Exhumed mantle is formed whenEarth's mantle rocks are exhumed byextensional tectonics such that they appear at the seabed. This occurs in two main settings, either duringseafloor spreading during the formation ofoceanic core complexes, or during therifting apart ofcontinental crust during break-up onnon-volcanic passive margins.
In normal rates of seafloor spreading, the space created by rifting along amid-ocean ridge is filled bymagma, forming the standardoceanic crust, with a centralmagma chamber, crystallising out asgabbros andutramaficcumulates, feeding volcanic rocks (typicallypillow lavas) via systems ofdykes. Such oceanic crust matches layers 2 nd 3 of the classicophiolite stratigraphy. Where spreading rates are intermediate or slow to ultraslow, magma does not necessarily reach the surface and extension involvesdetachment faulting in which mantle rocks in the fault footwall become exhumed at the seabed forming structures called "megamullions".[1]
Passive margins form during the break-up of existingcontinental masses as the product of progressiverifting. On some margins, there is very little magmatic activity during this process. On such non-volcanic passive margins, extension of the continental crust starts with fault block development within the upper part of the crust andductile thinning within the lower crust andlithospheric mantle. As the extension continues, the lower parts of the thinninglithosphere cool and start to be included in the brittle faulting, initially in the lower crust and finally in the lithospheric mantle. The final result of this is exemplified in the West Iberian margin, where a combination ofseismic reflection profiling and scientific drilling (ODP Legs 103, 149, and 173) have proved the presence ofserpentinized continental mantle immediately below post-riftsedimentary rocks, confirming that the mantle was exhumed at the seafloor by the time continental break-up was complete.[2]
Ophiolites within theAlpine orogenic belt are dominantly serpentinized peridotites with very little evidence ofpillow lavas andsheeted dykes and lessgabbro than would be expected if they represented slices of normal oceanic crust. Locally extensional detachments are exposed that juxtapose continental crustal rocks against serpentinized peridotite, while elsewhere lower continental crust is seen to thin and eventually to be cut out by detachment faults, a section interpreted as a preservedocean-continent transition.[2]