Plate Tectonics: The Unifying Theory Peter W. Sloss, NOAA-NESDIS-NGDC

Plate Tectonics Fundamental concept of geoscience Integrates from many branches First suggested based on geology and paleontology Fully embraced after evidence from geophysics

What tectonics theory explains • Distribution of earthquakes and volcanoes • Relationship of age and height of mountain belts • Age distribution of oceanic crust • Magnetic information in rocks

What is Plate Tectonics Theory? Earth’s upper mantle (lithosphere) is broken into rigid plates which move with respect to each other Plates rest on and move in the asthenosphere There are 3 kinds of plate margins that are marked by earthquakes and volcanoes Divergent Convergent Transform The movement is driven by uneven distribution of heat within Earth and the mechanism that drives plate movement is not well known

Mosaic of Earth’s Plates Peter W. Sloss, NOAA-NESDIS-NGDC

Theory evolved from 2 concepts: Continental Drift - by Alfred Wegener Fit of continents - Pangaea Ancient climatic evidence - glacial deposits Fossil evidence - Glossopteris & Mesosaurus Matched mineral zones & mountain chains (Problem - no acceptable method of motion)

“Fit” of the Continents

Anomalous Distribution of Fossils (Mesosaurus)

AND PALEOMAGNETIC EVIDENCE Polar Wandering - either the North Pole or the continents had moved both Europe and North America had apparently moved as a single continent for several hundred million years

Apparent polar wandering curves

Seafloor Spreading “Geopoetry” of Harry Hess & Robert Dietz New seafloor forms by upwelling at the center of MOR and moves laterally Older crust is destroyed in the subduction zones at the trenches Seafloor is younger than 200 MY Solved Continental Drift problem

Modern Proof of Seafloor Spreading Deep Sea Drilling Project - sampled sea floor sediment & crust Age & thickness increased with distance from MOR Ages were symmetrical about MOR Magnetic Anomalies - found weak & strong signal Alternating normal & reversed polarization Stripes || to & symmetrical about MOR

Magnetic Anomalies

Ocean crust records magnetic reversals

Age of Seafloor Crust Fig. 20.11 R. Dietmar Muller, 1997

More Proofs Hot Spots - deep, long-lived, stationary mantle magma sources Expressed at the surface by: linear chain of volcanoes aged with distance from hot spot Over 100 identified Used as fixed points against which plate motion is measured

Hawaiian-Emperor chain

Long-lived global hot spots

PUTTING IT ALL TOGETHER

I. Plates Group of rocks all moving in the same direction Can have both oceanic and continental crust or just one kind Plate interior are relatively quiet Most activity is located at plate margins

II. Types of plate boundaries • divergent: mid-ocean ridges • convergent: collision zones volcanic arcs • strike-slip: San Andreas Fault Alpine Fault, N.Z.

A. Divergent plate boundaries Usually begin within continents as hot spot burns thru crust and eventually grows to become ocean basin Two kinds of divergent margins Mid Ocean Ridges (MOR) Continental Rift Valleys

Features of Mid Ocean Ridges • Central rift valley (width is inversely proportional to the rate of spreading) • Region of high heat flow Shallow-focus earthquakes • Almost exclusively basalt volcanism

Earthquakes Associated with Divergent Margins

Rifting and Seafloor Spreading Along the Mid-Atlantic Ridge Peter W. Sloss, NOAA-NESDIS-NGDC

Rifting and Seafloor Spreading

Features of Continental Rifts • East Africa, Rio Grande rift • Beginning of ocean formation (may not get that far) • Rifting begins at a triple junction (two spreading centers get together to form ocean basin, one left behind). • Rock types: basalt and sandstone

Hot Spot induced rifting Hot spot burns thru crust -> 3 branched rift Divergence begins 2 branches are active 1 branch is not active Linear sea develops

Inception of Rifting Along the East African Rift System Peter W. Sloss, NOAA-NESDIS-NGDC

Inception of Rifting Within a Continent

Gulf of ‘Aqaba Gulf of Suez Nile Delta Gulf of ‘Aqaba Gulf of Suez Red Sea Earth Satellite Corp.

