Ocean floor: Technological progress has enabled a better mapping of the

1. Ocean floor: Technological progress has enabled a better mapping of the seafloor: direct observations and sampling, bathymetry, internal architecture, structure and geophysical properties The discovery made ca 40-50 years ago showing that oceanic lithosphere is generated by the magmatic and tectonic activity along the Mid-Ocean-Ridges is one of the most central elements of Plate-Tectonics. The Mid-Ocean-Ridges have a length of ca 60 000 km; they are a formidable system with major importance for the driving force of the plates and in the enery-budget of the plate-tectonics.

2. Mid-Ocean-Ridges Have these characteristics: TOPOGRAPHY (≈1000 km) broad ridges with narrow central rifts. BASALT VOLCANISITY mostly tholeitic composition 3. HIGH HEAT-FLOW 4. NEGATIVE GRAVITY ANOMALIES (melts) SEISMIC ACTIVITY (mainly shallow earthquakes) 6. MAGNETIC ANOMALIES oriented parallel with the ridges

3. Observations show that the width of a spreading ridge is proportional with the spreading velocity, illustrated below (NB scale: h/v = 1/60) A spreading centre comprises a rift-valley between normal faults. The rift is often sharply defined as a narrow (10-30km) zone. The lithosphere is at its thinnest above such a rift over en slik rift, and in many models, the astenosphere is considered to reach the seafloor. The crust and lithosphere thicken away from the rift. This is compensated by Isostasy and the crust uplifted in the rift-shoulders.

4. TOPOGRAPHY and gravity along mid-ocean-ridges provide important information about processes which are responsible for their formation. The high topography is a result of thermal expansion and lower density (). i.e. a mass-deficiency/volume which is isostatically compensated by the topography (mid-ocean-ridges ”float” high). Curve-fitting or empirical measurements show that the ocean depth (D) is: D = a√t + d0 t: ocean floor age in million years; d0:water depth at the spreading ridge (≈ 2.5 km) a: constant = 0.336 NB This equation is purely empirical and says nothing about the processes.

5. DEPTH: D = a√t + d0 t - age; depth- d0≈ 2.5 km); constant- a = 0.336 What is the water depth above 16 myr old ocean floor? D = 0.336√16 + 2.5km = 3.8 km The graph shows depth of the ocean as a function of age out from a ridge. Empirical studies show that the subsidence follows another curve: D = 6.4 - 3,2e-t/62,8when the ocean floor gets older than 70 myr. Again this is pure curve-fitting and not quantified from any process. So how Can this be done quantified using the physical laws? For t < 70 mill år --> D = a√t + d0 For t > 70 mill år --> D = 6.4 - 3,2e-t/62,8 Havdyp Alder

6. Depth of oceans: (z), coeff. of thermal expansion= 3 x 10-5K-1 w- water depth - density or mantle(m) (3.2) water (w) t- time T- temperature, T1=1280oC, Ts=0oC - thermal diffusivity 1) Density as function of temperature 2) Column A at compensation 3) Column B at compensation 4) (2), (3) and isostasy, see Stüwe p157 5) (4) first term after =, finds derivative with respect to z and wrights into integral and it gets form which says that water depth depends on the density sturcture as a function of depth 6) Inserting (1) into (5) where T(z) is the unknown (determined from heat conduction equation see Stüwe p 96) 7) Inserting heat conduction equation in (6), which simplifies to: 8) and to : 9) After taking constanst out of the integrall. If we introduce the constant n in (10) 10) We can take all the constants out to the integral and get: 11) Integral of the errorfunction is not know the 0 and z but is know for integration with Limit infinity, it is: 12) Substutuing this integral into (11) we get an expression for the water depth : NB! + w at the spreading ridge 13) Which after inserting standard values for all the constanst give : 14) The water depth in oceans is proportional to the square root of the age and 5.91 times 10-5

7. Mid-Ocean-Ridges Have these characteristics: TOPOGRAPHY (≈1000 km) broad ridges with narrow central rifts. TOPOGRAPHY (≈1000 km) broad ridges with narrow central rifts. BASALT VOLCANISITY mostly tholeitic composition BASALT VOLCANISITY mostly tholeitic composition 3. HIGH HEAT-FLOW 3. HIGH HEAT-FLOW 4. NEGATIVE GRAVITY ANOMALIES (melts) SEISMIC ACTIVITY (mainly shallow earthquakes) 6. MAGNETIC ANOMALIES oriented parallel with the ridges

8. ELEVATED HEAT-FLOW It is very hard to obtain accurate heat-flow measurements! Heat-flow is given in heat-flow-units defined as milliwatt/m2. A simplified empirical formula for heat flow (Q) is: Q = 473 t-1/2 t - age, t > 120 Ma Q = 33.5 + 67e-t/62.8 Also a purely empirical expression, how can we quantify ? Q= k(T1-Ts)[x]1/2 k-conductivity; T-temperatures, u rifting rate;  diffusivity; x-distance

9. HEAT-FLOW: Q = -k[(T+dT-T)/dz] = -k dT/dz (rate of flow pr unit area up through plate) where k - thermal conductivity (Wm-1oC-1) T- temp (T (z + dz) > Tz z - thickness of plate z + dz T + dT a k - thermal conductivity (Wm-1oC-1) z T Flow of heat Consider a small volume of height dz and cross-section “a” Change in temperature dT in time dt depends Flow og heat across the surface (net heat-flow in or out) Heat generated in the volume Thermal capasity (spesific heat) of the material One dimensional heat conduction equation: r - density, cP - spesific heat, A - heat production pr unit time Temp is assumed to be function of time and depth only, can be expanded to 3-d

