Numerical Modeling of Salt Tectonics at Rifted Continental Margins

This thesis applies finite element numerical modeling to a range of projects investigating salt tectonics at rifted continental margins. Both two-dimensional and fully three-dimensional models are used. The first project demonstrates that incorrect density scaling in physical analogue models of salt tectonics leads to: 1) overestimated buoyancy force and 2) underestimated pressure gradient and sediment strength. Numerical models (2D) show a shift in salt structures, from diapir-minibasin pairs to expulsion rollover, when density scaling errors are corrected. The second and third projects use 2D nested models, which allow for dynamic evolution of the continental margin scale system while providing high resolution visualization of salt basins, to study the interactions among syn-rift salt deposition and deformation, post-salt sedimentation, and ongoing rifting tectonics. This work makes the important advancement of allowing salt basin geometry to evolve dynamically. The second project considers intermediate width margins, and shows that the timing of salt deposition relative to rifting (early syn-rift vs late syn-rift salt) leads to distinct patterns of salt distribution and deformation. Key features of salt tectonics at the Central Red Sea are captured using a layered salt succession deposited in the mid to late syn-rift period. The third project considers wide continental margins, developed from weak continental crust. In addition to studying syn-rift salt tectonics, the impact of sedimentation on the style of rifting is investigated. A comparison is made to the central and northern Nova Scotia margin. The final project presents preliminary results from a fully 3D numerical modeling study of the formation of minibasins through uneven sediment loading over salt. This work compares the evolution of 2D-equivalent and fully 3D model designs, exploring the importance of studying even simple salt tectonic systems using a fully 3D approach. List of Abbreviations and Symbols Used Abbreviation/Symbol Description LS Large Scale SS Small Scale φ Internal angle of friction for clastic sediments L Length measurement in the model L Length measurement in nature t Time ρ Density g Acceleration due to gravity η Viscosity t Time measurement in the model t Time measurement in nature ρs Density of salt in the model ρs Density of salt in nature g Acceleration due to gravity in the model g Acceleration due to gravity in nature η Viscosity measurement in the model η Viscosity measurement in nature v Velocity v Velocity measurement in the model v Velocity measurement in nature σY Clastic sediment yield strength ρd Clastic sediment density σY Clastic sediment yield strength in the model σY Clastic sediment yield strength in nature σzz Buoyancy force per unit area σzz Buoyancy force per unit area in the model σzz Buoyancy force per unit area in nature ΔP Horizontal pressure gradient σ’ij Deviatoric stress tensor xi Spatial coordinate P Pressure (mean stress) vi Velocity component ε . ij Strain rate tensor h or h(x) Height above base of the model h1, h2, h3 Specific elevations above the base of the model W, W1, W2 Gaussian widths vprograde Progradation velocity ALE Arbitrary Lagrangian Eulerian J’2 Second invariant of the deviatoric stress Pf Hydrostatic pore fluid pressure ρg, ρgrain Grain density for compacting clastic sediments ρf, ρfluid Pore fluid density for compacting clastic sediments no Surface porosity for compacting clastic sediments c Compaction coefficient for compacting clastic sediments z Depth below sediment surface AP or AP-1 Analogue Properties NPL Natural Properties with Low sediment density NPH Natural Properties with High sediment density NPN Natural Properties with Neutral sediment density NPC Natural Properties with Compacting sediment density APS Analogue Properties with corrected sediment Strength APB Analogue Properties with corrected Bouyancy NPHW Natural Properties with High sediment density and Water effects APM Analogue Properties Marine P’ Effective pressure ρw Density of water PDSM polydimethylsiloxane T Temperature K, K(T) Thermal conductivity A Radioactive heat production per unit volume Or Pre-exponential scaling factor α Volumetric thermal expansivity WQz Wet Wuartzite DMD Dry Maryland Diabase WOl Wet Olivine f Scaling factor I . 2 ' Second invariant of the deviatoric strain rate tensor n Power law exponent Q Activation energy V Activation volume Or Volume R Universal gas constant φeff Effective internal angle of friction C Cohesion Cp Specific heat λ Wavelength RS or R Right Side LS or L Left Side AC Aggrading Clastic sediment ACV Aggrading Clastic sediment Variant ES Early syn-rift Salt LS Late syn-rift Salt NS Nova Scotia ECMA East Coast Magnetic Anomaly OCT Ocean Continent Transition SMART Scotian MARgin Transects NSC Nova Scotia Central margin NSNE Nova Scotia North Eastern margin PKL Pre-Kinematic Layer