degree of development of fractures along the margin of the fissure swarm and those in the center of the zone of active volcanism. A significant change in fracture strike at the rift margin is found to be due to the presence of a secondary stress field at the boundary between weak and strong crust. Data for Reykjanes Peninsula was compared to that for scaled models of oblique extension (α = 30°).