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GSC307 Introduction to Global Geophysics GSC307 Introduction to Global Geophysics Plate Tectonics at Work Today • Lecture: – Plate Boundaries and Vector Diagrams • Recommended reading: – Lowry 1.2 • Bring to Lab on Thursday: ruler, protractor, calculator, graph paper 4/14/15 Cal Poly Pomona GSC307 Introduction to Global Geophysics 4/14/15 Cal Poly Pomona GSC307 Introduction to Global Geophysics Three Types of Plate Boundaries Constructive PB: Vector Diagram 1. Divergent/ Constructive 2. Convergent/ Destructive Constructive (ridge) – Double line = ridge axis – Numbers in (a) = half spreading rate (will be explicitly stated otherwise) BvA = (vector) velocity of plate A with respect to plate B = -AvB 3. Transform/ Conservative 4/14/15 Cal Poly Pomona 4/14/15 Cal Poly Pomona 1 GSC307 Introduction to Global Geophysics GSC307 Introduction to Global Geophysics Constructive PB: Cartoon Constructive: • Pull-apart motion • Rocks fail in tension • Mainly small earthquakes (6 or greater is rare) and shallow (a few kms) 4/14/15 Constructive PB Features • For example: midocean ridge – Generates new ocean floor – Bathymetric high • Lithosphere spreads away from ridge, cools, contracts and sinks Cal Poly Pomona GSC307 Introduction to Global Geophysics 4/14/15 Cal Poly Pomona GSC307 Introduction to Global Geophysics Continental Rifting Iceland sits atop a divergent plate boundary where continental rifting occurs 4/14/15 Cal Poly Pomona 4/14/15 Cal Poly Pomona 2 GSC307 Introduction to Global Geophysics GSC307 Introduction to Global Geophysics Destructive PB: Cartoon Destructive PB: Vector Diagram Destructive (subduction zone) – Barbed (or hashed) line: barbs on side of overriding plate, pointing away from downgoing plate BvA Destructive: • Subduction zones or collisions • Greatest earthquakes, can be deep or shallow (down to about 700 km depth) = (vector) velocity of plate A with respect to plate B = -AvB 4/14/15 Cal Poly Pomona 4/14/15 GSC307 Introduction to Global Geophysics Cal Poly Pomona GSC307 Introduction to Global Geophysics Destructive PB Features • Denser plate sinks • May sink to great depths (CMB?) • Formation of continental or island arc in case of subduction • Deep trench a. Ocean-continent c. Continent-continent Subduction Zone Seismicity Earthquake locations define descending slab of oceanic lithosphere A B C D ,/7342%.'4( ).4%2-%$)!4%342%.'4( ()'(342%.'4( b. Ocean-ocean 4/14/15 Cal Poly Pomona 4/14/15 Cal Poly Pomona 3 GSC307 Introduction to Global Geophysics GSC307 Introduction to Global Geophysics Tonga Subducting Slab Conservative PB: Vector Diagram Conservative (transform fault) – Single line = transform fault BvA 4/14/15 Cal Poly Pomona GSC307 Introduction to Global Geophysics = (vector) velocity of plate A with respect to plate B = -AvB 4/14/15 Cal Poly Pomona GSC307 Introduction to Global Geophysics Conservative PB: Cartoon Conservative PB Features Conservative: • Slide-past, horizontal, motions • Large earthquakes • Usually connect ridge segments and represent most seismically active parts of ridge system • Transform faults often connect ridge segments • Away from ridge they extend as fracture zones 4/14/15 Cal Poly Pomona 4/14/15 Cal Poly Pomona 4 GSC307 Introduction to Global Geophysics GSC307 Introduction to Global Geophysics The Pacific Plate • Largest plate in the world • Created at spreading centers (E and S edges) • Long transform faults near western N America • Large subduction earthquakes (along N and W edges) https://youtu.be/MSbIKp571Rk 4/14/15 Cal Poly Pomona GSC307 Introduction to Global Geophysics 4/14/15 Cal Poly Pomona GSC307 Introduction to Global Geophysics Plate Motion Plotting Objects on a Velocity Line To better understand the past, present and future motion of plates, their relative motions are plotted on velocity diagrams, using vectors. 4/14/15 Cal Poly Pomona 4/14/15 Cal Poly Pomona 5 GSC307 Introduction to Global Geophysics GSC307 Introduction to Global Geophysics Plotting Objects on a Velocity Line Plate Motion in Velocity Space Plot BVG and GVB and B itself Relative motions of plates can be similarly described by plotting them in velocity space. Which of these three plate diagrams is inconsistent with the velocity space diagram above? 4/14/15 Cal Poly Pomona GSC307 Introduction to Global Geophysics 4/14/15 GSC307 Introduction to Global Geophysics Two Plates on a Flat Earth Western boundary of plate B is ridge that is spreading with half-rate of 2 cm/yr. What happens at other boundaries between B and A? 4/14/15 Cal Poly Pomona Cal Poly Pomona Three Plates on a Flat Earth Add third plate C, with plate A subducting under C at rate of 6 cm/yr. To determine relative rate of plate motion at B-C boundary, use vector addition: CvB = CvA + AvB Will B grow or disappear? 4/14/15 Cal Poly Pomona 6