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Fifteen-Twenty Fracture Zone

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1 Fifteen-Twenty Fracture Zone
“MAGMA STARVED” region in the Mid-Atlantic ridge exposes the Earth’s Mantle

2 “Magma Starved”??? MAGMA = derived from partial melting of pre-existing rocks in Earth’s mantle or crust At Mid Ocean Ridges, plates diverge, solid magma decompresses, gets squeezed = ADIABATIC MELTING …leaving some elements behind and cooling to form BASALT

3 MORPHOLOGIES Slow-Spreading: Intermediate-Spreading Fast-spreading
0.6-2 cm/yr Deep rift valley Highly variable relief ( m) Intermediate-Spreading 5–7 cm/yr Long, alternating spreading rates Fast-spreading 8-18 cm/yr Low axial highs Minimal rift mountain topography

4 Slow-Spreading Much thinner oceanic crust
Absence of volcanic activity on long segments of the ridge Abundant mantle peridotite

5 Gakkel = Ultraslow, 6 – 1.7 mm/yr, lies beneath the Arctic Ocean
Southwest Indian Ridge = Very slow, mm/yr, lies beneath the southern oceans Mid-Atlantic Ridge = Slow, 25 mm/year, FTFZ

6 Fifteen-Twenty Fracture Zone 15º20N, 45ºW
FTFZ African North American

7 Is there a HOLE in the Oceanic crust??!!
Basalts which form the crust are thin/absent Mantle is NOT partially melting Slow-spreading Minor crust = Faults EXPOSED MANTLE MATERIAL Olivine + Seawater = Serpentine + Magnetite + H2 H2 + seawater carbonate = CH4

8 CRUISES: 1988 - “Ridelente” 1992 - “Faranaut” 1998 - “MODE 98, Leg 1”
“RRS James Clark Ross JR63” “ODP Leg 209” “IFREMER Pourquoi Pas” “RRS James Cook JC007”

9 UNDERLYING EVIDENCE Bathymetry Transform fault Rugged Smooth


11 - Abyssal hill topography
Gravity = Bouguer and mantle anomalies + Thin basaltic crust, Rugged topography Serpentinized peridotites Transform walls - Abyssal hill topography Shallower avg. topography Transform valley

12 B. Rocks Thin crust, rugged terrain
Within/near discontinuous ridge segments = SERPENTINIZED PERIDOTITES Equipment = Robotic rock drill Dredging

13 UNSOLVED MYSTERIES… Why is the mantle not melting in this area (low magma supply)? Mantle is too cold Mantle is infertile from previous partial melting Melt is trapped in deep axial lithosphere Why is the oceanic crust so thin? Basaltic crust was never formed Crust was removed by tectonic

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