The Gulf of California Formed by Rifting of Baja California from Mainland Mexico Worldsat International/Photo Researchers

B. Convergent boundaries New crust created at MOR—old crust destroyed (recycled) at subduction zones 2 kinds: subduction & collision Relative important densities: continental crust ≈ 2.8 g/cm3 oceanic crust ≈ 3.2 g/cm3 asthenosphere ≈ 3.3 g/cm3

Convergent boundaries Three types: ocean–ocean Philippines ocean–continent Andes continent–continent Himalaya

Ocean–Ocean Island arcs: Chain of volcanic islands • Highly seismic tectonic belt of shallow to deep earthquakes • High heat flow arc of active andesitic volcanoes • Bordered by a submarine trench

Ocean–Ocean Subduction Zone

Ocean–Continent Continental arcs: • Magmatic belt of active volcanoes (andesite to rhyolite) • Often accompanied by compression of upper crust which builds mountains Bordered by a submarine trench

Ocean-Continent Subduction Zone

Continent–Continent Continent–continent boundaries, convergence is accommodated by • Folding (shortening and thickening) • Strike-slip faulting • Underthrusting (intracontinental subduction)

Continent–Continent Convergent Boundary

Continent-Continent Collision

Himalayas and Tibetan Plateau • Product of the collision between India and Asia. • Collision began about 45 M yr. ago, continues today. • Before collision, southern Asia looked something like the Andes do today.

Stages in the collision of India with Asia

C. Transform Boundary Offsetting Spreading Centers

III. Rates of plate motion Mostly obtained from magnetic anomalies on seafloor Slow spreading: 3 cm/year Fast spreading: 10 cm/year Very fast spreading: 17cm/year

Relative Velocity and Direction of Plate Movement Data from C. Demets, R.G> Gordon, D.F. Argus, and S. Sten, Model Nuvel-1, 1990

IV. Rock assemblages and plate tectonics Each plate tectonic environment produces a distinctive group of rocks. By studying the rock record of an area, we can understand the tectonic history of the region.

Layer of ophiolite suite.

Precambrian Ophiolite Suite Pillow basalt M. St. Onge/Geological Survey of Canada

V. Exotic or Microplate Terranes Large blocks that contrast sharply with surrounding area Wrong faulting, folding, fossils, rock types, metamorphism, magnatism Thought to be fragments of continents, seafloor, seamounts, island arcs that rafted in & docked in new place

Approaching Arc or Microcontinent

Collision

Accreted Microplate Terrane

Microplate terranes Added to Western North America Over the Past 200 Million Years After Hutchinson, 1992-1993

VI. Driving mechanism of plate tectonics Thought to be convection of the mantle. Friction at base of the lithosphere transfers energy from the asthenosphere to the lithosphere. Convection may have overturned asthenosphere 4–6 times.

Other factors • Trench pull Plate sliding • Ridge push

Three possible driving factors

Three possible mechanisms for the movement of lithosphere over the asthenosphere Fig. 17.17

VII. Tectonic reconstructions A variety of evidence traces the motion of continents over time: Paleomagnetism Deformational structures Environments of deposition Fossils Distribution of volcanoes

Assembly of Pangaea I.W.D. Dalziel, 1995

Breakup of Pangaea 200 million years ago After Dietz & Holden, 1970

Breakup of Pangaea 140 million years ago After Dietz & Holden, 1970

Breakup of Pangaea 65 million years ago After Dietz & Holden, 1970

Breakup of Pangaea Today After Dietz & Holden, 1970

Examining Deep-sea Drill Cores Texas A&M University

Questions about plate tectonics • What do we really know about convection cells in the mantle? • Why are some continents completely surrounded by spreading centers? • Why are tectonics in continental crust and oceanic crust so different?

Cross Section of Western Canada

Fig. 20.25a

Formation of Magnetic Anomalies Fig. 20.10

Himalayas and Tibetan Plateau Models • Underthrusting • Distributed shortening •Strike-slip faulting

Wilson cycle Plate tectonics repeats itself: rifting, sea- floor spreading, subduction, collision, rifting, … Plate tectonics (or something like it) seems to have been active since the beginning of Earth’s history.

After Hutchinson, 1992-1993 Fig. 20.22

Examples of Plate Boundaries O-C convergent O-O divergent C-C divergent O-O divergent O-O convergent O-O divergent O-C convergent Fig. 20.8a,b

Volcanic and Nonmarine sediments are deposited in rift valleys Fig. 20.17a

Cooling and subsidence of rifted margin allows sediments to be deposited Fig. 20.17b

Carbonate platform develops Fig. 20.17c

Continental margin continues to grow supplied from erosion of the continent Fig. 20.17d

Ocean–Continent Convergent Boundaries

Opening of the Atlantic by Plate Motion After Phillips & Forsyth, 1972 Fig. 20.13

Idealized Ophiolite Suite - Oc. Crust Deep-sea sediments Pillow basalt Gabbro Peridotite

Model for Forming Oceanic Crust at Mid-ocean Ridges

The growth of oceanic basin 1

The growth of oceanic basin 2

The growth of oceanic basin 3

Age of the Ocean Basins After map by Sclater & Meinke

Parts of an Ocean–Ocean Convergent Plate Boundary Fig. 20.18

Parts of an Ocean–Continent Convergent Plate Boundary Fig. 20.19

Continued Subduction Fig. 20.20a

Continent– Continent Collision Fig. 20.20b