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8. Ocean Crustal Structure and Seismic Reflection William Wilcock

OCEAN/ESS 410. 8. Ocean Crustal Structure and Seismic Reflection William Wilcock. Lecture/Lab Learning Goals. Know the basic structure of the oceanic crust as determined from ophiolite studies Understand how oceanic crustal structure is linked to mantle melting and volcanic processes

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8. Ocean Crustal Structure and Seismic Reflection William Wilcock

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  1. OCEAN/ESS 410 8. Ocean Crustal Structureand Seismic ReflectionWilliam Wilcock

  2. Lecture/Lab Learning Goals • Know the basic structure of the oceanic crust as determined from ophiolite studies • Understand how oceanic crustal structure is linked to mantle melting and volcanic processes • Understand what seismic impedance is and how it controls the amplitude of seismic • Know how seismic reflection data is collected • Know how reflection data is stacked and converted into a seismic record section • What can seismic record sections tell you? - LAB

  3. Melting beneath mid-ocean ridges The melt rises out of the mantle because it is buoyant (less dense than the mantle)

  4. Ophiolites

  5. What will happen when the Juan de Fuca Ridge collides with the North American Plate? 30

  6. Ophiolites around the world

  7. Ophiolites - Remnants of oceanic crust • The spreading center collides with a subduction zone. • As the two plates converge the spreading center is thrust on top of the downgoing plate to form the ophiolite. Oman

  8. Ophiolite Sequence

  9. Mantle

  10. Crust-Mantle Interface (“Moho”)

  11. Gabbro layer

  12. 0 Depth, km 3 6

  13. Gabbros (layered) - they can also be un-layered (massive)

  14. Sheeted dike layer • “feeder channels” for basaltic lava rising from below • The cracks extend over large distances beneath ridges

  15. Sheeted dike layer

  16. Sheeted dike layer

  17. Pillow basalt layer

  18. Pillow lavas

  19. Pillow basalt layer • Lavas extruded onto ocean floor at a spreading center

  20. Sediment layer Pelagic sediments deposited above oceanic crust basalts

  21. Seismic Reflection

  22. Reflections from Interfaces When a downgoing P-wave meets an interface, a portion of the wave is reflected.

  23. Acoustic (Characteristic) impedance • An inherent property of the medium • Depends on sound speed (or seismic velocity) and density • Pressure generated by vibration of molecules of a particular medium at a given frequency

  24. Amplitudes of Reflections for vertical rays The amplitude of the reflected and transmitted phase depends on the seismic velocity, V and the density, ρ in each layer. Larger contrasts in impedance result in large amplitude reflections A0 V1, 1 V2, 2

  25. Marine Reflection Seismology - Airgun Sources Reflection data is relatively easy to acquire in the oceans. Seismic sounds (shots) can be generated with arrays compressed air guns (airguns) towed behind the ship

  26. Marine Reflection Seismology - Hydrophone Streamers The airgun shots are recorded by arrays of hydrophones towed behind the ship in a streamer. The seismic streamers contain 1000’s of hydrophones and can be >10 km long. A modern 3-D seismic ship will tow several (the records is 20) streamers.

  27. Marine Reflection Seismology - Geometry The streamer records waves reflected from interfaces

  28. Marine Reflection Seismology - Geometry The streamer records waves reflected from interfaces

  29. Marine Reflection Seismology - Data The seismic data recorded for a particular shot will look display a geometric effect termed “normal moveout” (NMO) which reflects the increased distance the wave travels for as the source-receiver offset increases 0 Offset X Time, s Time

  30. Marine Reflection Seismology - Sorting Records The records are sorted so that they all have the same mid-point (Common Mid-Point - CMP)

  31. Marine Reflection Seismology - Airgun Sources The seismic records can be corrected for geometric affects and stacked (summed) to produce a single record for the reflections below each each point Before Geometric Correction After Geometric Correction Stacked (summed)

  32. Marine Reflection Seismology - Filled Wiggle Plots Stacked records are plotted on the same plot with the horizontal axis showing position along the profile. Rather than showing lines for each record the plots often show filled regions for positive (or negative) displacements Time, s Position

  33. A reflection profile across the East Pacific Rise Reflections come from the seafloor, the base of layer 2A (pillow basalts), the axial magma chamber (AMC) and the Moho (M)

  34. Intersecting Record Sections from the East Pacific Rise

  35. 0 Structure of a mid-ocean ridge crust Depth, km 3 6

  36. 2A molten Layered Gabbro Moho Peridotite (Mantle)

  37. Endeavour segment of Juan de Fuca Ridge TWTT (s) 2A/2B AMC

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