10. 3-dimentional heat conduction equation: or [using differential operator notation (Laplacianoperator)] Also considering movement of small volume of material with velocity uz conduction term; production term, advection term

11. NEGATIV GRAVITY ANOMALI (melts) The free air anomaly: gf = gobs- glat - gh where: glat = g(l)= geq(1 + asin2l +bsin4l) gh ≈ g0(1 - 2h/R) l = latitude The Bouger anomaly: gb = gf - dgb + dgt = gobs- glat + gh- dgb + dgter Where: dgb = 2pGrh (bouger correction) G - gravitational constant (6,673 x 10-11 m3kg-1s-2) dgt - terrain correction (deviations from horizontal) h - height r - density

12. NEGATIV GRAVITY ANOMALI (melts)

13. Mid-Ocean-Ridges Have these characteristics: TOPOGRAPHY (≈1000 km) broad ridges with narrow central rifts. TOPOGRAPHY (≈1000 km) broad ridges with narrow central rifts. BASALT VOLCANISITY mostly tholeitic composition 3. HIGH HEAT-FLOW 4. NEGATIVE GRAVITY ANOMALIES (melts) SEISMIC ACTIVITY (mainly shallow earthquakes) SEISMIC ACTIVITY (mainly shallow earthquakes) 6. MAGNETIC ANOMALIES oriented parallel with the ridges

14. SEISMIC ACTIVITY (mainly shallow earthquakes) Earthquakes last week Jan- first week Feb, 2004 Fast spreading East-Pasific Rise Intermediate spreading rate Mid-Atlantic Ridge and Southeast Indian Ridge

15. Earthquakes along the Mid-Atlantic Ridge near the Azores

16. Mid-Ocean-Ridges Have these characteristics: TOPOGRAPHY (≈1000 km) broad ridges with narrow central rifts. TOPOGRAPHY (≈1000 km) broad ridges with narrow central rifts. BASALT VOLCANISITY mostly tholeitic composition 3. HIGH HEAT-FLOW 4. NEGATIVE GRAVITY ANOMALIES (melts) SEISMIC ACTIVITY (mainly shallow earthquakes) 6. MAGNETIC ANOMALIES oriented parallel with the ridges 6. MAGNETIC ANOMALIES oriented parallel with the ridges

17. MAGNETIC ANOMALIES We know that the earth is a magnetic dipol, with magnetic north and south. The magnetic field varies in both intensity and orientation, but over time (105yr) the magnetic poles coinside with the rotation poles i.e. geographical north and south poles. Consequently the magnetic Field is vertical near the poles and horisontal near equator!

18. Reversals of the magentic field leads to periods of normal (present) and reverse magnetisation During seafloor- Spreading, the newly formed crust will function as a magnet tape-recorder where the alternating Normal and reverse Magnetizations will be preserved as intensity variations

19. The reversals produces periods of normal and (present) and reverse magnetization, which is preserved in the geo-record Reversals may also be studied on land in volcanic or sedimentary rocks. The reversals may be calibrated against mot stratigraphy and radiometric age-determinations; and magnetostratigraphy, is a dating method if the anomaly-sequence may be identified.

20. The figure shows the theoretical distribution of anomalies in a spreading ridge where the introduction of new magnetic material occur in a zone with width from 0 til 10 km. Even in a relatively broad volcanic zone there is an identifiable magnetic anomaly-pattern The magnetic anomalies are among the best evidence for seafloor spreading. It is hard to explain this pattern in other ways, and there is no other physio-chemical process than reversal that can explain the change in polarity.

21. The tectono-magmatic processes along spreading ridges gives a relatively uniform architecture of the oceanic lithosphere in time and space. Supra-custals, (basalts and sediments) Sheeted dyke complex Isotropic varied textured gabbros Layered gabbros Ultramafic cumulates Ultramafic mantle tectonites Magma composition: Tholteitic MORB (Mid-Ocean-Ridge-Basalt) Formed by relatively high degree of partial melting at relatively shallow level in the astenosphere

22. Intra-oceanic suspect/exotic terranes: Modern oceans contains large “anomalies” which have a different origin than spreading at ridges. These include: 1) Pieces of continents 2) Oceanic islands 3) Hot-spot traces and islands 4) Arc and back-arc compexes 5) Transform complexes Such terranes may end up inside suture zones of orogenic belts, in which case they represent suspect and/or exotic elements of the mountain belt. SUSPECT TERRANES: TECTONOSTRATIGRAPHIC TERRANES THAT HAVE UNSETTLED AFFINITY/ ORIGIN WITH RESPECT TO THE CONTINENT WHERE IT ENDS UP AFTER AN OROGENY EXOTIC TERRANES: TECTONOSTRATIGRAPHIC TERRANES THAT HAVE OUTBOARD ORIGIN WITH RESPECT TO THE CONTINENT WHERE IT ENDS UP AFTER AN OROGENY. EXAMPLES OPHIOLITES AND ISLAND ARC COMPLEXES, CONTINETAL FRAGMENTS WITH AN ORIGIN IN ANOTHER CONTINENT

23. Reconstruction of former plate motions. Based on magnetic anomaly-patterns in the oceans it is possible to reconstruct the face of the earth back in time for the period we have oceanic lithospere preserved We can see that the oldest ocean floor is ca ≈180Ma, how Can we reconstruct plate-motions before mid-Jurassic time? 180 155 130

24. Age of ocean floor,with plate-reconstructions for the past 130 Ma. Notice that the reconstruction also shows oceanic lithosphere that was destroyed during this time-span (from: www.geodynamic.no)