SIO 226: Introduction to Marine Geophysics Plate Tectonics Dave Chadwell
Plate Tectonics, Sea-Floor Spreading and Continental Drift
Main Parody
Plate Geography
Lineated Magnetic Anomalies in the Oceanic Crust From: R.G. Mason and A.D. Raff Magnetic Anomalies Off the West Coast Of North America GSA Bulletin 72, 1259 1961.
Three Interpretations of Magnetic Profiles Volcanic Intrusions Basement Faulting Variations in Magnetization
Horizontal Displacements in the Floor of the Pacific Ocean V. D. Vacquier, Magnetic Evidence for Horizontal Displacements in The Floor of the Pacific Ocean GSA Bulletin 72, 1250, 1961.
Magnetic Anomalies Offset by Deep-Sea Escarpments
Magnetic Anomalies Over Oceanic Ridges F. J. Vine and D. H. Matthews -- Nature 199, 947, 1963 1. Magnetic Field Reversals 2. Ocean Crust Records Field Direction 3. Sea Floor Spreading at Ridges
Magnetic Reversal Time Scale
Plate Tectonics: How it Works Cox and Hart Definitions and Symbols
Plate Tectonics How it Works Motion of B is oblique to all boundaries Motion of B is parallel to one boundary a transform.
Plate Tectonics How it Works Linear velocity of point b The velocity fields of (b) and (c) are both consistent with the assumed relative motion of plates A and B about Euler pole E.
Plate Tectonics How it Works The Velocity Field of a Geologically Realistic Plate
Plate Tectonics How it Works Two sets of plates with the same spreading rates, but different relative velocities
Plate Tectonics How it Works Find a velocity field consistent with all boundaries Which are impossible?
Juan de Fuca Plate Motions The Juan de Fuca Ridge is adjacent to a series of transforms that are not all parallel. See (1), (2), (3), (4) and (5). Is the JdF Ridge at the boundary between the Pacific Plate and the North America Plate?
Juan de Fuca Plate Motions Introducing the JDF Plate eliminates the inconsistency in the directions of the transforms. Taking the NA Plate as fixed, the velocity of the JDF Plate relative to NA is toward the northeast, a convergent margin
Mercator Projection Projection of a globe upon a cylinder Progressive distortion at high latitudes Great circle paths are generally NOT straight lines 1/9/02
Oblique Mercator Projection Defines a new pole for projection and equator of new pole is a straight line on the resulting map
Mercator Projection of Fracture Zones After LePichon et al.
Oblique Mercator Projection as a tool for finding poles relative motion Pole of projection for South Pacific 69-N 157-W
Absolute Motion: Hot Spot Tracks Model of formation of Hawaiian Island chain by Hot Spot Volcanism
Absolute Motion: Hot Spot Tracks Age in Ma for volcanoes in the Hawaiian-Emperor Chain
Comparison of Hawaiian and Louisville Hot Spot Chains Matching of hotspot chains in the North and South Pacific about inferred poles of Pacific plate motion After: Lonsdale JGR
Comparison of Age Progression
Plate Driving Forces: Thermal Convection Do Ridges and Trenches delimit the limbs of a convection cell?
Driving Forces Acting on Plates: After Forsyth and Uyeda Mantle and Continental Drag Forces proportional to the area and velocity of plate Ridge Push gravitational potential energy Slab Pull subducting slab density Transform Fault Resistance faulting occurs at critical yield so force is velocity independent Colliding Resistance faulting occurs, velocity independent Trench Suction flow of astenosphere related to subduction
Review of Plate Tectonic Model Oceanic crust is created at ridge crests Ridge crest is segmented by transform faults Oceanic crust is re-cycled at subduction zones
GPS-Acoustic positioning (A = D+C+E) Optical survey of phase center offsets Precise kinematic GPS positioning Acoustic survey of transponder relative positions array allows exploiting layering of sound speed to remove uniform fluctuations Acoustic range ship to PXP Array horizontal position in global frame. Observe horizontal seafloor motion with centimeter resolution by combing GPS with precision underwater acoustics (e.g., Spiess et al, 1998; Chadwell & Spiess, JGR, 2008). Seafloor motion vectors in same frame as GPS allows direct comparison to land-gps vectors and other seafloor vectors separated by 100s km.