SIO 226: Introduction to Marine Geophysics Plate Tectonics LeRoy Dorman Scripps Institution of Oceanography January, 2005

Early Tectonic Ideas Herodotus of Halicarnassos(484 BC-ca. 425 BC), now part of Turkey, observed shells in the hills of Egypt and concluded that they had lain beneath the sea. Herodotus of Halicarnassos(484 BC-ca. 425 BC), now part of Turkey, observed shells in the hills of Egypt and concluded that they had lain beneath the sea. Predecessors Anaximander and Xenophanes are quoted as making similar statements. Predecessors Anaximander and Xenophanes are quoted as making similar statements. Thus the first aspect of tectonics to be noted was vertical motion. Thus the first aspect of tectonics to be noted was vertical motion.

Wegener, in 1924, proposed a reconstruction of Pangaea, based primarily on geometric fit. From UCSG “This Dynamic Earth, Kious and Tilling,1996

Smith and Hallam, 1970, generated fits which were consistent with structural and paleontological data.

2/26/2004 Velocities from the Global Positioning System confirm that movement continues

2/26/2004 Main Parody

2/26/2004 Plate Geography

2/26/2004 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,

2/26/2004 Three Interpretations of Magnetic Profiles Volcanic Intrusions Volcanic Intrusions Basement Faulting Basement Faulting Variations in Magnetization Variations in Magnetization

2/26/2004 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.

2/26/2004 Magnetic Anomalies Offset by Deep-Sea Escarpments

2/26/2004 Magnetic Anomalies Over Oceanic Ridges F. J. Vine and D. H. Matthews -- Nature 199, 947, Magnetic Field Reversals 2. Ocean Crust Records Field Direction 3. Sea Floor Spreading at Ridges

2/26/2004 Magnetic Reversal Time Scale

2/26/2004 Plate Tectonics: How it Works –Cox and Hart Definitions and Symbols

2/26/2004 Plate Tectonics – How it Works Motion of B is oblique to all boundaries Motion of B is parallel to one boundary – a transform.

2/26/2004 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.

2/26/2004 Plate Tectonics – How it Works The Velocity Field of a Geologically Realistic Plate

2/26/2004 Plate Tectonics – How it Works Two sets of plates with the same spreading rates, but different relative velocities

2/26/2004 Plate Tectonics – How it Works Find a velocity field consistent with all boundaries … Which are impossible?

2/26/2004 Juan de Fuca Plate Motions The Juan de Fuca Ridge is adjacent to a series of transforms that are not all parallel. The Juan de Fuca Ridge is adjacent to a series of transforms that are not all parallel. See (1), (2), (3), (4) and (5). See (1), (2), (3), (4) and (5). Is the JdF Ridge at the Is the JdF Ridge at the boundary between the Pacific Plate and the North America Plate?

2/26/2004 Juan de Fuca Plate Motions Introducing the JDF Plate 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 Taking the NA Plate as fixed, the velocity of the JDF Plate relative to NA is toward the northeast, a convergent margin

2/26/2004 Mercator Projection Projection of a globe upon a cylinder Progressive distortion at high latitudes Great circle paths are generally NOT straight lines

2/26/2004 Oblique Mercator Projection Defines a new “pole” for projection and “equator” of new pole is a straight line on the resulting map

2/26/2004 Mercator Projection of Fracture Zones After LePichon et al.

2/26/2004 Oblique Mercator Projection as a tool for finding poles relative motion Pole of projection for South Pacific 69-N 157-W

2/26/2004 Absolute Motion: Hot Spot Tracks Model of formation of Hawaiian Island chain by Hot Spot Volcanism

2/26/2004 Absolute Motion: Hot Spot Tracks Age in Ma for volcanoes in the Hawaiian-Emperor Chain

2/26/2004 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 Matching of hotspot chains in the North and South Pacific about inferred poles of Pacific plate motion After: Lonsdale JGR

2/26/2004 Comparison of Age Progression

2/26/2004 Plate Driving Forces: Thermal Convection Do Ridges and Trenches delimit the limbs of a convection cell?

2/26/2004 Driving Forces Acting on Plates: After Forsyth and Uyeda Mantle and Continental Drag Forces – proportional to the area and velocity of plate Mantle and Continental Drag Forces – proportional to the area and velocity of plate Ridge Push – gravitational potential energy Ridge Push – gravitational potential energy Slab Pull – subducting slab density Slab Pull – subducting slab density Transform Fault Resistance – faulting occurs at critical yield so force is velocity independent Transform Fault Resistance – faulting occurs at critical yield so force is velocity independent Colliding Resistance – faulting occurs, velocity independent Colliding Resistance – faulting occurs, velocity independent Trench Suction – flow of astenosphere related to subduction Trench Suction – flow of astenosphere related to subduction

2/26/2004 Review of Plate Tectonic Model Oceanic crust is created at ridge crests Oceanic crust is created at ridge crests Ridge crest is segmented by transform faults Ridge crest is segmented by transform faults Oceanic crust is re-cycled at subduction zones Oceanic crust is re-cycled at subduction zones