Presentation on theme: "Geology of the Lithosphere 3. Formation and Destruction of Oceanic Lithosphere What is the structure of the oceanic lithosphere? What evidence supports."— Presentation transcript:
Geology of the Lithosphere 3. Formation and Destruction of Oceanic Lithosphere What is the structure of the oceanic lithosphere? What evidence supports a layered internal structure of the oceanic lithosphere? How does oceanic lithosphere form? How can the rates and directions of sea-floor spreading be calculated? Where & how is the oceanic lithosphere reabsorbed into the mantle? How do ocean basins evolve?
What is the structure of the oceanic lithosphere? Sediments Basaltic lavas Sheeted dykes Gabbro Peridotite Layer 1 Layer 2 Layer 3 Layer 4 Moho
a). Describe the composition and layered structure of the ocean lithosphere. b). Discuss the extent to which ophiolites show evidence of this composition and layered structure. (25) Geology of the Lithosphere LayerCompositionRock TypeDetails 1 Sediments Black shales Volcanic tuff Siltstones Sandstones 0-10km thick. Increases with distance from MOR. Very little over MOR & very thick near continental areas. MOR – pelagic sediments, wind blown ash. Continental edges – clastic material from rivers & turbidity currents. 2a MaficBasalt Up to ½km thick Pillow lavas (each one between 10-100cm diameter) Rounded, bulbous top, with pinched bases. Rapid cooling by seawater. 2b MaficDolerite Up to 1km thick Sheeted dykes (each one ½m to 1m wide) Highly fractured 3 MaficGabbro 5km thick Coarse-grained – slow cooling Magma chamber 4 UltramaficPeridotite Upper mantle – lithosphere Olivine-rich
What is the evidence for the structure of the oceanic lithosphere? LayerReason for Velocity Change 1 Increases as sediments become more consolidated into rock. 2 Large increase due to change from fragmental sedimentary rocks to crystalline igneous rocks. Increases through Layer 2 due to fewer fractures and vesicles with depth. 3 Gradual increase in velocity due to fewer fractures and less water in fractures. 4 Large increase where gabbro rests on peridotite – more rigid & less compressibe.
What is the evidence for the structure of the oceanic lithosphere? 1960’s - Deep Sea Drilling Project (DSDP) Glomar Challenger
What is the evidence for the structure of the oceanic lithosphere? 1985 - Ocean Drilling Program (ODP) JOIDES Resolution
What is the evidence for the structure of the oceanic lithosphere? Manned & unmanned submersiblese.g. Alvin
What is the evidence for the structure of the oceanic lithosphere? Ophiolites or Ophiolite Complexes
b). Discuss the extent to which ophiolites show evidence of this composition and layered structure.(25) Geology of the Lithosphere StrengthsWeaknesses Ophiolite layered sequence is similar in structure to that inferred for oceanic lithosphere from seismic and drilling studies of the ocean basins. Assumption they are old oceanic lithosphere, so may not represent true oceanic lithosphere. Most likely to have formed in back-arc settings. Similar rock types in ophiolites and modern ocean crust. Thinner accumulations of igneous rock are found in ophiolites than are inferred from studies of oceanic crust. Seismic velocities measured in rock samples from the upper parts (Layers 1,2 & 3) of the ophiolite sequence are similar than in the corresponding parts of oceanic lithosphere. Ophiolites with compositions comparable with mid-oceanic ridge basalt are rare Due to their long and arduous history (obduction) ophiolites have been affected by regional metamorphism as well as initial metamorphism due to circulation of hydrothermal fluids. Seismic velocities measured in rock samples from the bottom parts (Layer 4) of the ophiolite sequence are lower than in the corresponding parts of oceanic lithosphere.
