Oceanic Crust and Rocks Appendices

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Oceanic Crust and Rocks(Appendices)

• JFM505E Physics and Structure of the Earths
  Crust

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Comparision of Characteristics of Typical
Refraction and Reflection Techniques
Comparision of Characteristics of Typical
Refraction and Reflection Techniques for Deep
Crustal Structure Studies (Braile and Chiang,
1986)
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Summary of Advantages and Limitations of
Refraction and Reflection Techniques for Deep
Crustal Structure Studies (Braile and Chiang,
1986)
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SEISMIC RESOLUTION

• Vertical Resolution
• Horizontal Resolution
• Vertical Resolution
• For 2 reflections (one from the top and one from
  the bottom of a thin layer),
• there is a limit on how close they can be, yet
  still be separable (Yilmaz,
• 1989).
• The dominant wavelength of seismic waves is given
  by
• ?v / f,
• where v is velocity and f is the dominant
  frequency.
• The acceptable threshold for vertical resolution
  generally is a quarter of
• the dominant wavelength.

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• (Yilmaz, 1989)

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(Yilmaz, 1989)
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Lateral Resolution

• Lateral resolution refers to how close two
  reflecting points can be
• situated horizontally, yet be recognized as two
  separate points rather
• than one (Yilmaz, 1989).

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• The total energy arriving within the time
  interval (t1-t0), which equals half the dominant
  period (T/2), interferes constructively.
• AA is called a half-wavelength Fresnel zone
  (Hilterman, 1982) or the first Fresnel zone
  (Sheriff, 1984). Two reflecting points that fall
  within this zone generally are considered
  indistinguishable as observed from the Earths
  surface.
• The Fresnel zone width is a measure of lateral
  resolution.
• The Fresnel zone radius (rOA)
• r?(z? / 2)1/2(v / 2) (t / f)1/2
• t(2z / v)

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(Yilmaz, 1989)
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Layers of Oceanic Crust
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• Layer 1 Vp 1.6-2.5 km/s, Thickness 0.4 km,
  Rocks Sediment
• Layer 2 Vp 3.4-6.2 km/s, Thickness 1.4 km
• Layer 2A Vp 3.5 km/s, Rocks Fractured
  Basalt
• Layer 2B Vp 5.2 km/s, Rocks Massive Basalt
  with Dykes
• Layer 2C Vp 6.1 km/s, Rocks Dykes with
  Massive Basalt
• Layer 3 Vp 6.4-7.0 km/s, Thickness 5 km
• Layer 3A Vp 6.8 km/s, Rocks Metagabbros and
  gabbros with pockets of plagiogranite and
  protrusions of serpentine
• Layer 3B Vp 7.3 km/s, Rocks Gabbros and
  metagabbros with serpentinite protrusions and
  pockets of cumulate ultramafic
• Moho
• Upper Mantle Vp 7.4-8.6 km/s
• Rocks Serpentinized ultramafics lt 6.9 km/s
• Anomalous mantle (close to ridge axis) 7.8
  km/s
• Harzburgite and lherzolite 8.1 km/s
• (Kearey and Vine, 1992, p20)

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(Ehlers and Blatt, 1982)
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(Hamblin and Christiansen, 1998)
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(Hamblin and Christiansen, 1998)
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Ophiolites

• Description
• The term ophiolite refers to a distinctive rock
  assemblage containing ultramafic, gabbroic and
  basaltic rocks, often capped by layers of deep
  sea sediments.
• Locations
• Well described ophiolite suites are located Bay
  of Islands in Newfound land, Troodos in Cyprus,
  Semail in Oman and eastern Papua in New Guinea.
• Process (Obduction)
• Initial stage of collision, the oceanic plate is
  thrust over the continental plate.

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Possible mechanisms for obduction of ophiolite
sheets onto continental margin (Ehlers and Blatt,
1982)
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Ophiolitic rocks in Turkey (Piril, 2003)

• Within the Tethyan belt, Turkey is well known for
  extensive areas of
• ophiolitic rocks in the mountains of the Alpine
  chain.
• In NW Turkey, a complete ophiolite sequence has
  not been found.
• Izmir-Ankara-Erzincan suture zone (ophiolitic
  zone ) formed during Early
• Tertiary continental collision following
  northward subduction of Tethyan
• oceanic lithosphere
• The Izmir-Ankara-Erzincan suture zone separates
  the Pontides from the
• Anatolides-Taurides.
• It is the boundary between Gondwana and Laurasia,
  and two separate
• Oceanic lithospheres, Palaeo-Tethys and
  Neo-Tethys, have been
• consumed along the suture
• In contrast, the Tauride ophiolites in the south
  are larger and more
• complete, of mid- to late Cretaceous age.

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Tectonic map of the Eastern Mediterranean
(Robertson, 2000)
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Plate tectonic sketches showing the simlified
evolution of southern Turkey a) Late Triassic, b)
Late Cretaceous, c) Late Eocene (Robertson, 2002)
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Ophiolitic rocks in the Island of Cyprus

• The Troodos ophiloite in the Island of Cyprus
  formed at a spreading center
• above a subduction zone.

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Map of the Troodos Ophiolite (Staudigel et al.,
1999)
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• The Troodos ophiolite belongs to the
  mid-Cretaceous,
• and formed 91 my ago.
• A belt of ophiolites of this age stretches far to
  the east,
• ending with the well known Samail ophiolite in
  Oman.
• Uplift of Troodos to its present position took
  place
• episodically, but was initiated 20 My ago.

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Ophiolites in the Island of Cyprus. The vertical
slabs of rock are dikes intruding into lavas that
erupted on the seafloor. The transition from
lavas to sheeted dikes (Layer 2B)
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Ophiolitic Rocks in Oman

• http//www.angelfire.com/ms/snasir/page14.html

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• Ophiolites are very common in the Middle East.
  They occur in elongate belts that make up an
  integral part of the Alpine mountain chains.
• These ophiolites extend eastward and southward
  from Cyprus into Syira, the Turkey-Iran boerder
  fold belt, through Neyriz in Iran , then across
  the Arabian Gulf into Oman. The Semail ophiolite
  , Sultanate of Oman, is part of these ophiolits.
• It provides the best exposure in the world to
  study oceanic lithosphere.
• The Semail ophiolite crops out in a belt 600 km
  long and 150 km wide and between 5 and 10 km
  thick.
• The Sultanate of Oman forms the southeastern
  corner of the Arabian Plate (http//www.angelfire.
  com/ms/snasir/page14.html ).

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(Hamblin and Christiansen, 1998)
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(Hamblin and Christiansen, 1998)
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(Hamblin and Christiansen, 1998)
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(Hamblin and Christiansen, 1998)
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(Hamblin and Christiansen, 1998)
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(Hamblin and Christiansen, 1998)
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(Hamblin and Christiansen, 1998)