Mid -ocean ridge morphology and tectonics: Insights from numerical modeling of faults and dikes

• Main Author: Qin, Ran
• Format: Thesis
• Language: English
• Published: Columbia University 2008
• Subjects: Geophysics|Physical oceanography; Reykjanes; Iceland
• Online Access: http://pqdtopen.proquest.com/#viewpdf?dispub=3299362

Mid-ocean ridge morphology and tectonic features are surface expression of underlying plate tectonic processes. This thesis develops an advanced numerical model to simulate dike intrusions and faults, and investigates their role in the formation of ridge morphology and tectonic features. The formation of axial rift valleys at slow spreading ridges is modeled by FLAC, a code based on finite different method (FDM). I show that rift valleys and the bounding shoulders are formed by fault offsets. And the isostatic restoring force at the ridge axis forces the lithosphere to deform at a specific way that makes rift valley stable. The relief of a stable rift valley is a function of lithospheric geometry and strength. The formation of axial highs and rifted highs at fast and intermediate spreading ridges is partly controlled by dike intrusions. Firstly, TWODD, a code based on boundary element method (BEM) is modified to track dike propagation and calculate dike opening. After testing the numerical model on the crack penetration problem in the icy shell of Europa, I show that limited magma supply from crustal magma chamber is the key to create meter-wide dikes at mid-ocean ridges. Furthermore, I modified FLAC to simulate continued surface deformation following dike intrusions. At the time scale from months to years, I show that the ridge surface may continue to displace at a rate of a few centimeters to decimeters per year both vertically and horizontally, due to viscoelastic relaxation of the asthenosphere. This distinctive surface relaxation pattern may be detected at Iceland, Afar and other mid-ocean ridges. Finally I couple the numerical models for faults and dike intrusions, to simulate the variation of ridge topography and tectonic features. I show that the transition of ridge topography from axial highs to rift valleys is controlled by the relative dominance of faults and dikes, which is a function of axial lithosphere thickness, magma reservoir radius and diking interval. Fault offset is required to build the extrusive layer, which generally increases toward slow spreading ridges but is absent if the magma reservoir is small. Reykjanes Ridge, Iceland and other hotspot-influenced ridges are places where magma reservoir is abnormally large due to high mantle temperature, and their topography resembles axial highs.