How does the oceanic lithosphere form? Sea-floor spreading
Sea-Floor Spreading Oceanic lithosphere thins Oceanic lithosphere pulled apart Mobile asthenosphere rises to fill gap Partial melting of peridotite due to reduced pressure Slow cooling basaltic magma forms gabbro Fracturing of brittle oceanic lithosphere allows basaltic magma to be injected into crust Cooling & solidification forms sheeted dykes Any magma reaching the sea floor comes in contact with sea water and cools rapidly to form pillow lavas Sediment builds up over time
Linear magnetic “stripes” with abrupt changes from positive anomalies to negative anomalies Stripes extensive laterally – 100’s kms along ocean floor Pattern of anomalies displaced by distances of over 1000km at prominent fault-like zones Anomaly pattern appeared symmetrical across mid-oceanic ridges
Vine-Matthews Hypothesis Magnetite in rocks Earth’s magnetic field Palaeomagnetism Magnetic reversals Magnetic anomalies (patterns/stripes, extent, symmetry, effect of transform faults) Curie Point Relate to sea-floor spreading
5 million years 100 km 100 km in 5 My 20 km in 1 My 20 000 m in 1 000 000 years 2 000 000 cm in 1 000 000 years 20 000 000 mm in 1 000 000 years 20 mm in 1 year Lithosphere Asthenosphere Layer 1 Layer 2 Layer 3 Layer 4 Moho 20 Half spreading rate 20 40 mm in 1 year Full spreading rate
Age of oceanic crust (Ma) Depth (m) 020406080100120 1000 2000 3000 4000 5000 6000 Observed relationship between depth to ocean floor and age of oceanic crust Depth to ocean floor gives a rough age of oceanic crust If distance from MOR is known then a rate of plate motion can be calculated
~ 3000km ~ 2000km 3000km in 43 million years 3000 / 43 = 70mm / year 2000km in 35 million years 2000 / 35 = 57mm / year
Describe how the rate of seafloor spreading may be calculated. Evaluate the accuracy of any method(s) you describe. (25 marks) MethodAccuracy Constructive margin (half & full spreading rates) Rate = distance/time Distance measured from map/survey of magnetic stripes/GPS Time measured from age of sea-floor rock (basalt) using radiometric dating or magnetostratigraphy Figures/examples - Mid –Atlantic Ocean – 20mm/yr (half-spreading rate) & 40mm/yr (full spreading rate) Collection of specimens difficult on sea-floor, but use of magnetostratigraphy makes it easier. Radiometric dating +/-2% accuracy depending on method Distance very accurate with GPS, but hard to locate the exact ridge axis. Spreading may not be equal in both directions Hot spots Rate = distance/time (as above) Examples/figures – Hawaii (Emperor Chain) 70mm/yr Hot spots stationary? Use of sediment thickness & fossil contentCollection of specimens difficult on sea-floor
How do ocean basins evolve? J. Tuzo Wilson (Canadian Geologist) The Wilson Cycle is the generalised sequence of opening and closing of ocean basins. ~35 – 50 Ma 0 Ma ~150 Ma ~250-300 Ma ~350 Ma ~400 Ma
Stage 1: Embryonic Ocean East African rift valleys Crustal extension & uplift Rift valleys/normal faults Igneous activity associated with thinning of the lithosphere
2005 - European Space Agency ENVISAT satellite showed a huge rift, 37 miles long & 8 metres wide had opened up in the crust. Between 14 September and 4 October 2005, 163 earthquakes (magnitudes greater than 3.9) and a volcanic eruption occurred in the Afar region of the East African Rift valley
Stage 2: Young Ocean Red Sea/Gulf of California Subsidence & spreading No subduction Constructive plate margin formation Ocean crust development in rift & palaeomagnetic stripes High heat flows Narrow seas with parallel coasts & a central depression
Stage 4: Declining Ocean Pacific Ocean Spreading & shrinking Ocean basin with active spreading axes Subduction associated with marginal trenches Ocean ridge not central e.g. East Pacific Rise Rates of spreading fast due to slab pull
Oceanic lithosphere cools by conduction as it moves away from the oceanic ridge, leading to a thickening of oceanic lithosphere. Oceanic lithosphere is reabsorbed into the mantle by sinking at subduction zones. The subducting lithosphere however does not generally melt unless it is quite young (< 2-3 million years) and therefore hot. It is the dewatering of the slab which lowers the melting point of the overlying mantle wedge and causes partial melting of the asthenosphere. As the old, cold lithosphere descends into the asthenosphere less dense minerals such as water are lost increasing its density. So the older and colder the oceanic lithosphere the denser it will be and the faster it will subduct. Also at depth phase changes occur in the slab converting some minerals into denser varieties. At about 400km depth olivine transforms into a form in which the atoms are packed more closely, known as spinel, with a consequent increase in density of about 10%. There is further phase change at about 670km where the spinel structure transforms into an even-higher pressure form known as perovskite with another density increase of about 10%. These increases in density all increase slab pull forces, helping the oceanic lithosphere to subduct, as does the steepness of the descending plate (the steeper the subducting plate the quicker it subducts). These subducted slabs descend to 700km or sometimes all the way to the core-mantle boundary where they eventually melt and rise up again as mantle plumes to form hot spots.
Subduction Zones Orogenic Belts Cooling, thickening & becomes denser as it moves away from MOR Reabsorbed into mantle by subduction Does not melt unless young (<3 Ma) Dewatering of slab lowers MTP of mantle wedge causing it to partially melt Dehydration of slab cools it and increases its density 400km phase change olivine to spinel (10% denser) 670km phase change spinel to perovskite (10% denser) Slab pull forces increase